GEOLOGIJA 37, 38, 23-62 (1994/95), Ljubljana
Lower Jurassic beds with bivalves in south Slovenia
Spodnjejurske plasti s školjkami v južni Sloveniji
Stanko Buser
University in Ljubljana, Faculty of Natural Science and Engineering,
Geology Department, Aškerčeva 2, SI-1000 Ljubljana, Slovenia
Irena Debeljak
Ivan Rakovec Institute of Palaeontology, Scientific Research Centre,
Slovenian Academy of Sciences and Arts, Gosposka 13, SI-1000 Ljubljana, Slovenia
Abstract
The Lower Jurassic beds of south Slovenia outcrop on a surface of several
hundred km^ with their thickness in places exceeding 300 meters. They were
deposited on the Dinaric Carbonate Platform. In them rich accumulations of char-
acteristic bivalves occur that in Pliensbachian and Toarcian inhabited the wide
interconnected shallow water regions of the western and southern margins of Tethys
and the eastern Pacific. The most interesting are three large bivalve species:
Lithiotis problematica, Cochlearites loppianus and Lithiopedalion scutatus.
In addition, numerous other genera can be found: Gervilleiopema, Mytilus, Opi-
soma and Pachyrisma (with subgenera Pachymegalodon and Durga).
The bivalves formed in the region of south Slovenia, in the prevailingly qui-
et environment of the restricted shelf, sea-bottom mats or biostromes. Their shells
can be only rarely found in their growth position. The horizon with bivalves ("lithio-
tid horizon") in south Slovenia is attributed to Pliensbachian (Domerian). It is
up to 75 metres thick and it almost does not pinch out. Within it individual lu-
machelles of bivalves occur which are from several centimetres to ten metres thick.
They are almost exclusively associated with dark, micritic, in places marly lime-
stone and bituminous dolomite. The biodiversity in lumachelles is very low. The
intermediate beds that do not contain bivalves mostly consist of oolitic and bio-
sparitic limestone. In this article some localities from various areas of the car-
bonate platform are described. Considered are paleogeographical and paleoeco-
logical conditions that permitted the existence of this typical bivalve fauna.
Kratka vsebina
Spodnjejurske plasti južne Slovenije izdanjajo na površini več 100 km^ in
ponekod dosežejo debelino več kot 300 metrov. Nastale so na Dinarski karbonat-
ni platformi. V njih so bogata nahajališča značilnih školjk, ki so v pliensbachiju
in toarciju množično poselile obsežne, med seboj povezane plitvomorske predele
zahodnega in južnega obrobja Tetide ter vzhodnega Pacifika. Najzanimivejše so
tri vrste velikih školjk: Lithiotis problematica, Cochlearites loppianus in Lithio-
pedalion scutatus. Najdemo pa še številne vrste rodov Gervilleiopema, Myti-
lus, Opisoma in Pachyrisma (s podrodovoma Pachymegalodon in Durga).
24		Stanko Buser & Irena Debeljak
Školjke so na področju južne Slovenije, v pretežno mirnem okolju zaprtega
šelfa sestavljale podmorske trate ali biostrome. Njihove lupine le malokje najdemo
v življenjskem položaju. Horizont s školjkami (»litiotidni horizont«) v južni Slo-
veniji uvrščamo v pliensbachij (domerij). Debel je do 75 metrov in se skoraj ne
izklinja. V njem so posamezne lumakele školjk debele od nekaj centimetrov do
10 metrov in so vezane skoraj le na plasti temnega, mikritnega, ponekod lapor-
natega apnenca in bituminoznega dolomita. Biodiverziteta v njih je zelo nizka.
Vmesne plasti brez školjk najpogosteje sestavljata oolitni in biosparitni apnenec.
V našem prispevku so opisana posamezna nahajališča iz različnih predelov kar-
bonatne platforme. Podane so paleogeografske in paleoekološke razmere, ki so
pogojevale obstoj značilne školjčne favne.
Introduction
In south Slovenia which is situated in the northv^^esternmost part of the Dinar-
ides the Jurassic beds exceed the thickness of 1500 metres, and they consist pre-
dominantly of limestone. They w^ere deposited on the Dinaric Carbonate Platform, in
the Mediterranean part of the ancient Tethys. Since in them no ammonites were found,
they cannot be directly subdivided into stages or even zones. For more detailed dating
the researcher can use the remains of fossil bivalves, gastropods, brachiopods, hy-
drozoans, foraminifers and algae.
For subdivision of Liassic beds the fossil bivalves are the most important. Char-
acteristic is Lithiotis problematica Giimbel from the Lithiotidae family that is the
most famous and geographically the most widespread Lower Jurassic bivalve. Its name
is derived from "ear-like" sections that were noticed in north Italy already more than
250 years ago. They were described first as plant remains (Giimbel, 1874, 1890).
The species is still "problematic" for paleontologists, since many features connected
with the unusual shape of its shell cannot be explained (Reis, 1903; Accorsi Benini
& Broglio Loriga, 1977; Chinzei, 1982).
The most abundant bivalves in south Slovenia are Cochlearites loppianus (Tausch)
and Lithiopedalion scutatus (Dubar). Lithiotis problematica can be found only in
few places. All three species are distinguished by large, unusually shaped and high-
ly variable shells. Justified are question on the mode of life of these bivalves. Their
sections are clearly visible in the black Podpeč limestone that is among the most beatiful
ornamental stones of Slovenia (Buser, 1987b).
In the Liassic beds of south Slovenia also numerous other bivalves can be found
(S. Buser, 1965a; I. Buser, 1989). In preparation is the article with systematic
descriptions of individual species announced for the following issue of Geologija (De-
beljak & Buser). In the present paper, however, the important localities in south
Slovenia are described with the aim of reconstructing the paleoecological conditions
during Lower Jurassic on the Dinaric Carbonate Platform that enabled thriving of this
characteristic fauna for a relatively short time interval.
The Lower Jurassic bivalves are doubtlessly worth their attention. They are in-
teresting from the biologic, and also from stratigraphie aspects. Further, they per-
mit paleogeographic comparison v^dth other world knovm localities.
Lower Jurassic beds with bivalves in south Slovenia	25
Previous research
In 1890 Tausch von Gloeckelsthurn published his work on the fauna from
Liassic limestones in South Alps. It is interesting to note that in his work he repre-
sented several beautiful megalodontid specimens from the Podpeč quarry near Ljubljana.
In his treatise (1890, 28-29) he correctly established the age of the strata with bi-
valves that outcrop south of Ljubljana as Jurassic, and not Triassic, as it was consid-
ered earlier.
In the years 1959-65 S. Buser geologically mapped the territory of south Slove-
nia from the Italian-Slovenian border in the west, across Trnovski gozd, Hrušica, Nanos,
Logatec and Bloke Plateau with Javorniki, Krim-Mokrec hills and Lower Carniola, to
Suha krajina in the east. During this work he collected a rich fossil material, and achieved
the subdivision of Jurassic beds (Buser, 1965a,b). As first he determined the spe-
cies Lithiotis problematica and Cochlearites loppianus in Slovenia. He established
a new bivalve genus with multivincular ligament to which most of specimens from
south Slovenia belong. He named it Lithiopedalion in his doctoral thesis. He attri-
buted it together with genera Lithiotis and Cochlearites to the family Lithioti-
dae.Therefore in Slovenia for these characteristic bivalves the name Lithiotidae be-
came common, although they are nowadays attributed to various families. Bus er pre-
sented his work at the 42"^^ annual meeting of the Paleontological Society in Graz in
1972. However, after many years of cooperation, he was overtaken vvdth publication
by the Italian paleontologist Accorsi Bennini (1979) who described her genus as
Lithiopema, although the name Lithiopedalion was already used in the international
professional literature (Bosellini, 1972).
Localities of Lower Jurassic bivalves elsewhere in the world
The shallow marine Lower Jurassic beds with bivalves are in many places devel-
oped lithologically in a similar facies as in Slovenia. Upwards and downwards they
are usually not sharply limited. Beside more or less dark grey micritic and biosparit-
ic limestone and dolomite frequently occur marly layers and sheets, and complexes
of oolitic limestone.
The most typical faunistic element of the shallow marine Lower Jurassic beds is
bivalve Lithiotis problematica. It gave its name to the beds and to the facies in which
it occurs (e.g. Lithiotis limestones, Lithiotis facies), although other large, thick-shelled
bivalves are usually more abundant (Berti Cavicchi et al., 1971).
The most well known and best studied are localities in the Southern Alps of north
Italy (provinces of Verona, Trento and Vicenza). They occur in the upper part of grey
Liassic limestones called "Calcari grigi" (Böhm, 1884; Tausch von Gloeckelsthurn,
1890; Bosellini & Broglio Loriga, 1971; Bosellini, 1972).
The Lower Jurassic beds with characteristic bivalves extend across south Slove-
nia also to neighboring Croatia: Velika Kapela and Velebit (Grubić, 1961), Herze-
govina (Katzer, 1904) and Dalmatia (Schubert, 1906). They were found also at
Plitvice Lakes. The best preserved specimens of Lithiotis problematica and Coch-
learites loppianus come, however, from the locality Kopilje in Montenegro. Corre-
sponding data unfortunately have not been published.
Similar Lower Jurassic facies with characteristic bivalves appear in south Spain
(Turnšek et al., 1975; Geyer, 1977), in south and central Apennines (De Castro,
26		Stanko Buser & Irena Debeljak
1962) and in Morocco (Dubar, 1948; Agard & Du Dresnay, 1965; Lee, 1983).
Krumbeck (1923) thoroughly studied fauna on the Timor island in Indonesia. Broglio
Loriga and Neri (1976), Geyer (1977) and Nauss and Smith (1988) collect-
ed data from locahties in western France (Sarthe department), Switzerland (Grau-
bünden), Tunisia, Algeria, Albania, Greece, Turkey, Somalia (Mogadiscio), Oman, south
Iran, Iraq and in the Himalayas.
Von Hillebrandt (1981) reports genus Lithiotis from north ChiU and Peru.
In North America this genus occurs in California, Nevada and in east and central Oregon
(Nauss & Smith, 1988). Lithiotis from Oregon was first described under generic
name Plicatostylus (Lupher & Packard, 1930), which has since been recognized
as a junior synonym of Lithiotis (Buser, 1965a).
Fig. 1. Paleogeographic distribution of genus Lithiotis and presumed migration directions
(After Broglio Loriga & Neri, 1976, and Nauss & Smith, 1988)
SI. 1. Paleogeografska razširjenost rodu Lithiotis in predvidene migracijske smeri
(Po: Brogho Loriga & Neri, 1976 in Nauss & Smith, 1988)
Lower Jurassic beds with bivalves in south Slovenia	27
Such vñde extension of localities across the world indicates the existence of vdde
interconnected shallow marine regions during Lower Jurassic along the western and
southern margins of Tethys and the eastern Pacific. Here on carbonate platforms, in
the tropic belt or under the influence of warm oceanic currents, very similar pale-
oecological conditions were established.
The paleogeographic reconstruction of the enumerated localities appears in fig-
ure 1. It can be hypothesized that already in Pliensbachian Tethys and the ancestor
of the present Pacific were connected by a shallow marine corridor across the present
Central America. The cormection is called the Hispanic Corridor (Smith, 1983; Smith
& Tipper, 1986). It enabled the migration of fauna and flora between the two oceans.
In all enumerated regions, large thick-shelled bivalves abundantly occur. Almost
everywhere the most typical species is Lithiotis problematica (fig. 1). According to
certain authors the bivalves flourished on the sea bottom in the form of mats or bi-
ostromes, while others report genuine reefs or bioherms (Agard & Du Dresnay,
1965; Bosellini, 1972; Göhner, 1980; Nauss & Smith, 1988).
In most of enumerated localities besides bivalves the corals, large gastropods,
brachiopods and crinoids can be found. Also the microfauna is highly comparable.
The most characteristic species among the foraminifers is Orbitopsella praecursor
(Giimbel). In places also remains of terrestrial plants were found.
Finds of ammonites in shallow marine Lower Jurassic beds are very rare and
accidental. Somewhere they occur in under- or overlying beds. On the basis of am-
monites the beds wdth the mentioned fossil assemblage {Lithiotis facies) have been
attributed to Pliensbachian (mostly to its upper part: Domeñan), and somewhere (e.g.
in South America) also to the lower part of Toarcian.
Localities of Lower Jurassic bivalves in Slovenia
The Lower Jurassic bivalves occur in the "lithiotid horizon" of southern Slove-
nia, as it was named in 1965 by Bus er (1965 a) after the characteristic bivalves that
he then attributed owing to evident similarities to the same family: Lithiotidae. Their
long, narrow sections serve as a clear identification of the rocks in the field. Lithi-
otis problematica is the most well known, but rather rare. Much more frequent are
species Lithiopedalion scutatus and Cochlearites loppianus. Since nowadays the
mentioned three species are attributed to distinct families, the name lithiotides should
be abandoned. However, the terms as "lithiotid horizon" or "lithiotid limestone" can
be used principally in the sense of facies. Next to enumerated bivalves also the gen-
era Pachyrisma, Gervilleiopema, Mytilus and Opisoma are found.
