Euparkeria (; meaning "Parker's good animal", named in honor of W. K. Parker) is an extinct genus of archosauriform reptile from the Triassic of South Africa. Euparkeria is close to the ancestry of Archosauria, the reptile group that includes crocodilians, pterosaurs and dinosaurs (including birds).
Fossils of Euparkeria, including nearly complete skeletons, have been recovered from the Cynognathus Assemblage Zone (CAZ, also known as the Burgersdorp Formation), which hosts the oldest advanced archosauriforms in the fossil-rich Karoo Basin. Tentative dating schemes place the CAZ around the latest Early Triassic (late Olenekian stage) or earliest Middle Triassic (early Anisian stage), approximately 247 million years old.
Euparkeria is among the most heavily described and discussed non-archosaur archosauriforms. It was a small carnivorous reptile with a boxy skull, slender limbs, and two rows of tiny teardrop-shaped osteoderms (bony scutes) along its backbone. Euparkeria is a eucrocopod, meaning that it was among the reptiles most closely related to true crown group archosaurs, according to specializations of the ankle and hindlimbs. The hind limbs were slightly longer than its forelimbs, which has been taken as evidence that it may have been able to rear up on its hind legs as a facultative biped. This conception supplemented older studies which interpreted Euparkeria as a particularly close relative to fully bipedal early dinosaurs. Its normal movement was probably more quadrupedal, with limbs positioned in a semi-erect posture, analogous (but not identical) to a crocodilian high walk. Biomechanical analyses suggests that Euparkeria was incapable of even short periods of bipedal activity.
History and naming
In the late 1900s South African palaeontologist Alfred Brown collected a collection of small reptiles from beds believed to Upper Triassic around Aliwal North, South Africa. This collection was first studied in 1912 by British palaeontologist D. M. S. Watson who named all of the material Mesosuchus browni. However, the identification of Watson was quickly doubted by South African palaeontologist Robert Broom in 1913, who further examined the material and found that two separate taxa were present. One, showing similarities to the rhynchocephalian Howesia, was described in the most detail by Watson, so Broom established the skull of this form as the lectotype of Mesosuchus, while the other, believed to be a reptile close to Ornithosuchus, was named Euparkeria capensis by Broom. The genus name was in honour of Broom's former professor William Kitchen Parker, in combination with the Ancient Greek prefix ευ- ("eu-") meaning "true" or "good", and the Latin substantive suffix -ia, while the species name presumably refers to the Cape of Good Hope and its namesake the Cape Province. Broom described Euparkeria in much greater detail later in 1913, elaborating that it could be characterized by the nearly complete skull and most of a skeleton of its holotype, and considering it to be a thecodont possibly close to the origins of dinosaurs. The skull of Euparkeria had not yet been fully prepared when Watson studied it, so he would not have been able to see the obvious differences between the two taxa in Brown's collection. In the second 1913 paper Broom also named the new taxon Browniella africana for specimens from Brown's collection that were similar to, but much larger than, the main material of Euparkeria. The genus name for this taxon was in honour of Brown.
Following the death of Brown the material in his collection was acquired by the South African Museum where it was further prepared and studied by South African palaeontologist Sidney Henry Haughton in 1922. Most of the material could be identified as Euparkeria, including the pelvis and shoulder previously considered as part of Browniella, which as a result only preserved its femur. The holotype of Euparkeria was numbered SAM-PK-5867, with five other specimens being assigned, covering almost all of the regions of the skeleton. In addition to the material collected by Brown, in 1924 and 1925 the SAM acquired some of the material collected by Albert W. Higgins under the direction of Brown, while other specimens went to the Institut und Museum für Geologie und Paläontologie of the University of Tübingen, and one specimen went to the University College London. The complete array of Euparkeria specimens was only fully prepared with modern techniques by 1965 under the work of zoologist Rosalie F. Ewer. Ewer also attempted to relocate the original localities of the material of Euparkeria, which was labelled in the museums as coming from "Krielfontein", but even with the assistance of local authorities no such location could be found. Through the guidance of D. N. de Wet, who had known Brown as a child, Ewer was able to identify a probably locality of Broom's collecting as a layer of sandstone in the Cynognathus Assemblage Zone, with the fragmentation and material matching so close that it could be possible that all remains of Euparkeria had come from this single locality. Ewer also identified that the material of Browniella belonged to only one individual, and that following Haughton this individual could be assigned to Euparkeria, making Browniella a junior synonym.
Further rediscovery of location notes by Brown identify a different location from that proposed by Ewer. These notes detail that the first specimens were discovered in a stone quarry of Alexander Alcock, identified during preparation of slabs by a worker identified as "Mr. Gibbs" on 22 July 1907. This material was available for study by Watson and thus would have included the types of Mesosuchus and Euparkeria, but subsequent discoveries at the same quarry on 21 July 1912 were not. The notes also confirm the quarry was alongside "Krietfontein Spruit", which is recorded as a cluster of mineral springs in town records of Aliwal North. The location of the springs and quarry were confirmed by British and South African palaeontologists Roger M. H. Smith and Frederik P. Wolvaardt in 2019. The Cynognathus AZ in the Karoo Basin of South Africa is equivalent with the Burgersdorp Formation, as the youngest deposits of the Beaufort Group. Euparkeria is only known from a single locality within the Cynognathus AZ, as part of the subzone characterized by synapsids Trirachodon and Kannemeyeria. Through biostratigraphy, the Trirachodon–Kannemeyeria subzone can be correlated with the early Anisian and possibly latest Olenekian, establishing an approximate age for Euparkeria, which was found above the base of the subzone.
