GEOLOGIJA 40, 11-64 (1997), Ljubljana 1998
Lithiotid Bivalves in Slovenia and Their Mode of Life
Litiotidne školjke v Sloveniji in njihov način življenja
Irena Debeljak
Ivan Rakovec Institute of Palaeontology, Scientific Research Centre, Slovenian Academy
of Sciences and Arts, Gosposka 13, SI-1000 Ljubljana, Slovenia
Stanko Buser
Geology Department, Faculty of Natural Sciences and Engineering, University of Ljubljana,
Aškerčeva 2, SI-1000 Ljubljana, Slovenia
Key words: Lower Jurassic Bivalvia, Pliensbachian, Lithiotis, Cochlearites, Li-
thioperna (syn. Lithiopedalion), morphological adaptations, muddy substratum. Bi-
narie Carbonate Platform, Slovenia
Ključne besede: spodnjejurske školjke, pliensbachij, Lithiotis, Cochlearites, Li-
thiopema (syn. Lithiopedalion), morfološke adaptacije, blatni substrat, Dinarska
karbonatna platforma, Slovenija
Abstract
Lithiotid bivalves are a characteristic faunal element of the shallow marine fad-
es of Lower Jurassic beds in southern Slovenia. The horizon containing Middle Li-
assic bivalves, which is up to 75 m thick, is called the "lithiotid horizon" and is at-
tributed to the Pliensbachian or Domerian.
In Slovenia the name lithiotid bivalves represents three morphologically similar
genera or species of sessile monomyarian dysodont bivalves: Lithiotis problematica,
Cochlearites loppianus and Lithiopema scutata, which are systematically examined
in this paper The term lithiotid bivalves does not have any taxonomic significance,
since they are now classified in different families. The order is Pterioida.
Lithiotid bivalves lived in an upright position on soft lagoonal bottoms in a tight
aggregate of individuals crowded together which mutually supported one another
and simultaneously competed for living space and light. The sedimentation of cal-
careous mud was fairly rapid, thus throughout their lives they grew constantly in a
subvertical direction so that the small soft body space at the ventral end remained
above the level of the surrounding substratum. Lithiotid bivalves had peculiar, vari-
able shells adapted to the specific environment. They are very large, flat and di-
stinctly dorso-ventrally elongated. Their inner surface is tripartite; in the middle is
the central area, and at the sides there are feather-like areas. The mechanism of
opening and closing their valves has not yet been fully explained.
Kratka vsebina
Litiotidne školjke so značilen favnistični element plitvomorskega razvoja spo-
dnjejurskih plasti južne Slovenije. Do 75 metrov debeli horizont s srednjeliasnimi
18_Irena Debeljak & Stanko Buser
školjkami imenujemo "litiotidni horizont" in ga uvrščamo v pliensbachij oz. dome-
ri] •
Pod imenom litiotidne školjke v Sloveniji združujemo tri morfološko podobne
rodove oziroma vrste sesilnih, monomiarnih, disodontnih školjk: Lithiotis proble-
matica, Cochlearites loppianus in Lithiopema scutata, ki so sistematično obdelane
v tem prispevku. Izraz litiotidne školjke nima taksonomskega pomena, saj jih danes
uvrščamo v različne družine. Spadajo v red Pterioida.
Litiotidne školjke so v pokončnem položaju živele na mehkem lagunskem dnu, v
tesni združbi skupaj nagnetenih osebkov, ki so se med seboj podpirali in hkrati te-
kmovali za življenjski prostor Sedimentacija karbonatnega blata je bila precej hi-
tra, zato so litiotidne školjke vse življenje enakomerno rastle v navpični smeri, tako
da je njihov bivalni del na ventralnem koncu ostajal nad nivojem obdajajočega sub-
strata. Litiotidne školjke so imele nenavadne, variabilne, specifičnemu okolju prila-
gojene lupine. So zelo velike, sploščene in izrazito razpotegnjene v višino. Njihova
notranja površina je tridelna; v sredini je glavno oz. osrednje polje, ob straneh pa
peresasti polji. Mehanizem odpiranja in zapiranja njihovih lupin še danes ni docela
pojasnjen.
Introduction
In the Middle Liassic lithiotid bivalves inhabited the relatively calm muddy bot-
tom of the restricted shelf on the Binarie Carbonate Platform. The horizon with the
bivalves barely wedges out in the Lower Jurassic beds of southern Slovenia; its thic-
kness at some localities reaches 75 m. It is named the "lithiotid horizon" after these
characteristic bivalves. B u s e r (1965, 44-46) attributed it to the Pliensbachian, or
its upper section: to the Domerian. The most important localities of Lower Jurassic
bivalves in Slovenia, which extend for over one hundred kilometres in length and
several tens of kilometres in width, were presented in the issue of Geologija.
The palaeoecological conditions that enabled the characteristic bivalve fauna to flo-
urish for a relatively short period have also been examined (Buser & Debeljak,
1996).
This paper gives a systematic description of lithiotid bivalves and their mode of li-
fe. The term lithiotid bivalves is still used in Slovenia although it no longer has any
taxonomic meaning. It comprises three frequent and characteristic, and at first sight
similar genera: Lithioperna (syn. Lithiopedalion), Cochlearites and Lithiotis. All
three genera were widespread in the shallow marine regions of the western and sou-
thern margins of the Tethys and even the Eastern Pacific (genus Lithiotis). Lithiotid
bivalves therefore have great palaeogeographical, biostratigraphical and palaeoeco-
logical significance. They became well-known mainly because of their unusual sha-
pes, which still challenge palaeontologists to offer various explanations.
The remaining species of Lower Jurassic bivalves from southern Slovenia will be
presented on another occasion.
Previous Research into Lithiotid Bivalves in Slovenia
Between 1959 and 1965Buser(1965) geologically mapped the territory of Sou-
thern Slovenia. He subdivided Jurassic beds and collected rich fossil material, ma-
inly hundreds of specimens of unusually shaped large-shelled bivalves. Among them
he determined the species Lithiotis problematica and Cochlearites loppianus, which
were previously known from classic localities in northern Italy. He established that
the majority of the specimens in southern Slovenia belong to a new genus of bivalves
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
v^ith the multivincular type of ligament. In his doctoral dissertation he named it Li-
thiopedalion. Together with the genera Lithiotis and Cochlearites he classified it into
the family Lithiotidae because of their obvious similarities. Buser(1972) presented
his work at the international meeting of palaeontologists in Graz, but unfortunately
failed to publish it in the manner required by the International Code of Zoological
Nomenclature. The genus Lithiopedalion was later described under the name Lithio-
pema (Accorsi Benini, 1979).
Systematic Descriptions of Lithiotid Bivalves
Subclassis PTERIOMORPHIA Beurlen, 1944
Ordo Pterioida Newell, 1965
? Subordo Lithiotina Accorsi Benini & Broglio Loriga, 1977
? Superfamilia Lithiotacea Accorsi Benini &
Broglio Loriga, 1977
Familia Lithiotidae Reis, 1903
Genus Lithiotis Gümbel, 1871
Lithiotis problematica Gümbel, 1871
Pl. 1, figs. 1-3; Pl. 2, figs. 1-3
1871 Lithiotis problematica - G ü m b e 1, 48; Pl. 2, figs. 13, 14.
1890 Lithiotis problematica (= Lithiotis ostreacina, Ostrea lithiotis) -Gümbel, 64-
67, Textfigs. 1-4.
1892 Ostrea problematica Gümbel -Böhm, 71-73; Pl. 3, figs. 1-3.
1903 Lithiotis problematica Gümbel -Reis, 9-13, Textfig. 4; PI. 6, figs. 5-16; Pl. 7,
figs. 1-10.
1923 Lithiotis timorensis sp. nov. - Krumbeck, 80-81; PI. 5, fig. 9.
1923 Lithiotis sp. (?) äff. problematicae (Gümb.) - Krumbeck, 81-82; PI. 5, figs.
10a, b.
1923 Lithiotis problematica Gümbel - R e i s; PI. 6, figs. 17-22.
1930 Plicatostylus gregarius new species, new genus - Lupher & Packard,
204-207; Pl. 1-3, Pl. 4, fig. 1.
1962 Lithiotis problematica Gümbel - De Castro, 13-14; PI. 10, figs. 1-6.
1965 Lithiotis problematica Gümbel - B u s e r, 17-18; Pl. 8, figs. 1, 2; Pl. 9, figs. 1, 2;
Pl. 10, figs. 1-4; Pl. 11, figs. 1-4.
1969 Plicatostylus gregarius Lupher & Packard - Cox (Moore ed.), N866, Textfigs.
H2,4a-c.
1911 Lithiotis problematica Gümbel - Stenzel (Moore & Teichert eds.),
N1200, Textfigs. J150,la-c (kop. Reis, 1903).
1971 Lithiotis problematica Gümbel - Berti Cavicchi, Bosellini &
Broglio Loriga, 43-47, Textfigs. 1, 3; Pl. 1, figs. 1-5.
1977 Lithiotis proóíemaízca Gümbel - A C C o r s i Benini & Broglio Lo-
riga, 42-50, Textfigs. 1-9, 16-18; Pl. 1, figs. 1-4; Pl. 2, figs. 1-3; Pl. 3, fig. 2; Pl.
8, fig. 1; Pl. 9, figs. 1, 2.
1977 Plicatostylus gregarius Lupher & Packard - Accorsi Benini & Bro-
glio Loriga, Pl. 3, figs. 1,3, 4.
18_Irena Debeljak & Stanko Buser
1982 Lithiotis problematica Gümbel - C h i n z e i, 179-196, Textfigs. 2, 4, 6, 7, 10.
1988	Lithiotis Gümbel - Nauss & Smith, 258-259, Textfigs. 6-8.
1989	Lithiotis problematica Gümbel - B u s e r L, 20-24, Textfigs. 13-15; Pl. 1, figs. 1,
2; Pl. 2, fig. 1; Pl. 10, fig. 1.
1995 Lithiotis problematica Gümbel - Savazzi, 281-289, Textfigs. 1-5.
Material: Over one hundred specimens. Mainly they consist of shell fragments
several centimetres high, with clearly recognisable characteristics. In six cases the
apex was preserved. Only five specimens have part of the body space preserved.
Description: It is characteristic that in all cases only one of the valves was
preserved: that with which the bivalve was attached to the solid base. According to
Chinzei (1982) this was the right valve. The opposite valve was so thin that only
fragments have been preserved. Evidence for this are fragments of a 1 to 2 mm thick
limestone "crust", which can be found anywhere on the inner surface of the thicker
valve. Lithiotis problematica is ribbon shaped; narrow and distinctively dorso-ven-
trally elongated. It is thought that adult specimens measured 20 to 50 cm in height
(or even more in some cases) and were about 4 to 7 cm wide. The apex is pointed.
From the side the shell is flat, consequently its cross section is elliptical. Generally
the valve is 1 to 2 cm thick. The apex is vertical or bent in any direction. The shell is
sometimes straight, but sometimes fairly curved (PI. 2, figs. 1,2). The outer surface of
the attached valve is irregularly undulating, as with oysters. Only rarely can small
concentric growth lines (increments) be seen.
Figure 1 shows the shape of the attached valve and the characteristics of its interi-
or. The pear-shaped body space was exceptionally small in adult specimens when
compared to the total size of the shell. It was limited to the ventral end. The soft body
extended several centimetres into the hollow part of the shell, called the umbonal
notch or cavity. The internal buttress divided it into smaller and larger cavity. Along
the sides of the inner surface are two feather-like areas. They are composed of incre-
ments which are at times linked into sheaves (PI. 1, fig. 2). Between the feather-like
areas is a vertical, 2 to 4 cm wide furrowed plate, which is somewhat elevated above
the lateral feather-like areas. Parallel grooves with intermediate ridges, which split
into shallower grooves and ridges which again split into even shallower ones, run
throughout its height. Thus, depending on how well the specimen is preserved, the
number of grooves can range from less than 8 to over 50.
The microstructure of the shell is described in the following works: Accorsi
B e n i n i and Broglio Loriga (1977) and Chinzei (1982), summarized in
Buser 1.(1989).
Comparison: Despite their very pronounced variability, specimens of Lithio-
tis problematica extracted from rock have such specific characteristics that they can-
not be mistaken for any other species. The uniqueness of this bivalve is also seen in
that it is today the sole species of the family Lithiotidae, and together with the speci-
es Cochlearites loppianus it is classified as an independent suborder Lithiotina (ac-
cording to Accorsi Benini & Broglio L o r i g a, 1977). A possible relati-
on with oysters (Böhm, 1892) was rejected by the aforementioned Italian palaeonto-
logists on the basis of proof that the shells were originally made of aragonite, while
oysters have calcitic shells. Systematic classification of the genera Lithiotis and Co-
chlearites has not yet been fully resolved. Chinzei (1982) is of the opinion that
they are taxonomically close to the Pteriacea superfamily, and especially to the Iso-
gnomonidae family.
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
Fig. 1. Lithiotis problematica Gümbel. Left: interior side of the thicker, at-
tached (probably right) valve. Centre: longitudinal section. Right: cross sec-
tions at various heights. The thin, free valve is marked by the dashed line.
The growth position was subvertical, with the ventral end turned upwards
SI. 1. Lithiotis problematica Gümbel. Levo: notranja stran debelejše (verjet-
no desne) lupine, s katero se je školjka pritrdila na trdno podlago. Sredina:
vzdolžni prerez. Desno: prečni prerezi na različnih višinah. Tanko, prosto
lupino označuje prekinjena črta. Življenjski položaj je bil pokončen;
ventralni konec je bil pri tem obrnjen navzgor
18_Irena Debeljak & Stanko Buser
The possibility that Lithiotis and Plicatostylus (later described from Oregon) are
identical genera was raised by G r u b i c (1959; 1961). B u s e r (1965) confirmed this
assumption with a study of the original specimens which the American authors Lu-
pher and Packard sent to Professor Kühn in Vienna.
Lithiotis problematica can also be distinguished from the other lithiotid bivalves
by cross-sections in solid rock. These are always monovalve and of an elliptical shape
in the cross-section (see Buser & Debeljak, 1996, 36, figs. 9, 10). Especially
characteristic are the "ear-like" sections with an opening in the central part, after
which Lithiotis was named.
Localities: Lithiotis problematica is the rarest species of lithiotid bivalves in
Slovenia. B u s e r (1965) found it in a deserted quarry on the right bank of the river
Sušica, west of Dolenjske Toplice. Unfortunately this quarry is now filled in. Buser
found some specimens on the northern slope of Mokrec during the construction of a
new road. Today these outcrops are overgrown. A large number of specimens can still
be found east of the village Zafara near Žužemberk. Relatively well preserved shells
of L. problematica can be extracted from limestone in the vicinity of the Glijun
spring and on the Poljanica hill west of Bovec. Characteristic sections can be obser-
ved in the ornamental Podpeč limestone (Buser & Debeljak, 1996), but the
shells of L. problematica in Podpeč cannot be extracted from solid rock.