On the basis of field data and comparisons with similar localities in north Italy
Buser (1965a, 44-46) attributed the beds wdth characteristic bivalves to the up-
per part of Middle Lias: Domerian. In Trnovski gozd the characteristic bivalves
vanish 10-30 thickness meters below the beds that contain the Upper Liassic brachiopods,
and about 50-60 meters below the Lower Dogger beds that were proved by micro-
fossils (Bus er, 1979).
The lithiotid horizon in south Slovenia is constant and it almost does not wedge
out. It might be less than half a meter thick, but in places it attains even 75 meters.
The horizon reaches its maximal thickness at Podpeč, in the Krim-Mokrec hills and
in the Krka valley. Here the limestone beds are typically dark grey or even black in
color. Northwards the limestone becomes lighter, and the beds with bivalves thinner
28		Stanko Buser & Irena Debeljak
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Lower Jurassic beds with bivalves in south Slovenia	29
and thinner; in Trnovski gozd and Banjišice plateau they in places even pinch out.
Also towards the south the lithiotid horizon becomes thinner and it pinches out, es-
pecially in places of occurrences of black coal, i.e. at Loški Potok, Bloke, Cerknica
and Rakitnica. On most of Logatec plateau, Hrušica, Bloke plateau, Slivnica, Mala and
Vehka gora and in Kočevje area the bivalves are found in bituminous coarse grained
dolomite or in rare thin limestone intercalations within dolomite (Buser, 1965a; Savie
& Dozet, 1985).
Figure 2 shows the extension of Lower Jurassic beds that contain bivalves. Spe-
cially marked are those localities in which shells can be isolated from the softer marly
matrix.
Certain localities discovered by S. Buser during geological mapping more than
thirty years ago are unfortunately destroyed or overgrown. This is the case for the
abandoned quarry on the right bank of Sušica west of Dolenjske Toplice and for the
locality on Javornik (east part of Trnovski gozd). Also the road cuts in Krim-Mokrec
hills, southeast of Lož, north of Cerknica and at Grčarevec are almost totally over-
grown. Individual specimens can still be found in railroad cuts between Preserje and
Verd. Very numerous bivalves are found today in the quarry of Podpeč, in the east-
ern slope of Špik (north of Col in Trnovski gozd) and at the karst spring Globocec
(west of Zagradec). Newly discovered is the locality in the eastern slope of the Stražišče
hill east of Gorenje Jezero near Cerknica. The species Lithiotis problematica, which
is the least abundant, is found east of the village Zafara near Žužemberk.
In the following text are described in detail the localities Špik in Trnovski gozd,
the Podpeč quarry, Globočec near Zagradec, Grčarevec south of Logatec, Stražišče
at Gorenje Jezero, Ravne at Borovec in the Kočevje area, and Travna gora. These localities
are spatially distributed in the manner to represent distinct ancient environments.
Besides, at them geological sections could be measured (fig. 3).
Špik in Trnovski gozd
Špik is about 950m high peak in Trnovski gozd about 2 km north of Col. Well exposed
Upper Ti-iassic and Liassic beds gently dip towards the south. The Upper Triassic
dolomite with characteristic stromatolites gradually passes into the Lower Liassic massive
coarse grained dolomite of light grey color that is about 100m thick. Upwards fol-
low approximetely 30m of light grey to white micritic limestone which contains in
its upper part up to 3m thick intercalations of coarse grained dolomite. Lower Lias-
sic limestone beds form steep rocky steps in the woody slope. At approximately 900
meters altitude their dip is 180/45. The lithologie boundary with the Middle Liassic
beds was placed by Buser (1978) at the occurrence of brownish oolitic dolomite
which is at that place about 30 meters thick.
Several meters below the Špik summit appears an approximately 70 cm thick layer
of grey brown marly limestone that contains abundant various bivalves. Prevailing is
species Lithiopedalion scutatus. Its shells are up to 40cm long and intensely re-
crystallized. This species is accompanied by Gervilleiopema buchi (Zigno), G. ta-
ramellii (Böhm), Mytilus lepsii Tausch and M. mirabilis Lepsius. Very frequent are
also medium sized megalodontid shells that belong to a new genus and new species.
Various bivalve species were probably transported together after death. Their shells
are rather well preserved, an indication of short transport. In limestone next to bi-
valves also numerous small fragments of various fossils occur. The layer from which
shells can be separated extends for about 200 meters across the east slope of Špik.
30		Stanko Buser & Irena Debeljak
PODPEČ
Fig. 3. Stratigraphie columns of Middle Liassic beds in different localities
SI. 3. Stratigrafski stolpci srednjeliasnih plasti v različnih nahajališčih
Lower Jurassic beds with bivalves in south Slovenia	31
Above the layer with shells occur about 5 meters of light oolitic dolomite with
numerous fragments of crinoids, echinoids, bryozoans and solitary corals. Higher in
the section is an about one meter thick layer of light grey micritic limestone with
numerous, however not rock building bivalves of species L. scutatus. Some shells he
perpendicularily to the bedding, an indication of the preservation of their living po-
32		Stanko Buser & Irena Debeljak
sition also after death. This layer is followed by another meter of micritic limestone
without shells (fig. 3).
The area of Trnovski gozd in Middle Liassic belonged to the north distal margin
of the Dinaric Carbonate Platform. This part was opened to influence of waves and
currents from the northerly lying deep sea of the Slovenian Basin. The environment
with an increased water energy did not suit best to bivalves. At Špik they are con-
sequently found only in two relatively thin layers. It appears that only the enumer-
ated species could survive in the stressful environment.
Podpeč
Podpeč is situated in the south rim of the Ljubljana Moor. Well exposed in the
large quarry are the Lower Jurassic beds of the Podpeč limestone that was exploit-
ed already by the Romans for building the ancient Emona. From Middle Ages on, large
quantities of building and ornamental stone were extracted from this quarry. Nowa-
days, the quarry has been abandoned for about three decades. The Podpeč "marble"
decorates various buildings and monuments across Slovenia. Its value found its best
expression in creations of the architect Jože Plečnik (Prelovšek, 1987). Every visitor
of the National and Universitary Library in Ljubljana notice in the hall the wonder-
Fig. 4. The Podpeč ornamental limestone from the National and University Library in
Ljubljana with white recrystallized bivalve shells in black micritic matrix. In the lower part
of slab occur typical sections of genus Cochlearites, and in the middle part up to 60cm
long sections of genus Lithiopedalion
SI. 4. Okrasni podpeški apnenec iz Narodne in univerzitetne knjižnice v Ljubljani z belimi,
prekristaljenimi lupinami školjk v črni mikritni osnovi. V spodnjem delu plošče so značilni
preseki rodu Cochlearites, v sredini pa do 60cm dolgi preseki rodu Lithiopedalion
Lower Jurassic beds with bivalves in south Slovenia	33
ful black stone with long white shell sections (fig. 4). In Ljubljana the stone is at
display also in the old Scyscraper, in the former Chamber of Commerce, in the stair-
case of the Mining Department, in the Slovenian Parlament building, in Ljubljana Tovm
Hall, and on many places elsewhere.
The beds of the Middle Liassic limestone dip almost vertically southwards (180/
80). They are from 10cm to 2m thick. Well exposed they occur in the eastern and
western sides of the quarry (fig. 5, 6). The large exploited central part gives the idea
of the vast volume of the stone quarried. The limestone in the quarry is cut by sev-
eral faults. Along them individual blocks slid one along the other, as indicated by broad
smoothed fault planes with vertical striations. During diagenesis in limestone numerous
stylolithes formed.
Between the limestone beds are frequent marl coatings and sheets. In the lower
part of the section they are up to 15 cm thick. They are best visible in the western
part of the quarry in the fresh cut behind the new building. The surface of limestone
below the marl sheets is often bulbously corroded (fig. 7). Above this irregular sur-
face occurs a several millimetres thick violet red crust, followed by yellow brown or
reddish marl. X-ray analysis of the violet crust revealed the foUovidng minerals: muscovite,
illite, calcite, hematite and anatas resp. brookite. This mineralogical composition is,
according to M. Mišič (personal communication), the indication of in situ weather-
ing. Most probably the limestone was uplifted above the sea level and submerged again.
During the limestone weathering on land red karstic clays, or terra rossa, were formed
that later evolved into the mentioned brown and red marls. Such oscillations can be
observed only in the lower part of the section. M. Mišič recognized in the marl the
followdng minerals: muscovite, illite, calcite and goethite. The pollen analysis performed
by B. Jelen did not have positive results (personal communication).
The profile (fig. 3) was recorded in the eastern side of the quarry. The Uthio-
tid horizon in the Podpeč quarry is exposed in its entire thickness that amounts to
75 meters. It contains relatively thin beds with shells (0.1-1.5m), and thick inter-
mediate beds without shells. In the section more than 15 isolated lumachelles can
be found. Their total thickness is approximately 12 meters. The large number of
relatively thin lumachelles suggests the idea of frequent disturbance or even inter-
ruption of the growth of bivalves. The bivalves in the Podpeč quarry are found in
black micritic limestone, with the exception of the lower part of section where they
occur also in marly layers. The lumachelles are most often followed by sparitic limestone
with ooids, tiny rounded fragments of various shells (bioclasts) and intraclasts. Currents
and waves that delivered various debris and washed out carbonate mud most prob-
ably disturbed the growth of bivalves. During periods of unfavorable conditions the
animals survived somewhere in the neighborhood, and colonized the old places again
when circumstances changed. In the upper part of section the micritic limestone is
much more abundant than in the lower part. The environment at that time was quiter,
with the resulting thicker and more numerous lumachelles.
The most abundant species among the bivalves is Lithiopedalion scutatus (Dubar)
that is also the first to appear. In the lower part of the section fine specimens can
be separated out of the marly layers. In places the shells are densely packed, like
shards, with very little matrix inbetween (fig. 8). As demonstrated by sections in the
limestone, certain individuals attained the size of 3/4 meter. In the second part of
the section with indications of a quieter depositional environment also species Co-
chlearites loppianus is abundant. Here the shells cannot be separated out from the
rock, and the presence of the species is demonstrated by sections in the limestone
34		Stanko Buser & Irena Debeljak
Fig. 5. Eastern part of quarry at Podpeč. Middle Liassic layers at the blasting-powder
magazine with lumachelles of various bivalves
SI. 5. Vzhodni del kamnoloma v Podpeči. Srednjeliasne plasti v bližini skladišča za razstreli-
vo z lumakelami različnih školjk
Fig. 6. The west part of the Podpeč quarry. The layer with individual bivalve specimens of
genus Opisoma is marked
SI. 6. Zahodni del kamnoloma v Podpeči. Označena je plast s posamičnimi primerki školjk iz
rodu Opisoma
Lower Jurassic beds with bivalves in south Slovenia	35
Fig. 7. Bulbously weathered upper surface of limestone, covered by violet crust (fossil soil)
and overlain by a marl sheet. Podpeč quarry
SI. 7. Gomoljasto preperela zgornja površina apnenca. Prevlečena je z vijoličasto skorjo
(fosilna tla), sledi pa lapornata pola. Kamnolom v Podpeči
Fig. 8. Extraordinarily numerous shells of genus Lithiopedalion that can be separated from
marly matrix. Podpeč quarry
SI. 8. Izredno številne lupine rodu Lithiopedalion, ki jih je mogoče izluščiti iz lapornate
osnove. Kamnolom v Podpeči
36		Stanko Buser & Irena Debeljak
Fig. 9. Ornamental Podpeč limestone from hall of the Ljubljana scyscraper. 5-7 cm wide
cross sections of species Lithiotis problematica are closely packed together
SI. 9. Okrasni podpeški apnenec iz veže Ljubljanskega nebotičnika. 5-7 cm široki prečni
preseki vrste Lithiotis problematica so tesno nagneteni eden ob drugem
Fig. 10. Ornamental Podpeč limestone from staircase of Mining Department building.
"Scattered" remains of various bivalves; among them characteristic remains of genus
Lithiotis and of megalodontids can be recognized (scale presented by camera cover)
SI. 10. Okrasni podpeški apnenec s stopnišča Montanistike. »Razmetani« ostanki različnih
školjk; med njimi lahko prepoznamo značilne preseke rodu Lithiotis in megalodontid.
(Merilo je pokrovček fotoaparata)
Lower Jurassic beds with bivalves in south Slovenia	37
only. The species Lithiotis problematica is very rare at Podpeč; its characteristic
sections (fig. 9, 10) can be found only in two or three layers in the upper part of
the section.
In the quarry and in the field in general the sections of the mentioned bivalves
are not easy to distinguish owing to patina that covered the limestone, or ovñng to
tectonized surface. Fortunately, the orientation of bivalve shells can be studied on
polished sheets of ornamental Podpeč limestone. Mainly well preserved shells are chiefly
parallel wdth stratification. In the lumachelles usually a single species prevails. In cases
of irregular "scattering" of shells of various species in the limestone, the shells are
often damaged (fig. 10). Shells were often broken and in part dissolved also during
diagenesis.
