Product Code Database
Euparkeria (; meaning "Parker's good animal", named in honor of W. K. Parker) is an Extinction genus of archosauriform reptile from the Triassic of South Africa. Euparkeria is close to the ancestry of Archosauria, the reptile group that includes , , and (including ).
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 (bony scutes) along its backbone. Euparkeria is a Archosauriformes, 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 Bipedalism early dinosaurs. Its normal movement was probably more quadrupedal, with limbs positioned in a semi-erect posture, analogous (but not identical) to a high walk. Biomechanics analyses suggests that Euparkeria was incapable of even short periods of bipedal activity.
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.
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, Elachistosuchus 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.
Following these early and largely independent analysis, it was established that the consensus of archosaur evolution was that proterosuchids, erythrosuchids, Euparkeria, and proterochampsids formed a series of groups outside but leading to Archosauria, with Archosauria being separated into crocodile-line and bird-line groups. Phytosaurs would be the earliest members of the crocodile-line, and pterosaurs would be the earliest members of the bird-line, though the position of ornithosuchids along the crocodile-line was uncertain. Subsequent analyses largely reused the same characters and taxa of those from the 1990s, up to the 2011 study of archosaur origins by American palaeontologist Sterling Nesbitt. Like previous studies, Nesbitt found that Euparkeria was a close relative of archosaurs but not a true archosaur, though it was closer to the crocodile-bird divergence than proterochampsids. Phytosaurs were removed from the crocodile-line and instead placed as the closest relatives of archosaurs, between Euparkeria and Archosauria, with ornithosuchids as the earliest pseudosuchians (the name applied to crocodile-line archosaurs) and pterosaurs as the earliest (the name applied to bird-line archosaurs).
The 2016 phylogenetic analysis of archosauromorphs by Argentine palaeontologist Martín Ezcurra contrasted with the analyses of Nesbitt and Sookias in the placement of Euparkeria relative to Archosauria, finding it to be further from the group than proterochampsians, with phytosaurs as early pseudosuchians as in earlier studies. While Dorosuchus was included and found to be a non-euparkeriid as in the studies of Sookias, the Chinese euparkeriids were not analyses so the status of Euparkeriidae was left uncertain. At the same time, the new clade Eucrocopoda was named for all archosauromorphs more derived than erythrosuchids, encompassing Euparkeria and close relatives, proterochampsians, and archosaurs. Subsequent iteration of the Ezcurra analysis added Halazhaisuchus and Osmolskina for the purposes of assessing morphological disparity rather than phylogenetic relationships, resulting in their exclusion from analysis for assessment of relationships, though when included they can be unresolved, support a Euparkeriidae including Euparkeria, Osmolskina, Halazhaisuchus and the specimen NMQR 3570, or recover Euparkeria further from Archosauria than the other taxa. Analyses based on Nesbitt and Sookias recover a consistent Euparkeriidae, and have also found the eucrocopodan Marcianosuchus to be very close to the family. The consensus of Euparkeria relationships described by Sookias in 2020 is shown below.
A 2020 study of range of motion in the hindlimbs of Euparkeria found conflicting evidence for its posture. The structure of the femur (thigh bone) and hip socket suggest that the legs were capable of a very wide range of motion, ranging from a nearly vertical stance to a thigh which projects forwards, backwards, or outwards at a nearly horizontal angle. Rotation of the thigh was more limited, a factor that argues against a sprawling gait reliant on broad outward leg sweeps. Although the hip socket argues in favor of an upright 'pillar-erect' hindlimb stance, the structure of the tibia (inner shin bone) and ankle show that the lower legs and feet would have splayed outwards during normal usage, supporting a semi-erect rather than fully erect stance. The hindlimbs of Euparkeria have been used to argue that the evolution of a fully erect gait in true archosaurs was a stepwise process which first developed in bones closer to the hip.
A 2023 paper analyzed the possibility of facultative bipedalism and came to the conclusion that Euparkeria was quadrupedal at all times. Models of weight distribution found that the center of mass for Euparkeria was far in front of the hips, meaning that a body held horizontally during a bipedal stance would have to fight against a very large forward pitching moment. This pitching moment far exceeds that of modern long-limbed capable of facultative bipedalism. The pitching moment would only stabilize if the body was held up at an implausibly high angle (>60 degrees), regardless of how the tail was held. In addition, models of muscle activation indicate that the ankle plantarflexor group (the muscles which bend the foot down to maintain stability) would have been overexerted to the point of failure if a bipedal posture was attempted by the animal.
A recent comparative study of bone cross-sectional geometry also inferred a fully quadrupedal locomotion in Euparkeria.
The locality of Euparkeria in the quarry along Krietfontein Spruit is above the base of the Trirachodon- Kannemeyeria Subzone, in the region where the deposits change from low-sinuosity channels to a more meandering river system. The regional climate is considered to have been warm and semi-arid with seasonal rainfall during the lower Burgersdorp, becoming colder and wetter over time but maintaining seasonality. A bonebed of Euparkeria and Mesosuchus from slightly lower than the type locality containing approximately 50 individuals shows a wide diversity of taphonomy suggesting the accumulation of desiccation carcasses before burial alongside those that were buried rapidly and perhaps trampled, as a concentration during a flood event. Alternate specimens of Euparkeria associated with tree trunks and plant debris also corroborate a mass-drowning. The death accumulation of Euparkeria and Mesosuchus on top of a low point bar may indicate a preferred habitat of the riparian forests alongside watercourses, and the vegetated sandbars between watercourses, and at the same time Euparkeria may have been the principal scavengers of the carcasses causing the bone scatter and trampling.
|
|