During the Pliensbachian and Toarcian Lithiotis heavily populated the extensive
and interconnected shallow marine areas of the western and southern margins of the
Tethys and the Eastern Pacific. Various authors have reported finds of the species in
southern Spain (?) and western France (?), northern Italy, the south-central Apenni-
nes, Croatia (?), Herzegovina, Montenegro, Albania (?), Greece, Turkey (?), Marocco
(?), Somalia (?), Oman (?), Iran (?), Iraq (?), the Himalayas (?), the island of Timor in
Indonesia, the USA (Oregon, Nevada, California), Chile and Peru. A question mark
denotes that the locality is not reliably confirmed. The above data are collected from:
Broglio Loriga and Neri, 1976; Accorsi Benini and Broglio Lo-
riga, 1977; Geyer, 1997; Nauss and Smith, 1988; Buser and Debeljak,
1996.
Familia Cochlearitidae Accorsi Benini & Broglio Loriga, 1977
Genus Cochlearites Reis, 1903
Cochlearites loppianus (Tausch, 1890)
PI. 3, figs. 1-3
1890 Trichites Loppianus n. f. - Tausch von Gloeckelsthurn, 18-19; PI.
5, figs. 5-7.
1892 Ostrea Loppiana Tausch -Böhm, 67-71; PI. 2, figs. 1-4.
1892 Ostrea problematica Gümbel var. lithiotis - B ö h m, 74; PI. 4, fig. 1.
1903 Cochlearites Loppianus, nov gen. -Reis, 2-8, Textfigs. 1-3; PL 1, figs. 1-8; PL
2, figs. 1-10; Pl. 3, figs. 1-11; Pl. 4, figs. 1-11; Pl. 5, figs. 1-9; Pl. 6, figs. 1-4.
1965 Cochlearites loppianus (Tausch) - B u s e r, 19-20; Pl. 12, figs. 1, 2; Pl. 13, figs. 1-4.
1971 Coc/iíeantes loppianus (Tausch) - Cox (Moore ed.), N1200, Textfigs.
J150,2a,b (kop. Reis, 1903).
1971 Cochlearites gr. loppianus (Tausch) - Berti Cavicchi, Bosellini &
Broglio Loriga, 43-47, Textfigs. 2, 4; Pl. 2, figs. 1-4.
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
1911 Cochlearites loppianus {Tausch) - A c c o T s i Benini & Broglio Lo-
riga, 52-57, Textfigs. 12, 19; Pl. 4, figs. 1-5; Pl. 5, figs. 1-3; Pl. 6, figs. 1-3.
1911 Cochlearites loppianus t. A-Accorsi Benini & Broglio Loriga,
56, Textfigs. 14, 20; Pl. 6, fig. 4; Pl. 7, figs. 1-4; Pl. 8, fig. 2.
1982 Cochlearites loppianus (Tausch) -Chinzei, 179-196, Textfigs. 3, 5, 8, 11c, 13.
1989 Cochlearites loppianus (Tausch) - B u s e r L, 25-28, Textfigs. 16, 17; Pl. 2, figs.
2, 3a,b; Pl. 10, fig. 2.
Material: Approximately 80 specimens. Mostly fragments of left valves up to
10 cm large; fewer right valves. Ten of the specimens have both valves partially pre-
served. The apex of the shell and the body space are not preserved in any of the speci-
mens. The ligament groove is visible in six shells.
Description of Species: The shell is narrow, and strongly dorso-ven-
trally elongated with a tapered apex. Adult specimens measure from about 20 to mo-
re than 50 cm in height; their average width is 5 to 8 cm. The animal attached itself to
the solid base with its left valve, which is approximately 1 to 2 cm thick. The right
valve was free and thinner than the left; at its centre it measure approximately 0.5 to
1 cm. The shell is usually straight, but may be bent to the side (PL 3, fig. 2). Concen-
tric growth lines can sometimes be seen on the rough and irregular outer surface.
The shape and interior of the shell is shown in fig. 2. The inner side of the shell is
tripartite, which is characteristic of lithiotid bivalves. The central area is about 2 to 4
cm wide, and bordered by two feather-like areas. The feather-like appearance is cre-
ated by growth lines, which can be joined into sheaves. The valves usually gaped at
the edges of the feather-like areas. The soft body space of the shell was very small
compared to the total size of the shell. The central area where the valves were tightly
joined has a relief form. A more or less wide depression bordered by two ridges runs
down the centre of the left valve and the central crest of the right valve fits tightly
into it. Semicircular traces are often found on the central area; these are growth lines
which the edge of the mantle left behind as it moved in the ventral direction.
At the apical end approximately down the middle of the cardinal area of both val-
ves runs a deep and narrow groove (resilifer) in which the fibrous ligament was atta-
ched. The lamellar part of the ligament was attached at both sides of the groove. The
height of the ligament groove varies with the specimens. Usually it measures 3 to 6 cm.
Slovenian specimens usually have recrystallised shells, but during fossilisation the
parts with a different original microstructure were selectively coloured so that the
characteristic features of the basic structure can often be observed in the sections
(B u s e r L, 1989, Pl. 10; cf. C h i n z e i, 1982).
Comparison: The specimens of Cochlearites extracted from rock cannot be
mistaken for any other bivalve. Confusion can arise when an attempt is made to de-
termine the species from sections in the rock alone. They can be very similar to secti-
ons of Lithioperna scutata. However, in specimens of Cochlearites the left valve is
thicker than the right, whereas in Lithioperna the two shells are equally thick. Nor-
mally in Lithioperna both valves fit tightly around the edges; one valve follows the
other like a mirror image. In Cochlearites the valves often gape in the feather-like
areas. The easiest to identify are cross-sections with a characteristic central ridge on
the right valve and a corresponding depression on the left valve (Buser & De-
beljak, 1996, p. 32 - fig. 4, p. 41 - fig. 15).
Individual specimens of Cochlearites loppianus can differ considerably from one
another in the shape and size of the shell. Such variations have arisen owing to con-
18
Irena Debeljak & Stanko Buser
Fig. 2. Cochlearites loppianus (Tausch). Left: interior side of the right, fi-
xed valve. Centre: longitudinal section of both valves. Right: cross secti-
ons at various heights. The growth position was subvertical, with the
ventral end turned upwards
SI. 2. Cochlearites loppianus (Tausch). Levo: notranja stran desne, proste
lupine. Sredina: vzdolžni prerez čez obe lupini. Desno: prečni prerezi na
različnih višinah. Življenjski položaj je bil pokončen; ventralni konec je
bil pri tem obrnjen navzgor
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
stant adaptations to the environment and growth in tight communities or aggregates.
Practically no two specimens have identical central areas. According to the ligament
groove Reis (1903) identified three types of Cochlearites. Accorsi Benini
and Broglio Loriga (1977) admitted only two types: the normal type, in
which the ligament was connected to the mantle, and the abnormal type (forma A), in
which a short, stunted fibrous ligament no longer had any connection with the living
part of the bivalve, which can be ascertained by shifted mantle growth lines. C h i n -
z e i (1982, 193) showed that practically all adult specimens belong to the abnormal
type or form A, which results from the ligament becoming stunted sooner or later du-
ring the growth of the bivalve. This finding makes irrelevant considerations of diffe-
rent types or even subspecies with regard to the appearance of the ligament area.
The systematic position of the Lithiotis and Cochlearites genera has not been fi-
nally determined, and their possible relationship is similarly not yet clear.
Localities: Buser (1965) found the finest specimens south-west of Lož and
in the northern and southern parts of Mokrec. Today those localities are overgrown.
Individual specimens can still be obtained at the Globočec spring west of Zagradec,
and on the Stražišče hill east of Gorenje Jezero near Cerknica. Cochlearites loppia-
nus also occurs in the Podpeč quarry, in the surroundings of Grčarevec near Logatec,
at Borovec in the Kočevje region, and on Travna gora, as in these localities characte-
ristic sections have been observed in limestone or dolomite.
Elsewhere in the world C. loppianus was found in northern Italy, the south-central
Apennines, Montenegro, Greece and Morocco. It may possibly occur in western Fran-
ce, Somalia and on the island of Timor in Indonesia. (After: Broglio Loriga
& Neri, 1976; Accorsi Benini & Broglio L o r i g a, 1977; G e y e r,
1977; Bus er & D e b e 1 j a k, 1996).
Subordo Pteriina Newell, 1965
Superfamilia Pteriacea Gray, 1847
Familia Isognomonidae Woodring, 1925
Genus Lithiopema Accorsi Benini, 1979 - syn. Lithiopedalion Buser, 1965
Lithioperna scutata (Dubar, 1948)
PI. 4, fig. 1; Pl. 5, figs, la, b; Pl. 6, fig. 1; Pl. 7, figs. 1-3; Pl. 8, fig. 1; Pl. 9, figs la, b
? 1948 Perna scutata n. sp. - D u b a r, 158-159; Pl. 11, figs. 1-3.
? 1948 Perna sp. nov -Dubar, 159-161, Textfig. 51; Pl. 13, figs. 10a,b.
1965 Lithiopedalion kuehni n. sp., n. gen. - B u s e r, 21-22; Pl. 14, fig. 1.
1965 Lithiopedalion sp. - B u s e r, 20-21; Pl. 15, figs. 1-5; Pl. 16, figs. 1-3; Pl. 17, figs.
1, 2; Pl. 18, figs. 1-3.
1971 "Perna" - B e rt i Cavicchi, Bosellini & Broglio Loriga,
47-48, Textfigs. 5a,b; PI. 3, figs. 1-3.
1916 Lithiopedalion kuehni Buser - Br o gli o Loriga & N e r i, 656-657; Pl.
85, figs. 1, 2.
1979 Lithioperna scutata (Dubar), n. gen. - Accorsi Benini, 251-253, Textfigs.
1-12, 15; Pl. 1, figs. 1, 2; Pl. 2, figs. 1, 2; Pl. 3, figs. 1-4; Pl. 5, figs. 1, 2; Pl. 6, figs.
1, 2.
? 1979 Lithioperna scutata (Dubar) - Accorsi Benini, 251, Textfig. 14.
18_Irena Debeljak & Stanko Buser
1989 Lithiopema scutata (Dubar) - В u s e r L, 30-34, Textfigs. 18, 19, 25; Pl. 4, fig. 1;
Pl. 5, figs. 1-3; Pl. 11, fig. 1.
1996 Lithiopedalion scutatus (Dubar) - Buser & Debeljak, 25, Textfigs. 4, 8.
Material: About 80 specimens, most with both valves preserved. Fragments of
the apical parts of the central plate with ligament grooves are predominant in the
collection. The marginal parts are generally missing.
Description of Genus and Species: The shell is linguiform and
often very large. Sections in the limestone reveal that some specimens reached a he-
ight of three-quarters of a metre. The marginal parts of the shell are very thin, and
for this reason we have so far not succeeded in finding an undamaged intact speci-
men. Therefore, in citing size we have to rely on sections in the limestone. On avera-
ge, the shells are 30 to 70 cm high. The height is usually about twice the length. From
the side the shell is distinctly compressed. The two valves have the same shape, size
and thickness, and fit closely. Together they are 1 to 4 cm thick. The longitudinal sec-
tion often has an undulating appearance. The external surface of the valve is nor-
mally rough and irregular, and in some rare specimens concentric growth lines can be
seen on it.
Characteristics of the shell interior are shown in Figure 3. Flat feather-like areas
are well-formed at both sides with clearly visible increments. (These indicate the for-
mer lateral outline of the body cavity which shifted in the ventral direction with the
growth of the bivalve.) The anterior feather-like area is generally larger and often
bent into a knee shape. A byssal notch runs along its interior edge. Under the apex it
is shaped like a byssal groove, and towards the body space it is bordered by two folds
or edge lines. Between the two there is a ridge of greater or lesser width in the right
valve, and a corresponding depression in the left valve that can be shallow (PI. 5) or
quite deep (PI. 6).
Between the lateral feather-like areas is an even, fairly flat central area or plate,
which covers the largest part of the interior of the shell. The bivalve's mantle was spre-
ad over this surface, and here both valves were tightly joined. Under the apex a charac-
teristic ligament area is formed, with a straight or occasionally a semicircular upper
edge. The ligament was multivincular. The fibrous ligament was placed in several liga-
ment grooves or resilifers, while the lamellar ligament was attached in spaces between
them. As the bivalve grew the ligament shifted in the ventral direction and left behind
thin growth lines, which in the grooves are curved in a convex manner with respect to
the apex, but between the grooves they are concave. The ligament grooves are generally
sub-parallel. In several specimens the position and centre of gravity of the shell was
changed during growth, and the ligament later progressed in a different direction to the
original one. Sometimes the ligament grooves run so distinctly towards the anterior
part that they reach the anterior feather-like area (PI. 7, figs. 2, 3).
Even if the byssal notch on the anterior side is not preserved, it can be determined
whether the specimen is the left or the right valve. First, a vertical boundary is traced
between the central plate and the feather-like area, and then a straight line is imagi-
ned running through the ends of the ligament grooves. The angle made by the two li-
nes is less than 90° on the anterior side and more than 90° on the posterior. The left
and right valves can thus be distinguished.
The density of the ligament grooves can Vary. With regard to the width of the la-
mellar part of the ligament, that is of the intervals between individual ligament groo-
ves, two groups can be determined. In both groups the grooves are 2 to 3 mm wide.
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
Fig. 3. Lithioperna scutata (Dubar). Left: inner surface of the right valve. Right: characteristic
undulating longitudinal section of both valves. Bottom: cross sections through both valves
SI. 3. Lithiopema scutata (Dubar). Levo: notranja površina desne lupine. Desno: značilen, valo-
vit vzdolžni prerez obeh lupin. Spodaj: prečni prerez čez obe lupini
18_Irena Debeljak & Stanko Buser
The intermediate sections are 2 to 3 mm wide in the first group, and 8 to 10 mm wide
in the second. In the first group there are about 16 grooves on 10 cm of the hinge axis,
in the second about 8. In some specimens it is clear that some ligament grooves have
lagged behind while others have continued to develop at the normal rate (PI. 7, fig. 2;
PI. 9). It can be concluded that in the second group approximately every other liga-
ment groove with a fibrous ligament became stunted, and the lamellar part of the lig-
ament was attached in its place. The ligament grooves can range from just a few mil-
limetres in height to 8 cm. The amount of ligament area preserved depends on the
thickness of the valve beneath the apex: the thicker the valve, the longer the grooves.
In any case, the majority of specimens are very thin beneath the apex.