Classification
Ancestor of archosaurs
Brom's initial interpretation of Euparkeria was that it represented an early reptile very close to the common ancestor of crocodiles, dinosaurs, birds and pterosaurs, as a relative of rhynchocephalians, phytosaurs and gnathodonts. Broom refined this later in 1913 to the classification of Euparkeria as a member of Pseudosuchia, which he treated as an equivalent name for Thecodontia, and as an ancestral group of unspecialized reptiles that would give rise to dinosaurs, birds and pterosaurs. Watson in 1917 placed Euparkeria within Ornithosuchidae as a thecodont, alongside Ornithosuchus, Scleromochlus and Sphenosuchus, which he interpreted as direct ancestors of theropod dinosaurs. German palaeontologist Friedrich von Huene instead treated Thecodontia as a larger group than Pseudosuchia, introducing the new pseudosuchian family Euparkeriidae in 1920 for Euparkeria and Browniella. Huene would reiterate this classification of thecodonts and Euparkeria in 1922 and 1936, and elaborated in 1948 that euparkeriids would be descended from ornithosuchids and serve as precursors of saurischians. He considered them the closest relatives of Sphenosuchidae, the ancestors of crocodiles, and Scleromochlidae, the probable ancestors of pterosaurs. These three families would be the most specialized and final thecodonts before the group diverged.
The pre-cladistic schemes of classification of Euparkeria and other thecodonts largely followed Huene over the succeeding decades, with American palaeontologist Alfred Sherwood Romer maintaining Euparkeria (as the only member of Euparkeriidae) as a pseudosuchian thecodont close to Erpetosuchidae, Teleosauridae, Elastichosuchidae and Prestosuchidae in 1966, and then elaborating on this in 1972 with the introduction of Wangisuchus as a possible euparkeriid and the limiting of Pseudosuchia to euparkeriids, ornithosuchids and scleromochlids. Alternative suggestions for the placement of Euparkeria include intermediate between the thecodont groups Proterosuchia and Pseudosuchia or as a member of the proterosuchian family Erythrosuchidae. Argentine palaeontologist José Bonaparte treated euparkeriids as the ancestral taxa to dinosaurs, ornithosuchids and sphenosuchids in 1976, placing the family, limited to Euparkeria, as the ancestral member of the pseudosuchian infraorder Ornithosuchia from which the various groups evolved. Ornithosuchia was separated from Pseudosuchia by Indian palaeontologist Sankar Chatterjee in 1982, where it was believed that Euparkeria gave rise to ornithosuchids and Lagosuchus (within the separate suborder Lagosuchia). Dinosaurs were believed by Chatterjee to have been two separate unrelated groups, evolving separately from different thecodont ancestors. This classification was based on a misinterpretation of the ankle joint in dinosaurs, with British palaeontologists Arthur Cruickshank and Michael Benton showing in 1985 that dinosaurs shared the same ankle type, and while they evolved from reptiles similar to Ornithosuchus and Euparkeria, neither genus was a direct ancestor of later forms, with Euparkeria preceding the crocodile–dinosaur split, and Ornithosuchus as an early relative of dinosaurs.
Removal from Archosauria
Introductions of phylogenetic analysis to the study of the origins of archosaurs was begun in 1986 by American palaeontologist Jacques Gauthier, who evaluated the origins of birds relative to dinosaurs, crocodiles, and "thecodonts". As "thecodonts" were a paraphyletic group excluding their descendents, Gauthier advocated for abandoning it as a term, and he modified the contents of Pseudosuchia to represent the crocodile-line archosaurs. Euparkeria had previously been considered close to the origins of birds by Broom and others, as it was unspecialized in a way that could have allowed it to evolve into birds without reversals of features, but through analysis Gauthier instead found that birds were definitively descended from dinosaurs, and Euparkeria, of relevance to the divergence of crocodiles and birds, was found as the earliest member of the bird-line archosaurs he used the name Ornithosuchia for. The 1988 analysis of Benton and American palaeontologist James M. Clark found similar results for Euparkeria, but moved it just outside the crocodile-bird split. Benton and Clark did retain Euparkeria as an archosaur though, as while Gauthier used the name for just the crown group, Benton and Clark expanded its contents to also include extinct taxa that preceded the crocodile-bird divergence such as Euparkeria. Subsequent analyses throughout the 1990s supported Euparkeria as outside crown-group Archosauria, and also moved Ornithosuchidae from the bird line to crocodile line, advocating for the abandonment of Ornithosuchia as a name for the bird-line archosaurs as the group no longer included Ornithosuchus. They also used Archosauria as the name for the crown group, relegating Euparkeria to the status of a non-archosaurian archosauromorph, with their results also supporting Proterochampsidae between Euparkeria and archosaurs. Benton continued to use Archosauria as the larger group including Euparkeria in 1999, creating the new name Avesuchia for the crown group.