Eardasaurus is a genus of thalassophonean pliosaurid from the middle Jurassic Oxford Clay Formation. The animal would have measured over long (the tail was not fully preserved) and possessed a high amount of teeth relative to other pliosaurs. Its teeth show distinct ridges formed by the tooth enamel, some of which are very pronounced and similar to carinae, giving the teeth a cutting edge.
History and naming
The remains of Eardasaurus were discovered H. Philip Powell in the Oxford Clay Formation near Yarnton in 1994. The only known fossil and holotype, specimen OUMNH PAL-J.2247 (previously listed as OUMNH J.02247), was unearthed from the Peterborough Member of the formation, which indicates that it lived during the Callovian stage of the Jurassic. The specimen is fairly complete and partially articulated, preserving both the cranium and mandible, large parts of the vertebral column as well as ribs and gastralia as well as the remains of the left fore- and hindlimbs.
The genus name is composed of the Old English word "eard", which means "home" or "dwelling", and "saurus", Greek for lizard. The name of the nearby town of Yarnton is likewise derived from "eard". The species name on the other hand honors H. Philip Powell for discovering the fossil.
Description
The holotype specimen measures approximately long, however as the tail is only incompletely preserved the animal would have been larger in life.
Skull
Eardasaurus was a longirostrine animal with a roughly triangular skull in dorsal view. The skull measures long, with 54% of the length composed of the section in front of the eyes. Among the Oxford Clay pliosaurs this ratio is closest to Liopleurodon with a preorbital skull length that makes up 55% of the total length. While the rostrum itself is well preserved, the back of the skull is much more fragmented, rendering it difficult to identify the bone sutures of the region. The anterior-most tip of the rostrum is made up of the paired premaxilla, which bear a series of foramina along their surface. The premaxillae contain five and six teeth respectively, in line with the other pliosaurids it shared its habitat with. The teeth themselves are irregular (anisodont), with the largest being the penultimate alveolous. The smallest on the other hand are the first and last teeth, which show a notable size decrease compared to the other teeth around them. Still, the last tooth in the premaxilla is not as much reduced as observed in other pliosaurs, which may show a size decrease of over 50%. All teeth of the premaxilla are closely spaced except for the area where the premaxilla contacts and interlocks with the maxilla. Here a toothless area (diastema) is present. Such a diastema is also present in Liopleurodon, but absent in most other middle Jurassic pliosaurs. More widespread is the presence of a constriction in the same area. The dorsal contact between the premaxillae is almost straight but interrupted by a slit-like opening surrounded by rugose texturing. While such an opening is present in a more pronounced fashion in rhomaleosaurids, in the case of Eardasaurus it is more than likely a pathology. The premaxillae stretch over the skull deep between the maxillae, only ending at approximately the same level as the anterior edge of the orbits, where they meet the parietal bones.