Lithiotid Bivalves in Slovenia and Their Mode of Life

Lithiotid bivalves are a characteristic faunal element of the shallow marine f a d es of Lower Jurassic beds in southern Slovenia. The horizon containing Middle Liassic bivalves, which is up to 75 m thick, is called the "lithiotid horizon" and is attributed to the Pliensbachian or Domerian. In Slovenia the name lithiotid bivalves represents three morphologically similar genera or species of sessile monomyarian dysodont bivalves: Lithiotis problematica, Cochlearites loppianus and Lithiopema scutata, which are systematically examined in this paper The term lithiotid bivalves does not have any taxonomic significance, since they are now classified in different families. The order is Pterioida. Lithiotid bivalves lived in an upright position on soft lagoonal bottoms in a tight aggregate of individuals crowded together which mutually supported one another and simultaneously competed for living space and light. The sedimentation of calcareous mud was fairly rapid, thus throughout their lives they grew constantly in a subvertical direction so that the small soft body space at the ventral end remained above the level of the surrounding substratum. Lithiotid bivalves had peculiar, variable shells adapted to the specific environment. They are very large, flat and distinctly dorso-ventrally elongated. Their inner surface is tripartite; in the middle is the central area, and at the sides there are feather-like areas. The mechanism of opening and closing their valves has not yet been fully explained.


Introduction
In the Middle Liassic lithiotid bivalves inhabited the relatively calm muddy bottom of the restricted shelf on the Binarie Carbonate Platform. The horizon with the bivalves barely wedges out in the Lower Jurassic beds of southern Slovenia; its thickness at some localities reaches 75 m. It is named the "lithiotid horizon" after these characteristic bivalves. B u s e r (1965,(44)(45)(46) attributed it to the Pliensbachian, or its upper section: to the Domerian. The most important localities of Lower Jurassic bivalves in Slovenia, which extend for over one hundred kilometres in length and several tens of kilometres in width, were presented in the issue of Geologija. The palaeoecological conditions that enabled the characteristic bivalve fauna to flourish for a relatively short period have also been examined (Buser & Debeljak, 1996). This paper gives a systematic description of lithiotid bivalves and their mode of life. The term lithiotid bivalves is still used in Slovenia although it no longer has any taxonomic meaning. It comprises three frequent and characteristic, and at first sight similar genera: Lithioperna (syn. Lithiopedalion), Cochlearites and Lithiotis. All three genera were widespread in the shallow marine regions of the western and southern margins of the Tethys and even the Eastern Pacific (genus Lithiotis). Lithiotid bivalves therefore have great palaeogeographical, biostratigraphical and palaeoecological significance. They became well-known mainly because of their unusual shapes, which still challenge palaeontologists to offer various explanations.
The remaining species of Lower Jurassic bivalves from southern Slovenia will be presented on another occasion. The possibility that Lithiotis and Plicatostylus (later described from Oregon) are identical genera was raised by G r u b i c (1959; 1961). B u s e r (1965) confirmed this assumption with a study of the original specimens which the American authors Lupher and Packard sent to Professor Kühn in Vienna.
Lithiotis problematica can also be distinguished from the other lithiotid bivalves by cross-sections in solid rock. These are always monovalve and of an elliptical shape in the cross-section (see Buser & Debeljak, 1996, 36, figs. 9, 10). Especially characteristic are the "ear-like" sections with an opening in the central part, after which Lithiotis was named.
Localities: Lithiotis problematica is the rarest species of lithiotid bivalves in Slovenia. B u s e r (1965) found it in a deserted quarry on the right bank of the river Sušica, west of Dolenjske Toplice. Unfortunately this quarry is now filled in. Buser found some specimens on the northern slope of Mokrec during the construction of a new road. Today these outcrops are overgrown. A large number of specimens can still be found east of the village Zafara near Žužemberk. Relatively well preserved shells of L. problematica can be extracted from limestone in the vicinity of the Glijun spring and on the Poljanica hill west of Bovec. Characteristic sections can be observed in the ornamental Podpeč limestone (Buser & Debeljak, 1996), but the shells of L. problematica in Podpeč cannot be extracted from solid rock.
Material: Approximately 80 specimens. Mostly fragments of left valves up to 10 cm large; fewer right valves. Ten of the specimens have both valves partially preserved. The apex of the shell and the body space are not preserved in any of the specimens. The ligament groove is visible in six shells.