In addition to the genera Lithiopedalion, Cochlearites and Lithiotis occur in
Podpeč also the following bivalve species: Gervilleiopema buchi, G. taramellii,
G. timorensis Krumbeck, Opisoma cf. excavatum Böhm, O. cf. menchikoffi Du-
bar and Pachyrisma {Pachymegalodon) chamaeforme (Schlotheim). Interesting is
also the approximately 40 cm thick layer of marly limestone with very small shells
that probably belong to genus Astarte. This layer is exposed just at the blasting-
powder magazine. The sample sites of various bivalves are marked on figures. All of
the enumerated bivalves are rock-forming, with the exception of genus Opisoma that
occurs individually (fig. 11), in biosparitic limestone full of organic detritus that was
spherically overgrown by cyanobacteria. It is interesting that in lumachelles with other
bivalves no oncoids are present, although they are very frequent elsewhere in the
section. Evidently, the cyanobacteria were not able to thrive in association with bi-
valves.
Buser (1965a) described from the Podpeč quarry two brachiopod species: Ter-
ebratula rotzoana Schauroth and T. renieri Catullo. Gastropods are moderately nu-
merous, but they cannot be prepared out of the tough rock. In thin sections numer-
ous tiny tests may be found, and in the ornamental quality Podpeč limestone gastropods
with sections measuring up to 10cm across (fig. 12). Corals are rather rare. Individ-
ual specimens were fixed also on shells of the megalodontid bivalves. It is interest-
ing that nearby, at Gornja Brezovica near Preserje, in the lithiotid horizon also a small
coral patch reef can be seen. It is 35 meters long, a few meters wide and 5 meters
thick. It laterally passes into dark grey oolitic limestone.
The most characteristic for the Middle Liassic is next to bivalves the large fo-
raminifer Orbitopsella praecursor (fig. 13). It is rock-building in some layers of the
Podpeč limestone. The microforaminifers are present almost in all beds, therefore they
are not specially marked in the lithological column (fig. 3). They are the most abun-
dant in the pelbiomicritic limestone (fig. 14). L. Šribar (manuscript report) deter-
mined the following foraminifer families: Ammonidiscidae {Glomospira sp.), Lituol-
idae (genera Lituosepta, Orbitopsella, Haurania, Pseudocyclammina), Verneuilinidae,
Textularidae and Nubeculariidae (Opthalmidium sp.). Present are also dasycladaceans
Thaumatoporella parvovesiculifera (Raineri) and Palaeodasycladus mediterraneus
(Pia), codiaceans and problematica Aeolissacus sp. The microfossils from Podpeč were
determined also by R. Radoičič (manuscript report). In the upper parts of the sec-
tion R. Radoičič determined also the species Labyrinthina recoarensis. According
to her opinion, several lowermost beds in the quarry belong to the upper part of Lower
Lias. In the uppermost part of section that is rather heavily tectonized the bounda-
ry between the Middle and the Upper Lias could be expected.
The most frequent benthic foraminifer is Glomospira. It is very abundant in the
38		Stanko Buser & Irena Debeljak
Fig. 11. Ornamental Podpeč limestone in the National and University Library. About lOxlOcm
large section of bivalve of genus Opisoma, most probably species O. excavatum. In limestone
also characteristic sections of large foraminifers Orbitopsella praecursor occur
SI. 11. Okrasni podpeški apnenec v Narodni in univerzitetni knjižnici. Približno lOxlOcm ve-
lik presek školjke iz rodu Opisoma. Po vsej verjetnosti gre za vrsto O. excavatum. V apnen-
cu so tudi značilni preseki velikih foraminifer Orbitopsella praecursor
Fig. 12. Ornamental Podpeč limestone from National and University Library. Sections of gas-
tropods are up to 10 cm in diameter. Fragments of bivalve shells of genus Opisoma
SI. 12. Okrasni podpeški apnenec v Narodni in univerzitetni knjižnici. Preseki polžev so veliki
do 10 cm. Nekaj odlomkov školjčnih lupin rodu Opisoma
Lower Jurassic beds with bivalves in south Slovenia	39
Fig. 13. Dismicritic limestone full of organic detritus, pellets and intraclasts. Above sections
of foraminifer O. praecursor a section of dasycladacean. Podpeč
SI. 13. Dismikritni apnenec, poln organskega detritusa, peletov in intraklastov. Nad preseko-
ma foraminifere O. praecursor je presek dazikladaceje. Podpeč
Fig. 14. Passage of biomicritic limestone into sparitic limestone full of rounded fragments
(bioclasts), ooids and intraclasts. Podpeč
SI. 14. Prehod biomikritnega apnenca v sparitni apnenec, ki je poln zaobljenih odlomkov
(bioklastov), ooidov in intraklastov. Podpeč
40		Stanko Buser & Irena Debeljak
entire section, even there w^here no other microfossil appears. Although the above
mentioned calcareous algae are quite frequent in the Podpeč quarry, they м^еге not
detected in the bivalve lumachelles. In general is the biodiversity very low in beds
where the genera Lithiopedalion, Cochlearites, Gervilleiopema and Lithiotis were
identified.
The characteristic bivalves in the upper part of section vanish suddenly, with-
out gradual passage. However, according to lithology, the deposition at that did not
change considerably. Possible changes in the foraminiferal assemblages have not been
investigated.
Globočec near Zagradec
Globočec is a large karst source southwest of Zagradec. In this area the Upper
Triassic Main Dolomite with stromatolites passes into light grey micritic Lower Lias-
sic limestone. At the Globočec source layers of dark grey to black Middle Liassic lime-
stone are very well exposed. They are from 10 to 100 cm thick and they dip towards
southwest (220/15). In the wooded slope they form rocky steps that may be lateral-
ly traced for 50 m. Exposed are only the upper 20 meters of the lithiotid horizon that
is in the Krka valley about 50 meters thick.
In the lower part of section (fig. 3) at Globočec appear 5m of dark grey micritic
limestone with rock-forming bivalves of genus Cochlearites. Inbetween are only two
thin parts that contain just a few sections of shells. Limestone in the lower part is
in places marly, therefore shells are easily separated from the rock. Certain speci-
mens are typical representatives of species Cochlearites loppianus, whereas others,
found about one meter higher, are much larger and of different shape. The latter belong
to a new species, but according to the characteristic valve structure the genus is the
same. In this part of section also individual specimens of solitary corals were found
fixed upon thick bivalve shells.
Higher follow two meters of sparitic limestone with intraclasts, individual ooids,
fragments of shells and crinoids. This is the only larger intercalation that does not
contain shells. It is overlain by 7 meters of limestone packed wdth rock-building shells
of Cochlearites. Sections of genus Lithiopedalion are rare. In this part the shells
cannot be separated out of the hard rock. After this layer the shells vanish sudden-
ly, without any gradual transition. In black micritic limestone of the last 30 meters
of exposed beds not a single specimen can be found.
The type of limestone in the Globočec locality is an indication of quiet sedimen-
tational environment. Bivalves grew almost completely undisturbed. The shells are
mostly undamaged, and appear in certain beds even in vertical, i.e. living position.
Most of them, however, were overturned on sea bottom after death, so that the fos-
sil shells are oriented parallel to bedding. Very often also wedge-shaped agglomera-
tions consisting of numerous shells can be observed. In such a case after death, or
still during life, several shells were tilted together, so that they support each other
in the inclined position.
Grčarevec
The small village of Grčarevec hes south of Logatec. About 1km towards north-
west the Middle Liassic beds are well exposed. They dip 300/15. Prevails dark grey
Lower Jurassic beds with bivalves in south Slovenia	41
Fig. 15. Characteristic sections of species Cochlearites loppianus in limestone northwest
of Grčarevec
SI. 15. Značilni preseki vrste Cochlearites loppianus v apnencu severozahodno od
Grčarevca
coarse grained dolomite containing individual bivalve lumachelles that are up to
2 meters thick.
In a limestone lens intercalated weithin dolomite is at the last summer houses
northvi^est of Grčarevec a fresh road cut. In three separated layers that are up to 70 cm
thick are v^ell visible sections of rock-building bivalves. Species Cochlearites loppi-
anus is here absolutely prevailing (fig. 15). Much less frequent are sections that most
probably belong to genus Lithiopedalion, and in the middle layer also to genera
Gervilleiopema and Mytilus. Limestone is micritic and dark grey to black in color.
It is overlain by dolomite still containing in its lov^er part characteristic shells that
higher up disappear.
At the sharp curve of the old road north of Grčarevec Bus er (1978, 396) found
in lithiotid horizon w^ell preserved plant remains. This locality is novi^ overgrown. The
remains of terrestrial plants indicate the temporary existence of smaller land islands
on the carbonate platform.
Stražišče near Gorenje Jezero
Above the Klance village in the Stražišče hill east of Gorenje Jezero near Cerknica,
also west of Stari trg, one of the most perfectly exposed sections in the lithiotid horizon
can be found.
42		Stanko Buser & Irena Debeljak
In more than 20 meters of exposed section about 10 bivalve lumachelles can be
numbered. They are from 0.5 to 1 meter, and at most 2.5 meters thick. Genus Co-
chlearites is prevailing. Its characteristically shaped shells appear in places in lens-
like dispositions in the rock, the lumachelle pinching out in-between. In certain beds
the shells are so numerous that there is practically no cement between them. As
a rule they occur in dark grey micritic limestone. Owing to the marly admixture they
can be in places isolated out of the rock. The intermediate beds with no shells mainly
consist of oolitic and biosparitic limestone.
In the section the sudden massive appearance of bivalves, and also their sudden
disappearance can be observed, both without gradual transitions. The lithiotid hori-
zon is followed upwards by sparitic limestone with ooids and bioclasts. It forms the
upper part of the hill in the form of an erosion remnant.
Travna gora
A couple of kilometers southeast of Travna gora in the wooded slope a more than
20 meters thick lithiotid horizon can be traced. It is entirely developed in dolomite
(fig. 3). The shells can be isolated from the rock in this locality only from one 20cm
thick marl layer in the lower part of section. In dolomite only three real bivalve lu-
machelles can be found. The intermediate beds also contain individual shells that are
more or less frequent and intensely recrystalhzed. With certainty only the genus
Cochlearites was identified. In the lower part of the section its shells measure up
to 30 cm, and are similar to those in the Globočec locality; they probably belong to
a new species. In the uppermost part of the lithiotid horizon the bivalve lumachelle
is about three meters thick. In the first half it consists of recrystallized shells that
could be attributed to genera Cochlearites and Lithiopedalion, although the spec-
imens here are much smaller and thinner than usual (valve length 5 cm, thickness
not more than 1cm). In the upper part of the mentioned lumachelle again predom-
inate the typical representatives of genus Cochlearites. About 40 meters higher the
dolomite is overlain by oolitic dark grey Upper Liassic limestone that alternates with
thin sheets of black micritic limestone.
Ravne near Borovec
The Middle Liassic beds in the wider surroundings of Kočevje largely consist of
coarse grained bituminous dolomite (fig. 2). This area originally belonged to the
southerly, most restricted parts of lagoon, or to the inner part of the Dinaric Car-
bonate Platform.
The locality Ravne near Borovec is the southernmost in Slovenia. In the lower
part of the northern slope of Borovska gora outcrops the black, bituminous thin-bedded
limestone. It is overlain by a 1.5 meters thick layer of black micritic limestone that
contains relatively small, about 10 cm long shells of genus Cochlearites. Upwards layers
of bituminous dolomite follow. In the upper part, a 4 meters thick bank of dolomite
occurs which is full of unusually thin specimens of genus Cochlearites. This bank
protrudes from the slope as a steep rocky step which owes its higher resistance to
numerous contained bivalve shells.
Lower Jurassic beds with bivalves in south Slovenia	43
Living conditions of bivalves on the territory of southern Slovenia during
the Lower Jurassic
On the territory of present southern Slovenia the v^dde Dinaric Carbonate Plat-
form was situated during the Lower Jurassic. This shallow marine environment bor-
dered in the north on the deep sea of the Slovenian Basin (Buser, 1987a), while
in the south it continued across the present Croatia. Reconstruction of the ancient
relations appears in figure 16.
It is interesting to note that Lithiotis problematica occurs also in the surroundings
of Bovec (on the Poljanica hill and at the karstic source Glijun), consequently on the
territory that with certainty belonged to the Julian Carbonate Platform that was sit-
uated north of the Slovenian Basin (fig. 16). It appears almost impossible that the
shallow marine fauna could migrate from the Dinaric Carbonate Platform to the northerly
lying Julian Carbonate Platform across the deep marine see of the Slovenian Basin.
Our opinion is that the Slovenian Basin pinched out in the area of the present cen-
tral part of the Soča valley, at Trnovo west of Kobarid. It follows that this deep marine
Fig. 16. Present extension of the ancient Juhan and Dinaric Carbonate Platforms and the
intermediate Slovenian Basin
SI. 16. Današnja razprostranjenost nekdanje Julijske in Dinarske karbonatne platforme in
vmesnega Slovenskega bazena
44		Stanko Buser & Irena Debeljak
basin did not continue farther westward into the Belluno Basin of northern Italy. Owing
to pinching out of the Slovenian Basin, the Dinaric and the Julian Carbonate Plat-
forms directly bordered on each other in the mentioned area during Liassic.
Let us have a look at the conditions of those times in the region of south Slov-
enia, on the Dinaric Carbonate Platform (fig. 17):
The northern border of the platform intercepted most of the striking power of
waves and currents that arrived from the open deep sea of the Slovenian Basin. Their
destructive power is evidenced by limestone breccias on Banjšice and in north Low-
er Carniola. Othenvise in the northern parts of the Dinaric Carbonate Platform (Trnovski
gozd, Banjšice) oolitic and crinoid limestones of light color predominate. They were
deposited in vdde shoals v^dth warm water only 1-3 meters deep. Cooler waters flowed
in from deeper parts of the Slovenian Basin. Owing to increased temperature, part
of carbonates precipitated from them around tiny grains that floated in turbulent water.