The body space of the bivalve with a single muscle scar is small in comparison to
the overall size of the shell. It occupies only the ventral section of the shell and part
of the space between the anterior border folds or lines. The depression for the soft
body is very shallow.
An analysis of extremely well-preserved specimens from around Verona has shown
that Lithioperna shells were composed of aragonite (Accorsi Benini, 1979,
228). In many specimens from Slovenia the characteristic structure of the shell can be
seen with the naked eye, i.e. an alternation of lighter and darker laminae or layers
parallel to each other and to the external surface of the valve (B u s e r L, 1989, Pl.
11, fig. 1). The light, glittering layers once had a prismatic microstructure, and the
darker, opaque layers had a nacreous microstructure (Accorsi Benini, 1979).
Comparison: The Lithiopema genus is now classified with the Isognomoni-
dae. In these bivalves the hinge teeth are absent, but a large multivincular ligament
compensates for them. The classification among the Isognomonidae is still somewhat
uncertain because the shell structure of L. scutata is uniquely developed (Accorsi
B e n i n i, 1979). Among the Isognomonidae Lithioperna could be compared with
Isognomon Solander in Lightfoot, 1786 (syn. Perna Bruguiére, 1789), in which the
shells were attached to the firm base by a byssus as well. However there is no large
central plate in Isognomon. The feather-like areas on the sides are not present, the
valve is biconvex, and there is no alternation of two types of laminae.
Lithiopema can also be recognised from sections in rock (Buser & Debe-
Ij ak, 1996, 32, fig. 4). Typical sections are very long and thin, and usually undulate
gently. The two valves are of equal thickness and fit closely on all edges. In general
only the very thin soft body space gapes (fig. 3).
At present the genus contains only the species L. scutata, although individual spe-
cimens differ considerably. Variations in shell size proportions and shape are very
common, and the appearance of the ligament area also differs. No two specimens are
the same. However, only one specimen, described below, is essentially different,
which is not sufficient to describe a new species.
Remarks: The name Lithiopedalion was given to the genus by B u s e r (1965) in
his doctoral thesis according to the common characteristics of Lithiotis and Pedalion
Dillwyn, 1817 (an old synonym for Isognomon). The new genus and species was pre-
sented at the 421^'i annual meeting of the Palaeontological Society in Graz (B u s e r,
1972). Unfortunately this work was not published in the way required by internatio-
nal rules, but the name Lithiopedalion was nevertheless used in Slovenia and in the
literature elsewhere (cf. Bosellini, 1972; Broglio Loriga & Neri,
1976). After many years of collaboration he was overtaken in publication by the Itali-
an palaeontologist Accorsi Benini (1979), who described the genus under the
name: Lithiopema.
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
Together with the genus B u s e r (1965) described a new species Lithiopedalion
kuehni, but Accorsi Benini (1979) equated it with the species Perna scutata
from Morocco as already described by D u b a r (1948), and included it in Lithioper-
na. Given that Benini studied original material from Dubar's collection, for the time
being her classification of these as the same species must be trusted. However, despi-
te this we think that three modest figures showing poorly preserved specimens do not
exhibit all the characteristics typical of specimens from Slovenia and northern Italy.
According to their shape they are considerably reminiscent of Gervilleioperna Krum-
beck, 1923; this applies in particular to Dubar's specimen that Accorsi Benini
(1979, 251, fig. 14) presented as a paralectotype in describing the new genus. Judging
by the description and pictures the anterior feather-like area is formed as a lunule,
while the byssal notch is very deep and just under the apex spreads and deepens into
the body cavity. The Slovenian and Italian specimens have a significantly more dor-
so-ventrally elongated shell, a higher ligament area, a shallower byssal notch in most
cases, and, above all, a larger central plate where the valves were tightly joined. In
the future it would certainly be recommenc able to make a revision of the Lithioperna
scutata species using the Dubar material, ^hich is kept in France (Lille).
Localities: In the lithiotid horizon of Middle Liassic beds in Slovenia (Trnov-
ski gozd, Hrušica, Nanos, Logaška planota, Krim-Mokrec hills, Dolenjska) Lithioper-
na scutata is the most common species, found in almost all localities (B u s e r &
Debeljak, 1996, 28, fig. 2; presented in this article as Lithiopedalion scutatus).
Specimens that can be extracted from marly layers between limestone can now be fo-
und at Špik, north of Col in Trnovski gozd, in the Podpeč quarry, and along the rail-
way between the stations Preserje and Verd. B u s e r (1965) found numerous speci-
mens on the Krim-Mokrec hills; the outcrops are now almost entirely overgrown. The
locality on Javomik and those north of Cerknica and south of Lož are also over-
grown. The quarry on the right bank of the Sušica west of Dolenjske Toplice is now
filled in.
Elsewhere in the world Lithioperna can be found in Liassic beds of northern Italy
(Berti Ca vic chi et al., 1971; Brogli o Loriga & N e r i, 1976), the cen-
tral Apennines, Albania, Greece, France and Morocco (after Accorsi Benini,
1979). Rey (1990; 1997) reports it from southeastern Spain. We estimate that the Li-
thioperna is present elsewhere, but different authors have designated it under the na-
me Perna or Isognomon, or in some places have perhaps classified it with the oysters
(e.g. Pemostrea Munier-Chalmas, 1864 with the multivincular type of ligament).
? Lithioperna sp.
PL 9, fig. 1
Material and Site: One poorly preserved specimen with both valves. Only
the ligament area, part of the central plate, and the upper part of the byssal notch are
preserved. The specimen was found by a forest road north of Mokrec.
Description and Comparison: Extremely narrow ligament grooves
are present on the ligament area, approximately 1 mm wide. On the hinge axis (in so
far as it has been preserved), which is 11 cm long there are 36 ligament grooves. That
is at least twice as many as is common for Lithioperna scutata. The height (more than
4 cm) and density of the ligament grooves could fit the Isognomon genus or the Hip-
pochaeta Philippi, 1844 subgenus, which is known only from Tertiary strata. Howe-
18_Irena Debeljak & Stanko Buser
ver in Isognomon the body cavity begins under the ligament, but in the specimen de-
scribed, as in Lithiopema scutata, the ligament grooves end in the flat central plate,
where the valves were tightly joined.
The specimen is too poorly preserved for a more detailed determination, or for the
description of a new species.
Mode of Life of Lithiotid Bivalves
In southern Slovenia during the Middle Liassic lithiotid bivalves flourished in the
mainly quiet environment of a more or less restricted shelf on the Dinaric Carbonate
Platform. Individual species created monocolonies in the form of sea-bottom mats or
biostromes with several lens-like accumulations. The moderate influence of the pela-
gic sea could be felt from the north. Sedimentation in the lagoon was rapid, and the
substratum on the sea bed was principally composed of mud, which was determined
with regard to the properties of the matrix in bivalve lumachelles. Biodiversity in
such lumachelles is very low. Bivalves were able to build shells of three-quarters of a
metre in length only under tropical or sub-tropical conditions. The palaeogeographi-
cal and palaeoecological conditions that enabled the existence of the characteristic
bivalve fauna and the distribution of various species have already been described
(Bus er & D eb el j a k, 1996).
A description of the living habits and special adaptations of individual genera is
given below.
Lithiotis
Although the first investigators in Italy discovered Lithiotis 250 years ago, it still
presents a puzzle to palaeontologists today. The specimens extracted from rock do
not really resemble present-day bivalves, and it is not surprising that they were first
described as plant remains (Gümbel, 1871; cf. 1890). It is supposed that the life of
such bivalves developed as described below.
The larvae first swam or floated in water until they found a suitable attachment
point. On the muddy sea floor the firmest substratum was presented by the shells of
other bivalves, usually adult Lithiotis individuals. Juveniles fixed themselves to the-
se with a sp cial adhesive substance, oriented in such a way that they could grow
vertically u':) vards. Numerous Lithiotis specimens testify to this, having identically
oriented you iger individuals fixed to their surfaces (B u s e r, 1965, Pl. 8, 9). It can al-
so be seen from sections in the limestone that certain adults were literally cemented
to each other (Euser & Debeljak, 1996, fig. 9). Chinzei (1982) suggested
that the attachn ent area was small, and that cementation was possible only in juve-
niles. On the be sis of our own observations we have concluded that these bivalves re-
tained the capacity for cementation throughout their lives, thus the construction of
their aggregates was very firm, and after death they often remained in their life posi-
tion.
In any case Lithiotis bivalves lived in large groups, giving one another support and
spreading out in bunches (fig. 4). Sedimentation of carbonate mud took place quite
rapidly, so that the shell was permanently anchored in it. The body space increased
only in young individuals, afterv^ards the shell grew only in height, in a subvertical
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
Fig. 4. Aggregate of Lithiotis shells; characteristic branching in subverti-
cal direction
SI. 4. Združba školjk rodu Lithiotis; značilno razraščanje v subvertikalni
smeri
direction (cf. G ö h n e r, 1980; C h i n z e i, 1982; C h i n z e i et al., 1982; S e i 1 a -
cher, 1984). Much can be learned about the growth from the incremental lines on
the exterior surface and on the feather-like areas. They are combined into sheaves
that on well-preserved specimens repeat approximately every centimetre. C h i n z e i
(1982, 189-192) concluded that periods of faster and slower growth alternated, resul-
ting in the appearance of annual rings. By counting the growth lines and measuring
the size of the shell he reached the conclusion that it took about forty years for the
shell to attain a height of 50 cm. During all this time mud gathered around it. Indivi-
duals that did not grow quickly enough became buried in mud. This often happened
to juveniles, which owing to the crowd did not succeed in gaining living space. Whe-
never an individual became dangerously tilted, it was still able to correct its position
during further grov^rth. Such specimens with "bent knee" shells are very common (PI.
2, fig. 1).
It is interesting that in Lithiotis it was only the thicker valve that was preserved,
i.e. the valve cemented to the base. The opposite, free valve must have been extremely
thin. Only pieces of thin crust on the thicker valve remained of it. In limestone too
only single-valve Lithiotis specimens can be observed. Sometimes thin traces that co-
uld be fragments of the thinner valve are scattered among them (Buser & De-
be 1 j a k, 1996, 36). Today it is still not known whether this valve was as big as the
thicker, attached valve (Reis, 1903, 11; C h i n z e i, 1982, 181; S a v a z z i, 1996,
287), or whether it covered the bivalve's soft body only in the form of an operculum
(Lupher & P a c k a r d, 1930; A c c o r s i Benini & Broglio Lori-
g a, 1977, 21). The thicker attached valve was hollow under the lowermost part of the
furrowed plate. Here the soft body found support, so that its weight did not burden
the thin, free valve.
The mechanism of opening and closing the shell has not yet been fully explained.
Usually in bivalves the valves are opened by a ligament functioning in the opposite
way to the adductor muscle. Chinzei (1982, 193) suggested that there was no
functional ligament in Lithiotis, and that the valves were able to open and close
18_Irena Debeljak & Stanko Buser
due to the elasticity of the thin valve. This would have been bent over the ventral
edge of the furrowed plate (in this case it would be the hinge axis) whenever the
adductor muscle was contracted. When the muscle was relaxed, the thin valve
stretched out and the valves gaped open. The question with this hypothesis is what
(apart from mud) held both valves together at the dorsal end if there was no liga-
ment or hinge teeth. The valves themselves were very large but the body space with
the adductor muscle was small. If the valves were really the same height, then du-
ring opening they might become displaced and irruption of mud might occur. In
this case it is indeed more likely that the thin valve was merely some sort of oper-
culum over the body space of the thick valve. This supposition is somewhat contra-
dicted by the fact that remnants of the thin valve are found at all heights on the
thick shell. It is possible that the thin shell gradually broke up and fragmented in
the dorsal area while the shell grew in height and the body space shifted in the ven-
tral direction.
Also unsolved is the question of what function the central furrowed plate ser-
ved. It can not be that this was just a frick of nature. There are no traces on it left
behind by the mantle. The soft part of the bivalve did not extend to this area.
Böhm (1892) even thought that the characteristic grooves arose secondarily,
owing to weathering. Gümbel (1890, 65) and B u s e r (1965, 18) described the
furrowed area as the place for the ligament. Reis (1903, 43) asserted that the li-
gament was stunted and that the indentation of the furrowed area acted as a sort
of hinge. A c c o r s i Benini and Broglio L o r i g a (1977, 21-24) sugge-
sted that the furrowed area was just a superstructure that covered special internal
tubes in the shell where thin mantle appendages, the centres of calcification, were
located. The grooves would have originated as these internal tubes opened out-
wards. Recently S a v a z z i (1996) found evidence for the presence of the ligament
on the furrowed plate, and suggested that in Lithiotis the ligament was active
throughout the height of the furrowed area and not only on the ventral end or hin-
ge axis, as is normal for bivalves with the multivincular ligament. In Lithiotis the
ligament structure would have developed in a highly original way. S a v a z z i fo-
und that the ligament fibres were oriented and inserted in such a manner that they
allowed small changes in the reciprocal distance of the valves. This increased the
length over which the free valve flexed when the bivalve closed. Stress was thus
more evenly distributed; tension was reduced, as was the possibility that the thin
shell would break up or become damaged. Thus according toSavazzi the free
valve was able to close by flexing, as established by Chinzei (1982), but not by
articulating over the hinge axis.
Chinzei (1982; 1986) compared the shell shape and mode of growth of Lithiotis
and Cochlearites with certain oysters that are also strongly elongated and live or
used to live in a vertical position on soft, muddy ground: Saccostrea Dollfus & Daut-
zenberg, 1920, which live along the coast of east Africa and whose shape is remini-
scent of rudists - the thin free valve is shaped like an operculum (Stenzel, 1971);
and Konbostrea Chinzei, 1986, an Upper Cretaceous oyster from northern Japan,
which is most similar to Lithiotis in its shape and growth pattern.
We assume that Lithiotis problematica was very selective in its choice of habitat.
Data from Slovenian localities indicate that it formed monocolonies; it required its
own living space which was not shared with other lithiotid bivalves and excluded the
majority of other organisms as well (Buser & Debeljak, 1996). The Lithiotis
genus is the rarest among lithiotid bivalves in Slovenia.
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
Cochlearites
Cochlearites is very common in the lithiotid horizon in Slovenia, particularly in
those places where restricted parts of shelf were spread.
The Cochlearites shell grew similarly to Lithiotis shells (see previous section). Owing
to its narrow and compressed shape many individuals of the same species were able to
crowd together Their living position was vertical, as seen today in Pinna Linné, 1758.
