Pliosaurus brachydeirus
Age
Jurassic（Kimmeridgian）
157.3-152.1 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Plesiosauria
Family: Pliosauridae
Genus: Pliosaurus
Species: Pliosaurus brachydeirus
Morphological description
Pliosaurus brachydeirus is currently known with confidence only from the holotype, OXFUM/OUMNH J.9245, and its associated material, which includes portions of the skull, mandible, axial skeleton, and limbs. The teeth are fully trihedral in cross-section, with a flat, smooth labial surface and a convex lingual surface bearing longitudinal enamel ridges. A 2013 reassessment estimated that each dentary contained approximately 36–37 alveoli, of which about 12–13 were situated within the mandibular symphysis. The entire mandible therefore probably carried more than 70 teeth, representing one of the highest mandibular tooth counts documented in Pliosaurus. The premaxilla possessed six closely spaced alveoli, and the distalmost premaxillary alveolus was not substantially reduced in size, indicating the absence of the anisodont premaxillary dentition seen in some other species. A diastema was present between the premaxillary and maxillary tooth rows, while the mediolateral expansion of the premaxillary and maxillary caniniform regions was relatively slight. The premaxilla–parietal suture was positioned approximately level with the anterior region of the orbit.
Early studies differed considerably in their estimates of mandibular symphysis length. The symphysis was once reconstructed as containing approximately 12 pairs of symphyseal teeth, although an estimate of only seven pairs was also proposed. Because the posterior portion of the symphysis is incomplete, a 2012 revision could establish only that it extended posterior to the seventh tooth and diagnosed the species as possessing approximately 8–9 or more pairs of symphyseal teeth. A subsequent reassessment in 2013 estimated the complete count at approximately 12–13 pairs.
The retroarticular process at the posterior end of the mandible is less than half the width of the mandibular glenoid and tapers posteriorly to a sharp point in dorsal view. The cervical vertebrae provide another distinctive feature. Their ventral surfaces lack the rugose ornamentation present in most other species of Pliosaurus, and the two ventral surfaces converge medially to form a distinct longitudinal ventral ridge.
The proximal articular surfaces of the radius or tibia are relatively flat. The 2013 study suggested that this condition may reflect the incomplete skeletal maturity of the holotype and therefore may not represent a stable species-level feature in fully grown individuals.
Etymology
The generic name Pliosaurus is derived from the Greek plio-, meaning "more" or "greater," and sauros, meaning "lizard."
The specific name brachydeirus is derived from the Greek brachy-, meaning "short," and deirus, referring to the "neck," in reference to the animal's short-necked morphology.
Biological description
The holotype of Pliosaurus brachydeirus was discovered at Market Rasen in Lincolnshire, England. It comes from the lower part of the Kimmeridge Clay Formation and dates to the early Kimmeridgian. The currently recognised type material includes OXFUM/OUMNH J.9245, J.9247–J.9301, and J.10453 as listed in the 2013 revision, together comprising a partial skull, mandible, and elements of the postcranial skeleton.
A large-bodied, short-necked plesiosaurian specimen from Market Rasen in the collection of William Buckland had already been mentioned in 1824, when short-necked plesiosaurians of this general form were provisionally referred to as Plesiosaurus giganteus. Richard Owen formally erected Plesiosaurus (Pleiosaurus) brachydeirus in 1841 and introduced the genus Pliosaurus the following year. Owen explicitly stated that P. brachydeirus referred to the Market Rasen specimen, making this material central to the taxonomic history of the genus.
There is an important historical discrepancy in the literature concerning the catalogue number of the holotype. Judyth Sassoon and colleagues recognised P. brachydeirus as the type species of Pliosaurus in 2012, but listed OXFUM J.10454 as its type specimen in their systematic palaeontology section. In contrast, the detailed taxonomic revisions by Espen M. Knutsen in 2012 and Roger B. J. Benson and colleagues in 2013 consistently treated OUMNH/OXFUM J.9245 and its associated material as the holotype of P. brachydeirus. The 2013 study further identified this material as the only specimen that could be confidently referred to the species. Under these more recent revisions, J.9245 and its associated remains therefore constitute the appropriate type material for P. brachydeirus.
The individual represented by OUMNH/OXFUM J.9245 was evidently not fully mature when it died. The distal end of the femur retains a rounded margin, and the femoral capitulum and trochanter are not yet clearly differentiated, suggesting a relatively young individual. The 2013 study recorded a mandibular length of approximately 132 cm, substantially smaller than the skulls of many large Late Jurassic species of Pliosaurus. Its comparatively small size therefore appears to reflect incomplete growth rather than the adult dimensions of the species.
Ecologically, the 2013 study interpreted Late Jurassic species of Pliosaurus as a relatively morphologically and ecologically conservative assemblage of macropredators. These animals generally possessed robust skulls, large body sizes, and stout trihedral teeth suited to feeding on relatively large prey. Pliosaurid diversity had declined by the Late Jurassic compared with the Middle Jurassic, leaving an assemblage dominated by these large-bodied macropredatory forms.
Pliosaurus brachydeirus also occupies an early position in the evolutionary history documented for the genus. Comparative studies suggest that the total number of mandibular teeth decreased from the Kimmeridgian toward the Tithonian, while forms with long mandibular symphyses disappeared by the Tithonian. The early Kimmeridgian P. brachydeirus possessed one of the highest mandibular tooth counts documented in the genus together with a relatively long mandibular symphysis, representing an earlier stage in this apparent trend of cranial evolution.
（Author: Shui-Ye You）
References
Benson RBJ et al. (2013). A Giant Pliosaurid Skull from the Late Jurassic of England. PLOS One.
Knutsen EM. (2012). A taxonomic revision of the genus Pliosaurus (Owen, 1841a) Owen, 1841b. Norwegian Journal of Geology.
Sassoon J et al. (2012). Cranial anatomy, taxonomic implications and palaeopathology of an Upper Jurassic Pliosaur (Reptilia: Sauropterygia) from Westbury, Wiltshire, UK. Palaeontology.
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