Description of Species: The shell is narrow, and strongly dorso-ventrally elongated with a tapered apex. Adult specimens measure from about 20 to more than 50 cm in height; their average width is 5 to 8 cm. The animal attached itself to the solid base with its left valve, which is approximately 1 to 2 cm thick. The right valve was free and thinner than the left; at its centre it measure approximately 0.5 to 1 cm. The shell is usually straight, but may be bent to the side (PL 3, fig. 2). Concentric growth lines can sometimes be seen on the rough and irregular outer surface.
The shape and interior of the shell is shown in fig. 2. The inner side of the shell is tripartite, which is characteristic of lithiotid bivalves. The central area is about 2 to 4 cm wide, and bordered by two feather-like areas. The feather-like appearance is created by growth lines, which can be joined into sheaves. The valves usually gaped at the edges of the feather-like areas. The soft body space of the shell was very small compared to the total size of the shell. The central area where the valves were tightly joined has a relief form. A more or less wide depression bordered by two ridges runs down the centre of the left valve and the central crest of the right valve fits tightly into it. Semicircular traces are often found on the central area; these are growth lines which the edge of the mantle left behind as it moved in the ventral direction.
At the apical end approximately down the middle of the cardinal area of both valves runs a deep and narrow groove (resilifer) in which the fibrous ligament was attached. The lamellar part of the ligament was attached at both sides of the groove. The height of the ligament groove varies with the specimens. Usually it measures 3 to 6 cm.
Slovenian specimens usually have recrystallised shells, but during fossilisation the parts with a different original microstructure were selectively coloured so that the characteristic features of the basic structure can often be observed in the sections (B u s e r L, 1989, Pl. 10; cf. C h i n z e i, 1982).
Comparison: The specimens of Cochlearites extracted from rock cannot be mistaken for any other bivalve. Confusion can arise when an attempt is made to determine the species from sections in the rock alone. They can be very similar to sections of Lithioperna scutata. However, in specimens of Cochlearites the left valve is thicker than the right, whereas in Lithioperna the two shells are equally thick. Normally in Lithioperna both valves fit tightly around the edges; one valve follows the other like a mirror image. In Cochlearites the valves often gape in the feather-like areas. The easiest to identify are cross-sections with a characteristic central ridge on the right valve and a corresponding depression on the left valve (Buser & Debeljak, 1996, p. 32 -fig. 4, p. 41 -fig. 15).
Individual specimens of Cochlearites loppianus can differ considerably from one another in the shape and size of the shell. Such variations have arisen owing to con-  Reis (1903) identified three types of Cochlearites. Accorsi Benini and Broglio Loriga (1977) admitted only two types: the normal type, in which the ligament was connected to the mantle, and the abnormal type (forma A), in which a short, stunted fibrous ligament no longer had any connection with the living part of the bivalve, which can be ascertained by shifted mantle growth lines. C h i nz e i (1982,193) showed that practically all adult specimens belong to the abnormal type or form A, which results from the ligament becoming stunted sooner or later during the growth of the bivalve. This finding makes irrelevant considerations of different types or even subspecies with regard to the appearance of the ligament area.
The systematic position of the Lithiotis and Cochlearites genera has not been finally determined, and their possible relationship is similarly not yet clear.
Localities: Buser (1965) found the finest specimens south-west of Lož and in the northern and southern parts of Mokrec. Today those localities are overgrown. Individual specimens can still be obtained at the Globočec spring west of Zagradec, and on the Stražišče hill east of Gorenje Jezero near Cerknica. Cochlearites loppianus also occurs in the Podpeč quarry, in the surroundings of Grčarevec near Logatec, at Borovec in the Kočevje region, and on Travna gora, as in these localities characteristic sections have been observed in limestone or dolomite.
Description of Genus and Species: The shell is linguiform and often very large. Sections in the limestone reveal that some specimens reached a height of three-quarters of a metre. The marginal parts of the shell are very thin, and for this reason we have so far not succeeded in finding an undamaged intact specimen. Therefore, in citing size we have to rely on sections in the limestone. On average, the shells are 30 to 70 cm high. The height is usually about twice the length. From the side the shell is distinctly compressed. The two valves have the same shape, size and thickness, and fit closely. Together they are 1 to 4 cm thick. The longitudinal section often has an undulating appearance. The external surface of the valve is normally rough and irregular, and in some rare specimens concentric growth lines can be seen on it.