Its shells are also very elongated in height, but dtiring growth the soft body part increa-
ses in size. Using its foot Pinna can bury itself in sandy or muddy sediment, after which
it uses a bunch of byssal threads to fix itself to any solid base, for example a buried sto-
ne (C o X, 1969, N8-N10). Cochlearites and Lithiotis were unable to bury themselves, as
the small soft body occupied only the ventral end of the bivalve; in addition, one valve
was immobile, always cemented to a fixed base. We think that the shell cementation
capability in Cochlearites was much more limited than that of Lithiotis, and so the
construction of their aggregates was not as firm. The sedimentation of calcareous mud
in which the bivalve was anchored took place quite rapidly, thus the valve in any case
had sufficient support. A considerable amount of fecal mud accumulated around the
bivalves (suspension-feeders) themselves, because they continuously filtered large qu-
antities of water in which small particles of nutritives floated. It can be imagined that
the sedimentation environment must have been fairly calm. Strong waves and currents
would have washed away the mud which supported the bivalves, and scattered them
around the sea floor, where they would have perished.
Chinzei (1982, 193) considered that in Cochlearites the ligament was active
only in very young individuals, but then became stunted and no longer had any con-
nection with the mantle, that is with the living part of the body. The majority of bi-
valves need the ligament as like a spring it opposes the action of the adductor muscle,
thus opening the valves whilst also connecting them. In Cochlearites the relief struc-
ture of the cardinal area assumed the role of the hinge teeth (which lithiotid bivalves
do not have), so that the valves did not become displaced. Most of the shell was stuck
in the mud. Therefore along its entire height the shell should not open, because mud
would irrupt. In the cardinal area the valves were in close contact all the time, but in
the body space they gaped somewhat, and thus remained flat and parallel during
growth. The solid valves may have been elongated into the conchiolin-rich lamellae
(Chinzei, 1982, 194). With the help of these elastic lamellae the bivalve was able
to close hermetically like some of today's bivalves. When the adductor muscle rela-
xed, the lamellae flattened and left a narrow slit at the end. According to C h i n z e i
the elasticity of the ventral parts of the valves replaced the ligament.
As in Lithiotis, the soft body was very small, and always remained above the level
of the surrounding mud. The bivalve probably extended its mantle through the nar-
row slit at the ventral end, intercepting and absorbing the substances it needed to
build a large shell.
Lithioperna
Of all the lithiotid bivalves in Slovenia Lithioperna was the most common. Its
shells usually lie in limestone parallel to the bedding plane. Only rarely are layers
with numerous subvertically oriented sections found, which tell us much about their
original life position.
18
Irena Debeljak & Stanko Buser
Originally Lithiopema shells were fixed to a firm object with the byssus, like
other isognomonids. The byssal gland secreted a special sticky fluid that solidified in
water into byssal threads. The byssus emerged from the anterior side of the valve.
The anterior feather-like area is usually more strongly developed than that on the po-
sterior. The anterior side of the shell was turned towards the weak currents that bro-
ught nutrients to the bivalve (fig. 5). In most specimens the ligament grooves run so-
mewhat obliquely towards the anterior margin. The hinge axis runs through their lo-
wer ends. The obliqueness of the hinge axis can be explained on the assumption that
the posterior side of the valve was more sunk in the soft sediment than the anterior.
The hinge axis had to stay parallel to the surface of the substratum, otherwise mud
would irrupt into the posterior during opening (fig. 5).
Fig. 5. Growth position in Lithiopema (syn. Lithiopedalion). Mostly youn-
ger individuals grew vertically (left), providing mutual support, and bran-
ched radially. Some gradually tilted towards the soft sea floor, or even fell
over (right). The right valve is not shown in two specimens so that the inner
surface with ligament area can be seen
SI. 5. Naravni položaj pri rodu Lithiopema (syn. Lithiopedalion). Pred-
vsem mlajši osebki so rastli navpično (levo) in se medsebojno podpirali ter
razraščali. Nekateri so se sčasoma nagnili ali pa celo prevrnili na mehko
morsko dno (desno). Desna lupina pri dveh primerkih ni narisana, zato da
se vidi notranja površina z ligamentnim poljem
Like ali lithiotid bivalves, Lithiopema grew rapidly in height. The effectiveness of
the byssus was gradually diminished, and it became more difficult for it to hold the
shell in an upright position. Adjacent individuals provided one another with support,
like books on a book shelf. In addition they were anchored in the mud. Those indivi-
duals whose support was not solid enough eventually overturned, and spent the re-
mainder of their life lying flat on the sea floor. According to Accorsi Benini
(1979, 245-246) the shell was able to adapt its growth so that the ventral end of the
valve with the soft body always remained above the level of the mud. The changes in
the direction of growth gave the specimen a characteristic wave-like appearance.
This undulation also helped maintain effective mutual contact between the two val-
ves. The large, flat shell was stable on the sea floor and did not sink into the soft se-
diment. From time to time weak tidal currents washed mud away from it. Lithioper-
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
na bivalves were probably also capable of cleaning themselves, as certain oysters do:
by rapidly contracting the adductor muscle they can squirt a strong jet of water out
of the mantle cavity to clean their surface (Stenzel, 1971, N1001).
Lithioperna bivalves lived in such shallow water that occasionally they may have
found themselves on dry ground. In such cases the valves sealed hermetically. They
also closed in such a manner when the decay of organic matter caused oxygen-deple-
ted conditions on the sea floor. In such periods the bivalve was forced to breath anae-
robically. According to Accorsi Benini (1979, 228-242) this is reflected in the
shell microstructure: During growth, when the valves gaped open the mantle absor-
bed Ca2+ ions and secreted nacre on its external surface. During periods of anaerobic
respiration acidic products accumulated in the extrapallial liquid between the man-
tle lobe and the shell wall and reacted with the shell. To neutralise the acidity, calci-
um ions were released. During this process the shell's internal layer, which previously
had a nacreous microstructure, took on a new pseudoprismatic appearance. This
happened repeatedly, and the shell consequently has two types of alternating laminae
or layers with a different microstructure.
Owing to the size and undulating shape of the shell the ligament was under consi-
derable stress during opening. The fibrous part of the ligament was located in several
ligament grooves (i.e. resilifers). When the adductor muscle contracted, the fibres
were compressed, and when it relaxed, the fibres stretched out like a spring. During
this process the valves opened somewhat. The lamellar part of the ligament, which
was attached between the grooves, connected the two valves. Owing to the mechani-
cal load the ligament was permanently cracked and decayed at the dorsal end. As it
was composed of organic matter, it was also destroyed by bacteria. Therefore only the
ventral part of the ligament, continuously excreted by the mantle isthmus, was acti-
ve. This occurs in many bivalves that have a similar type of ligament, i.e. multivincu-
lar (S t e n z e 1, 1979, N971-974). The hinge axis also shifted in the ventral direction
together with the ligament. In certain specimens the ligament grooves ceased gro-
wing and the lamellar ligament attached itself in their place. This probably occurred
because effective contact between the two valves was essential. The fibrous ligament,
used for opening, was not so important. Moreover, at the ventral end the solid valves
gaped open by a few millimetres for most of the time, and thus during growth remai-
ned flat and parallel; they did not become curved as in most other bivalves. Lithio-
pema bivalves may have secreted poisonous mucus from the ventral slit as a defence
measure. It is supposed that the opening was sealed when necessary by thin elastic
scales or lamellae that fringed the greatly thinned ventral edge of the two valves, as
in Lithiotis and Cochlearites. Such lamellae have not been preserved as fossils, beca-
use they were poorly calcificated and rich in organic matter (cf. Stenzel, 1971,
N977). In numerous present-day oysters such conchiolin lamellae are semi-translu-
cent and dark brown to olive in colour, with a horny appearance. Even during the li-
fetime, they are prone to split and decay in the old parts.
The largest part of the shell interior in Lithioperna is occupied by the central pla-
te. The bivalve's mantle spread over its entire surface. The mantle was also a respira-
tory organ, relieving the gills. From the water it absorbed oxygen and the calcium
ions needed to build the shell (Stenzel, 1971, N967). The larger mantle surface in
Lithioperna provided a larger respiratory capacity, which was needed to enable the
bivalve to thrive and build an immense shell.
18_Irena Debeljak & Stanko Buser
Conclusion
The period in which lithiotid bivalves flourished was relatively short. They mostly
disappeared in the transition from the Pliensbachian to the Toarcian. Their extincti-
on was hastened by extensive tectonic movements, changes in the sea level causing
fundamental changes in habitat and the environment which highly specialised orga-
nisms could not successfully respond to (Buser & Debeljak, 1996).
The similarity of the sessile mode of life in a specific environment (muddy substra-
tum, rapid sedimentation) and in dense aggregates, and the associated morphological
properties and adaptations characteristic of the convergent and contemporary genera
Lithiotis, Cochlearites and Lithioperna, justify the use of a common name, although
the name lithiotid bivalves does not have any taxonomic significance.
All three genera are distinguished by their very large, unusually shaped and re-
markably variable shells. Their principal characteristic is their flatness and height
elongation, i.e. in the direction of growth, which made more or less constant progress
during their entire lifetime. The body space of lithiotid bivalves was unusually small:
it occupied only the far ventral end of an otherwise very large shell. The valves were
tightly joined over almost the entire surface. All three genera have a tripartite inter-
nal construction in common, with feather-like areas on the sides and a central area
with a very different shape in the middle. Lithiotid bivalves had a considerable capa-
city for twisting as they grew.
The great variability of lithiotid bivalves can be attributed to the constant adapta-
tion of the growth of the shell to changes in the environment and in the close commu-
nity in which hundreds of individuals of the same species crowded together, thus pro-
viding mutual support and spreading out in bunches, similarly to plants in their se-
arch for sunlight.
Lithiotid bivalves are undoubtedly one of the most interesting and distinctive fos-
sil groups in Slovenia. Since their first discovery (B u s e r, 1965) a systematic de-
scription has long been delayed. This paper also presents their mode of life, which
was reconstructed using the authors' own observations and the findings of numerous
researchers from around the world. Many questions remain unanswered, and new
findings on these unusual bivalves can be expected in the future.
Acknowledgements
The photographs of lithiotid bivalves were made by Franci Cimerman and Marjan
Grm. Alba Debeljak translated papers from Italian. The translation of the Slovene
text into English was done by Dan Ryan. The authors would like to express their gra-
titude to all those who assisted in the paper's development in any way.
Litiotidne školjke v Sloveniji in njihov način življenja__	^
Litiotidne školjke v Sloveniji in njihov način življenja
Uvod
V	srednjem liasu so školjke množično poseljevale pretežno mirno, blatno dno zapr-
tega šelfa na Dinarski karbonatni platformi. Horizont s školjkami se v spodnjejur-
skih plasteh južne Slovenije skorajda ne izklinja; njegova debelina na nekaterih me-
stih doseže kar 75 metrov. Po značilnih školjkah ga imenujemo "litiotidni horizont".
B u s e r (1965, 44-46) ga je uvrstil v pliensbachij oziroma njegov zgornji del - dome-
rij. V predzadnji številki Geologije so bila predstavljena najpomembnejša nahajališča
spodnjejurskih školjk v Sloveniji, ki se sicer razprostirajo v dolžini več kot sto in v ši-
rini več deset kilometrov. Podane so bile paleoekološke razmere, ki so omogočile raz-
cvet značilne školjčne favne (Buser & Debeljak, 1996).
V	tokratnem prispevku so sistematično opisane litiotidne školjke in njihov način
življenja. Izraz litiotidne školjke je pri nas še vedno v veljavi, čeprav nima več takso-
nomskega pomena. Združuje tri zelo pogostne in značilne, na prvi pogled podobne
rodove Lithioperna (syn. Lithiopedalion), Cochlearites in Lithiotis. Vsi trije rodovi so
bili široko razširjeni po obrobnih plitvomorskih predelih zahodnega in južnega obro-
bja Tetide in celo vzhodnega Pacifika (rod Lithiotis). Litiotidne školjke so zato pa-
leogeografsko, biostratigrafsko in paleoekološko zelo pomembne. Zaslovele pa so
predvsem po zaslugi svoje nenavadne oblike, ki paleontologe še danes izziva k razli-
čnim znanstvenim razlagam.
Preostale vrste spodnjejurskih školjk iz južne Slovenije bodo predstavljene ob dru-
gi priložnosti.
Dosedanje raziskave litiotidnih školjk v Sloveniji
V	letih 1959-65 je B u s e r (1965) geološko kartiral ozemlje južne Slovenije. Pri
tem je razčlenil jurske plasti in nabral bogat fosilni material, predvsem na stotine
primerkov nenavadno oblikovanih velikolupinskih školjk. Med njimi je določil vrsti
Lithiotis problematica in Cochlearites loppianus, ki so ju prej poznali s klasičnih na-
hajališč severne Italije. Ugotovil je, da večina primerkov v južni Sloveniji pripada
novemu rodu školjk z večveznim ligamentom. V svoji doktorski disertaciji ga je poi-
menoval Lithiopedalion. Skupaj z rodovoma Lithiotis in Cochlearites ga je zaradi
očitne podobnosti uvrstil v družino Lithiotidae. B u s e r (1972) je svoje delo predsta-
vil na mednarodnem srečanju paleontologov v Gradcu, vendar ga žal ni objavil na
način, kot zahteva Kodeks zoološke nomenklature. Rod Lithiopedalion je bil kasneje
opisan pod novim imenom Lithioperna (Accorsi Benini, 1979).
Sistematski opisi litiotidnih školjk
Subclassis PTERIOMORPHIA Beurlen, 1944
Ordo Pterioida Newell, 1965
? Subordo Lithiotina Accorsi Benini & Broglio Loriga, 1977
? Superfamilia Lithiotacea Accorsi Benini & Broglio Loriga, 1977
18_Irena Debeljak & Stanko Buser
Familia Lithiotidae Reis, 1903
Genus Lithiotis Gümbel, 1871
Lithiotis problematica Gümbel, 1871
Tab. 1, si. 1-3; tab. 2, si. 1-3
Material: Več kot sto primerkov. Večinoma so to nekaj centimetrov visoki deli
lupin z lepo razpoznavnimi značilnostmi. Pri šestih je ohranjen vrh. Le pet primer-
kov ima ohranjen del bivalne votline.
Opis: Značilno je, da je vedno ohranjena le ena lupina, tj. tista, s katero se je
školjka pritrdila na trdno podlago. Po C h i n z e i-ju (1982) je to desna lupina. Druga
lupina je bila tako tanka, da se je ohranila le fragmentarno. O njej pričajo koščki 1-2
mm debele apnenčeve "skorje", ki jih najdemo kjerkoli na notranji površini debelejše
lupine. Vrsta Lithiotis problematica je trakaste oblike, ozka in izrazito dorzo-ven-
tralno razpotegnjena. Sklepamo, da so odrasli primerki merili v višino 20-50 cm (naj-
brž pa tudi več) in so bili široki okoli 4-7 cm. Vrh je koničasto priostren. Od strani je
lupina sploščena, zato je prečni presek eliptičen. Pri večini primerkov je lupina debe-
la 1-2 cm. Vrh je pokončen ali pa nagnjen v katerokoli stran. Lupina je včasih ravna,
včasih pa precej upognjena (tab. 2, si. 1, 2). Zunanja površina pritrjene lupine je ne-
pravilno valovita; takšna, kakršno imajo ostrige. Zelo redko opazimo na njej drobne
koncentrične prirastnice.
