''Eorasaurus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

archosauromorph Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) than to lepidosaurs (such as tuataras, lizards, a ...

reptile known from the middle

late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

(late

Capitanian In the geologic timescale, the Capitanian is an age or stage of the Permian. It is also the uppermost or latest of three subdivisions of the Guadalupian Epoch or Series. The Capitanian lasted between and million years ago. It was preceded by t ...

or early

Wuchiapingian In the geologic timescale, the Wuchiapingian or Wujiapingian (from in the Liangshan area of Hanzhong, Shaanxi Province ) is an age or stage of the Permian. It is also the lower or earlier of two subdivisions of the Lopingian Epoch or S ...

age) of

Tatarstan Tatarstan, officially the Republic of Tatarstan, sometimes also called Tataria, is a Republics of Russia, republic of Russia located in Eastern Europe. It is a part of the Volga Federal District; and its capital city, capital and largest city i ...

European Russia European Russia is the western and most populated part of the Russia, Russian Federation. It is geographically situated in Europe, as opposed to the country's sparsely populated and vastly larger eastern part, Siberia, which is situated in Asia ...

. It contains a single species, ''Eorasaurus olsoni''. When originally described by Sennikov (1997), ''Eorasaurus'' was identified as an early archosauromorph and assigned to the family Protorosauridae,

Ezcurra Ezcurra is a surname. Notable people with the surname include: * Alma Ezcurra (born 1986), Spanish politician * Bautista Ezcurra (born 1995), Argentine rugby union player * Encarnación Ezcurra (1795–1838), Argentine political activist * Exequiel ...

''et al.'' (2014) and Ezcurra (2016) later reclassified ''Eorasaurus'' and placed it within the group Archosauriformes. ''Eorasaurus'' is based solely on scant fossil material from the neck region, and is thus considered an unstable taxon in phylogenetic analyses. If ''Eorasaurus'' is an archosauriform, it would be the oldest known member of the group and would pre-date the previous record holder (the

proterosuchid Proterosuchidae is an early family of basal archosauriforms whose fossils are known from the Late Permian and the Early Triassic. The highest diversity of genera is known from European Russia, but fossils are also known from South Africa, India, ...

Archosaurus ''Archosaurus'' (meaning "ruling lizard") is an extinct genus of carnivorous proterosuchid archosauriform reptile. Its fossils are dated to the latest Permian of Russia and Poland, it is one of the earliest known archosauriforms. The type and ...

'').

Discovery

''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. These four specimens were discovered in the 1930s near the

Volga River The Volga (, ) is the longest river in Europe and the longest endorheic basin river in the world. Situated in Russia, it flows through Central Russia to Southern Russia and into the Caspian Sea. The Volga has a length of , and a catchment ...

in Tatarstan, Russia. They consist of nine vertebrae as well as one dorsal rib and two limb bones. Two cervicals (neck vertebrae), were termed specimen PIN 156/109 and designated as the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

. Specimen PIN 156/108 represents three cervicals from the base of the neck, PIN 156/110 represents a rib and four dorsals (back vertebrae) near the neck, while PIN 156/111 is a pair of long bones and other fragments possibly from a limb. The material was collected from the Semin Ovrag locality of the upper Severodvinian Horizon (also known as the North Dvina Gorizont) in the Volga River Basin.

Description

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 intercentra, additional wedge-like bones between its cervical vertebrae. This differs from Sennikov's original description, which reported a lack of intercentra. The

neural arch Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

es 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. The main rib facets (diapophyses) are connected to other parts of the vertebrae via bony ridges called prezygodiapophyseal and centrodiapophyseal laminae. Laminae are characteristic of archosauromorph vertebrae, along with an irregular polygon-like shape of the cervicals. One

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a Synapomorphy, derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or Outgroup (cladistics), outgroup taxa, not ...

(unique derived 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, a deep pit which lies in front of the centrodriapophyseal lamina.

Classification

Based on the structure of its vertebrae, ''Eorasaurus'' is almost certainly a member of a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of reptiles called Archosauromorpha, which includes

crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

ns and

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s as modern representatives. It is a basal taxon that falls outside

crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...