Characteristics of the shell interior are shown in Figure 3. Flat feather-like areas are well-formed at both sides with clearly visible increments. (These indicate the former lateral outline of the body cavity which shifted in the ventral direction with the growth of the bivalve.) The anterior feather-like area is generally larger and often bent into a knee shape. A byssal notch runs along its interior edge. Under the apex it is shaped like a byssal groove, and towards the body space it is bordered by two folds or edge lines. Between the two there is a ridge of greater or lesser width in the right valve, and a corresponding depression in the left valve that can be shallow (PI. 5) or quite deep (PI. 6).
Between the lateral feather-like areas is an even, fairly flat central area or plate, which covers the largest part of the interior of the shell. The bivalve's mantle was spread over this surface, and here both valves were tightly joined. Under the apex a characteristic ligament area is formed, with a straight or occasionally a semicircular upper edge. The ligament was multivincular. The fibrous ligament was placed in several ligament grooves or resilifers, while the lamellar ligament was attached in spaces between them. As the bivalve grew the ligament shifted in the ventral direction and left behind thin growth lines, which in the grooves are curved in a convex manner with respect to the apex, but between the grooves they are concave. The ligament grooves are generally sub-parallel. In several specimens the position and centre of gravity of the shell was changed during growth, and the ligament later progressed in a different direction to the original one. Sometimes the ligament grooves run so distinctly towards the anterior part that they reach the anterior feather-like area (PI. 7, figs. 2, 3).
Even if the byssal notch on the anterior side is not preserved, it can be determined whether the specimen is the left or the right valve. First, a vertical boundary is traced between the central plate and the feather-like area, and then a straight line is imagined running through the ends of the ligament grooves. The angle made by the two lines is less than 90° on the anterior side and more than 90° on the posterior. The left and right valves can thus be distinguished.
The density of the ligament grooves can Vary. With regard to the width of the lamellar part of the ligament, that is of the intervals between individual ligament grooves, two groups can be determined. In both groups the grooves are 2 to 3 mm wide.
The intermediate sections are 2 to 3 mm wide in the first group, and 8 to 10 mm wide in the second. In the first group there are about 16 grooves on 10 cm of the hinge axis, in the second about 8. In some specimens it is clear that some ligament grooves have lagged behind while others have continued to develop at the normal rate (PI. 7, fig. 2; PI. 9). It can be concluded that in the second group approximately every other ligament groove with a fibrous ligament became stunted, and the lamellar part of the ligament was attached in its place. The ligament grooves can range from just a few millimetres in height to 8 cm. The amount of ligament area preserved depends on the thickness of the valve beneath the apex: the thicker the valve, the longer the grooves. In any case, the majority of specimens are very thin beneath the apex.
The body space of the bivalve with a single muscle scar is small in comparison to the overall size of the shell. It occupies only the ventral section of the shell and part of the space between the anterior border folds or lines. The depression for the soft body is very shallow.
An analysis of extremely well-preserved specimens from around Verona has shown that Lithioperna shells were composed of aragonite (Accorsi Benini, 1979, 228). In many specimens from Slovenia the characteristic structure of the shell can be seen with the naked eye, i.e. an alternation of lighter and darker laminae or layers parallel to each other and to the external surface of the valve (B u s e r L, 1989, Pl. 11, fig. 1). The light, glittering layers once had a prismatic microstructure, and the darker, opaque layers had a nacreous microstructure (Accorsi Benini, 1979).
Comparison: The Lithiopema genus is now classified with the Isognomonidae. In these bivalves the hinge teeth are absent, but a large multivincular ligament compensates for them. The classification among the Isognomonidae is still somewhat uncertain because the shell structure of L. scutata is uniquely developed (Accorsi B e n i n i, 1979). Among the Isognomonidae Lithioperna could be compared with Isognomon Solander in Lightfoot, 1786 (syn. Perna Bruguiére, 1789), in which the shells were attached to the firm base by a byssus as well. However there is no large central plate in Isognomon. The feather-like areas on the sides are not present, the valve is biconvex, and there is no alternation of two types of laminae.
Lithiopema can also be recognised from sections in rock (Buser & Debe-Ij ak, 1996, 32, fig. 4). Typical sections are very long and thin, and usually undulate gently. The two valves are of equal thickness and fit closely on all edges. In general only the very thin soft body space gapes ( fig. 3).