Slika 1 shematično ponazarja obliko pritrjene lupine in značilnosti njene notra-
njosti. Hruškasto oblikovani bivalni del je bil pri odraslih osebkih v primerjavi s ce-
lotno velikostjo lupine izredno majhen; omejen je bil le na njen ventralni konec. Meh-
ko telo školjke je za nekaj centimetrov segalo tudi v izvotljeni del lupine, ki ga ime-
nujemo umbonalna votlina. Notranji opornik jo deli na manjši in večji del. Ob stra-
neh notranje površine debelejše lupine sta peresasti polji. Sestavljajo ju prirastnice,
ki se včasih združujejo v snope (tab. 1, si. 2). Med peresastima poljema je navpično,
okoli 2-4 cm široko brazdasto polje, ki je nekoliko dvignjeno nad stranski peresasti
polji. Po vsej njegovi višini potekajo vzporedne brazde z vmesnimi grebeni, ki se
cepijo v plitvejše brazde in grebenčke in ti v še plitvejše. Tako v osrednjem delu ra-
zločimo, odvisno od ohranjenosti, od manj kot 8 do več kot 50 brazd.
Struktura oziroma mikrostruktura lupine je opisana v naslednjih delih: Accor-
si Benini in Broglio L o r i g a (1977) in C h i n z e i (1982) ter povzeta v:
Buser L (1989).
Primerjav a: Iz kamnine izluščeni primerki vrste Lithiotis problematica imajo
kljub zelo izraženi variabilnosti tako svojstvene značilnosti, da jih ne moremo zame-
njati z nobeno drugo vrsto. O enkratnosti te školjke priča tudi dejstvo, da jo danes
kot edino vrsto uvrščamo v družino Lithiotidae in skupaj z vrsto Cochlearites loppia-
nus v samostojen podred Lithiotina (po Accorsi Benini & Broglio Lo-
riga, 1977). Možno povezavo z ostrigami (Böhm, 1892) sta omenjeni italijanski pa-
leontologinji ovrgli na podlagi dokazov, da so bile lupine prvotno aragonitne, medtem
ko imajo ostrige kalcitne lupine. Sistematska uvrstitev rodov Lithiotis in Cochleari-
tes še ni dokončno rešena. Chinzei (1982) meni, da sta taksonomsko blizu naddru-
žini Pteriacea, še posebno družini Isognomonidae.
Na možno istovetnost rodov Lithiotis in Plicatostylus, slednji je bil kasneje opisan
iz Oregona, je opozoril že G r u b i ć (1959 oz. 1961). B u s e r (1965) je to domnevo
potrdil s študijem originalnih primerkov, ki sta jih ameriška avtorja Lupher in Pac-
kard poslala profesorju Kühnu na Dunaj.
Litiotidne školjke v Sloveniji in njihov način življenja_	^
Od drugih litiotidnih školjk lahko vrsto Lithiotis problematica ločimo tudi po pre-
sekih v trdni kamnini. Ti so vedno enolupinski in imajo v prečnem prerezu eliptično
obliko (glej Buser & Debeljak, 1996, 36, si. 9, 10). Posebno značilni so "uše-
sasti" preseki z odprtino v sredini, po katerih je rod Lithiotis dobil svoje ime.
Nahajališča: Vrsta Lithiotis problematica je med vsemi litiotidnimi školjka-
mi v Sloveniji naj redkejša. Buser(1965)joje našel v opuščenem kamnolomu na de-
snem bregu Sušice zahodno od Dolenjskih Toplic. Na žalost je danes ta kamnolom
zasut. Nekaj primerkov je Buser našel tudi na severnem pobočju Mokrca, ko so tam
gradili novo cesto. Danes so ti useki preraščeni. Zelo številne primerke še vedno naj-
demo vzhodno od vasi Zafara pri Žužemberku. Razmeroma dobro ohranjene lupine
L. problematica je mogoče izluščiti iz apnenca nedaleč od izvira Glijun in na hribu
Poljanica zahodno od Bovca. V okrasnem podpeškem kamnu lahko opazujemo zna-
čilne preseke te vrste (Buser & Debeljak, 1996), vendar školjk L. problemati-
ca v Podpeči ne moremo izluščiti iz trdnega apnenca.
Lithiotis je v pliensbachiju in toarciju množično poselil obsežne, med seboj pove-
zane plitvomorske predele zahodnega in južnega obrobja Tetide ter vzhodnega Paci-
fika. Različni avtorji poročajo, da so ga našli v južni Španiji (?), zahodni Franciji (?),
severni Italiji, na Apeninih, Hrvaškem (?), v Hercegovini, Črni gori, Albaniji (?), Grči-
ji, Turčiji (?), Maroku (?), Somaliji (?), Omanu (?), Iranu (?), Iraku (?), na Himalaji (?),
na otoku Timor v Indoneziji, v ZDA (Oregon, Nevada, Kalifornija), Čilu in Peruju.
Nahajališča, ob katerih je postavljen vprašaj, niso zanesljivo potrjena. Podatki so po-
vzeti po: B r o g 1 i o Loriga in N e r i, 1976; A c C o r s i Benini in Bro-
glio Loriga, 1977; Geyer, 1977; Nauss in Smith, 1988; Bus er in
Debeljak, 1996.
Familia Cochlearitidae Accorsi Benini & Broglio Loriga, 1977
Genus Cochlearites Reis, 1903
Cochlearites loppianus (Tausch, 1890)
Tab. 3, si. 1-3
Material: Okoli 80 primerkov. Večidel so to do 10 cm veliki kosi levih lupin;
desnih lupin je manj. Deset primerkov ima delno ohranjeni obe lupini. Vrh lupine in
bivalna votlina nista ohranjeni pri nobenem primerku. Na šestih lupinah je vidna li-
gamentna brazda.
Opis vrste: Lupina je ozka, močno dorzo-ventralno razpotegnjena in ima koni-
čast vrh. Odrasli primerki merijo v višino od približno 20 do več kot 50 cm; široki so
povprečno 5 do 8 cm. Z levo lupino se je žival pritrdila na stabilno podlago; debela je
približno 1 do 2 cm. Desna lupina, ki je bila prosta, je tanjša od leve; v sredini meri
okoli 0.5 do 1 cm. Školjka je ponavadi ravna, lahko pa je tudi upognjena vstran (tab. 3,
si. 2). Na grobi, nepravilni zunanji površini včasih opazimo koncentrične prirastnice.
Obliko in notranjost lupine ponazarja slika 2. Notranja stran lupine ima za litioti-
dne školjke značilno tridelno zgradbo. Osrednje polje je široko okoli 2 do 4 cm, ome-
jujeta pa ga peresasti polji. Peresast videz ustvarjajo prirastnice, ki se lahko zdru-
žujejo v snope. Na obrobju peresastih polj sta lupini običajno zevali. Bivalni del
školjke je bil v primerjavi s celotno velikostjo lupine zelo majhen. Osrednje oz. glav-
no polje, kjer se lupini tesno stikata, je reliefno oblikovano. Po sredini leve lupine po-
teka bolj ali manj široka globel, ki jo omejujeta dva grebena. Vanjo se prilega osrednji
18_Irena Debeljak & Stanko Buser
greben desne lupine. Vsi omenjeni reliefi blizu telesne votline poplitvijo in niso več
tako izraziti. Na osrednjem polju pogosto opazimo polkrožne sledi; to so prirastnice,
ki jih je plaščni rob puščal za seboj, ko se je pomikal v ventralni smeri.
Pod vrhom, približno po sredini glavnega polja obeh lupin, poteka globoka, ozka
brazda (resilifer), v kateri je imel sedež vlaknati oziroma fibrozni del ligamenta. La-
melami del ligamenta se je pritrjal ob straneh te brazde. Ligamentna brazda je pri
različnih osebkih različno visoka. Ponavadi meri od 3 do 6 cm.
Slovenski primerki imajo večinoma prekristaljene lupine, med fosilizacijo pa so se
deli s prvotno različno mikrostrukturo selektivno obarvali, tako da se v presekih po-
gosto razločijo značilne poteze osnovne zgradbe lupine (B u s e r L, 1989, tab. 10; cf.
Chinzei, 1982).
Primerjav a: Iz kamnine izluščeni primerki rodu Cochlearites so tako značilni,
da jih ne moremo zamenjati z nobeno drugo školjko. Do zamenjave lahko pride, če
skušamo ugotoviti vrsto le po presekih v kamnini. Ti so lahko precej podobni prese-
kom vrste Lithioperna scutata. A pri rodu Cochlearites je leva lupina debelejša od
desne, medtem ko ima Lithiopema obe lupini enako debeli. Običajno se lupini rodu
Lithiopema ob vseh robovih tesno prilegata; ena lupina kot negativ sledi dmgi. Pri
rodu Cochlearites lupini na obrobju peresastih polj večinoma zevata. Najbolj značilni
in nedvoumni pa so prečni preseki z značilnim osrednjim grebenom v desni in z ustre-
zno vboklino v levi lupini (B u s e r & Debeljak, 1996, 32-si. 4, 41-si. 15).
Posamezni primerki vrste Cochlearites loppianus se lahko med seboj precej razli-
kujejo tako po obliki kakor po velikosti lupine. Takšne variacije so nastale zaradi
stalnega prilagajanja okolju in zaradi rasti v tesnih združbah. Praktično niti dva pri-
merka nimata enakega osrednjega polja. Glede na videz ligamentne brazde je R e i s
(1903) ločil tri tipe rodu Cochlearites. Accorsi Benini in Broglio Loriga
(1977) sta priznali le dva tipa. Normalni tip naj bi bil tisti, pri katerem je ligament
imel zvezo s plaščem, nenormalni tip (forma A) pa naj bi imel kratek, zakrnel vlakna-
ti ligament, ki ni imel več povezave z živim delom školjke, kar lahko ugotovimo po
odmaknjenih plaščevih prirastnicah. Chinzei (1982, 193) je dokazal, da praktično
vsi odrasli osebki pripadajo nenormalnemu tipu oziroma formi A, kar je posledica te-
ga, da je ligament pri rasti školjke prej ko slej zakrnel. S to ugotovitvijo odpadejo ra-
zmišljanja o različnih tipih ali celo podvrstah glede na videz ligamentnega polja.
Sistematska uvrstitev rodov Lithiotis in Cochlearites še ni dokončno rešena, prav
tako ni pojasnjena njuna morebitna sorodnost.
Nahajališč a: Buser (1965) je najlepše primerke našel jugozahodno od Loža
in na severnem in južnem pobočju Mokrca. Danes so ta nahajališča zaraščena. Posa-
mezne primerke še vedno dobimo pri izviru Globočec zahodno od Zagradca in na hri-
bu Stražišče vzhodno od Gorenjega Jezera pri Cerknici. Vrsta Cochlearites loppianus
zagotovo nastopa tudi v podpeškem kamnolomu, v okolici Grčarevca pri Logatcu, pri
Borovcu na Kočevskem in na Travni gori, kajti v teh nahajališčih smo opazili značil-
ne preseke v apnencu oziroma dolomitu.
Drugod v svetu so vrsto C. loppianus našli v sevemi Italiji, Apeninih, Čmi gori,
Grčiji in Maroku. Morda nastopa tudi v zahodni Franciji, Somaliji in na otoku Timor
v Indoneziji. (Po: Broglio Loriga & Neri, 1976; Accorsi Benini &
Broglio Loriga, 1977; Geyer, 1977; Buser & Debeljak, 1996.)
Litiotidne školjke v Sloveniji in njihov način življenja_	^
Subordo Pteriina Newell, 1965
Superfamilia Pteriacea Gray, 1847
Familia Isognomonidae Woodring, 1925
Genus Lithioperna Accorsi Benini, 1979 - syn. Lithiopedalion Buser, 1965
Lithioperna scutata (Dubar, 1948)
Tab. 4, sl. 1; tab. 5, si. la, b; tab. 6, si. 1; tab. 7, si. 1-3; tab. 8, si. 1; Tab. 9, si. la, b
Material: Približno 80 primerkov; večina med njimi ima ohranjeni obe lupini.
V zbirki prevladujejo fragmenti vrhnjih delov glavnega polja z ligamentnimi brazda-
mi. Obrobni deli v splošnem niso ohranjeni.
Opis rodu in vrste: Lupina ima jezičasto obliko in je pogosto izrazito ve-
lika. Preseki v apnencu pričajo, da so nekateri osebki dosegli v višino kar 3/4 metra.
Obrobni deli lupine so zelo stanjšani, zato nam doslej še ni uspelo najti nepoškodova-
nega, v celoti ohranjenega primerka. Pri navajanju velikosti smo se zato prisiljeni
opirati na preseke v apnencu. V povprečju merijo lupine v višino 30 do 70 cm. Višina
je ponavadi približno dvakrat večja od dolžine. Od strani je školjka izrazito splošče-
na. Obe lupini sta po obliki, velikosti in debelini enaki ter se tesno prilegata. Skupaj
sta debeli 1 do 4 cm. Vzdolžni presek ima pogostokrat valovit videz. Zunanja površi-
na lupine je ponavadi vegasta in nepravilna. Pri redkih primerkih pa se na njej vidijo
koncentrične prirastnice.
Značilnosti notranjosti lupine ponazarja slika 3. Ob straneh sta razviti ploski pe-
resasti polji z razločno vidnimi prirastnicami. (Prirastnice kažejo nekdanji stranski
obris telesne votline, ki se je z rastjo školjke pomikala v ventralni smeri). Sprednje
peresasto polje je praviloma večje in velikokrat kolenasto upognjeno. Ob njegovem
notranjem robu poteka bisusna zajeda. Pod vrhom je oblikovana kot bisusna brazda,
proti bivalnemu delu lupine pa jo omejujeta sprednji obrobni gubi oziroma obrobni
liniji. Med njima je v desni lupini bolj ali manj izrazit širok greben, v levi lupini pa
ustrezna poglobitev, ki je lahko plitva (tab. 5) ali precej globoka (tab. 6).
