Ozimek (named after the Polish town of the same name) is a genus of archosauromorph reptile, known from Late Triassic deposits in Poland, likely closely related to Sharovipteryx from Kyrgyzstan. It contains one species, O. volans, named in 2016 by Jerzy Dzik and Tomasz Sulej. Like Sharovipteryx, Ozimek had long, slender limbs with the hindlimbs longer than the forelimbs; the hindlimbs and probably the forelimbs likely supported gliding membranes as fossilized in Sharovipteryx. Another unusual characteristic was the shoulder girdle, where the massive coracoids formed a shield-like structure covering the bottom of the shoulder region that would have limited mobility. In other respects, such as its long neck, it was a typical member of the non-natural grouping Protorosauria. Initially classified in the family Sharovipterygidae, phylogenetic analysis has indicated that it, possibly along with Sharovipteryx, may have been an unusual member of the protorosaur group Tanystropheidae, although further study of its anatomy is needed to resolve its precise relationships.
Five articulated skeletons and 30 fragmentary specimens constitute the known fossil material of Ozimek, which comes from the Krasiejów clay pit near Opole. Its remains were found uniformly distributed within a rock layer containing fossils of terrestrial animals. During the Late Triassic, the Krasiejów region would have been a lake surrounded by coniferous forests, where Ozimek would have glided between trees feeding on insects such as cupedid beetles. However, individuals of Ozimek still fell into the lake on occasion, where they were possibly subjected to scavenging by amphibians such as Cyclotosaurus and reptiles such as Parasuchus before burial.
Discovery and naming Starting in 1974, the Krasiejów clay pit near Opole, Poland was excavated by the cement plant Strzelce Opolskie, with the excavated clay being mixed with water for cement production. In the 1980s, amateur fossil collectors discovered bone fragments in the outcrops exposed by these excavations and brought them to the attention of Magdalena Borsuk-Białynicka at the Institute of Paleobiology of the Polish Academy of Sciences (ZPAL). The locality was not found until 1993, when Jerzy Dzik discovered a skull of Paleorhinus. More extensive excavations took place from 2000 to 2002, which were sponsored by the Górażdże cement plant that owned the pit by this point. The clay made the fossils easy to extract, but they were also fragile and needed to be protected by a plaster jacket (for large fossils) or cyanoacrylate glue (for small fossils). Two bone-bearing rock layers exist at Krasiejów: one mainly preserving lake-dwelling animals, and the other mainly preserving terrestrial animals. Based on comparisons of its fauna with sites in Germany (including the presence of Paleorhinus), these layers likely date to the Late Carnian age of the Triassic, approximately 230 million years ago, as part of the Grabowa Formation.
Most of the terrestrial fossils were found towards the north of the clay pit, which was likely the shore of a lake during the Late Carnian. These consist of a number of large tetrapod skeletons, as well as uniformly distributed fossils from smaller reptiles. At some point before 2007, a single, extremely elongated vertebra was discovered among these small reptile fossils; it was compared to the neck vertebrae of Tanystropheus, or the tail vertebrae of pterosaurs. Many additional specimens of this animal were recognized afterwards, with it being the most common among these small reptile fossils. An almost complete, articulated skeleton missing the head was designated as the holotype of a new genus and species, and catalogued as ZPAL AbIII/2512. Other articulated skeletons included ZPAL AbIII/2012, which preserves a crushed head, a nearly complete neck, and partial limbs; ZPAL AbIII/2511, which is the most complete specimen (lacking only the head) but is disarticulated; ZPAL AbIII/3191, which is almost complete with some disarticulated skull bones; and UOPB (University of Opole) 1148, which preserves parts of the neck and limbs. Another 30 fragmentary specimens in the ZPAL collection include neck and back vertebrae, neck ribs, gastralia ("belly ribs"), humeri, femora, tibiae, fibulae, and tarsi (ankle bones).
In 2016, Dzik and Tomasz Sulej named this new genus and species Ozimek volans. The genus name, Ozimek, is derived from Ozimek, the nearby town of the same name. The species name volans (Latin "flying") refers to its possible gliding lifestyle (see §Paleobiology).
Description
Ozimek was a rather small reptile, with an estimated total length of about .
Skull and vertebrae
The skull of Ozimek is only known from fragmentary fossils; from the identifiable fragments, the skull appears to have been diapsid, broad, and compact, unlike the elongated skulls of Protorosaurus and other small archosauromorphs. In particular, like Langobardisaurus, the squamosal bone of the skull roof was large. By contrast, the likewise poorly preserved skull of Sharovipteryx was interpreted as having been narrow and deep by David Unwin and colleagues in 2000, although Dzik and Sulej suggested that its skull was actually similar to Ozimek. The preserved portions of the jaws show numerous small, sharp teeth. Based on the shape and arrangement of teeth in the maxilla, Ozimek may not have had an antorbital fenestra (opening in front of the eye socket). In the eye socket, the scleral ring may have been fossilized. On the side of the skull, the jugal bone would have formed a complete arch.