In shallows large amounts of ooids, small tests and fragments of various organisms
accumulated that were rounded by wave action. In places, accumulations of this debris
reached the sea level. Wide beaches and dunes were formed. Together vdth small coral
and hydrozoan patch reefs (Trnovski gozd) they represented proper dams in front
of the northerly lying open deep sea, and they protected the southerly lying lagoon.
At high tides and at storms the fresh water overflowed the barriers and flooded
into the lagoon. It left behind beds of fine "oolitic sand". Therefore also in the areas
of former lagoon a lot of dark oolitic limestone can be found. Biosparitic limestone
is common as well. Prevailing, however, is the micritic limestone that was formed from
Fig. 17. Schematic representation of various sedimentation environments in lower Jurassic
with characteristic types of rocks on the Dinaric Carbonate Platform
SI. 17. Shematski prikaz razhčnih sedimentacijskih okolij v spodnji juri z značilnimi tipi
kamnin na Dinarski karbonatni platformi
Lower Jurassic beds with bivalves in south Slovenia	45
the lagoon carbonate mud. It contains a profusion of tiny tests of foraminifers that
used to live at the sea bottom (benthos). In places of steady infolov^^ of fresh water
also corals, crinoids and green algae (dasycladaceans) lived. Otherwise in the lagoon
most of the living environment on the muddy sea bottom was occupied by bivalves.
They formed submarine biostromes with individual lense-Uke thickenings. In places
also gastropods and brachiopods settled. The carbonate deposition was the fastest
in this central area, and the lithiotid horizon attained the maximal thickness.
Towards the south the influence of the northerly deep marine region became
progressively weaker. Waves and currents reached only to a certain distance. Farther
extended the restricted and quiet parts of lagoon. Much oxygen was used for decom-
position of organic matter. At the bottom reducing conditions were established. Rocks
formed in restricted lagoon are highly bituminous and of characteristic dark grey to
black color. Evaporation and with that also salinity of sea water were highest here.
The pore water in these sediments contained high magnesium, and consequently they
were most likely subjected to dolomitization during diagenesis. It may be conjectu-
red that the alternation of drought periods wdth prevailing evaporation and rainy periods
with heavy rainfall controlled the salinity variation in the restricted lagoon to a high
degree.
In the larger part of the lagoon, or inner shelf, see water was only a few meters
deep (subtidal). Indubitably existed also wider areas that were dry during low tide
(intertidal); however, desiccation cracks and stromatolites are in Lower Jurassic beds
extremely rare, a distinction that separates them from the Upper Triassic shallow marine
beds. The fossil Mytilus bivalves, that can be used as indicators for such environ-
ment, are quite frequent in south Slovenia.
Occurrences of coal and remains of land plants in rare localities (Bloke, Loški
Potok, Grčarevec) prove the periodic existence of small land islands that were probably
surrounded by swamps.
The rocks and the organisms bear evidence of tropical or at least subtropical
climate.The paleogeographic distribution of the present territories was essentially
different in the Lower Jurassic times. The entire "Adriatic plate" was situated more
to the south, in lower latitutes, in the then existing tropical belt (fig. 1).
Distribution of species and mode of life of bivalves
It appears that the northern, most exposed parts of the carbonate platform were
not favorable for growth of bivalves. The animals from time to time colonized also
these areas, but frequently their growth was soon interrupted. Strong waves could
damage their shells or carry them away The turbulent environment was the best endured
by megalodontid bivalves, and also by genera Lithiopedalion, Mytilus and Gervil-
leiopema. Especially the megalodontid shells appear in the northwestern part of
Trnovski gozd even in places where all other bivalve species already disappear.
In the more restricted parts of the shelf that, however, were nevertheless reached
by fresh water, rich in oxygen and nutrients, the massive thriving of bivalves was the
most frequent. The conditions for their growth were in this part the most favorable,
but still rather unstable. Numerous, relatively thin lumachelles indicate that the growth
of bivalves was often interrupted. The matrix in lumachelles is as a rule micritic. The
sedimentological and paleontological characteristics of the deposits between lumachelles
suggest that at times of interruption of bivalve growth, most often increases of wa-
46		Stanko Buser & Irena Debeljak
ter energy occurred. Then sediments deposited from which oolitic, oncolitic and bio-
sparitic limestones were formed. Periodically, perhaps during heavier storms, the currents
brought so much material to lagoon that the bivalves were literally covered by vari-
ous debris. On the other hand, strong waves could wash out all sediment that sur-
rounded large shells of the characteristic bivalve species and provided support for
them, and scattered them across the sea bottom where they perished. During the times
of unfavorable conditions the bivalves survived elsewhere, and later they colonized
the previous localities again. The base for the first colonization respectively fixation
of juvenile bivalves could have been any firm bottom, various detritus or shells of
decayed bivalves. In the central part of lagoon that was under influence of deep marine
basin Lithiopedalion prevailed. Genus Cochlearites was frequent as well. More or
less numerously were represented all above enumerated bivalves, also the genus
Lithiotis.
For the moment, the data show that the quiet, muddy bottom in more southern
parts of lagoon was mostly inhabited by genus Cochlearites. In the Middle Liassic
dolomite often sections of valves are found that could be ascribed to genera Lithio-
pedalion and Cochlearites, but they are extraordinarily small. The shell size attains
at most 10cm and its thickness at most 1cm, and usually even less. It is possible that
the mentioned shells grew under very unfavorable living conditions (increased salinity,
decreased influx of fresh water), and they remained small, or that they represent distinct
species. Unfortunately, such specimens could not be separated out of the dolomitic
rock at any place. We presume that the living conditions in the most restricted parts
of the lagoon were in general less favorable than in its central part that used to be
under steady, moderate influence of the pelagial. On the other hand, the environment
in the most protected parts was much more stable, as proved by the rather thick (up
to 4 meters), although not very frequent lumachelles.
At reconstructing various depositional environments on the carbonate platform
it must be considered that the area of maximum influences from the deep marine
region changed with time. This is the most expressed at the transition from basin to
platform. In all parts of the shelf sea, respectively the lagoon, the water energy was
subjected to changes, as well as the sea level, the sea bottom and the sediments that
were deposited on it.
All enumerated Lower Jurassic bivalves formed submarine biostromes where
hundreds of individuals of the same species were crowded together. The bivalves pre-
dominantly grew in very shallow water (intertidal-subtidal), since they were able to
reach the maximum of nutrients near the sea surface.
The genus Gervilleiopema lived in a similar manner as the recent genus Isog-
nomon. These bivalves were attached on a firm base with a bunch of byssus fibers.
The same is valid for representatives of genus Mytilus. This is the same genus that
even at present inhabits all shallow seas except for the extreme north. Characteris-
tical for it is the colonization of the tidal environment. At low tide it can survive on
the air for hours, so that it hermetically closes the valves. The organism has good
resistance to changes of water salinity. It is the best adapted to brackish water in
the near shore belt (Cox, 1969, N5-N15). The megalodontid bivalves lived with their
apex anchored in the soft sea bottom. The valves and the hinge were very strong,
and the round shape enabled the animals to be rolled without damage across the sea
bottom in case of stronger waves.
There are still many unsolved questions about life mode of the most character-
istic Lower Jurassic bivalves: Lithiotis, Cochlearites and Lithiopedalion. There is
Lower Jurassic beds with bivalves in south Slovenia	47
no recent bivalves similar to them and we could hardly compare them also v^^ith some
other fossil group. Their unusual shape and high degree of variability already for
themselves rouse numerous questions concerning their biology (more in: Debeljak
& Bus er, in prep.). The animals were entirely adapted to the muddy sea bottom,
fast sedimentation in the environment and sessile hfe in densely packed assemblage
of specimens of the same species. With regard to relatively small size of their soft
bodies they developed enormously large shells. They used to grow throughout their
lives, so that the upper part of the shell remained above the level of the surround-
ing sediment, and that they obtained as much sunlight as possible. Such permanent
and uniform growth in height is an extraordinary property, as the growth rate of animals
usually exponentially diminishes with age. In deposits where the shells remained in
their growth positions after death, we can observe that the individual specimens
supported each other, and grew upwards in a tuft-like manner, similar to plants. Frequent
are knee-shapedly bent individuals, at which most probably the valve was tilted too
dangerously, so that the animal afterwards corrected its position by subsequent ver-
tical growth. Considering such cases it is not surprising that the shape of the shell
in the same species can be so variable. The external similarity of the mentioned three
genera, which otherwise belong to distinct famihes, is a result of convergence, of
adaptation to similar, very specific living environment. The shells of Lithiotis, Co-
chlearites and Lithiopedalion might have measured even 0.5 meters. Mussels of such
size can be at present found in the equatorial belt or in regions of warm currents
where the sea water temperature is constantly above at least 23 °C. Owing to the
decrease of CO2 concentration in water the precipitation of calcium carbonate and
skeleton building is facilitated.
It is characteristic that the biodiversity in individual bivalve lumachelles is ex-
traordinarily low. Different species of bivalves evidently did not live together in the
same place. In the same manner, next to bivalves practically no other organisms became
preserved as fossils. The reason can partly be ascribed to the selective influence of
specific environment (muddy substrate, fast sedimentation) that required from or-
ganisms special adaptation. Probably also the bivalves themselves rendered impossi-
ble the existence of their competitors with fast overgrowth and production of large
amounts of organic mud. Photosynthesis in muddy water was practically impossible,
what excluded from the environment many organisms, especially the green algae and
cyanobacteria. Similar circumstances can be observed nowadays in large oyster farms.
Certain species of oysters are able to filter 40 liters of water per hour. In addition,
they are genuine accumulators of sediment rich in organic matter that has a putrid
smell and that decomposing produces reducing conditions. On the water surface H2S
oxidizes into sulfuric acid. At the top of the sediment the environment is acidic which
results into often corroded shells (Stenzel, 1971, N1000-N1003), a phenomenon ob-
served also on fossil remains.
The genera Lithiotis, Cochlearites and Lithiopedalion formed submarine mats
or biostromes on the territory of present south Slovenia. In Slovenia there is no ev-
idence of the existence of reef structures. Smaller lense-like thickenings of general-
ly thin bivalve lumachelles are most probably beds of shells after a short transport.
Lumachelles laterally do not pinch out, they only thin and thicken. The micritic matrix
suggests a low water energy during the time of deposition of sediment. With respect
to their shape, the shells are relatively well preserved. The shells of isolated speci-
mens are usually not eroded. The biodiversity in the lumachelles is low. All this sug-
gests generally short transport distances. We beheve that the characteristic bivalves
48		Stanko Buser & Irena Debeljak
grew on large surfaces, and that after death they were most often only somewhat
overturned by the waves.
The blossom time of the characteristic Lower Liassic bivalves (genera Lithiotis,
Cochleañtes and Lithiopedalion) lasted only around 5-10 miUion years. On the territory
of Slovenia, however, they did not last even that long. In all Slovenian localities at
the end of the lithiotid horizon that is attributed to Domerian the mussels suddenly
disappeared without gradual diminishing.
During the Upper Liassic in the region of Tethys extensive tectonic movements
occurred. In the territory of northern Slovenia, on the former Juhan Carbonate Plat-
form, a short land phase took place, and afterwards it rapidly subsided and disinte-
grated (Buser, 1987a). The influence of these events undoubtedly reached also the
Dinaric Carbonate Platform. In the areas of Stična, Šentvid, Radohova vas, in suroundings
of Trebnje and Veliki Gaber the Malm beds transgressively overlie the Middle Liassic
beds. At Loški Potok in the Upper Liassic coal ocurrences are known. All this may
be understood as evidence of uplift of the territory after the deposition of Middle
Liassic beds with bivalves. Tectonic activity was reported also from the neighboring
Italy. On the Trento Carbonate Platform the shallow marine beds with bivalves are
overlain by deep marine limestone of the ammonitico rosso type (Bosellini & Broglio
Loriga, 1971).
It can be only conjectured about the causes that led to the sudden extinction of
the characteristic Middle Jurassic bivalves. Most probably the reason was in the co-
incidence of various causes. It is known that these fossils were extremely special-
ized organisms that could not respond to the environmental change with new adap-
tations. Of the ancient life, only the interesting fossil record remained.
Acknowledgement
Dr. Stevo Dozet geologically studied the Lower Jurassic beds with bivalves in the
wider area of Kočevje. He presented in the field several characteristical localities.
Ljudmila Šribar and Dr. Rajka Radoičič determined microfauna in limestone. Msc. Miha
Mišič did x-ray determination of marls from Podpeč. Arian Debeljak made graphic parts
of the article. The authors are very grateful to them.
Spodnjejurske plasti s školjkami v južni Sloveniji
Uvod
V južni Sloveniji, ki zajema skrajni severozahodni del Dinaridov, so jurske plasti
debele prek 1500 metrov; pretežno jih sestavlja apnenec. Usedale so se na Dinarski
karbonatni platformi, v mediteranskem delu nekdanje Tetide. V njih ne najdemo amonitov,
zato jih ne moremo neposredno razdeliti na stopnje ali celo na cone. Pri natančnejšem
določanju starosti si lahko pomagamo s fosilnimi školjkami, polži, brahiopodi, korala-
mi, hidrozoji, foraminiferami in algami.