Med stranskima peresastima poljema je gladko, bolj ali manj ravno osrednje polje,
ki zavzema največji del notranjosti lupine. Po tej površini se je raztezal školjkin plašč
in tu sta se lupini tesno stikali. Pod vrhom je razvito značilno ligamentno polje z rav-
nim oziroma redkeje polkrožnim zgornjim robom. Ligament je bil večvezen. Vlaknati
ali fibrozni del ligamenta se je pritrjal v več ligamentnih brazdah, med katerimi je
imel sedež lamelami del ligamenta. Z rastjo školjke se je ligament selil v ventralni
smeri in puščal za sabo drobne prirastnice, ki so v brazdah upognjene konveksno gle-
de na vrh, med brazdami pa so konkavne. Ligamentne brazde so običajno približno
vzporedne. Pri nekaterih primerkih pa je med rastjo prišlo do spremembe v legi ozi-
roma težišču lupine in ligament je poslej napredoval v drugi smeri kakor na začetku.
Ligamentne brazde včasih potekajo tako izrazito proti sprednjemu delu, da lahko do-
sežejo sprednje peresasto polje (tab. 7, sl. 2, 3).
Tudi če bisusna zajeda na sprednji strani nekega primerka ni ohranjena, lahko
ugotovimo, ali gre za desno ali levo lupino. Mejo med osrednjim in peresastim poljem
postavimo navpično. Skoz konce ligamentnih brazd si zamislimo premico. Kot, ki ga
oklepa z navpičnico, je na sprednji strani manjši od 90 stopinj, na zadnji pa večji. S
tem sta določeni leva in desna lupina.
Gostota ligamentnih brazd je lahko različna. Glede na širino lamelarnega dela li-
gamenta, to je presledkov med posameznimi ligamentnimi brazdami, ločimo dve
18_Irena Debeljak & Stanko Buser
osnovni skupini. Brazde so v obeh skupinah široke 2-3 mm. Razmiki med njimi so v
prvi skupini široki 2-3 mm, v drugi pa 8-10 mm. V prvi skupini naštejemo približno
16 brazd na 10 cm sklepnega roba, v drugi pa jih je približno 8. Pri nekaj primerkih
je razvidno, da so posamezne ligamentne brazde zaostale v rasti, medtem ko so druge
rastle naprej (tab. 7, si. 2; tab. 9). Sklepamo lahko, da je v drugi skupini približno
vsaka druga brazda z vlaknatim ligamentom zakrnela in se je na njeno mesto pritrjal
lamelami del ligamenta. Ligamentne brazde so lahko visoke le nekaj mm pa tja do 8
cm. Od debeline lupine pod vrhom je odvisno, koliko ligamentnega polja se je ohrani-
lo. Debelejša je lupina, daljše so brazde. Večina primerkov pa je pod vrhom močno
stanjšana.
Bivalni del školjke z enim mišičnim odtiskom je v primerjavi s celotno velikostjo
lupine majhen. Zavzema le ventralni del lupine in deloma prostor med sprednjima
obrobnima gubama. Depresija za mehko telo je zelo plitva.
Analiza izredno dobro ohranjenih primerkov iz okolice Verone je pokazala, da so
bile lupine rodu Lithioperna prvotno aragonitne (Accorsi Benini, 1979, 228).
Pri mnogih primerkih iz Slovenije že s prostim očesom opazimo značilno zgradbo lu-
pine, to je menjavanje svetlejših in temnejših lamin oziroma plasti, ki so vzporedne
med seboj in z zunanjo površino lupine (B u s e r L, 1989, tab. 11, si. 1). Svetle, ble-
ščeče lamine so imele nekoč prizmatsko mikrostrukturo, temnejše, motne plasti pa
mikrostrukturo biseme matice (Accorsi Benini, 1979).
Primerjava: Rod Lithioperna danes uvrščamo med Isognomonidae. Sklepne
zobce pri teh školjkah nadomešča velik večvezni ligament. Accorsi Benini-
j e v a (1979) je pred uvrstitvijo med izognomonide postavila vprašaj, ker je struktura
lupine pri vrsti L. scutata razvita zelo samosvoje. Med izognomonidami bi lahko z ro-
dom Lithioperna primerjali rod Isognomon Solander in Lightfoot, 1786 (syn. Perna
Bruguiére, 1789), pri katerem so se lupine pritrjale na podlago prav tako z bisusom.
Vendar, pri rodu Isognomon ni velikega osrednjega stičnega polja. Peresasti polji ob
straneh nista razviti, lupina je bikonveksna in v strukturi lupine se ne menjavata dve
vrsti lamin.
Rod Lithioperna lahko spoznamo tudi po presekih v kamnini (Buser & De-
beljak, 1996, 32, si. 4). Značilni preseki so zelo dolgi, tanki in ponavadi rahlo valo-
viti. Obe lupini sta enako debeli in se na vseh robovih tesno prilegata. Običajno zeva
le močno stanjšani bivalni del lupine (slika 3).
Rod Lithioperna zaenkrat obsega le vrsto L. scutata, čeprav se posamezni primer-
ki med seboj precej razlikujejo. Variacije v razmerjih dimenzij in obliki lupin so izre-
dno številne. Različno je tudi ligamentno polje. Niti dva primerka nista enaka. Ven-
dar, bistveno odstopa le primerek, opisan v nadaljevanju, ki pa ne zadošča za opis no-
ve vrste.
Pripombe: Ime rodu Lithiopedalion je postavil B u s e r (1965) v svoji doktor-
ski disertaciji, in sicer po skupnih značilnostih rodu Lithiotis in Pedalion Dillwyn,
1817 (star sinonim za rod Isognomon). Novi rod in vrsto je predstavil na 42. letnem
srečanju Paleontološkega društva v Gradcu (Buser, 1972). Tega dela žal ni objavil
na način, kakor to zahtevajo mednarodna pravila, vendar se je ime Lithiopedalion
vseeno uveljavilo v Sloveniji in se uporabljalo tudi v tuji strokovni literaturi (cf. Bo-
sellini, 1972; Broglio Loriga & N e r i, 1976). Z objavo ga je po dolgole-
tnem sodelovanju prehitela italijanska paleontologinja Accorsi Benini (1979)
in opisala rod pod drugim imenom: Lithioperna.
Buser (1965) je hkrati z rodom opisal tudi novo vrsto Lithiopedalion kuehni,
vendar jo je Accorsi Benini (1979) poistovetila z vrsto Perna scutata, ki jo je
Litiotidne školjke v Sloveniji in njihov način življenja_	^
opisal Dubar (1948) iz Maroka, ter jo vključila v rod Lithioperna. Glede na to, da je
Beninijeva preučila originalni material iz Dubarjeve zbirke, moramo zaenkrat verjeti
njenim ugotovitvam, da gre za isto vrsto. Vendar avtorja kljub temu meniva, da tri
skromne upodobitve slabo ohranjenih primerkov ne kažejo vseh značilnosti tipičnih
primerkov iz Slovenije ali severne Italije. Po obliki precej spominjajo na rod Gerville-
ioperna Krumbeck, 1923; predvsem velja to za Dubarjev primerek, ki ga je pri opisu
novega rodu kot paralektotip upodobila Accorsi Benini (1979, 251, sl. 14). Po
opisu in slikah sodeč je sprednje peresasto polje razvito kot lunula, bisusna zajeda je
zelo globoka in se kmalu pod vrhom razširi in poglobi v telesno votlino. Naši in itali-
janski primerki imajo precej bolj dorzoventralno razpotegnjeno lupino, višje liga-
mentno polje, večinoma plitvejšo bisusno zajedo in predvsem večje osrednje polje,
kjer sta se lupini tesno stikali. V bodočnosti bi bilo vsekakor dobro opraviti revizijo
vrste Lithioperna scutata na originalnem Dubarjevem materialu, ki je sedaj v Franci-
ji (Lille).
Nahajališč a: V litiotidnem horizontu srednjeliasnih plasti v Sloveniji (Trnov-
ski gozd, Hrušica, Nanos, Logaška planota, Krimsko-Mokrško hribovje. Dolenjska) je
Lithioperna scutata najpogostnejša vrsta, prisotna skorajda na vseh nahajališčih
(Buser & Debeljak, 1996, 28, sl. 2, v tem članku predstavljena kot Lithiope-
dalion scutatus). Primerke, ki se dajo izluščiti iz lapornatih plasti med apnencem, da-
nes najdemo na Špiku severno od Cola na Trnovskem gozdu, v kamnolomu v Podpeči
ter ob železnici med postajama Preserje in Verd. B u s e r (1965) je našel številne pri-
merke na Krimsko-Mokrškem hribovju; useki takrat narejenih cest pa so danes sko-
raj docela zaraščeni. Zaraščena so tudi nahajališče na Javorniku in nahajališči sever-
no od Cerknice in južno od Loža. Kamnolom na desnem bregu Sušice zahodno od Do-
lenjskih Toplic pa je danes zasut.
Drugod po svetu so rod Lithioperna našli v liasnih plasteh severne Italije (Berti
C a v i C C h i et al., 1971; B r o g 1 i o Loriga & N e r i, 1976), na Apeninih, v Al-
baniji, Grčiji, Franciji in Maroku (po Accorsi Benini, 1979). Rey (1990; 1997)
jo navaja tudi iz jugovzhodne Španije. Ocenjujemo, da je rod Lithioperna prisoten še
marsikje na obrobju nekdanje Tetide, le da so ga avtorji označili z imenom Perna ali
Isognomon, ponekod pa so ga morda pripisali ostrigam (npr. rodu Pernostrea Munier-
Chalmas, 1864 z večveznim ligamentom).
? Lithioperna sp.
Tab. 10, sl. 1
Material, najdišče: En sam, slabo ohranjeni primerek z obema lupinama;
ohranjeni so le ligamentno polje, del osrednjega stičnega polja in zgornji del bisusne
zajede. Primerek je bil najden ob gozdni cesti severno od vrha Mokrec.
Opis in primerjav a: Na ligamentnem polju so razvite izredno tanke liga-
mentne brazde, široke okoli en mm. Na 11 cm dolgem sklepnem robu (toliko ga je
ohranjenega) lahko naštejemo kar 36 ligamentnih brazd. To je vsaj enkrat več, kakor
je običajno za vrsto Lithioperna scutata. Gostota in višina ligamentnih brazd (več kot
4 cm) bi lahko ustrezala rodu Isognomon oz. podrodu Hippochaeta Philippi, 1844, ki
pa ga poznamo le iz terciarnih plasti. Toda pri rodu Isognomon se pod ligamentom
začne bivalna votlina školjke, pri opisanem primerku pa se podobno kot pri vrsti Li-
thioperna scutata ligamentne brazde končujejo na osrednjem, ravnem stičnem polju,
kjer sta se lupini tesno prilegali.
18_Irena Debeljak & Stanko Buser
Za natančnejšo določitev oziroma za opis nove vrste je primerek preslabo ohra-
njen.
Način življenja litiotidnih školjk
Na področju sedanje južne Slovenije so v srednjem liasu litiotidne školjke množi-
čno uspevale v pretežno mimem okolju bolj ali manj zaprtega self a na Dinarski kar-
bonatni platformi. Posamezne vrste so sestavljale monokolonije v obliki podmorskih
trat (biostrome) s posameznimi lečastimi odebelitvami. S severa je bilo čutiti zmeren
vpliv pelagiala. Sedimentacija v laguni je bila hitra, substrat na morskem dnu pa je
sestavljalo predvsem blato, kar smo ugotovili glede na lastnosti prikamnine v školj-
čnih lumakelah. Biodiversiteta v takšnih lumakelah je zelo nizka. Do 3/4 metra viso-
ke lupine so školjke lahko gradile le v tropskih ali subtropskih pogojih. Paleogeo-
grafske in paleoekološke razmere, ki so pogojevale obstoj značilne školjčne favne, so
bile že opisane; prav tako sta bili že opisani razširjenost in razporeditev različnih
vrst (Bus er & Debel j a k, 1996).
Sledi opis načina življenja in posebnih prilagoditev pri posameznih rodovih.
Rod Lithiotis
Čeprav so ga prvi raziskovalci v Italiji poznali že pred več kot 250 leti, je za paleon-
tologe še danes velika uganka. Iz kamnine izluščeni primerki res ne spominjajo na da-
našnje školjke in prav nič čudno ni, da so jih sprva opisali kot rastlinske ostanke
(Gümbel, 1871; cf. 1890). Domnevamo, da se je življenje teh školjk razvijalo takole:
Ličinke so sprva plavale oziroma lebdele v vodi, dokler niso našle za pritrditev
primernega mesta. Na blatnem morskem dnu so najtrdnejšo podlago predstavljale lu-
pine drugih školjk; ponavadi odrasli osebki rodu Lithiotis. Juvenilne školjke so se
nanje pritrdile s posebno, lepljivo snovjo, obrnjene tako, da so lahko rastle navpično
navzgor. O tem pričajo številni primerki rodu Lithiotis, ki imajo na svoji površini pri-
trjene enako orientirane mlajše osebke (B u s e r, 1965, tab. 8, 9). Tudi po presekih v
apnencu lahko opazimo, da so posamezni odrasli primerki dobesedno cementirani
eden ob drugega (Buser & Debeljak, 1996, si. 9). C h i n z e i (1982) je menil,
da je bilo mesto pritrditve majhno, oziroma da je bila cementacija možna le pri juve-
nilnih osebkih. Avtorja pa sva na podlagi svojih opazovanj prišla do sklepa, da so te
školjke obdržale sposobnost cementiranja vse življenje, tako da je bila konstrukcija
njihovih agregatov zelo trdna in so tudi po odmrtju pogosto ostale v življenjskem po-
ložaju.
Školjke rodu Lithiotis so vsekakor živele v velikih skupinah, si med seboj dajale
oporo in se šopasto razraščale (si. 4). Sedimentacija karbonatnega blata je bila precej
hitra, tako da je bila lupina vseskozi zasidrana v njem. Bivalni del se je povečeval le
pri mladih osebkih, potem pa je lupina rastla le še v višino, v subvertikalni smeri (cf.
G Ö h n e r, 1980; Chinzei, 1982; C h i n z e i et al., 1982; S e i 1 a c h e r, 1984). O
rastnem vzorcu nam mnogo povedo prirastnice na zunanji površini in peresastih po-
ljih. Združujejo se v snope, ki si pri dobro ohranjenih primerkih sledijo približno na
centimeter Chinzei (1982, 189-192) je sklepal, da je šlo za menjavanje obdobij hi-
trejše in počasnejše rasti; nekakšne letnice torej. S štetjem prirastnic in merjenjem
velikosti lupine je prišel do sklepa, da je v približno štiridesetih letih lupina dosegla
Litiotidne školjke v Sloveniji in njihov način življenja_	^
višino 50 cm. V vsem tem času se je okoli nje nabiralo blato. Osebke, ki niso dovolj
hitro rastli, je blato zasulo. To se je večkrat zgodilo z juvenilnimi primerki, ki si zara-
di gneče niso uspeli izboriti življenjskega prostora. Ko se je kak osebek nevarno na-
gnil, je z nadaljnjo rastjo še vedno lahko popravil svoj položaj. Takšni primerki s ko-
lenasto upognjenimi lupinami so zelo pogosti (tab. 2, sl. 1).