At present the genus contains only the species L. scutata, although individual specimens differ considerably. Variations in shell size proportions and shape are very common, and the appearance of the ligament area also differs. No two specimens are the same. However, only one specimen, described below, is essentially different, which is not sufficient to describe a new species.
Remarks: The name Lithiopedalion was given to the genus by B u s e r (1965) in his doctoral thesis according to the common characteristics of Lithiotis and Pedalion Dillwyn, 1817 (an old synonym for Isognomon). The new genus and species was presented at the 421^' i annual meeting of the Palaeontological Society in Graz (B u s e r, 1972). Unfortunately this work was not published in the way required by international rules, but the name Lithiopedalion was nevertheless used in Slovenia and in the literature elsewhere (cf. Bosellini, 1972;Broglio Loriga & Neri, 1976). After many years of collaboration he was overtaken in publication by the Italian palaeontologist Accorsi Benini (1979), who described the genus under the name: Lithiopema.
Together with the genus B u s e r (1965) described a new species Lithiopedalion kuehni, but Accorsi Benini (1979) equated it with the species Perna scutata from Morocco as already described by D u b a r (1948), and included it in Lithioperna. Given that Benini studied original material from Dubar's collection, for the time being her classification of these as the same species must be trusted. However, despite this we think that three modest figures showing poorly preserved specimens do not exhibit all the characteristics typical of specimens from Slovenia and northern Italy. According to their shape they are considerably reminiscent of Gervilleioperna Krumbeck, 1923; this applies in particular to Dubar's specimen that Accorsi Benini (1979, 251, fig. 14) presented as a paralectotype in describing the new genus. Judging by the description and pictures the anterior feather-like area is formed as a lunule, while the byssal notch is very deep and just under the apex spreads and deepens into the body cavity. The Slovenian and Italian specimens have a significantly more dorso-ventrally elongated shell, a higher ligament area, a shallower byssal notch in most cases, and, above all, a larger central plate where the valves were tightly joined. In the future it would certainly be recommenc able to make a revision of the Lithioperna scutata species using the Dubar material, ^hich is kept in France (Lille).
Localities: In the lithiotid horizon of Middle Liassic beds in Slovenia (Trnovski gozd, Hrušica, Nanos, Logaška planota, Krim-Mokrec hills, Dolenjska) Lithioperna scutata is the most common species, found in almost all localities (B u s e r & Debeljak, 1996, 28, fig. 2; presented in this article as Lithiopedalion scutatus). Specimens that can be extracted from marly layers between limestone can now be found at Špik, north of Col in Trnovski gozd, in the Podpeč quarry, and along the railway between the stations Preserje and Verd. B u s e r (1965) found numerous specimens on the Krim-Mokrec hills; the outcrops are now almost entirely overgrown. The locality on Javomik and those north of Cerknica and south of Lož are also overgrown. The quarry on the right bank of the Sušica west of Dolenjske Toplice is now filled in.
Elsewhere in the world Lithioperna can be found in Liassic beds of northern Italy (Berti Ca vic chi et al., Brogli o Loriga & N e r i, 1976), the central Apennines, Albania, Greece, France and Morocco (after Accorsi Benini, 1979). Rey (1990;1997) reports it from southeastern Spain. We estimate that the Lithioperna is present elsewhere, but different authors have designated it under the name Perna or Isognomon, or in some places have perhaps classified it with the oysters (e.g. Pemostrea Munier-Chalmas, 1864 with the multivincular type of ligament).
PL 9, fig. 1 Material and Site: One poorly preserved specimen with both valves. Only the ligament area, part of the central plate, and the upper part of the byssal notch are preserved. The specimen was found by a forest road north of Mokrec.
Description and Comparison: Extremely narrow ligament grooves are present on the ligament area, approximately 1 mm wide. On the hinge axis (in so far as it has been preserved), which is 11 cm long there are 36 ligament grooves. That is at least twice as many as is common for Lithioperna scutata. The height (more than 4 cm) and density of the ligament grooves could fit the Isognomon genus or the Hippochaeta Philippi, 1844 subgenus, which is known only from Tertiary strata. Howe-ver in Isognomon the body cavity begins under the ligament, but in the specimen described, as in Lithiopema scutata, the ligament grooves end in the flat central plate, where the valves were tightly joined.