Spodnjejurske plasti s školjkami v južni Sloveniji	49
Za členitev liasnih plasti so najpomembnejše fosilne školjke. Značilna je vrsta
Lithiotis problematica Gümbel iz družine Lithiotidae, ki je najbolj znamenita in geo-
grafsko najbolj razširjena spodnjejurska školjka. Ime je dobila po »ušesastih« presekih,
ki so jih v severni Italiji opazili že pred več kot 250 leti. Sprva so jih opisali kot rast-
linske ostanke (Gümbel, 1874, 1890). Vrsta je za paleontologe še vedno »proble-
matična«, saj si marsičesa v zvezi z nenavadno obliko lupine še vedno ne znamo razložiti
(Reis, 1903; Accorsi Benini & Broglio Loriga, 1977; Chinzei, 1982).
V	južni Sloveniji sta najpogostnejši školjčni vrsti Cochlearites loppianus (Tau-
sch) in Lithiopedalion scutatus (Dubar). Vrsto Lithiotis problematica najdemo le
na poredkih mestih. Vse tri vrste se odlikujejo po velikih, nenavadno oblikovanih in
močno variabilnih lupinah. Najbrž se marsikdo vpraša, kakšen je bil način življenja
teh školjk. Njihovi preseki se jasno razločijo v črnem podpeškem apnencu, ki je eden
naših najlepših okrasnih kamnov (Buser, 1987b).
V	liasnih plasteh južne Slovenije najdemo tudi številne druge školjke (S. Buser,
1965a; I. Buser, 1989). V pripravi je članek s sistematskimi opisi posameznih vrst,
ki bo izšel v naslednji številki Geologije (Debeljak & Buser). Tokrat pa so obde-
lana pomembna nahajališča v južni Sloveniji, s pomočjo katerih lahko sklepamo na
paleoekološke razmere, ki so v spodnji juri vladale na Dinarski karbonatni platformi
in za razmeroma kratek čas omogočile množično uspevanje značilne favne.
Spodnjejurske školjke vsekakor zaslužijo našo pozornost. Zanimive so v biološkem
in nič manj pomembne v stratigrafskem pogledu. Omogočajo pa nam tudi paleogeo-
grafsko primerjavo z drugimi svetovnimi nahajališči.
Dosedanje raziskave
Leta 1890 je Tausch von Gloeckelsthurn objavil svoje delo o favni liasnih
apnencev v Južnih Alpah. Zanimivo je, da je upodobil in opisal nekaj lepih primerkov
megalodontid iz podpeškega kamnoloma pri Ljubljani. V svoji razpravi (1890, 28-29)
je pravilno ugotovil, da pripadajo plasti s školjkami, ki izdanjajo južno od Ljubljane,
juri in ne triasu, kakor so domnevali pred njim.
V letih 1959-65 je S. Buser geološko kartiral ozemlje južne Slovenije od italijan-
sko-slovenske meje na zahodu prek Trnovskega gozda, Hrušice, Nanosa, Logaške in
Bloške planote z Javorniki, Krimsko-Mokrškega hribovja in Dolenjske do Suhe kraji-
ne na vzhodu. Pri tem je razčlenil jurske plasti in nabral bogat fosilni material (Bu-
ser, 1965a,b). Prvi je pri nas določil vrsti Lithiotis problematica in Cochlearites
loppianus. Ugotovil je, da večina primerkov v južni Sloveniji pripada novemu rodu
školjk z večveznim ligamentom. V svoji doktorski disertaciji ga je poimenoval Lithio-
pedalion. Skupaj z rodovoma Lithiotis in Cochlearites ga je uvrstü v družino Lithiotidae.
Tako se je pri nas za te značilne školjke uveljavilo ime litiotide, čeprav jih danes uvrščamo
v različne družine. Bus er je svoje delo predstavil na 42. letnem srečanju Paleontološkoga
društva v Gradcu leta 1972. Po dolgoletnem sodelovanju pa ga je z objavo prehitela
italijanska paleontologinja Accorsi Benini (1979) in opisala svoj rod Lithioper-
na, čeprav se je ime Lithiopedalion že uporabljalo v tuji strokovni literaturi (Bo-
seilini, 1972).
50		Stanko Buser & Irena Debeljak
Nahajališča spodiijejurskih školjk drugod po svetu
Plitvomorske spodnjejurske plasti s školjkami so marsikje litološko podobno razvite
kot v Sloveniji. Navzgor in navzdol navadno niso ostro omejene. Poleg bolj ali manj
temno sivega mikritnega in biosparitnega apnenca in včasih dolomita so pogostne tudi
lapornate plasti in pole ter kompleksi oolitnega apnenca.
Najbolj značilen favnistični element plitvomorskih spodnjejurskih plasti je vrsta
Lithiotis problematica. Po njej so dobile ime plasti in facies, v katerem se pojavlja
(npr. Lithiotis-KaXke, Lithiotis facies), čeprav so druge velike, debelolupinske školjke
v teh plasteh ponavadi pogostnejše (Berti Cavicchi et al., 1971).
Najbolj znamenita in najbolje raziskana so nahajališča v Južnih Alpah severne Italije
(province Verona, Trento in Vicenza). Nastopajo v zgornjem delu sivih liasnih apnencev,
imenovanih »Calcari grigi« (Böhm, 1884; Tausch von Gloeckelsthurn, 1890;
Bosellini & Broglio Loriga, 1971; Bosellini, 1972).
Spodnjejurske plasti z značilnimi školjkami se prek južne Slovenije nadaljujejo tudi
v sosednjo Hrvaško: Veliko Kapelo in Velebit (Grubič, 1961), Hercegovino (Katzer,
1904) ter Dalmacijo (Schubert, 1906). Našli so jih tudi pri Plitvicah. Najlepše ohranjeni
primerki vrste Lithiotis problematica in Cochlearites loppianus pa so iz nahajališča
Kopilje v Črni gori. Podatki o tem najdišču žal še niso bili objavljeni.
Podobni spodnjejurski razvoji z značilnimi školjkami so v južni Španiji (Turnšek
et al., 1975; Geyer, 1977), južnih in centralnih Apeninih (De Castro, 1962) in Maroku
(Dubar, 1948; Agard & Du Dresnay, 1965; Lee, 1983). Krumbeck (1923) je
temeljito obdelal favno na otoku Timor v Indoneziji. Broglio Loriga in Neri (1976),
Geyer (1977) ter Nauss in Smith (1988) so zbrali podatke o nahajališčih v zahodni
Franciji (dept. Sarthe), Švici (Graubünden), Tuniziji, Alžiriji, Albaniji, Grčiji, Turčiji,
Somaliji (Mogadiš), Omanu, južnem Iranu, Iraku in na Himalaji.
Von Hillebrandt (1981) poroča o rodu Lithiotis iz severnega Čila in Peruja.
V Severni Ameriki pa se ta rod poleg drugih školjk pojavlja v Kaliforniji, Nevadi ter
vzhodnem in centralnem Oregonu (Nauss & Smith, 1988). Lupher in Packard
(1930) sta rod Lithiotis iz Oregona prvič opisala pod rodovnim imenom Plicatostylus.
Kasneje se je izkazalo, da gre za isti rod (Buser, 1965a).
Takšna razprostranjenost najdišč po svetu priča, da so v spodnji juri na zahod-
nem in južnem obrobju Tetide in vzhodnega Pacifika obstajala obsežna, med seboj
povezana plitvomorska področja. Tu so na karbonatnih platformah, v tropskem pasu
ali pod vplivom toplih oceanskih tokov vladale zelo podobne paleoekološke razmere.
Paleogeografska rekonstrukcija naštetih nahajališč je na sliki 1. Domnevamo lahko,
da je že v pliensbachiju obstajal neki plitvomorski pas, ki je prek današnje Centralne
Amerike povezoval Tetido in prednika današnjega Pacifika. Imenujemo ga Hispanijski
koridor (Smith, 1983; Smith & Tipper, 1986). Omogočil je migracijo favne in flore
med obema oceanoma.
Na vseh naštetih področjih so pogostne velike, debelolupinske školjke. Skoraj povsod
je najznačilnejša vrsta Lithiotis problematica (si. 1). Po mnenju nekaterih avtorjev
so školjke množično poseljevale morsko dno v obliki podmorskih trat oziroma biostrom,
drugi pa poročajo o pravih grebenih oziroma biohermah (Agard & Du Dresnay,
1965; Bosellini, 1972; Göhner, 1980; Nauss & Smith, 1988).
V večini naštetih nahajališčih so poleg školjk prisotne korale, veliki polži, brahiopodi
in krinoidi. Tudi mikrofavna je zelo podobna. Med foraminiferami je najznačilnejša vrsta
Orbitopsella praecursor (Giimbel). Ponekod najdemo tudi ostanke kopenskih rastlin.
Najdbe amonitov so v plitvomorskih spodnjejurskih plasteh zelo redke in naključne.
Spodnjejurske plasti s školjkami v južni Sloveniji	51
Ponekod jih vsebujejo višje- oziroma nižjeležeče plasti. Z njihovo pomočjo plasti
z'omenjeno fosilno združbo {Lithiotis facies) uvrščajo v pliensbachij (večinoma v njegov
zgornji del: domerij) in ponekod (npr. v Južni Ameriki) tudi v spodnji del toarcija.
Nahajališča spodnjejurskih školjk v Sloveniji
Spodnjejurske školjke se pri nas množično pojavljajo v »litiotidnem horizontu« južne
Slovenije. Tako gaje leta 1965 poimenoval Bus er (1965a) po izredno značilnih školjkah,
ki jih je takrat zaradi očitnih podobnosti pripisal isti družini: Lithiotidae. Njihovi dolgi,
ozki preseki dajejo kamninam na terenu zelo razpoznaven značaj. Vrsta Lithiotis
problematica je najbolj znana, vendar precej redka. Veliko pogostnejši sta vrsti Li-
thiopedalion scutatus in Coclearites loppianus. Omenjene tri vrste danes prištevamo
v različne družine, zato bi morali ime litiotide opustiti. Izraze kot so »litiotidni hori-
zont« аИ »litiotidni apnenec« pa lahko uporabljamo predvsem v smislu faciesa. Po-
leg naštetih školjčnih vrst najdemo tudi rodove Pachyrisma, Gervilleiopema, Mytilus
in Opisoma.
Glede na terenske podatke in s pomočjo primerjave s podobnimi nahajališči v severni
Italiji je Bus er (1965 a, 44-46) plasti z značilnimi školjkami uvrstil v zgornji del sre-
dnjega liasa: domerij. Na Trnovskem gozdu značilne školjke izginejo 10-30 debe-
linskih metrov pod plastmi, ki vsebujejo zgornjeliasne brahiopode in okoli 50-60 me-
trov pod spodnjedoggerskimi plastmi, ki so bile dokazane z mikrofosili (Buser, 1979).
Litiotidni horizont je v južni Sloveniji stalen in se skoraj ne izklinja. Debel je lahko
manj kot pol metra, ponekod pa znaša celo 75 metrov. Največjo debelino doseže pri
Podpeči, na Krimsko-Mokrškem hribovju in v dolini Krke. Tu so plasti apnenca značilno
temno sive ali celo črne barve. Proti severu postane apnenec svetlejši, plasti s školjkami
pa so vse tanjše; na Trnovskem gozdu in Banjški planoti se ponekod celo izklinjajo.
Tudi proti jugu se litiotidni horizont močno stanjša in izklinja predvsem na mestih,
kjer so pojavi črnega premoga, to je pri Loškem Potoku, na Blokah, pri Cerknici in
Rakitnici. Na večjem delu Logaške planote, v Hrušici, na Bloški planoti, v Slivnici, na
Mali in Veliki gori in na Kočevskem najdemo školjke v bituminoznem debelozrnatem
dolomitu аИ pa v redkih, tankih apnenčevih vložkih med dolomitom. (Buser, 1965a,
39-47; Savie & Dozet, 1985).
Slika 2 kaže razprostranjenost spodnjejurskih plasti s školjkami. Posebej so označena
nahajališča, kjer školjke lahko izluščimo iz mehkejše lapornate osnove.
Nekatera izmed nahajališč, ki jih je S. Buser našel pri geološkem kartiranju pred
več kot tridesetimi leti, so žal uničena аИ zaraščena. To velja predvsem za nekdanji
kamnolom na desnem bregu Sušice zahodno od Dolenjskih Toplic in za nahajališče
na Javorniku (vzhodni del Trnovskega gozda). Tudi useki cest na Krimsko-Mokrškem
hribovju, jugozahodno od Loža, severno od Cerknice in pri Grčarevcu so skorajda povsem
zaraščeni. Posamezne primerke še dobimo ob železniški progi med Preserjem in Ver-
dom. Zelo številne školjke danes najdemo v kamnolomu v Podpeči, na vzhodnem pobočju
Špika (severno od Cola na Trnovskem gozdu) in ob kraškem izviru Globočec (zaho-
dno od Zagradca). Na novo je odkrito najdišče na vzhodnem pobočju hriba Stražišče
vzhodno od Gorenjega Jezera pri Cerknici. Vrsto Lithiotis problematica, ki je sicer
najredkejša, najdemo vzhodno od vasi Zafara pri Žužemberku.
