Monday 6 June 2022

Eardasaurus powelli: A new Pliosaurid from England's Middle Jurassic Oxford Clay Formation.

The earliest known Pliosaurid Plesiosaurians date from rocks of the Earliest Jurassic. Long-snouted forms showing adaptations to for a specialist piscivoran diet appearing by the end of the Early Jurassic. Classical 'pliosauromorph' forms with large bodies, short necks, and heads which form a high proportion of the total body-length, appearing by the Middle Jurassic, and reaching sizes of 10-12 m by the Late Jurassic. This bodyform then remained essentially unchanged until the group disappeared in the early Late Cretaceous.

One of the most important deposits for the study of this group is the Oxford Clay Formation of England, which has been studied since the nineteenth century, and has produced a large and diverse assemblage of Plesiosaurs, with at least five valid described species: Simolestes vorax, Liopleurodon ferox, Peloneustes philarchus, 'Pliosaurus' andrewsi, and Marmornectes candrewi (a sixth, Pachycostasaurus dawni, is problematic, since it was described from a juvenile specimen that could potentially belong to several of the other species). Many of these are known from multiple specimens, notably Peloneustes philarchus, of which 21 examples have been found.

Many of these specimens, as well as other spectacular fossils from the same formation, were collected from pits which primarily extracted clay for the brick-making industry. The heyday of this industry was in the second half of the nineteenth century, when the brothers Charles and Alfred Leeds collected numerous Plesiosaur (and other) specimens from the clay pits around Peterborough in Cambridgeshire, including the holotypes of Simolestes vorax and 'Pliosaurusandrewsi. The brick-making industry has declined greatly since this time, with a subsequent decline in the amount of activity at clay pits, and therefore the number of fossils recovered from them, but spectacular new finds are still reported from time to time, with the most recently described Oxford Clay Plisiosaur, Marmornectes candrewi, having been recovered from a clay pit in Bedfordshire in 1998.

In a paper published in the journal Acta Palaeontologica Polonica on 23 May 2022, Hilary Ketchum of the Oxford University Museum of Natural History, and Roger Benson of the Department of Earth Sciences at the University of Oxford, describe a new species of Pliosaurid from the Peterborough Member of the Oxford Formation, collected from the former ARC Cassington gravel pit near Yarnton, Oxfordshire.

The new Pliosaur is named Eardasaurus powelli, where 'Eardasaurus' is a combination of the  Old English 'eard', meaning 'native soil or dwelling', from which the village name Yarnton was derived, and '-saurus' the Greek for Lizard, commonly used as a suffix for large Mesozoic Animals, while 'powelli' honours Philip Powell, former Assistant Curator at the Oxford University Museum of Natural History, who took part in the excavation of the specimen from which the species is described in 1994, and carried out preparation work on its skull and postcranial skeleton.

The species is described from a single specimen, OUMNH PAL-J.2247, which was discovered during the construction of a deep silt pond. This specimen was partially embedded in six concretions, which enclosed the posterior two-thirds of the cranium, parts of the vertebral series, and the hindlimb.

Holotype of the Pliosaurid Plesiosaur Eardasaurus powelli (OUMNH PAL-J.2247) from the Middle Jurassic of Yarnton, Oxfordshire, UK. Photograph of the specimen on display in Oxford University Museum of Natural History. Skeleton laid out as discovered, with the exception of the left hindlimb (highlighted in black), which was originally disarticulated. An artist’s reconstruction of Eardasaurus powelli  is partially visible in the top right-hand side of the image. Ketchum & Benson (2022).

Specimen OUMNH PAL-J.2247  measures approximately 4.7 m long from the tip of the snout to the end of the (incomplete) tail, as preserved, and includes the cranium, lower jaw, four scleral ossicles, two hyoids, more than 40 teeth and tooth fragments (including both disarticulated and in situ teeth), atlas-axis complex, 57 postaxial vertebrae (comprising 18 cervicals, 3 pectorals, 24 dorsals, 4 sacrals, and 8 caudals, along with disarticulated neural arches, neural spines, ribs, and chevrons), and 11 gastralia. The preserved appendicular skeleton includes the distal end of the right humerus plus a partial left forelimb discovered in articulation, which includes the radius, ulna, proximal and distal carpals, and 17 phalanges. There is a partial left hindlimb compri sing the femur, proximal and distal tarsals. In total 40 complete phalanges, plus six partial phalanges were found disarticulated and cannot confidently be assigned to any particular limb. Of the limb girdles, only the interclavicle and a clavicle are preserved.

Eardasaurus powelli is a Longirostrine Pliosaurid with a high number (35-40) of alveoli on its maxilla, a jugal with fluted ornamentation on its orbital margin, a step-like contact between the jugal and the squamosal, a 'lacrimal' bone formed by the neomorphic ossification in the anteroventral orbit margin (and not homologous to the lacrimal of other Tetrapods) which is dorsoventrally slender with a posterior rod-like projection.

Eardasaurus powelli also has a pattern of enamel ridges on its larger (mesial) teeth, something also seen in Peloneustes philarchus, but absent in other Pliosaurids from the Oxford Clay. It also has five to six premaxillary teeth, a  prominent mediolateral constriction of the rostrum at the premaxilla–maxilla suture, a diastema (gap) between premaxillary and maxillary dentitions, a  maxilla excluded from contact with the medial margin of external naris by an anterior extension of the frontal, a proportionally elongate posterior interpterygoid vacuities compared to other Middle Jurassic Pliosaurids, a mediolaterally expanded mandibular symphysis, nine pairs of dentary teeth adjacent to the mandibular symphysis, a  splenial which encloses the posterior margin of the anterior opening of Meckel’s canal, and a coronoid exposed on the lateral surface of the mandible, a strongly convex medial expansion of the surangular in dorsal view.

Holotype of the Pliosaurid Pleasiosaur Eardasaurus powelli (OUMNH PAL-J.2247) from the Middle Jurassic of Yarnton, Oxfordshire, UK. Cranium in dorsal view. Photograph (A₁), interpretive drawing (A₂), light grey, broken bone; mid grey filler; dark grey, matrix. Ketchum & Benson (2022).

A phylogenetic analysis carried out by Ketchum and Benson recovered Eardasaurus powelli as an early-diverging Thalassophonean Pliosaurid, forming the sister taxon to the derived group comprising 'Pliosaurus' andrewsi, Simolestes vorax, Liopleurodon ferox, Pliosaurus spp., and Brachaucheninae.

Phylogenetic topology for Pliosauridae, focussing on Early–Middle Jurassic representatives, based on Bayesian Mkv analysis. Numbers indicate posterior support for nodes demonstrating strong support for many aspects, but weaker support for relationships among Peloneustes philarchus, Anguanax zignoi, and Eardasaurus powelli, which are tentatively recovered as grade leading to more derived Pliosaurids. Ketchum & Benson (2022).

Eardasaurus powelli has a pattern of deep ridges on its teeth which would have formed additional deep cutting surfaces in life. Such ridges are ecologically significant, as they are typically associated with macropradatory behaviour (i.e. attacking large prey). Such ridges are found in a range of taxa, although they are rare (but not unknown) in Pliosaurids, although Ketchum & Benson suggest that this may be due to under-reporting rather than a true absence.

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