The specimen is too poorly preserved for a more detailed determination, or for the description of a new species.

Mode of Life of Lithiotid Bivalves
In southern Slovenia during the Middle Liassic lithiotid bivalves flourished in the mainly quiet environment of a more or less restricted shelf on the Dinaric Carbonate Platform. Individual species created monocolonies in the form of sea-bottom mats or biostromes with several lens-like accumulations. The moderate influence of the pelagic sea could be felt from the north. Sedimentation in the lagoon was rapid, and the substratum on the sea bed was principally composed of mud, which was determined with regard to the properties of the matrix in bivalve lumachelles. Biodiversity in such lumachelles is very low. Bivalves were able to build shells of three-quarters of a metre in length only under tropical or sub-tropical conditions. The palaeogeographical and palaeoecological conditions that enabled the existence of the characteristic bivalve fauna and the distribution of various species have already been described (Bus er & D eb el j a k, 1996).
A description of the living habits and special adaptations of individual genera is given below.

Lithiotis
Although the first investigators in Italy discovered Lithiotis 250 years ago, it still presents a puzzle to palaeontologists today. The specimens extracted from rock do not really resemble present-day bivalves, and it is not surprising that they were first described as plant remains (Gümbel, 1871;cf. 1890). It is supposed that the life of such bivalves developed as described below.
The larvae first swam or floated in water until they found a suitable attachment point. On the muddy sea floor the firmest substratum was presented by the shells of other bivalves, usually adult Lithiotis individuals. Juveniles fixed themselves to these with a sp cial adhesive substance, oriented in such a way that they could grow vertically u':) vards. Numerous Lithiotis specimens testify to this, having identically oriented you iger individuals fixed to their surfaces (B u s e r, 1965, Pl. 8, 9). It can also be seen from sections in the limestone that certain adults were literally cemented to each other (Euser & Debeljak, 1996, fig. 9). Chinzei (1982) suggested that the attachn ent area was small, and that cementation was possible only in juveniles. On the be sis of our own observations we have concluded that these bivalves retained the capacity for cementation throughout their lives, thus the construction of their aggregates was very firm, and after death they often remained in their life position.
In any case Lithiotis bivalves lived in large groups, giving one another support and spreading out in bunches ( fig. 4). Sedimentation of carbonate mud took place quite rapidly, so that the shell was permanently anchored in it. The body space increased only in young individuals, afterv^ards the shell grew only in height, in a subvertical  cher, 1984). Much can be learned about the growth from the incremental lines on the exterior surface and on the feather-like areas. They are combined into sheaves that on well-preserved specimens repeat approximately every centimetre. C h i n z e i (1982,(189)(190)(191)(192) concluded that periods of faster and slower growth alternated, resulting in the appearance of annual rings. By counting the growth lines and measuring the size of the shell he reached the conclusion that it took about forty years for the shell to attain a height of 50 cm. During all this time mud gathered around it. Individuals that did not grow quickly enough became buried in mud. This often happened to juveniles, which owing to the crowd did not succeed in gaining living space. Whenever an individual became dangerously tilted, it was still able to correct its position during further grov^rth. Such specimens with "bent knee" shells are very common (PI. 2, fig. 1). It is interesting that in Lithiotis it was only the thicker valve that was preserved, i.e. the valve cemented to the base. The opposite, free valve must have been extremely thin. Only pieces of thin crust on the thicker valve remained of it. In limestone too only single-valve Lithiotis specimens can be observed. Sometimes thin traces that could be fragments of the thinner valve are scattered among them (Buser & Debe 1 j a k, 1996, 36). Today it is still not known whether this valve was as big as the thicker, attached valve (Reis, 1903, 11 Lorig a, 1977, 21). The thicker attached valve was hollow under the lowermost part of the furrowed plate. Here the soft body found support, so that its weight did not burden the thin, free valve.