V nadaljevanju so podrobneje opisana nahajališča Špik na Trnovskem gozdu, kam-
nolom v Podpeči, Globočec pri Zagradcu, Grčarevec južno od Logatca, Stražišče pri
52		Stanko Buser & Irena Debeljak
Gorenjem Jezeru, Ravne pri Borovcu na Kočevskem in Travna gora. Ta nahajališča so
razporejena po obravnavanem območju tako, da zastopajo nekdanja različna okolja.
Poleg tega je bilo tam možno posneti geološke profile (si. 3).
Spik na Trnovskem gozdu
Špik je okoli 950 metrov visok vrh na Trnovskem gozdu, ki leži približno 2 km severno
od Cola. Lepo odkrite zgornjetriasne in liasne plasti položno vpadajo proti jugu. Zgor-
njetriasni dolomit z značilnimi stromatoliti postopoma prehaja v spodnjeliasni masiv-
ni, debelozrnati dolomit svetlo sive barve, kije debel približno lOOm. Navzgor sledi
približno 30m svetlo sivega do belega mikritnega apnenca, ki vsebuje v zgornjem delu
do 3m debele vložke debelozrnatega dolomita. Plasti spodnjehasnega apnenca sesta-
vljajo strme skalne stopnje v gozdnem pobočju. Na približno 900 metrih višine znaša
njihov vpad 180/45. Litološko mejo s srednjeliasnimi plastmi postavlja Bus er (1978)
tam, kjer se pojavi rjavkasti oolitni apnenec. Tega je tu približo 30 metrov.
Nekaj metrov pod vrhom Špika je približno 70 cm debela plast sivo rjavega lapor-
natega apnenca, ki je poln različnih školjk. Prevladuje vrsta Lithiopedalion scuta-
tus. Njene lupine so dolge do 40cm in močno prekristaljene. Pridružujejo se ji Ger-
villeiopema buchi (Zigno), G. taramellii (Böhm), Mytilus lepsii Tausch in M.
mirabilis Lepsius. Zelo veliko je tudi srednje velikih megalodontidnih školjk, ki pri-
padajo novemu rodu in vrsti. Različne vrste školjk so bile verjetno nanesene skupaj
šele po odmrtju. Njihove lupine so precej dobro ohranjene, kar kaže na to, da tran-
sport ni bil dolg. V apnencu je poleg školjk vse polno drobnih odlomkov različnih fosilov.
Ta plast, iz katere lahko luščimo školjke, se vleče po vzhodnem pobočju Špika pri-
bližno 200 metrov daleč.
Nad plastjo s školjkami je približno 5 metrov svetlega oolitnega apnenca s šte-
vilnimi odlomki krinoidov, igUcami morskih ježkov, briozoji in posameznimi korala-
mi. Višje v profilu je približno en meter debela plast svetlo sivega mikritnega apnenca
s številnimi, vendar ne kamnotvornimi školjkami vrste L. scutatus. Nekatere lupine
leže pravokotno na plastnatost, kar kaže, da so školjke po odmrtju ostale v življenjskem
položaju. Nad to plastjo sledi še meter mikritnega apnenca brez školjk (si. 3).
Območje Trnovskega gozda je v srednjem Uasu pripadalo severnemu, distalnemu
robu Dinarske karbonatne platforme. Ta del je bil na udaru valov in tokov, ki so prihajali
iz severno ležečega globokega morja Slovenskega bazena. Okolje s povečano vodno
energijo školjkam ni najbolj ustrezalo. Na Špiku jih zato najdemo le v dveh, razme-
roma tankih plasteh. Kaže, da so naštete vrste še najbolje prenašale razburkano okolje.
Podpeč
Podpeč leži na južnem obrobju Ljubljanskega barja. V velikem kamnolomu so lepo
odkrite spodnjejurske plasti podpeškega apnenca, ki so jih že Rimljani izkoriščah za
gradnjo nekdanje Emone. Od srednjega veka naprej so iz tega kamnoloma pridobili
velike količine gradbenega in okrasnega kamna. Zdaj že več kot trideset let apnenca
ne lomijo več. Podpeški »marmor« po vsej Sloveniji krasi razUčne stavbe in spome-
nike. Najlepše je zaživel v stvaritvah arhitekta Jožeta Plečnika (Prelovšek, 1987).
Vsakdo, ki obišče Narodno in univerzitetno knjižnico v Ljubljani, opazi v njeni avh
čudoviti črni kamen z dolgimi, belimi preseki školjk (si. 4). V Ljubljani si ga lahko
Spodnjejurske plasti s školjkami v južni Sloveniji	53
ogledamo tudi v starem Nebotičniku, v nekdanji Trgovinski zbornici, na stopnišču Monta-
nistike, v zgradbi Slovenskega parlamenta, na Magistratu in še marsikje.
Plasti srednjeliasnega apnenca skoraj navpično vpadajo proti jugu (180/80). De-
bele so od 10 cm do 2 metra. Lepo so odkrite na vzhodni in zahodni strani kamnolo-
ma (si. 5, 6). Vmesni odkopani del priča, kako vehke količine kamna so tu pridobili.
Apnenec v kamnolomu prečka več prelomov. Ob njih so posamezne grude drsele ena
ob drugi, tako da so nastale velike zglajene prelomne ploskve z navpičnimi razami.
Med diagenezo so v apnencu nastaU številni stilolitni šivi.
Med plastmi apnenca so zelo pogostne lapornate prevleke in plasti. V spodnjem
delu profila so le-te debele tudi do 15 centimetrov. Najlepše jih vidimo v zahodnem
delu kamnoloma, kjer je svež odkop za novo halo. Površina apnenca pod polarni la-
porja je pogosto gomoljasto razjedena (si. 7). Nad to nepravilno površino je nekaj
milimetrov debela, vijoličasto rdeča skorja, sledi pa rumeno rjav ali rdečkast lapor.
Rentgenska analiza je pokazala, da vsebuje vijoHčna skorja naslednje minerale: mu-
skovit, illit, kalcit, hematit in anataz oziroma brookit. Mineraloška sestava po mnenju
M. Mišiča (ustno sporočilo) kaže na preperevanje in situ. Verjetno je bil apnenec večkrat
dvignjen nad morsko gladino in potem zopet spuščen. Ob preperevanju apnenca na
kopnem so nastale rdeče kraške gline oziroma terra rossa, ki je bila kasneje spremenjena
v omenjene rjave in rdeče laporje. Takšne oscilacije lahko opazimo le v spodnjem delu
profila. M. Mišič je v laporju določil naslednje minerale: muskovit, illit, kalcit in goe-
thit. Analiza na pelod, ki jo je opravil M. Jelen, ni dala pozitivnih rezultatov (ustno
sporočilo).
Profil (si. 3) je bil posnet na vzhodni strani kamnoloma. Litiotidni horizont je
v podpeškem kamnolomu odkrit v vsej debelini, ki znaša kar 75 metrov. V njem so
plasti s školjkami razmeroma tanke (0,l-l,5m), vmesne prekinitve brez školjk pa velike.
V profilu lahko naštejemo več kot petnajst med seboj ločenih školjčnih lumakel. Njihova
skupna debelina je približno 12 metrov. Veliko število razmeroma tankih lumakel kaže
na to, da je bila rast školjk velikokrat motena ali celo prekinjena. Školjke v podpeš-
kem kamnolomu najdemo v črnem mikritnem apnencu in v spodnjem delu profila
tudi v lapornatih plasteh. Školjčnim lumakelam najpogosteje sledi sparitni apnenec
z ooidi, drobnimi zaobljenimi odlomki različnih lupinic (bioklasti) in intraklasti. To-
kovi in valovi, ki so v laguno nanašaU različen drobir in spirali karbonatno blato, so
najbrž motili rast školjk. Školjke so v času neugodnih pogojev preživele nekje v soseščini
in se v ugodnejših razmerah spet naselile na prejšnjih mestih. V zgornji polovici profila
je precej več mikritnega apnenca kot v spodnji. Okolje je bilo takrat mirnejše, zato
so lumakele debelejše in številčnejše.
Med školjkami je daleč najpogostnejša vrsta Lithiopedalion scutatus (Dubar),
ki se tudi prva pojavi. V spodnjem delu profila lahko iz lapornatih plasti izluščimo
lepe primerke. Ponekod so lupine nagnetene ena ob drugi kot črepinje z zelo malo
veziva (si. 8). Po presekih v apnencu sodeč, so nekateri primerki dosegli velikost
kar 3/4 metra. V drugi polovici profila, ki kaže na mirnejše sedimentacijsko, okolje,
je pogostna tudi vrsta Cochlearites loppianus. Tu je školjke nemogoče izluščiti iz
kamnine, zato prisotnost te vrste dokazujejo le preseki v apnencu. Vrsta Lithiotis proble-
matica je v Podpeči zelo redka; njene značilne preseke (si. 9, 10) najdemo le v dveh
ali treh plasteh v zgornjem delu profila.
Preseki omenjenih školjk se v kamnolomu, na terenu zelo slabo razločijo, ker je
plasti prekrila patina ali pa je površina tektonizirana. Na srečo lahko orientiranost
školjčnih lupin opazujemo na zglajenih ploščah okrasnega podpeškega apnenca. Večinoma
lepo ohranjene lupine leže vzporedno s plastnatostjo. V takšnih lumakelah ponavadi
54__Stanko Buser & Irena Debeljak
prevladuje le ena vrsta. V primerih, ko so školjke razhčnih vrst nepravilno »razmeta-
ne« v apnencu, so lupine večkrat poškodovane (si. 10). Lupine so bile velikokrat razlom-
Ijene in deloma raztopljene tudi med diagenezo.
Poleg rodov Lithiopedalion, Cochlearites in Lithiotis nastopajo v Podpeči tudi
naslednje školjčne vrste: Gervilleiopema buchi, G. taramellii, G. timorensis Krumbeck,
Opisoma cf. excavatum Böhm, O. cf. menchikoffi Dubar in Pachyrisma {Pachy-
megalodon) chamaeforme (Schlotheim). Zanimiva je tudi približno 40cm debela plast
lapornatega apnenca z drobnimi školjkami, ki verjetno pripadajo rodu Astarte. Ta plast
je tik ob skladišču za razstrelivo. Na slikah je označeno, kje so bile najdene različne
školjke. Od naštetih vrst so prav vse kamnotvorne, samo primerke rodu Opisoma
najdemo posamično (si. 11), in sicer v biosparitnem apnencu, polnem organskega
detritusa, ki so ga sferično obraščale modrozelene cepljivke. Zanimivo je, da v lumakelah
z drugimi rodovi školjk ni onkoidov, čeprav so sicer v profilu zelo pogostni. Očitno
je, da modrozelene cepljivke v družbi s školjkami niso mogle uspevati.
Bus er (1965 a) je iz podpeškega kamnoloma opisal dve vrsti brahiopodov: Tere-
bratula rotzoana Schauroth in T. renieri Catullo. Polži so precej številni, vendar
jih je iz trdne kamnine nemogoče izluščiti. V zbruskih najdemo številne drobne hišice,
v okrasnem podpeškem apnencu pa so preseki polžev veliki tudi do 10 cm (si. 12).
Korale so precej redke. Posamezni primerki so se pritrjah tudi na lupine megalodon-
tidnih školjk. Zanimivo je, da je v bližini, v Gornji Brezovici pri Preserju v litiotid-
nem horizontu razvit tudi manjši koralni greben (patch reef). Dolg je 35 metrov, širok
nekaj metrov in debel 5 metrov. Lateralno prehaja v temno sivi oolitni apnenec.
Za srednji lias je poleg školjk najznačilnejša velika foraminifera Orbitopsella pra-
ecursor (si. 13). V podpeškem apnencu je v nekaterih plasteh kamnotvorna. Mikro-
foraminifere so prisotne skorajda v vseh plasteh, zato na shematskem profilu (si. 3)
niso posebej označene. Najštevilnejše so v pelbiomikritnem apnencu (si. 14). L. Šri-
bar (rokopisno poročilo) je določila naslednje družine foraminifer: Ammodiscidae {Glo-
mospira sp.), Lituolidae (rodovi Lituosepta, Orbitopsella, Haurania, Pseudocyclam-
mina), Verneuilinidae, Textularidae in Nubeculariidae {Opthalmidium sp.). Prisotne
so tudi dazikladaceje Thaumatoporella parvovesiculifera (Raineri) in Palaeodasycla-
dus mediterraneus (Pia), kodiaceje in problematični fosil Aeolissacus sp. Mikrofo-
sile iz Podpeči je določala tudi R. Radoičič (rokopisno poročilo). V višjih delih profi-
la je R. Radoičič določila tudi vrsto Labyrinthina recoarensis. Po njenem mnenju
pripada nekaj najnižjih plasti v kamnolomu zgornjemu delu spodnjega liasa. V najvišjem
delu profila, ki je tektonsko precej prizadet, pa bi lahko pričakovali mejo med sred-
njim in zgornjim liasom.
Najpogostnejša bentoška foraminifera je Glomospira. Zelo številna je v celem profilu,
tudi tam, kjer ni drugih mikrofosilov. Čeprav so zgoraj omenjene apnenčeve alge
v podpeškem kamnolomu precej pogostne, jih v školjčnih lumakelah nismo zaslediU.