Archosauria (the clade defined by the

most recent common ancestor A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...

of birds and crocodilians). When ''Eorasaurus olsoni'' was first described in 1997, Sennikov identified it as a close relative of ''

Protorosaurus speneri ''Protorosaurus'' (from , 'earlier' and , 'lizard') is an extinct genus of reptile. Members of the genus lived during the late Permian period in what is now Germany and Great Britain. Once believed to have been an ancestor to lizards, ''Protoro ...

'', a species of " protorosaur" that lived at around the same time as ''Eorasaurus'' (the middle Wuchiapingian) in what is now western Europe. Protorosauria is a group within Archosauromorpha that includes a variety of long-necked Triassic species. Some studies have considered the most basal major clade of archosauromorphs, close to the split between Archosauromorpha and its

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

Lepidosauromorpha Lepidosauromorpha (in PhyloCode known as Pan-Lepidosauria) is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which contains tw ...

, whose modern representatives are lizards and snakes. The monophyly of Protorosauria is debatable, with many studies considering it a

grade Grade most commonly refers to: * Grading in education, a measurement of a student's performance by educational assessment (e.g. A, pass, etc.) * A designation for students, classes and curricula indicating the number of the year a student has reach ...

of early archosauromorphs rather than a natural

. However, when Ezcurra ''et al.'' (2014) included ''Eorasaurus olsoni'' for the first time in a

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis, they found it to be a member of

Archosauriformes Archosauriformes (Ancient Greek, Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles encompassing Archosaur, archosaurs and some of their close relatives. It was defined by Jacques Gauthier (1994) as the clade stemmin ...

, 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 Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

. If ''Eorasaurus'' is an archosauriform rather than a more basal archosauromorph, it would the oldest archosauriform by several million years. The second oldest would be the

'' Archosaurus rossicus,'' from the latest Permian (

Changhsingian In the geologic time scale, the Changhsingian or Changxingian is the latest age or uppermost stage of the Permian. It is also the upper or latest of two subdivisions of the Lopingian Epoch or Series. The Changhsingian lasted from to 251.9 Ma a ...

) of Poland and Russia. ''Eorasaurus'''s position within Archosauriformes would also mean that archosauromorph groups more basal than it (

allokotosaurs Allokotosauria is a clade of early archosauromorph reptiles from the Middle Triassic, Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of spec ...

rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaurs ...

s, prolacertids, proterosuchids, and possibly ''

Euparkeria ''Euparkeria'' (; meaning "Parker's good animal", named in honor of W. K. Parker) is an Extinction, extinct genus of archosauriform reptile from the Triassic of South Africa. ''Euparkeria'' is close to the ancestry of Archosauria, the reptile gro ...

'' and

erythrosuchid Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named by ...

s based on Ezcurra ''et al.s analysis) originated long before the Permo-Triassic extinction event. This suggests long

ghost lineage A ghost lineage is a hypothesized ancestor in a species lineage that has left no fossil evidence, but can still be inferred to exist or have existed because of gaps in the fossil record or genomic evidence. The process of determining a ghost line ...

s within these groups, since most fossils from these groups do not appear until well after the extinction. ''Eorasaurus'' was also utilized in Ezcurra (2016)'s phylogenetic analysis. It was one of several unstable "

proterosuchia Proterosuchia is one of the suborders of the paraphyletic group Thecodontia, containing the most primitive and ancestral forms. These were primitive, vaguely crocodile-like, archosauriforms that mostly lived during the Early Triassic epoch. Th ...

n"-grade taxa which formed a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...

at the base of Archosauriformes. This polytomy was resolved only after the removal of ''Eorasaurus'' and its fellow wildcard taxa from the analysis. Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from Ezcurra ''et al.'' (2014) showing their interpretation of ''Eorasaurus'' as an archosauriform:

References

{{Taxonbar, from=Q15727693 Fossil taxa described in 1997 Permian reptiles of Europe Prehistoric reptile genera