The mechanism of opening and closing the shell has not yet been fully explained. Usually in bivalves the valves are opened by a ligament functioning in the opposite way to the adductor muscle. Chinzei (1982, 193) suggested that there was no functional ligament in Lithiotis, and that the valves were able to open and close due to the elasticity of the thin valve. This would have been bent over the ventral edge of the furrowed plate (in this case it would be the hinge axis) whenever the adductor muscle was contracted. When the muscle was relaxed, the thin valve stretched out and the valves gaped open. The question with this hypothesis is what (apart from mud) held both valves together at the dorsal end if there was no ligament or hinge teeth. The valves themselves were very large but the body space with the adductor muscle was small. If the valves were really the same height, then during opening they might become displaced and irruption of mud might occur. In this case it is indeed more likely that the thin valve was merely some sort of operculum over the body space of the thick valve. This supposition is somewhat contradicted by the fact that remnants of the thin valve are found at all heights on the thick shell. It is possible that the thin shell gradually broke up and fragmented in the dorsal area while the shell grew in height and the body space shifted in the ventral direction.
Also unsolved is the question of what function the central furrowed plate served. It can not be that this was just a frick of nature. There are no traces on it left behind by the mantle. The soft part of the bivalve did not extend to this area. Böhm (1892) even thought that the characteristic grooves arose secondarily, owing to weathering. Gümbel (1890,65) and B u s e r (1965,18) described the furrowed area as the place for the ligament. Reis (1903,43) asserted that the ligament was stunted and that the indentation of the furrowed area acted as a sort of hinge. A c c o r s i Benini and Broglio L o r i g a (1977,(21)(22)(23)(24) suggested that the furrowed area was just a superstructure that covered special internal tubes in the shell where thin mantle appendages, the centres of calcification, were located. The grooves would have originated as these internal tubes opened outwards. Recently S a v a z z i (1996) found evidence for the presence of the ligament on the furrowed plate, and suggested that in Lithiotis the ligament was active throughout the height of the furrowed area and not only on the ventral end or hinge axis, as is normal for bivalves with the multivincular ligament. In Lithiotis the ligament structure would have developed in a highly original way. S a v a z z i found that the ligament fibres were oriented and inserted in such a manner that they allowed small changes in the reciprocal distance of the valves. This increased the length over which the free valve flexed when the bivalve closed. Stress was thus more evenly distributed; tension was reduced, as was the possibility that the thin shell would break up or become damaged. Thus according toSavazzi the free valve was able to close by flexing, as established by Chinzei (1982), but not by articulating over the hinge axis. Chinzei (1982;1986) compared the shell shape and mode of growth of Lithiotis and Cochlearites with certain oysters that are also strongly elongated and live or used to live in a vertical position on soft, muddy ground: Saccostrea Dollfus & Dautzenberg, 1920, which live along the coast of east Africa and whose shape is reminiscent of rudists -the thin free valve is shaped like an operculum (Stenzel, 1971); and Konbostrea Chinzei, 1986, an Upper Cretaceous oyster from northern Japan, which is most similar to Lithiotis in its shape and growth pattern.
We assume that Lithiotis problematica was very selective in its choice of habitat. Data from Slovenian localities indicate that it formed monocolonies; it required its own living space which was not shared with other lithiotid bivalves and excluded the majority of other organisms as well (Buser & Debeljak, 1996). The Lithiotis genus is the rarest among lithiotid bivalves in Slovenia.

Cochlearites
Cochlearites is very common in the lithiotid horizon in Slovenia, particularly in those places where restricted parts of shelf were spread.
The Cochlearites shell grew similarly to Lithiotis shells (see previous section). Owing to its narrow and compressed shape many individuals of the same species were able to crowd together Their living position was vertical, as seen today in Pinna Linné, 1758. Its shells are also very elongated in height, but dtiring growth the soft body part increases in size. Using its foot Pinna can bury itself in sandy or muddy sediment, after which it uses a bunch of byssal threads to fix itself to any solid base, for example a buried stone (C o X, 1969, N8-N10). Cochlearites and Lithiotis were unable to bury themselves, as the small soft body occupied only the ventral end of the bivalve; in addition, one valve was immobile, always cemented to a fixed base. We think that the shell cementation capability in Cochlearites was much more limited than that of Lithiotis, and so the construction of their aggregates was not as firm. The sedimentation of calcareous mud in which the bivalve was anchored took place quite rapidly, thus the valve in any case had sufficient support. A considerable amount of fecal mud accumulated around the bivalves (suspension-feeders) themselves, because they continuously filtered large quantities of water in which small particles of nutritives floated. It can be imagined that the sedimentation environment must have been fairly calm. Strong waves and currents would have washed away the mud which supported the bivalves, and scattered them around the sea floor, where they would have perished.
