Temporal range: Late Permian, 255 Ma
Eorasaurus is an extinct genus of archosauromorph known from the middle Late Permian (late Capitanian or early Wuchiapingian age) of Tatarstan, European Russia. It contains a single species, Eorasaurus olsoni. Eorasaurus was named by Andrey G. Sennikov in 1997. It is known from four specimens representing a single individual, PIN 156/108 through PIN 156/111, out of which PIN 156/109 was designed as the holotype. The material was collected from the Semin Ovrag locality, from the upper Severodvinian Horizon of the Volga River Basin. Sennikov (1997) assigned this genus to Protorosauridae, but Ezcurra et al. (2014) reclassified it as an archosauromorph.
PIN 156/108 through PIN 156/111 were discovered near the Volga River in Tatarstan, Russia in the 1930s and consist of nine cervical (neck) and dorsal (back) vertebrae as well as one dorsal rib and two limb bones. The body or centrum of each of the cervical vertebrae is irregularly shaped with many ridges (laminae) and pits (fossae). Ezcurra et al. (2014) note that Eorasaurus has additional bones called intercentra between its cervical vertebrae, conflicting with Sennikov's original reporting of a lack of intercentra. The neural arches of the specimens are fully fused with the centra, indicating that the individual was an adult when it died. A combination of features in the vertebrae of Eorasaurus distinguish it from other reptiles, including the presence of plates of bone on the vertebrae called prezygodiapophyseal and centrodiapophyseal laminae, the presence of cervical intercentra, and the overall irregular polygon-like shape of the vertebrae. An autapomorphy or unique characteristic of Eorasaurus is the presence of an additional lamina that splits off from the underside of the centrodiapophyseal lamina and crosses the centrodiapophyseal fossa.
Based on the structure of its vertebrae, Eorasaurus is almost certainly a member of a clade of reptiles called Archosauromorpha, which includes crocodilians and birds as modern representatives. It is a basal taxon that falls outside crown group Archosauria (the clade defined by the most recent common ancestor of birds and crocodilians). When Eorasaurus olsoni was first described in 1997, Sennikov identified it as a close relative of Protorosaurus speneri, a species of protorosaur that lived at about the same time as Eorasaurus (the middle Wuchiapingian) in what is now western Europe. Protorosauria is a basal group within Archosauromorpha that includes a variety of mostly Triassic species. Many recent studies consider it the most basal clade of archosauromorphs, close to the split between Archosauromorpha and its sister group Lepidosauromorpha, whose modern representatives are lizards and snakes. However, when Ezcurra et al. (2014) included Eorasaurus olsoni for the first time in a phylogenetic analysis, they found it to be a member of Archosauriformes, a more derived clade within Archosauromorpha. Support for its placement within Archosauriformes was weak, based on a single character (the presence of posterior centrodiapophyseal laminae on cervical vertebrae) that is also present in protorosaurs through evolutionary convergence. If Eorasaurus is an archosauriform rather than a more basal archosauromorph, it would the oldest member of the group by several million years (the second oldest would be the proterosuchid Archosaurus rossicus from the latest Permian (Changhsingian) of Poland and Russia). Its position within Archosauriformes would also mean that archosauromorphs more basal than it (trilophosaurids, rhynchosaurs, prolacertids, proterosuchids, and possibly Euparkeria and erythrosuchids based on Ezcurra et al.'s analysis) originated long before the Permo-Triassic extinction event even though most members of these groups do not appear until well after the extinction.
Below is a cladogram from Ezcurra et al. (2014) showing the phylogenetic position of Eorasaurus within Archosauromorpha:
- Sennikov, Andrey G. (1997). "An enigmatic reptile from the Upper Permian of the Volga River Basin". Paleontologicheskii Zhurnal. 1997 (1): 95–103.
- Ezcurra, M. N. D.; Scheyer, T. M.; Butler, R. J. (2014). "The Origin and Early Evolution of Sauria: Reassessing the Permian Saurian Fossil Record and the Timing of the Crocodile-Lizard Divergence". PLoS ONE. 9 (2): e89165. doi:10.1371/journal.pone.0089165.