Zanimivo je, da se je pri rodu Lithiotis vedno ohranila le debelejša, verjetno desna
lupina, to je tista, s katero se je školjka cementirala na podlago. Druga, prosta lupina
je morala biti izredno tanka. O njej pričajo le koščki tanke skorje na debelejši lupini.
Tudi v trdni kamnini lahko opazimo le enolupinske preseke rodu Lithiotis. Med njimi
so včasih "razmetane" tanke sledi, ki bi lahko predstavljale polomljene ostanke dru-
ge, tanke lupine (Buser & Debeljak, 1996, 36). Še danes ne vemo, ali je bila
ta lupina enako velika kot debela, tj. pritrjena lupina (Reis, 1903, 11; Chinzei,
1982, 181; S a v a z z i, 1996, 287) ali pa je pokrivala mehko telo školjke le v obliki
pokrovčka (L u p h e r & P a c k a r d, 1930; A c c o r s i Benini & Broglio
Loriga, 1977, 21). Debela, pritrjena lupina je bila pod spodnjim delom brazdastega
polja izvotljena. Tu je mehko telo našlo oporo, tako da s svojo težo ni obremenjevalo
tanke, proste lupine.
Mehanizem odpiranja in zapiranja lupin še ni docela pojasnjen. Navadno pri
školjkah odpira lupini ligament, s tem da deluje kot nasprotje zapiralni mišici.
Chinzei (1982, 193) je menil, da funkcionalnega ligamenta pri rodu Lithiotis sploh
ni bilo in da sta se lupini lahko odpirali in zapirali zaradi prožnosti tanjše lupine. Ta
naj bi bila upognjena prek spodnjega roba brazdastega polja (v tem primeru bi bil to
sklepni rob), kadar je bila zapiralna mišica skrčena. Ko je mišica popustila, se je tan-
ka lupina zravnala in školjka je zazevala. Pri tej hipotezi se lahko vprašamo, kaj (ra-
zen blata) je lupini na dorzalnem koncu držalo skupaj, če ligamenta in sklepnih zob-
cev ni bilo. Velikost školjke je bila zelo velika, bivalni del z zapiralno mišico pa maj-
hen. Če sta bili lupini res enako visoki, bi pri odpiranju zlahka prišlo do zamika lupin
in vdora blata. Pri tem je torej res bolj verjetno, da je tanjša lupina oblikovala le ne-
kakšen pokrovček nad bivalnim delom debelejše lupine. Tej domnevi pa nasprotuje
dejstvo, da najdemo ostanke tanke lupine po vsej višini druge lupine. Morda je tanjša
lupina na dorzalnem delu postopoma razpadala in se lomila, medtem ko je školjka
rastla v višino in se je bivalni del vse bolj odmikal.
Nerešeno je tudi vprašanje, čemu je služilo osrednje brazdasto polje. Težko je ver-
jeti, da je šlo le za igro narave. Na njem ni nobenih sledi, ki bi jih za sabo pustil plašč.
Mehki del školjke na to polje torej ni segal. Böhm (1892) je celo menil, da so značil-
ne brazde nastale sekundarno, zaradi preperevanja. Gümbel (1890, 65) in B u s e r
(1965, 18) sta brazdasto polje opisala kot sedež ligamenta. Reis (1903, 43) je trdil,
da je ligament zakrnel in da je nazobčanost brazdastega polja delovala kot nekakšen
sklep. Italijanski paleontologinji Accorsi Benini in Broglio Loriga
(1977, 21-24) sta domnevali, da je brazdasto polje le nadgradnja, ki prekriva posebne
kanalčke v lupini, kjer so se nahajali tanki priveski plašča - centri kalcifikacije.
Brazde naj bi nastale tako, da so se ti kanalčki odpirali navzven. Pred kratkim je
S a v a z z i (1996) našel dokaze o prisotnosti ligamenta na osrednjem polju. Menil je,
da je bil ligament pri rodu Lithiotis aktiven po vsej višini brazdastega polja in ne le
na ventralnem oziroma sklepnem robu, kot je to običajno pri školjkah z večveznim li-
gamentom. Ligamentna struktura naj bi bila pri rodu Lithiotis zelo samosvoje razvi-
ta. S a v a z z i je ugotovil, da so bila vlakna ligamenta oblikovana in pritrjena tako,
da so dopuščala majhne razlike v medsebojni razdalji obeh lupin. S tem se je poveča-
la površina, prek katere se je upognila tanjša prosta lupina, kadar se je školjka zapr-
18_Irena Debeljak & Stanko Buser
la. Obremenitev se je tako enakomerne j e porazdelila; zmanjšala se je napetost in
hkrati možnost, da bi se tanka lupina polomila ali poškodovala. Prosta lupina se je
torej po mnenju Savazzija zapirala z upogibanjem, kakor je ugotovil že C h i n -
z e i (1982), vendar ne z artikulacijo prek sklepne osi.
Chinzei (1982; 1986) je primerjal obliko lupine in način rasti rodov Lithiotis in
Cochlearites z nekaterimi ostrigami, ki so prav tako močno razpotegnjene v višino in
živijo, oziroma so živele v vertikalnem položaju na mehkih, blatnih tleh. Rod Sacco-
strea Dollfus & Dautzenberg, 1920, ki živi ob obali vzhodne Afrike, po obliki spomi-
nja na rudiste; tanko prosto lupino ima oblikovano kot pokrovček (S t e n z e 1, 1971).
Konbostrea Chinzei, 1986, zgornjekredna ostriga iz severne Japonske, po obliki in ra-
zrasti še najbolj spominja na rod Lithiotis.
Domnevamo, da je bila vrsta Lithiotis problematica pri izbiri habitata zelo izbir-
čna. Podatki s slovenskih nahajališč kažejo, da je sestavljala monokolonije; zahtevala
je samostojen življenjski prostor, ki ga ni delila z drugimi litiotidnimi školjkami in je
izključeval tudi večino drugih organizmov (Buser & Debeljak, 1996). Rod Li-
thiotis je med litiotidnimi školjkami v Sloveniji najredkejši.
Rod Cochlearites
Cochlearites je v litiotidnem horizontu Slovenije zelo pogost, predvsem tam, kjer
so se nekoč razprostrirali zatišni deli šelfa.
Lupina rodu Cochlearites je rastla podobno kot pri rodu Lithiotis (glej prejšnje po-
glavje). Zaradi ozke in sploščene oblike se je skupaj lahko nagnetla množica istovr-
stnih primerkov. Življenjski položaj posameznih osebkov je bil navpičen, takšen, kot
ga danes na morskem dnu zavzema rod Pinna ali leščur Tudi ta ima lupino zelo po-
daljšano v višino, vendar se z rastjo veča tudi bivalni del školjke z mehkim telesom.
Leščur se lahko z mišičasto nogo sam zakoplje v peščeni ali blatni sediment, nakar se
s šopom bisusovih vlaken pritrdi na kakršnokoli trdno podlago, na primer na poko-
pani kamen (C o x, 1969, N8-N10). Rodova Cochlearites in Lithiotis se nista mogla
sama vkopati, saj je majhno mehko telo zavzemalo le ventralni konec školjke; poleg
tega je bila ena lupina nemobilna, vseskozi cementirana na trdno podlago. Pri tem
avtorja meniva, da je bila sposobnost cementiranja lupine pri rodu Cochlearites mno-
go bolj omejena kakor pri rodu Lithiotis, zato konstrukcija njihovih združb ni bila
tako trdna. Sedimentacija karbonatnega blata, v katerem je bila školjka zasidrana, je
bila precej hitra, tako da je imela lupina vseeno dovolj opore. Precej blata so okoli se-
be akumulirale tudi školjke same, saj so neprestano filtrirale velike količine kalne
vode, v kateri so lebdeli drobci hranilnih snovi. Lahko si predstavljamo, da je moralo
biti sedimentaci j sko okolje kolikor toliko mirno. Močni valovi in tokovi bi lahko iz-
prali blato, ki je dajalo školjkam oporo, in jih razmetali po morskem dnu, kjer bi bile
obsojene na propad.
Chinzei (1982, 193) je ugotovil, da je bil ligament rodu Cochlearites aktiven le
pri zelo mladih osebkih, potem pa je zakrnel in ni imel več povezave s plaščem oziro-
ma z živim delom telesa. Večina školjk ligament potrebuje, ker kot nekakšna vzmet
nasprotuje delovanju zapiralne mišice in tako odpira lupini, hkrati pa ju tudi pove-
zuje. Pri rodu Cochlearites je reliefno oblikovano glavno polje prevzelo vlogo skle-
pnih zobcev (ki jih litiotidne školjke nimajo), tako da ni prihajalo do zamika lupin.
Večji del lupine je tičal v blatu. Po vsej višini se torej lupina ni smela odpirati, ker bi
vanjo vdrlo blato. Na osrednjem polju sta se lupini ves čas tesno stikali, v bivalnem
Litiotidne školjke v Sloveniji in njihov način življenja__^
delu pa nekoliko zevali, zato sta pri rasti ostajali sploščeni in vzporedni. Trdni lupini
sta se morda podaljševali v dolge konhiolinske lamele (Chinzei, 1982, 194). S po-
močjo teh prožnih lamel se je školjka lahko neprodušno zaprla, podobno kot nekatere
današnje školjke (cf. Stenzel, 1971, N977-978). Ko je zapiralna mišica popustila,
so se lamele zravnale in na koncu je zazevala ozka reža. Prožnost ventralnih delov lu-
pin je tako po mnenju C h i n z e i a nadomestila ligament.
Podobno kot pri rodu Lithiotis je bilo mehko telo zelo majhno in je vseskozi osta-
jalo nad ravnijo obdajajočega blata. Skozi ozko režo na ventralnem koncu je školjka
verjetno iztegovala plašč. Z njim je lovila in absorbirala snovi, potrebne za gradnjo
velike lupine.
Rod Lithioperna
Med vsemi litiotidnimi školjkami v Sloveniji je rod Lithioperna najpogostejši. Nje-
gove lupine leže v apnencu večidel vzporedno s plastnatostjo. Le redko najdemo pla-
sti s številnimi, navpično orientiranimi preseki, ki nam zgovorno kažejo, kakšna je
bila njihova življenjska lega.
Lupine rodu Lithioperna so bile prvotno z bisusom pritrjene na podlago, podobno
kot pri drugih izognomonidah. Bisusna žleza je izločala posebno snov, ki se je v vodi
strdila v bisusne nitke. Bisus je izhajal iz sprednje strani lupine. Sprednje peresasto
polje je običajno močneje razvito od zadnjega. Sprednja stran lupine je bila obrnjena
proti šibkim tokovom, ki so školjki prinašali hranljive snovi (sl. 5). Pri večini primer-
kov potekajo ligamentne brazde nekoliko poševno proti sprednjemu robu. Skozi
njihove spodnje konce poteka sklepna os. Poševnost sklepnega robu si lahko razloži-
mo tako, da se je zadnja stran lupine bolj pogreznila v mehak sediment kakor spre-
dnja. Sklepni rob je pri tem moral ostati vzporeden s površino substrata, sicer bi pri
odpiranju lupine v zadnji del vdrlo blato (sl. 5).
Tako kot vse litiotidne školjke, je tudi rod Lithioperna zelo hitro rastel v višino.
Učinkovitost bisusa je bila vse manjša in vse težje je držal veliko lupino v navpični
legi. Posamezni osebki so se med seboj podpirali, podobno kot knjige na knjižni poli-
ci. Poleg tega so bili zasidrani v blatu. Tisti primerki, ki niso imeli dovolj trdne opore,
pa so se prej ko slej zvrnili. Ostanek življenja so potem preživeli tako, da so plosko le-
žali na morskem dnu. Accorsi Benini (1979, 245-246) je sklepala, da je lupina
lahko prilagajala svojo rast, tako da je ventralni konec lupine z mehkimi deli školjke
vedno ostajal nad nivojem blata. Zaradi sprememb v smeri rasti ima školjka značilen
valovit videz. Takšna valovitost je dodatno pomagala ohranjati učinkovit medsebojni
stik obeh lupin. Velika, ploska lupina je bila na morskem dnu stabilna in ni potonila
v mehak sediment. Šibki plimski tokovi so z nje od časa do časa sprali blato. Najbrž
so se bile škojke rodu Lithioperna sposobne tudi same očistiti, podobno kot to počne-
jo nekatere ostrige. Z naglim skrčenjem zapiralne mišice iz plaščeve votline izbrizga-
jo močan curek vode, ki očisti njihovo površino (Stenzel, 1971, N1001).
Školjke rodu Lithioperna so živele v tako plitvi vodi, da so se od časa do časa mor-
da celo znašle na suhem. V tem primeru se je lupina hermetično zaprla. Prav tako se
je zaprla, kadar so na morskem dnu zaradi razpada organske snovi zavladali reduk-
cijski pogoji. V takšnih obdobjih je bila školjka prisiljena dihati anaerobno. To se po
mnenju paleontologinje Accorsi Benini (1979, 228-242) odraža tudi v mikro-
strukturi lupine. V času rasti, ko je lupina zevala, je plašč iz vode absorbiral Ca2+ io-
ne in na svoji zunanji površini izločal biserno matico. V obdobjih anaerobnega diha-
18_Irena Debeljak & Stanko Buser
nj a pa so se v ekstrapalialni tekočini med zunanjo površino plašča in lupino nakopi-
čili kisli produkti, ki so reagirali z lupino. Da bi se nevtralizirala kislost, so se spro-
ščali Ca^"^ ioni. Pri tem dogajanju je notranja plast lupine, ki je imela prej strukturo
biseme matice, prevzela nov psevdoprizmatski videz. To se je večkrat ponovilo, zato
se v lupini menjavata dve vrsti lamin oz. plasti z različno mikrostrukturo.