Chinzei (1982,193) considered that in Cochlearites the ligament was active only in very young individuals, but then became stunted and no longer had any connection with the mantle, that is with the living part of the body. The majority of bivalves need the ligament as like a spring it opposes the action of the adductor muscle, thus opening the valves whilst also connecting them. In Cochlearites the relief structure of the cardinal area assumed the role of the hinge teeth (which lithiotid bivalves do not have), so that the valves did not become displaced. Most of the shell was stuck in the mud. Therefore along its entire height the shell should not open, because mud would irrupt. In the cardinal area the valves were in close contact all the time, but in the body space they gaped somewhat, and thus remained flat and parallel during growth. The solid valves may have been elongated into the conchiolin-rich lamellae (Chinzei, 1982, 194). With the help of these elastic lamellae the bivalve was able to close hermetically like some of today's bivalves. When the adductor muscle relaxed, the lamellae flattened and left a narrow slit at the end. According to C h i n z e i the elasticity of the ventral parts of the valves replaced the ligament.
As in Lithiotis, the soft body was very small, and always remained above the level of the surrounding mud. The bivalve probably extended its mantle through the narrow slit at the ventral end, intercepting and absorbing the substances it needed to build a large shell.

Lithioperna
Of all the lithiotid bivalves in Slovenia Lithioperna was the most common. Its shells usually lie in limestone parallel to the bedding plane. Only rarely are layers with numerous subvertically oriented sections found, which tell us much about their original life position.
Originally Lithiopema shells were fixed to a firm object with the byssus, like other isognomonids. The byssal gland secreted a special sticky fluid that solidified in water into byssal threads. The byssus emerged from the anterior side of the valve. The anterior feather-like area is usually more strongly developed than that on the posterior. The anterior side of the shell was turned towards the weak currents that brought nutrients to the bivalve ( fig. 5). In most specimens the ligament grooves run somewhat obliquely towards the anterior margin. The hinge axis runs through their lower ends. The obliqueness of the hinge axis can be explained on the assumption that the posterior side of the valve was more sunk in the soft sediment than the anterior. The hinge axis had to stay parallel to the surface of the substratum, otherwise mud would irrupt into the posterior during opening ( fig. 5). Mostly younger individuals grew vertically (left), providing mutual support, and branched radially. Some gradually tilted towards the soft sea floor, or even fell over (right). The right valve is not shown in two specimens so that the inner surface with ligament area can be seen SI. 5. Naravni položaj pri rodu Lithiopema (syn. Lithiopedalion). Predvsem mlajši osebki so rastli navpično (levo) in se medsebojno podpirali ter razraščali. Nekateri so se sčasoma nagnili ali pa celo prevrnili na mehko morsko dno (desno). Desna lupina pri dveh primerkih ni narisana, zato da se vidi notranja površina z ligamentnim poljem Like ali lithiotid bivalves, Lithiopema grew rapidly in height. The effectiveness of the byssus was gradually diminished, and it became more difficult for it to hold the shell in an upright position. Adjacent individuals provided one another with support, like books on a book shelf. In addition they were anchored in the mud. Those individuals whose support was not solid enough eventually overturned, and spent the remainder of their life lying flat on the sea floor. According to Accorsi Benini (1979,(245)(246) the shell was able to adapt its growth so that the ventral end of the valve with the soft body always remained above the level of the mud. The changes in the direction of growth gave the specimen a characteristic wave-like appearance. This undulation also helped maintain effective mutual contact between the two valves. The large, flat shell was stable on the sea floor and did not sink into the soft sediment. From time to time weak tidal currents washed mud away from it. Lithioper-na bivalves were probably also capable of cleaning themselves, as certain oysters do: by rapidly contracting the adductor muscle they can squirt a strong jet of water out of the mantle cavity to clean their surface (Stenzel, 1971. Lithioperna bivalves lived in such shallow water that occasionally they may have found themselves on dry ground. In such cases the valves sealed hermetically. They also closed in such a manner when the decay of organic matter caused oxygen-depleted conditions on the sea floor. In such periods the bivalve was forced to breath anaerobically. According to Accorsi Benini (1979,(228)(229)(230)(231)(232)(233)(234)(235)(236)(237)(238)(239)(240)(241)(242) this is reflected in the shell microstructure: During growth, when the valves gaped open the mantle absorbed Ca2+ ions and secreted nacre on its external surface. During periods of anaerobic respiration acidic products accumulated in the extrapallial liquid between the mantle lobe and the shell wall and reacted with the shell. To neutralise the acidity, calcium ions were released. During this process the shell's internal layer, which previously had a nacreous microstructure, took on a new pseudoprismatic appearance. This happened repeatedly, and the shell consequently has two types of alternating laminae or layers with a different microstructure.