Nasploh je biodiverziteta v plasteh, kjer so bih ugotovljeni rodovi Lithiopedalion,
Cochlearites, Gervilleiopema in Lithiotis, zelo nizka.
Značilne školjke v zgornjem delu profila dokončno izginejo nenadoma, brez po-
stopnega prehoda. Po litologiji sodeč, se sedimentacija pri tem ni bistveno spreme-
nila. Preučiti pa bi bilo treba, če je nastala kaka sprememba v združbi foraminifer.
Spodnjejurske plasti s školjkami v južni Sloveniji	55
Globočec pri Zagradcu
Globočec je velik kraški izvir jugozahodno od Zagradca. Na tem območju prehaja
zgornjetriasni glavni dolomit s stromatoliti v svetlo sivi, mikritni spodnjeliasni apne-
nec. Ob izviru Globočec so lepo odkrite plasti temno sivega do črnega srednjeliasne-
ga apnenca. Debele so od 10 do lOOcm in vpadajo proti jugozahodu (220/15). V gozdnem
pobočju sestavljajo skalne stopnje, ki jim lateralno lahko sledimo na dolžini 50m. Tu
je odkritih le vrhnjih 20 metrov litiotidnega horizonta, ki je sicer v dolini Krke debel
окоИ 50 metrov.
V	spodnjem delu profila (si. 3) pri Globočcu je več kot 5m temno sivega mikrit-
nega apnenca s kamnotvornimi školjkami rodu Cochlearites. Vmes sta samo dve tanki
prekinitvi z le posameznimi preseki lupin. Apnenec je v spodnjem delu ponekod la-
pornat, zato je školjke mogoče luščiti iz kamnine. Nekateri primerki so tipični pred-
stavniki vrste Cochlearites loppianus, drugi, ki smo jih našU kak meter višje, pa so
precej večji in drugačne oblike. Pripadajo novi vrsti, po značilni strukturi lupine sodeč
pa gre za isti rod. V tem delu profila so bili najdeni tudi posamezni primerki solitar-
nih koral, ki so bile prirasle na debele lupine školjk.
Višje sledi dva metra debel sparitni apnenec z intraklasti, posameznimi ooidi, odlomki
lupin in krinoidov. To je edina večja prekinitev brez školjk. Nad tem je 7 metrov apnenca,
ki je poln kamnotvornih školjk iz rodu Cochlearites. Preseki rodu Lithiopedalion so
redki. V tem delu školjk ne moremo izluščiti iz trdne kamnine. Zatem školjke nena-
doma, brez postopnega prehoda izginejo. V črnem mikritnem apnencu, v zadnjih 30
metrih odkritih plasti ne najdemo niti enega primerka več.
Ћр apnenca v nahajališču Globočec priča o mirnem sedimentacijskem okolju. Rast
školjk je bila skorajda nemotena. Njihove lupine so večinoma nepoškodovane in jih
v nekaterih plasteh najdemo celo v navpični, to je življenjski legi. Večina pa se jih
je po odmrtju prevrnila na morsko dno, tako da so preseki orientirani vzporedno s
plastnatostjo. Zelo pogosto lahko opazimo tudi nekakšne zagozde, sestavljene iz šte-
vilnih lupin. V tem primeru se je po odmrtju ali pa še za življenja več primerkov
nagnilo skupaj, tako da se podpirajo v poševni legi.
Grčarevec
Naselje Grčarevec leži južno od Logatca. Približno 1 km proti severozahodu so lepo
odkrite srednjeUasne plasti. Njihov vpad znaša 300/15. Prevladuje temno sivi debe-
lozrnati dolomit s posameznimi školjčnimi lumakelami, ki so debele do 2 metra.
V	leči apnenca, ki leži med dolomitom, je pri zadnjih počitniških hišah, severo-
zahodno od Grčarevca sveže odkopani usek. V treh ločenih plasteh, debelih približno
70 cm, se lepo vidijo preseki kamnotvornih školjk. Močno prevladuje vrsta Cochlea-
rites \oppianus (si. 15). Neprimerno manj je presekov, ki verjetno pripadajo rodu
Lithiopedalion, v srednji plasti pa tudi rodovoma Gervilleiopema in Mytilus.
Apnenec je mikriten in temno sive do črne barve. Nad apnencem sledi dolomit, ki
v spodnjem delu še vsebuje značilne školjke, navzgor pa teh ni več.
V	ostrem ovinku stare ceste severno od Grčarevca je Buser (1978, 396) v litio-
tidnem horizontu našel lepo ohranjene rastlinske ostanke. To nahajahšče je danes
preraščeno. Ostanki kopenskih rastlin dokazujejo, da so na karbonatni platformi od
časa do časa obstajah manjši kopni otoki.
56		Stanko Buser & Irena Debeljak
Stražišče pri Gorenjem Jezeru
Iz vasi Klance zavijemo v hrib Stražišče, ki se nahaja vzhodno od Gorenjega Je-
zera pri Cerknici oziroma zahodno od Starega trga. Tu naletimo na enega najlepše
odkritih profilov v litiotidnem horizontu.
V več kot 20 metrih odkritega profila lahko naštejemo okoli 10 školjčnih lumakel,
ki so debele okoli 0,5-1 metra in največ 2,5 metra. Prevladuje rod Cochlearites. Njegove
značilno oblikovane lupine so ponekod lečasto razporejene, vmes pa se lumakela izklinja.
V nekaterih plasteh so lupine tako številne, da vmes skorajda ni veziva. Praviloma
nastopajo v temno sivem mikritnem apnencu. Zaradi lapornate primesi jih v nekate-
rih delih lahko izluščimo iz kamnine. Vmesne plasti brez školjk v glavnem sestavljata
oolitni in biosparitni apnenec.
Školjke se v profilu nenadoma množično pojavijo in tudi izginejo nenadoma, brez
postopnega prehoda. Nad litiotidnim horizontom sledi sparitni apnenec z ooidi in bio-
klasti, ki kot erozijska krpa sestavlja vrhnji del hriba.
Travna gora
Dobra dva kilometra jugovzhodno od Travne gore lahko v gozdnem pobočju sle-
dimo vsaj 20 metrov debelemu litiotidnemu horizontu, ki je v celoti razvit v dolomi-
tu (si. 3). Školjke je mogoče izluščiti iz ene same, 20cm debele lapornate plasti
v spodnjem delu profila. V dolomitu najdemo le tri prave školjčne lumakele, vmesne
plasti pa vsebujejo tudi posamezne, bolj ali manj pogostne, močno prekristaljene lu-
pine. Zanesljivo gre predvsem za rod Cochlearites. V spodnjem delu opazimo naj-
prej do 30 cm velike preseke tega rodu, ki so podobni tistim v nahajališču Globočec
in verjetno pripadajo novi vrsti. V najvišjem delu litiotidnega horizonta je lumakela
s školjkami debela kar tri metre. V prvi polovici jo sestavljajo prekristaljene lupine,
ki bi jih lahko pripisali rodovoma Cochlearites in Lithiopedalion, vendar so prime-
rki precej manjši in drobnejši kot običajno (dolžina lupine 5cm, debelina do lem).
V zgornji polovici omenjene lumakele pa spet prevladuje tipični predstavniki rodu
Cochlearites. 40 metrov višje, nad dolomitom, sledi ooliten, temno sivi zgornjeliasni
apnenec, ki se menjava s tankimi ploščami črnega mikritnega apnenca.
Ravne pri Borovcu
Srednjeliasne plasti v širši okolici Kočevja sestavlja predvsem debelozrnat, bitu-
minozen dolomit (si. 2). To je območje, ki je nekdaj pripadalo južnim, najbolj zapr-
tim delom lagune oziroma notranjega dela Dinarske karbonatne platforme.
Najdišče Ravne pri Borovcu je najjužnejše nahajališčev Sloveniji. Na spodnjem delu
severnega pobočja Borovske gore izdanja črni, bituminozni, drobno ploščasti apne-
nec. Nad njim je 1,5 metra debela plast črnega mikritnega apnenca z razmeroma majh-
nimi, približno lOcm dolgimi lupinami rodu Cochlearites. Višje sledijo plasti bitumi-
noznega dolomita. V vrhnjem delu je 4 metre debela plast dolomita, ki je polna neobičajno
drobnih primerkov rodu Cochlearites. Ta plast na pobočju izstopa kot strma skalna
stopnja, ker je zaradi številnih školjčnih lupin mehansko bolj odporna.
Spodnjejurske plasti s školjkami v južni Sloveniji	57
Življenjske razmere školjk v spodnji juri v južni Sloveniji
V južni Sloveniji se je v spodnji juri razprostirala obsežna Dinarska karbonatna
platforma. Ta plitvomorski prostor je proti severu mejil na globoko morje Slovenske-
ga bazena (Buser, 1987a), proti jugu pa se je nadaljeval v današnjo Hrvaško. Rekon-
strukcija nekdanjih razmer je na sliki 16.
Zanimivo je, da najdemo školjke vrste Lithiotis problematica tudi v okolici Bo-
vca (na hribu Poljanica in pri kraškem izviru Glijun), torej na ozemlju, ki zanesljivo
pripada Julijski karbonatni platformi, ki se je razprostirala severno od Slovenskega
bazena (si. 16). Skorajda nemogoče je, da bi plitvomorska favna migrirala z Dinar-
ske karbonatne platforme na severnejšo Julijsko karbonatno platformo kar preko glo-
bokomorskega korita Slovenskega bazena. Sklepamo, da se je Slovenski bazen na območju
današnjega osrednjega dela doline Soče pri Trnovem, zahodno od Kobarida, izklinil.
Ta globokomorski bazen se torej ni nadaljeval proti zahodu v Belunski bazen sever-
ne Italije. Zaradi izklinitve Slovenskega bazena na omenjenem področju sta Dinarska
in Julijska karbonatna platforma v hasu neposredno mejiU ena na drugo.
Poglejmo zdaj, kakšne razmere so nekoč vladale v južni Sloveniji na Dinarski
karbonatni platformi (si. 17):
Severno obrobje platforme je prestreglo največjo udarno moč valov in tokov, ki
so prihajali z globljega morja Slovenskega bazena. O njihovi rušilni moči pričajo apnenčeve
breče na Banjšicah in severnem Dolenjskem. Sicer pa na severnem delu nekdanje
Dinarske karbonatne platforme (Trnovski gozd, Banjšice) prevladuje oolitni in kri-
noidni apnenec svetle barve. Usedal se je v obsežnih plitvinah s toplo vodo, globoko
le 1-3 metre. Iz globljih predelov Slovenskega bazena je sem prihajala bolj mrzla voda.
Zaradi zvišanja temperature se je iz nje izločil del karbonatov in se odlagal okoli drobnih
zrnc, ki so lebdela v nemirni vodi. V plitvinah so se kopičile vehke kohčine ooidov,
drobnih hišic in odlomkov razhčnih organizmov, ki so jih zaoblili valovi. Ponekod se
je nabralo toliko tega drobirja, da je dosegel vodno gladino. Nastajale so obsežne peščene
sipine in plaže. Skupaj z majhnimi koralnimi in hidrozojskimi grebenčki (Trnovski gozd)
so predstavljale prave pregrade pred globokim morjem na severu in zapirale južneje
ležečo laguno.
Ob plimi in neurjih je sveža morska voda prestopila pregrade in po širokih bibavičnih
kanalih vdrla v laguno. Za njo so ostajale naplavine drobnega »oolitnega peska«. Tako
tudi na področju nekdanje lagune najdemo precej temnega oolitnega apnenca. Pri-
družuje se mu biosparitni apnenec. Večinoma pa prevladuje mikritni apnenec, ki je
nastal iz lagunskega karbonatnega blata. V njem kar mrgoli drobnih hišic foraminifer,
ki so včasih živele na morskem dnu (bentos). Na mestih stalnega dotoka sveže vode
so uspevale tudi korale, krinoidi in zelene alge (dazikladaceje). Sicer pa so v laguni
največ življenjskega prostora na blatnem morskem dnu zasedale prav školjke. Sesta-
vljale so prave podmorske trate (biostrome) s posameznimi lečastimi odebehtvami.
Ponekod so se naselih tudi polži in brahiopodi. Karbonatna sedimentacija je bila
v tem, osrednjem predelu najhitrejša, zato tudi litiotidni horizont doseže tu največjo
debelino.
Proti jugu je bilo čedalje manj vpliva s severnega globokomorskega predela. Va-
lovi in tokovi skorajda niso več dosegah tega področja. To so bih zaprti in mirni deh
lagune. Veliko kisika je bilo porabljenega pri razpadanju organske snovi. Na dnu so
tako zavladah redukcijski pogoji. Kamnine, ki so nastale v zaprti laguni, so močno bi-
tuminozne in značilne temno sive do črne barve. Evaporacija in s tem tudi slanost
vode sta bih tu največji. Poma voda v sedimentih je vsebovala veliko magnezija in
58		Stanko Buser & Irena Debeljak
so zato le-ti med diagenezo najlažje zapadli dolomitizaciji. Sklepamo lahko, da so menjave
sušnih obdobij s prevladujočo evaporacijo in deževnih obdobij z močnimi nalivi pre-
cej vplivale na spremembe slanosti v zaprti laguni.
