Omphalosaurus

Omphalosaurs were medium in size, stocky sea reptiles ^[1] , they are known primarily because of their specialized teeth, not like the teeth of other ichthyosaurs. These teeth are button-like, practically round in cross section and resembling a dome when viewed from the side ^[2] , the enamel is rough, resembling an orange peel ^[3] . Each tooth does not exceed 12 mm in diameter ^{[3]; the} teeth on the premaxilla (in the anterior part of the maxilla) and on the mandible are flat. The well-preserved smooth palatine bone of O. nevadanus suggests that omphalosaurs, most likely, did not have palatine teeth (unlike placodonts ) ^[1] . Nevertheless, the number of teeth in an omphalosaurus could reach several hundred ^[4] . Each species of the genus differed in the structure and arrangement of the teeth, but the O. nevadanus teeth were the most accurate and lined up in regular rows, despite the slight difference in size. Scientists tried to calculate the number of rows of teeth in other species of this genus, but their teeth were located more chaotically ^[5] .

A number of upper teeth formed a convex surface, a number of lower - concave. Initially, scientists thought that omphalosaurs had wide, short jaws and powerful jaw muscles, but recent reconstructions show that the jaws of these animals were long, in the shape of the Latin letter “V” ^[1] . The reconstructed lower jaw of O. nevadanus would exceed half a meter in length ^[4] .

Fragments of the jaws show that omphalosaurs possessed “dental batteries” adapted to permanent wear: worn teeth were replaced with new ones, and quickly ^{[1] [6]} . Omphalosaurs are unusual in that their replacement teeth differed from normal teeth already developed and working in the microstructure of the tooth enamel . Like other ichthyosaurs, omphalosaurs enamel mature teeth was grainy, while the replacement teeth had enamel, consisting of small columns. At present, it is not known how this transformation took place ^[3] .

Very few other Omphalosaur fossils - except for the teeth - were found: only a few edges and spinal fragments belonging to O. wolfi ^[7] . These ribs are thick and hollow, which is generally characteristic of amniotes who have returned to aquatic life; each vertebra has both ends concave ^[5] . Ophalosaurus vertebrae are devoid of neural arches located in the ancestors of these animals from above the center of the vertebra ^[1] . The bone tissue is fibrous, indicating a rapid growth of the bones during ontogenesis ^[8] .

Power

The highly specialized teeth of omphalosaurs say that these animals fed on invertebrates, which had hard shells or shells ^[5] . The teeth of these animals are adapted precisely to this method of nutrition, which results in rapid wear, computed tomography of fossils shows that the teeth of the Ophalosaurus very quickly replaced each other - the place of worn teeth was taken by new ones. However, the omphalosaurus jaws obviously did not allow tightly compressing prey: their narrow jaws with teeth located in the back of their mouth are unlike those of short jaws typical of shellfish invertebrates, which allowed the shells and shells to split. The combination of heavily worn teeth and a small — judging by the estimates — bite force is more characteristic of herbivorous dinosaurs (including ornithopods). As with herbivorous dinosaurs, omphalosaurs are characterized by rapid teeth replacement and a smooth contact surface between the teeth of the lower and upper jaws, but the low prevalence in the average triassic of fibrous marine plants makes the herbivores of these animals unlikely. But ammonites and pseudoplankton bivalves were during this period, on the contrary, quite common, including in the habitat of omphalosaurs; what is important: the shells of these invertebrates were strong, but relatively thin. Sander and Faber (Sander and Faber) hypothesized that omphalosaurs could have muscular cheeks, which allowed these animals to suck clams from their shells, rather than chop and grind the shells with their teeth ^[1] . Newly discovered evidence suggests that omphalosaurs fed primarily on ammonites rather than on leaflets — they were the main prey of placodons ^[9] .

Decompression Disease

As in the case of other early ichthyosaurs, omphalosaurus fossils do not find traces of ischemic necrosis of the bones ; this means that these animals also did not suffer from decompression sickness arising from a sharp ascent from great depths. Rothschild and others explain this by the fact that in the early Triassic there were few large sea predators from which omphalosaurs would have had to flee at great depths. Early ichthyosaurs, most likely, dived and emerged slowly or had physiological mechanisms that prevented decompression sickness ^[10] .

The first fossils of the omphalosaurus were found in 1902 by the paleontologist V.K. Osmont (VC Osmont) in Nevada in the southwest United States; They were first described in 1906 by John C. Merriam ^[11] . Merriam was unable to identify O. nevadanus as an animal belonging to ichthyosaurs: he thought it could be a placodont or rhinhozavr ^[2] . The first to realize that Omphalosaurs belonged specifically to ichthyosaurs, was Kuhn (1934), Mazin (Mazin) in 1983 confirmed this classification. In 1997 and 2000, Motani (Motani) expressed his objections to it, saying that for omphalosaurs signs are not known, which are synapomorphies for all ichthyopterigies ^[12] ; this researcher suggested that omphalosaurs should be considered semipantilions ^{[1] [5] [6]} . However, in 2010, Maisch (Maisch) described a new species of omphalosaurs and re-confirmed their belonging to ichthyosaurs ^[13] .

At present, it is believed that the omphalosaurs were just ichthyosaurs - medium-sized or medium-sized. Like other ichthyosaurs, their vertebrae were concave at both ends and flat, with no noticeable transverse ribs. The ribs of the omphalosaurs, like all other ichthyosaurs from the family of Shastasauri (Shastasauridae) , articulate on the dorsal and ventral sides; O. wolfi has the same structure of cancellous bone tissue as other ichthyosaurs, as well as a number of other secondary aquatic vertebrates ^[5] . One of the main features that makes it possible to attribute omphalosaurs to ichthyosaurs is the microgranular structure of the enamel of their fully formed teeth, which is very rare in other reptiles ^[7] .