Owing to the size and undulating shape of the shell the ligament was under considerable stress during opening. The fibrous part of the ligament was located in several ligament grooves (i.e. resilifers). When the adductor muscle contracted, the fibres were compressed, and when it relaxed, the fibres stretched out like a spring. During this process the valves opened somewhat. The lamellar part of the ligament, which was attached between the grooves, connected the two valves. Owing to the mechanical load the ligament was permanently cracked and decayed at the dorsal end. As it was composed of organic matter, it was also destroyed by bacteria. Therefore only the ventral part of the ligament, continuously excreted by the mantle isthmus, was active. This occurs in many bivalves that have a similar type of ligament, i.e. multivincular (S t e n z e 1, 1979, N971-974). The hinge axis also shifted in the ventral direction together with the ligament. In certain specimens the ligament grooves ceased growing and the lamellar ligament attached itself in their place. This probably occurred because effective contact between the two valves was essential. The fibrous ligament, used for opening, was not so important. Moreover, at the ventral end the solid valves gaped open by a few millimetres for most of the time, and thus during growth remained flat and parallel; they did not become curved as in most other bivalves. Lithiopema bivalves may have secreted poisonous mucus from the ventral slit as a defence measure. It is supposed that the opening was sealed when necessary by thin elastic scales or lamellae that fringed the greatly thinned ventral edge of the two valves, as in Lithiotis and Cochlearites. Such lamellae have not been preserved as fossils, because they were poorly calcificated and rich in organic matter (cf. Stenzel, 1971, N977). In numerous present-day oysters such conchiolin lamellae are semi-translucent and dark brown to olive in colour, with a horny appearance. Even during the lifetime, they are prone to split and decay in the old parts.
The largest part of the shell interior in Lithioperna is occupied by the central plate. The bivalve's mantle spread over its entire surface. The mantle was also a respiratory organ, relieving the gills. From the water it absorbed oxygen and the calcium ions needed to build the shell (Stenzel, 1971, N967). The larger mantle surface in Lithioperna provided a larger respiratory capacity, which was needed to enable the bivalve to thrive and build an immense shell.

Conclusion
The period in which lithiotid bivalves flourished was relatively short. They mostly disappeared in the transition from the Pliensbachian to the Toarcian. Their extinction was hastened by extensive tectonic movements, changes in the sea level causing fundamental changes in habitat and the environment which highly specialised organisms could not successfully respond to (Buser & Debeljak, 1996).
The similarity of the sessile mode of life in a specific environment (muddy substratum, rapid sedimentation) and in dense aggregates, and the associated morphological properties and adaptations characteristic of the convergent and contemporary genera Lithiotis, Cochlearites and Lithioperna, justify the use of a common name, although the name lithiotid bivalves does not have any taxonomic significance.
All three genera are distinguished by their very large, unusually shaped and remarkably variable shells. Their principal characteristic is their flatness and height elongation, i.e. in the direction of growth, which made more or less constant progress during their entire lifetime. The body space of lithiotid bivalves was unusually small: it occupied only the far ventral end of an otherwise very large shell. The valves were tightly joined over almost the entire surface. All three genera have a tripartite internal construction in common, with feather-like areas on the sides and a central area with a very different shape in the middle. Lithiotid bivalves had a considerable capacity for twisting as they grew.
The great variability of lithiotid bivalves can be attributed to the constant adaptation of the growth of the shell to changes in the environment and in the close community in which hundreds of individuals of the same species crowded together, thus providing mutual support and spreading out in bunches, similarly to plants in their search for sunlight.
Lithiotid bivalves are undoubtedly one of the most interesting and distinctive fossil groups in Slovenia. Since their first discovery (B u s e r, 1965) a systematic description has long been delayed. This paper also presents their mode of life, which was reconstructed using the authors' own observations and the findings of numerous researchers from around the world. Many questions remain unanswered, and new findings on these unusual bivalves can be expected in the future.