Zaradi velikosti in nagubanosti lupin je bil ligament pri odpiranju školjke močno
obremenjen. Vlaknati del ligamenta je bil nameščen v več ligamentnih brazdah. Ka-
dar je bila zapiralna mišica skrčena, so bila vlakna stisnjena, ko pa je popustila, so
se vlakna iztegnila kot vzmet. Pri tem sta lupini nekoliko zazevali. Lamelami del li-
gamenta, ki se je pritrjal med brazdami, je spajal obe lupini. Zaradi mehanskih
obremenitev se je ligament na dorzalnem koncu stalno lomil in uničeval. Ker je bil
sestavljen iz organske snovi, so ga napadale tudi bakterije. Aktiven je bil torej le
ventralni del ligamenta, ki ga je sproti izločal rob plašča. To se dogaja pri vseh
školjkah, ki imajo podoben, to je večvezni tip ligamenta (S t e n z e 1, 1979, N971-
974). Skupaj z ligamentom se je v ventralni smeri selila tudi sklepna os. Pri nekate-
rih primerkih so posamezne ligamentne brazde nehale rasti in se je na njihovo mesto
pritrjal lamelami del ligamenta. Verjetno je prišlo do tega zato, ker je školjka nujno
potrebovala učinkovit stik obeh lupin. Vlaknati del ligamenta, ki služi za odpiranje,
ni bil tako zelo pomemben. Trdni lupini sta na ventralnem koncu tako ali tako veči-
no časa za nekaj milimetrov zevali, zato sta pri rasti ostajali sploščeni in vzporedni
ter se nista bočili tako kot pri večini drugih školjk. Iz ventralne reže je Lithioperna
morda izločala strupeno sluz, da se je ubranila pred morebitnimi sovražniki. Če je
bilo potrebno, so odprtino domnevno zaprle tanke prožne luske ali lamele, ki so resa-
sto obrobljale močno stanjšani ventralni rob obeh lupin, podobno kot pri rodovih Li-
thiotis in Cochlearites. Morebitne takšne lamele se fosilno niso ohranile, ker so bile
slabo kalcificirane in bogate z organsko snovjo (cf. S t e n z e 1, 1971, N977). Pri šte-
vilnih današnjih ostrigah so konhiolinske lamele prosojne, temno rjave do olivne bar-
ve, na videz podobne roževini in se že za življenja na starejših delih sproti cepijo in
propadajo.
Največji del notranjosti lupine pri rodu Lithiopema zavzema osrednje polje. Po
vsej tej površini se je razprostiral školjkin plašč. Plašč je tudi dihalni organ, ki raz-
bremenjuje škrge. Iz vode absorbira kisik in kalcijeve ione, potrebne za gradnjo lu-
pine (S t e n z e 1, 1971, N999). Povečana površina plašča je rodu Lithiopema poveča-
la dihalno kapaciteto, ki je bila potrebna, da so te školjke tako dobro uspevale in gra-
dile izjemno velike lupine.
Sklep
Cvetoča doba litiotidnih školjk je bila razmeroma kratka. Večidel so izginile na
meji med pliensbachijem in toarcijem. Njihovo izumrtje so pospešila obsežna tekton-
ska dogajanja, spremembe v višini vodne gladine ter s tem povezane bistvene spre-
membe življenjskega prostora in okolja, na kar se visoko specializirani organizmi ni-
so mogli uspešno odzvati (B u s e r & D e b e 1 j a k, 1996).
Podoben način sesilnega življenja v specifičnem okolju (blatni substrat, hitra sedi-
mentacija) in v gostih združbah ter s tem povezane morfološke posebnosti oziroma
prilagoditve, ki so značilne za konvergentne in istodobne rodove Lithiotis, Cochleari-
tes in Lithiopema, opravičujejo uporabo skupnega imena, čeprav izraz litiotidne
školjke nima taksonomskega pomena.
Litiotidne školjke v Sloveniji in njihov način življenja_	^
Vsi trije rodovi: Lithiotis, Cochlearites in Lithiopema se odlikujejo po zelo velikih,
nenavadno oblikovanih in močno variabilnih lupinah. Njihova bistvena značilnost je,
da so izrazito sploščene in razpotegnjene v višino, to je v smeri rasti, ki je bolj ali
manj konstantno napredovala vse življenje. Bivalni del litiotidnih školjk je bil nena-
vadno majhen; zavzemal je le skrajni ventralni konec sicer zelo velike lupine. Lupini
sta se tesno stikali skorajda po vsej površini. Vsem trem rodovom je skupna tridelna
notranja zgradba s peresastima poljema ob straneh in z različno oblikovanim osre-
dnjim poljem. Litiotidne školjke so imele med rastjo precejšno sposobnost upogiba-
nja in zvijanja.
Veliko variabilnost litiotidnih školjk lahko pripišemo temu, da se je rast lupine
stalno prilagajala spremembam v okolju in tesni skupnosti, v kateri se je nagnetlo na
stotine primerkov iste vrste, ki so se med seboj podpirali in se šopasto razraščali - po-
dobno kot rastline v potrebi za sončno svetlobo.
Litiotidne školjke so nedvomno ena najbolj zanimivih in markantnih fosilnih sku-
pin v Sloveniji. Od prvih odkritij (B u s e r, 1965) smo kar predolgo odlašali z njihovo
sistematično obdelavo. V pričujočem prispevku je predstavljen tudi njihov način živ-
ljenja, ki smo ga rekonstruirali s svojimi opažanji in ugotovitvami številnih razisko-
valcev z različnih koncev sveta. Odprtih pa je ostalo še veliko vprašanj in v bodočno-
sti lahko o teh nenavadnih školjkah pričakujemo nove izsledke.
Zahvala
Fotografije litiotidnih školjk sta izdelala Franci Cimerman in Marjan Grm. Alba
Debeljak je prevedla članke iz italijanščine. Vsem, ki so kakorkoli pomagali pri na-
stanku tega prispevka, se avtorja najlepše zahvaljujeva.
18_Irena Debeljak & Stanko Buser
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86, Milano.
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of bivalves living as infaunal secondary soft bottom dwellers. - N. Jb. Geol. Paläont. Abh. 164,
229-244, 6 figs., Stuttgart.
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leont. N, Mollusca 6, V. 1. - Geol. Soc. Amer. & Univ. Kansas Press, N2-N129, New York.
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(eds.). Treatise on Invert. Paleont. N, Mollusca 6, v. 3. - Geol. Soc. Amer. & Univ. Kansas Press,
N1199-N1200, New York.
De Castro, P., 1962: 11 Giura-Lias dei Monti Lattari e dei rilievi ad ovest della valle
dell'Imo e della Piana di Montoro. - Boll. Soc. Nat. Napoli 71, 1-34, 19 Pl., Napoli.
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Mém. Serv. Géol. Maroc. 68, 1-250, 30 PL, Lille.
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Geol. Paläont. Abh. 153, 304-340, 10 figs., Stuttgart.
G ö h n e r, D., 1980: "Covel dell'Angiolono" - ein mittellissisches Lithiotis-Schlammhioherm
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G ü m b e L C. W., 1871: Anhang: Lithiotis problematica. In: Die sogenannten Nulliporen (Li-
thothamnium und Dactylopora) und ihre Betheiligung an der Zusammensetzung der Kalkgeste-
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G ü m b e 1, C. W., 1890: Lithiotis problematica Gümb. eine Muschel. - Verh. k. k. geol. Reich-
sanstalt 1890, 64-67, 1 fig., Wien.
K r u m b e c k, L., 1923: Zur Kenntniss des Juras der Insel Timor, sowie des Aucellen Hori-
zontes von Seran und Buru. I. - Paläont. Timor 12, 1-120, 2 figs., 6 PL, Stuttgart.
Lupher, R. L. & Packard, E. L., 1930: The jurassic and cretaceous Rudistids
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13 figs., Amsterdam.
Lithiotid Bivalves in Slovenia and Their Mode of Life_15
Reis, O . M., 1903: Über Lithiotiden. - Abh. Geol. Reichsanstalt J7/6, 1-44, 4 figs., 7 Pl.,
Wien.
Reis, O . M., 1923: Nachträge zur Kenntnis der Gestaltung und Stellung der Lithiotiden.
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18_Irena Debeljak & Stanko Buser
Plate 1
Lithiotis problematica Gümbel, 1871
Natural size
1	Part of the shell near the apex. Fragments of "crust" on the inner surface are remnants of the
free, very thin valve. Zafara near Žužemberk
2	Central part of the shell with the long, flat furrowed plate. On some sections larger ridges
split into smaller ones. Dolenjske Toplice
3	Fragment of the central part of the shell. Zafara near Žužemberk
Tabla 1
Lithiotis problematica Gümbel, 1871
Naravna velikost
1	Del lupine blizu vrha. Fragmenti "skorje" na notranji površini so ostanki proste, zelo tanke
lupine. Zafara pri Žužemberku
2	Osrednji del lupine z dolgim, ravnim brazdastim poljem. Na nekaterih odsekih so večji gre-
benčki razcepljeni v manjše. Dolenjske Toplice
3	Fragment osrednjega dela lupine. Zafara pri Žužemberku
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
18_Irena Debeljak & Stanko Buser
Plate 2
Lithiotis problematica Gümbel, 1871
Natural size. Dolenjske Toplice
1 Central part of a knee-shaped specimen
2, 3 Two specimens with part of the umbonal cavity
Tabla 2
Lithiotis problematica Gümbel, 1871
Naravna velikost. Dolenjske Toplice
1 Osrednji del kolenasto upognjene lupine
2, 3 Dva primerka z delom telesne oz. umbonalne votline
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
18_Irena Debeljak & Stanko Buser
Plate 3
Cochlearites loppianus (Tausch, 1890)
Natural size. Globočec near Zagradec
la Fragment of the left valve; inner surface. In the middle is the depression in which the ridge
of the right valve fits (fig. lb). Fragment of the right valve joins in the lower section
lb Piece of the right valve from the same individual. A prominent ridge runs along the middle
of the inner surface
2	Upper part of the left valve; inner surface. The ligament groove is present under the apex.
The posterior feather-like area in this specimen is more strongly developed than the anterior
3	Central part of the left valve. Mantle growth lines are visible on the cardinal area
Tablas
Cochlearites loppianus (Tausch, 1890)
Naravna velikost. Globočec pri Zagradcu
la Fragment leve lupine; notranja stran. V sredini je globel, v katero se prilega greben desne lu-
pine (s slike Ib). V spodnjem delu se je drži fragment desne lupine
Ib Košček desne lupine istega osebka. Po sredini notranje površine poteka izrazit greben
2	Zgornji del leve lupine; notranja stran. Pod vrhom poteka ligamentna brazda. Zadnje pere-
sasto polje je pri tem primerku močneje razvito od sprednjega
3	Osrednji del leve lupine. Na glavnem polju se vidijo plaščeve prirastnice
Lithiotid Bivalves in Slovenia and Their Mode of Life
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18_Irena Debeljak & Stanko Buser
Plate 4
Lithioperna scutata (Dubar, 1948)
Natural size. Podpeč quarry
1 Left valve; inner side
Lithiopema scutata (Dubar, 1948)
Naravna velikost. Kamnolom v Podpeči
1 Leva lupina; notranja površina
Tabla 4
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
18_Irena Debeljak & Stanko Buser
Plate 5
Lithiopema scutata (Dubar, 1948)
Natural size. Javornik
la Fragment of the left valve; inner side with shallow byssal notch. Growth lines can be clearly
seen on the ligament area
lb Corresponding part of the right valve of the same individual
Tabla 5
Lithiopema scutata (Dubar, 1948)
Naravna velikost. Javomik
la Fragment leve lupine; notranja stran s plitvo bisusno zajedo. Na ligamentnem polju se lepo
vidijo prirastnice
Ib Ustrezen del desne lupine istega osebka
Lithiotid Bivalves in Slovenia and Their Mode of Life
15
18_Irena Debeljak & Stanko Buser
Plate 6
Lithioperna scutata (Dubar, 1948)
Natural size. Špik in Trnovski gozd
1 Inner surface of the left valve. The anterior feather-like area is knee-shaped. The byssal notch
is very deep
Tabla 6
Lithiopema scutata (Dubar, 1948)
Naravna velikost. Špik v Trnovskem gozdu
1 Notranja površina leve lupine. Sprednje peresasto polje je kolenasto upognjeno. Bisusna zaje-
da je močno poglobljena
Lithiotid Bivalves in Slovenia and Their Mode of Life
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18_Irena Debeljak & Stanko Buser
Plate 7
Lithiopema scutata (Dubar, 1948)
Natural size
1	Fragment of the left valve; inner surface with regularly spaced ligament grooves and a deep
byssal notch. Špik in Trnovski gozd
2	Fragment of the right valve; inner surface with somewhat oblique ligament grooves, two of
which have lagged during growth. At the side is part of the anterior feather-like area, on
which the increments can be clearly seen. Podpeč
3a Fragment of the left valve; inner surface. The ligament grooves are angled towards the ante-
rior feather-like area. Between Borovnica and Verd
3b Corresponding part of the right valve
Tablai
Lithiopema scutata (Dubar, 1948)
Naravna velikost
1	Fragment leve lupine; notranja površina z enakomernimi ligamentnimi brazdami in poglo-
bljeno bisusno zajedo. Špik v Trnovskem gozdu
2	Fragment desne lupine; notranja površina z nekoliko poševnimi ligamentnimi brazdami. Dve
od njih sta zaostali v rasti. Ob strani je del sprednjega peresastega polja, na katerem se lepo
vidijo prirastnice. Podpeč
3a Fragment leve lupine; notranja površina. Ligamentne brazde so usmerjene poševno proti
sprednjemu peresastemu polju. Ob železnici Borovnica-Verd
3b Ustrezen del desne lupine
Lithiotid Bivalves in Slovenia and Their Mode of Life
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18_Irena Debeljak & Stanko Buser
Plate 8
Lithioperna scutata (Dubar, 1948)
Reduced 0.75X. Podpeč quarry, Slovenia
1 Inner side of the right valve. Only the ventral i.e. body space is missing. Longitudinal traces
run from the ligament grooves over the central plate. The anterior feather-like area is more
strongly developed than the posterior. Along this is the byssal notch
Tabla 8
Lithioperna scutata (Dubar, 1948)
Pomanjšano 0,75x. Kamnolom v Podpeči
1 Notranja stran desne lupine. Manjka le ventralni, bivalni del. Od ligamentnih brazd po osre-
dnjem polju potekajo vzdolžne sledi. Sprednje peresasto polje je močneje razvito od zadnjega.
Ob njem je bisusna zajeda
Lithiotid Bivalves in Slovenia and Their Mode of Life
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18_Irena Debeljak & Stanko Buser
Plate 9
Lithiopema scutata (Dubar, 1948)
Natural size. Špik in Trnovski gozd
la Fragment of the left valve; inner surface
lb Fragment of the right valve of the same specimen
Tabla 9
Lithiopema scutata (Dubar, 1948)
Naravna velikost. Špik v Trnovskem gozdu
la Fragment leve lupine; notranja površina
Ib Fragment desne lupine istega primerka
Lithiotid Bivalves in Slovenia and Their Mode of Life
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18
Irena Debeljak & Stanko Buser
Plate 10 - Tabla 10
? Lithiopema sp.
Natural size. Mokrec
1 Left valve of the specimen with very dense ligament grooves
? Lithiopema sp.
Naravna velikost. Mokrec
1 Leva lupina primerka z zelo gostimi ligamentnimi brazdami