V večjem delu lagune oziroma šelfnega morja je bila voda globoka le nekaj me-
trov (subtidal). Zagotovo so obstajala tudi večja območja, ki so bila med oseko na
suhem (intertidal); vendar so izsušitvene pore in stromatoliti v spodnjejurskih pla-
steh izredno redki, po čemer se ločijo od zgornjetriasnih plitvomorskih plasti. Školjke
rodu Mytilus (klapavice), ki so lahko tudi indikatorji za takšno okolje, pa so v južni
Sloveniji precej pogostne.
Pojavi premoga in ostanki kopenskih rastlin na redkih mestih (Bloke, Loški Po-
tok, Grčarevec) dokazujejo, da so na platformi od časa do časa obstajali manjši kop-
ni otoki, ki so jih najbrž obrobljala močvirja.
Kamnine in organizmi kažejo, da je bilo podnebje tropsko ali pa vsaj subtropsko.
Paleogeografska razporeditev sedanjih ozemelj je bila v spodnji juri bistveno drugačna.
Celotna »Jadranska plošča« se je nekdaj nahajala južneje, v nižjih geografskih širi-
nah in tedanjem tropskem pasu (si. 1).
Razporeditev vrst in način življenja školjk
Kaže, da severni, najbolj izpostavljeni deli karbonatne platforme za rast školjk niso
bili ugodni. Školjke so od časa od časa poselile tudi to področje, vendar je bila njihova
rast vsakokrat kmalu prekinjena. Močni valovi so lahko njihove lupine odnašali ali
poškodovali. Nemirno okolje so še najbolje prenašale megalodontidne školjke, pa tudi
rodovi Lithiopedalion, Mytilus in Gervilleiopema. Predvsem megalodontidne školjke
se v severozahodnem delu Trnovskega gozda pojavljajo celo tam, kjer vse druge školjčne
vrste že izginejo.
V bolj zatišnih delih šelfa, ki pa jih je vseeno dosegla sveža voda, bogata s ki-
sikom in hranilnimi snovmi, so školjke uspevale najbolj množično. Razmere za njiho-
vo rast so bile v tem delu najugodnejše, vendar tudi precej nestabilne. Številne, raz-
meroma tanke, po vertikali ločene lumakele pričajo, da je bila rast školjk velikokrat
prekinjena. Prikamnina je praviloma mikritna; sedimentološka in paleontološka slika
med njimi ležečih plasti pa kaže, da je ob prekinitvah rasti školjk največkrat prišlo
do povišane vodne energije. Takrat so se usedaU sedimenti, iz katerih so nastali oo-
litni, onkolitni in biosparitni apnenci. Občasno, morda ob večjih neurjih, so tokovi v
laguno nanesli toliko materiala, da so školjke dobesedno zasuli z različnim drobirjem.
Po drugi strani so lahko močni valovi izprali ves sediment, ki je obdajal velike lupine
najbolj značilnih školjčnih vrst in jim dajal oporo, ter jih razmetah po morskem dnu,
kjer so propadle. Ob neugodnih razmerah so školjke preživele nekje drugje in se ob
priliki spet naselile na prejšnjem mestu. Osnovo za prvo poselitev oziroma pritrditev
juvenilnih školjk je lahko predstavljala trdna podlaga, različni detritus ali pa kar lu-
pine odmrlih školjk. V osrednjem predelu lagune, ki je bil pod vplivom globokomor-
skega bazena, je prevladoval rod Lithiopedalion, zelo pogostni pa so tudi primerki
rodu Cochlearites. Bolj ali manj številčno so bile zastopane vse že prej naštete školjke,
med njimi tudi rod Lithiotis.
Podatki zaenkrat kažejo, da je mirno, blatno dno v južnejših, najbolj zaprtih de-
lih lagune poseljeval predvsem rod Cochlearites. V srednjeliasnem dolomitu pogosto
naletimo na preseke lupin, ki bi jih lahko pripisali rodovoma Lithiopedalion in Co-
chlearites, vendar so neobičajno majhni. Velikost lupine doseže največ 10 cm in de-
Spodnjejurske plasti s školjkami v južni Sloveniji	59
belino do lem, ponavadi pa še manj. Možno je, da so omenjene školjke rastle ob tako
slabih pogojih (povečana slanost, zmanjšan dotok sveže vode), da so ostale majhne,
lahko pa da gre za posebne vrste. Žal se takšnih primerkov nikjer ne da izluščiti iz
dolomitne kamnine, pa tudi lupine so diagenetsko močno spremenjene. Predvideva-
mo, da so bile razmere v najbolj zaprtih delih lagune za školjke v splošnem manj ugodne
kakor v osrednjem delu, ki je bil pod stalnim, zmernim vplivom iz pelagiala. Po dru-
gi strani pa je bilo okolje v najbolj zatišnih delih bolj stabilno, na kar kaže precejšnja
debelina sicer redkih školjčnih lumakel (do 4 metre).
Pri ugotavljanju razhčnih sedimentacijskih okolij na karbonatni platformi se mo-
ramo zavedati, da se je težišče vplivov iz globokomorskega predela sčasoma spreminjalo.
To je bilo najbolj izrazito na prehodu iz bazena v platformo. Na vseh področjih šelf-
nega morja oziroma lagune pa so se vsaj deloma spreminjali vodna energija, nivo morske
gladine in morsko dno, vključno s sedimenti, ki so se usedah na njem.
Za vse naštete spodnjejurske školjke velja, da so sestavljale prave podmorske trate,
tako da se je skupaj nagnetlo na stotine osebkov iste vrste. Večinoma so školjke živele
v zelo plitvi vodi (intertidal-subtidal), saj so bhzu vodne gladine dosegle največ hra-
nilnih snovi.
Rod Gervilleiopema je živel podobno, kot še danes živi rod Isognomon. Na tr-
dno podlago so se te školjke pritrjale s šopom lepljivih bisusovih vlaken. Enako velja
za predstavnike rodu Mytilus. To je edini rod, ki je še danes razširjen v vseh plitvih
morjih, razen na skrajnem severu. Značilno zanj je, da poseljuje območje plimovanja.
Ob oseki lahko več ur preživi na suhem, tako da hermetično zapre lupini. Dobro prenaša
tudi spremembe v slanosti vode. Najbolj mu ustreza brakična voda v priobalnem pasu
(Cox, 1969, N5-N15). Megalodontidne školjke so živele z vrhom zasidrane v mehko
morsko dno. Lupina in sklep sta bila zelo močna, okrogla obhka pa jim je omogočala,
da so se ob močneših vaiovih in tokovih brez škode kotalile po morskem dnu.
O načinu življenja najznačilnejših spodnjejurskih rodov Lithiotis, Cochlearites
in Lithiopedalion je še veliko nejasnosti. Podobne školjke danes ne živijo več, pa
tudi s kako fosilno skupino bi jih le težko primerjali. Njihova nenavadna obhka in
vehka stopnja variabilnosti že sami po sebi zastavljata številna biološka vprašanja (več
v: Debeljak & Buser, in prep.). Bile so docela prilagojene na blatno morsko dno,
hitro sedimentacijo v okolju in sesilno življenje v gosti združbi istovrstnih primerkov.
Glede na majhno velikost mehkega telesa so gradile izjemno velike lupine. Rastle so
vse življenje, tako da je školjka ostajala nad nivojem obdajajočega sedimenta, in da
si je v gosti družbi sosedov priborila dovolj sončne svetlobe. Takšna nenehna, enako-
merna rast v višino je prava posebnost, saj se rast živali običajno eksponentno zmanjšuje
s starostjo. V plasteh, kjer so lupine po odmrtju ostale v življenjskem položaju, lahko
opazujemo, kako so se posamezni primerki med seboj podpirah in se navzgor šopa-
sto razraščah; podobno kot rastline. Pogostni so »kolenasto« upognjeni primerki, pri
katerih se je lupina morda nevarno nagnila in potem spet popravila svoj položaj z navpično
rastjo. Pri vsem tem ni čudno, da pri isti vrsti obhka močno variira. Zunanja podob-
nost omenjenih treh rodov, ki sicer pripadajo razhčnim družinam, je rezultat konver-
gence: prilagoditve na podobno, zelo specifično življenjsko okolje. Lupine rodov Li-
thiotis, Cochlearites in Lithiopedalion so lahko merile kar 0,5 metra. Tako velike
školjke danes najdemo predvsem v ekvatorialnem pasu ah pa na območjih s toplimi
tokovi, kjer je temperatura vode ves čas najmanj 23°C. Zaradi zmanjšane vsebnosti
CO2 v vodi je tam olajšano izločanje kalcijevega karbonata oziroma gradnja skeleta.
Značilno je, da je biodiverziteta v posameznih školjčnih lumakelah izredno nizka.
Očitno se posamezne vrste školjk med seboj niso mešale. Prav tako med školjkami
60		Stanko Buser & Irena Debeljak
skorajda ni drugih organizmov, ki bi se fosilno ohranili. Deloma lahko to pripišemo
selektivnemu vplivu specifičnega okolja (blatni substrat, hitra sedimentacija), ki je
zahtevalo od organizmov posebno prilagojenost. Najbrž pa so tudi same školjke
onemogočale svoje tekmece s hitrim razraščanjem in produkcijo velikih količin or-
ganskega blata. V kalni vodi je bila fotosinteza praktično onemogočena, kar je iz tega
prostora izključilo veliko organizmov, predvsem pa zelene alge in modrozelene ce-
pljivke. Podobne razmere danes opažamo na vehkih ostrižiščih. Nekatere vrste ostrig
lahko v eni uri prefiltrirajo 40 litrov vode. Pri tem so pravi akumulatorji sedimenta,
bogatega z organsko snovjo, ki zaudarja in ob razpadanju povzroča redukcijske po-
goje. Na vodni gladini se H2S oksidira v žvepleno kisUno. Na vrhu sedimenta je kislo
okolje, zato so lupine pogosto korodirane (Stenzel, 1971, N1000-N1003), kar lahko
opazimo tudi na fosilnih ostankih.
Rodovi Lithiotis, Cochlearites in Lithiopedalion so v južni Sloveniji ustvarjali
podmorske trate ali biostrome. Pri nas ni dokazov za obstoj grebenskih tvorb. Manjše
lečaste odebelitve sicer razmeroma tankih školjčnih lumakel bolj verjetno predstavljajo
nasutine po krajšem transportu prenesenih lupin. Lumakele se lateralno ne izklinjajo;
kvečjemu tanjšajo in debelijo. Mikritna prikamnina kaže med usedanjem sedimenta
na nizko vodno energijo. Glede na svojo obliko so lupine razmeroma dobro ohranjene.
Lupine izoliranih primerkov navadno niso erodirane. Biodiverziteta v lumakelah je nizka.
Vse to kaže, da običajno ni prihajalo do daljšega transporta. Menimo, da so značilne
školjke uspevale na velikih površinah in da so jih valovi po odmrtju najpogostneje le
nekoliko premetaU.
Cvetoča doba značilnih spodnjejurskih školjk (rodovi Lithiotis, Cochlearites in
Lithiopedalion^ je trajala le окоИ 5-10 milijonov let; na področju južne Slovenije pa
se niso obdržale niti tako dolgo. V vseh naših nahajahščih ob koncu litiotidnega ho-
rizonta, ki ga uvrščamo v domerij, školjke izginejo nenadoma, brez postopnega pojemanja.
V zgornjem liasu je na področju Tetide prišlo do obsežnih tektonskih premikov.
V severni Sloveniji, na nekdanji Julijski karbonatni platformi, je prišlo do kratke okopnitve,
potem pa se je naglo pogreznila in razpadla (Buser, 1987a, 316). Vpliv teh dogajanj
je gotovo segel tudi na Dinarsko karbonatno platformo. Na območju Stične, Šentvi-
da, Radohove vasi, v okolici Trebnjega in Velikega Gabra ležijo malmske plasti trans-
gresivno na srednjeliasnih. Pri Loškem Potoku pa so iz zgornjega liasa pojavi premo-
ga. Vse to kaže, da je po odložitvi srednjeliasnih plasti s školjkami prišlo do dvigovanja
ozemlja. Tektonske spremembe so se dogajale tudi v sosednji Italiji. Na Trento kar-
bonatni platformi leže nad plitvomorskimi plastmi s školjkami globokomorski apnen-
ci tipa ammonitico rosso (Bosellini & Broglio Loriga, 1971).
O vzrokih, ki so pripeljali do nenadnega izumrtja značilnih spodnjejurskih školjk,
lahko samo ugibamo. Po vsej verjetnosti je šlo za splet različnih okoliščin. Vemo, da
so bili to skrajno specializirani organizmi, ki se spremembi okolja niso mogli odzvati
z novimi prilagoditvami. O nekdanjem življenju, ki nas vedno znova preseneča s svojo
raznolikostjo, je ostal le zanimiv fosilni zapis.
Zahvala
Dr. Stevo Dozet, ki je geološko raziskal spodnjejurske plasti s školjkami na širšem
območju Kočevja, je avtorjema članka na terenu predstavil nekaj nahajališč. Ljudmi-
la Šribar in dr. Rajka Radoičić sta določili mikrofavno v apnencu. Mag. Miha Mišič je
opravil rentgenske analize laporjev iz Podpeči. Arian Debeljak je izdelal grafične dele
prispevka. Vsem se avtorja najlepše zahvaljujeta.
Lower Jurassic beds with bivalves in south Slovenia	61
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