Like Langobardisaurus and Tanytrachelos but unlike Tanystropheus, Ozimek had vertebrae that were procoelous (i.e., with articulating surfaces that were concave in front and convex behind). Like the latter two, the neural spines were low. It appears that Ozimek had 9 neck vertebrae, 16 back vertebrae, 3 hip vertebrae (in contrast to the 4 to 6 estimated for Sharovipteryx), and at least 7 tail vertebrae. The neck vertebrae were very elongated and thinly walled, with the fourth, fifth, and sixth being the longest; the initially discovered elongated vertebra was one of them. The neck ribs were also long, with some of them extending for the length of three vertebrae. Elongated neck vertebrae and ribs were a common characteristic among protorosaurs. Like other tanystropheids, the neural spine of the axis (second neck vertebra) sloped upwards at the front. For the back vertebrae, the wide neural spines with straight, thickened top margins were similar to Macrocnemus. The gastralia suggest that the underside of the animal was gently convex. In the tail, the transverse processes of the first few vertebrae were very long like Tanytrachelos, which would have given the base of the tail a rhomboidal shape.
Limb girdles and limbs
An anatomical characteristic that distinguished Ozimek from other protorosaurs was the coracoids in its shoulder girdle, which were large and plate-like, and formed a convex shield covering the bottom of the shoulder region. This structure probably originated by the coracoids fusing with the sterna; Tanytrachelos in particular had large, quadrangular sterna, but they were unfused to the coracoids. Overall, the fused scapulocoracoid complex was the largest bone among fossils attributable to Ozimek. Each coracoid bore two openings in front of each other. The front opening may have been homologous with the coracoid foramina found in other animals, but the origin of the rear opening is unclear. Another distinguishing characteristic is that its scapulae were low (like Tangasaurus) but crescent-shaped (like Tanystropheus and Tanytrachelos). The glenoid fossa where the humerus articulated with the scapulocoracoid seemed to have been limited to the coracoids. The pelvic girdle of Ozimek was more typical of protorosaurs, being similar to Prolacerta, Malerisaurus, and Pamelaria. As in Sharovipteryx, the portion of the ilium in front of the acetabulum (hip socket) was well-developed.
The limbs of Ozimek were exceptionally elongated compared to all other members of the Protorosauria, with the hindlimbs being generally longer than the forelimbs and the femur being 18 times as long as its diameter at its top end. Like Sharovipteryx, the tibia of Ozimek was longer than its trunk; however, Ozimek had proportionally shorter legs relative to its trunk (1.2 times as long, compared to 2.1 times in Sharovipteryx) but thinner femora (14.5 times as long as the top end diameter in Sharovipteryx). The specimens ZPAL AbIII/2511 and ZPAL AbIII/2512 have similarly sized torsos but differ notably in limb proportions; the latter, probably a juvenile specimen, had smaller forelimbs in relation to its hindlimbs along with disproportionately thin humeri but thicker femora. Its feet were also large, with ZPAL AbIII/2511 suggesting that they were twice the size of the hands. Like Azendohsaurus and also other protorosaurs such as Prolacerta, however, the fifth metatarsal in the heel was robust, curved, and hooked, an adaptation related to the stiffening of the foot for more efficient standing and locomotion in diapsids. Like Sharovipteryx, there was a pulley-like projection at the bottom of the femur.
Classification
Protorosauria is a diverse group of Permian and Triassic archosauromorph reptiles generally characterized by long necks, especially so in specialized members of the group such as tanystropheids and dinocephalosaurids. Along with its long neck, other features that allowed Dzik and Sulej to refer Ozimek to the Protorosauria included the curvature of its scapulae and the procoelous vertebrae. However, the poor preservation of its skull inhibited them from comparing it to many other protorosaurs. Dzik and Sulej also recognized the many shared features between Ozimek and Sharovipteryx: the elongated hyoid bones, the expansion of the ilium in front of the acetabulum, the femoral projection, and the elongated tibia. These were previously listed as distinguishing characteristics of Sharovipteryx by Unwin; Dzik and Sulej therefore assigned Ozimek to the Sharovipterygidae, which otherwise contained only Sharovipteryx.
The relationship between Sharovipterygidae and Protorosauria is related to uncertainty over the definition of Protorosauria. In 1945, Charles Camp recognized a close relationship between Protorosaurus and Prolacerta; he defined the Protorosauria as being divided into the Prolacertiformes (including Prolacerta), Trachelosauria (i.e., Tanystropheidae), Thalattosauria, and Acrosauria (i.e., Pleurosauridae). By the 1980s, it was recognized that Protorosauria and Prolacertiformes were functionally equivalent groups, with the former taking priority but the latter being preferred due to uncertainty over the position of Protorosaurus. An emerging consensus in the 1980s and 1990s held that Sharovipteryx belonged to the Prolacertiformes, albeit based on characteristics that were either not determinable or widely present among diapsids. In 2000, Unwin and colleagues provided a list of characteristics to better support a prolacertiform attribution: long neck vertebrae and ribs, low neural spines on neck vertebrae, straight femora, tibiae longer than femora, long second phalanges in the fifth digits of the feet, seven or less neck vertebrae, and fourth metatarsals less than three times the length of the fifth. However, in subsequent years, first Prolacerta and then Protorosaurus were increasingly recognized as no longer belonging to the same group as other traditional members by phylogenetic analyses, thus making both Prolacertiformes and Protorosauria non-monophyletic (i.e., composed of groups that were not sister groups). The poor preservation of Sharovipteryx itself prevented its inclusion in these analyses.