The most important sign that caused a discussion about the systematic position of omphalosaurs is the location of their teeth in the jaw. Unlike other ichthyosaurs, whose teeth are arranged in regular rows, the omphalosaurus teeth were irregularly arranged. In addition, in no other ichthyosaur the chewing surfaces of the teeth are at right angles to each other. The crown of the tooth is lower and more irregular in shape than other ichthyosaurs that fed on armored invertebrates, and the texture of the enamel resembles an orange peel - unlike the teeth of other ichthyosaurs covered with longitudinal ribs of durability. In addition, the ribs and humerus of the omphalosaurs were hollow and bore prominent longitudinal ridges, both of which were never observed in other ichthyosaurs ^[5] .

Types of Omphalosaurs

• Omphalosaurus nevadanus is a type species of the genus, one of two species of omphalosaurs, whose fossils were found in the Pride formation in the Humboldt Mountains ( Nevada , USA ). O. nevadanus lived in the Anisian Middle Triassic Age (247.2–242 million years ago); first described by Merriam in 1906 ^[5] . Among the fossils found are: the lower part of the skull with several vertebrae from the back of the neck, as well as a piece of the jaw, devoid of a joint (the jaw was broken even before its fusion). On this piece, the jaws are clearly visible: angular and supraarthal bones, teeth, and lamellar bone ^[11] . Unlike other species, this omphalosaurus has rounded back edges of teeth. O. nevadanus was much larger than O. nettarhynchus , he also had more teeth ^[5] .
• Omphalosaurus nettarhynchus is the second species found in the Pride formation ; it existed in the Olenek Age of the Middle Triassic. First described by Mazin and Bucher in 1987. It is known only by a fragment of the lower jaw, which is smaller than that of other species of omphalosaurs; the teeth in the jaw are relatively small, but they are large; maxillary symphysis extends laterally ^[5] .
• Omphalosaurus merriami was discovered in marine sediments in the Svalbard archipelago and described by Maisch in 2010. He lived at the beginning of the Triassic. Fragments of the jaws are known, which at first were found to belong to the ichthyosaur Pessopteryx nisseri , found there. There are always three rows of teeth with smooth enamel on these fragments of the jaw, the roots of these teeth contain Plycidentin ^[13] .
• The humerus of some kind of omphalosaurs was found in Middle Triassic deposits in Franconia , Germany, namely in Hohenlohe . This omphalosaurus lived at the end of the Ladin Age , discovered by Hagdorn in 1984, described by Sander and Faber in 1998. The humerus found demonstrates a spongy structure typical of ichthyosaurs, but it is not yet clear to which particular animal it belonged ^[5] .

Species whose existence is disputed

• Omphalosaurus wolfi lived in the Middle Triassic (at the beginning of the Ladinian century ); found in the mountains of Hida (northern Japanese Alps) , Japan , also found in Germany ^[5] . Discovered by G. Wolf in 1991, described by Tichy in 1995 ^[14] . Available fossils include: several pre-sacral vertebrae (flat and with concave ends), ribs (thick and hollow), as well as fragments of the skull ^[5] and lower jaw ^[1] . The teeth of O. wolfi are similar to those of O. nisseri, the lower jaw more closely resembles that of O. nevadanus , the only other type of omphalosaurs, for which a decently preserved skull is known ^[5] . Also found ten unidentified cranial bones ^[1] and another bone, which may be the humerus ^[5] . Sander and Faber (Sander and Faber) in 2003 expressed the view that O. wolfi may actually be O. cf. nevadanus , however, Maisch argues with them, arguing that it is necessary to re-examine the existing cranial bones, and before conducting this study, consider O. wolfi as a separate species ^[13] .
• Omphalosaurus peyeri lived in the middle of the Anisian century . Its fossils are found in sediments of porous calcareous sandstone in the Shaumkalk formation in the region of Rüdersdorf (Brandenburg, east of Germany); it is believed that during the life of this animal, these places were sea shoals near the coast, and O. peyeri was most likely a coastal animal. Found fossils represent the back of the lower jaw with three working teeth in the form of fungi and several teeth growing to replace them, which are not yet ready for work. Unlike other omphalosaurs, O. peyeri possessed only one (and not several) number of working teeth, but he also had “replacement” teeth ^[15] . Because of this, he was first mistaken for a placodonte, and only in 2002, Maesch and Lehmann (Maisch and Lehmann) proved that he was an omphalosaurus ^[16] , relying on the shape of his teeth, typical of omphalosaurs, and the structure of the tooth enamel resembling an orange the crust. O. peyeri is considered to be a basal group in relation to the rest of the omphalosaurs ^[15] , however, its belonging to omphalosaurs is still disputed due to the structure of the enamel of his teeth, characteristic of placodonts ^{[3] [9]} .

Species whose identity has been revised

• Pessopteryx nisseri was found on the Svalbard archipelago in 2010; the fragment of the jaw originally attributed to him is now attributed to the omphalosaurus O. merriami ^[13] . Wiman and Mazin believed that this fragment of the jaw was O. nisseri , but by now other fossils (limb skeleton and shoulder belt fragment) have shown that this animal belonged to a separate species and even to its own genus Pessopteryx in the detachment ichthyosaurs ^[17] .

• List of ichthyosaurs