Pliosaurus carpenteri
Age
Jurassic（Kimmeridgian）
152-151 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Plesiosauria
Family: Pliosauridae
Genus: Pliosaurus
Species: Pliosaurus carpenteri
Morphological description
Pliosaurus carpenteri was a giant pliosaurid known from a specimen with a skull and mandible approximately 1.8 m long. Its most distinctive feature occurs in the surangular: the dorsal surface is broad, faces dorsally, and lacks the groove or fossa present in most other Late Jurassic species of Pliosaurus. This condition represents an autapomorphy of the species. Its parietal crest is relatively low, contrasting with the tall, anteroposteriorly extensive crest of Pliosaurus westburyensis.
The teeth are fully trihedral, with a broad, flattened labial surface lacking enamel ridges. The premaxilla bears six closely spaced alveoli, with the distalmost premaxillary alveolus reduced relative to the more mesial alveoli, producing an anisodont premaxillary dentition. The caniniform regions of the premaxilla and maxilla are markedly expanded mediolaterally. A diastema separates the premaxillary and maxillary tooth rows. The dentary contains 27 alveoli in total, including nine within the mandibular symphysis and 18 postsymphysial alveoli. In the well-preserved symphysial region, the pattern of alveolar size reverses around the sixth alveolus, and the two largest symphysial alveoli are approximately 5 cm in mesiodistal length.
The premaxilla–parietal suture is located approximately level with the anterior region of the orbit and probably extends posteriorly to a point anterior to orbital midlength, although damage to the orbital and interorbital regions prevents this condition from being confirmed with certainty. The internal naris is located at approximately the level of the ninth maxillary alveolus. Posteriorly, the paired squamosals form a broad, posteriorly projecting convexity. The surface of the occipital condyle is marked by several coarse grooves and lacks a notochordal pit, while the basal tubera of the basioccipital project primarily laterally or slightly ventrolaterally. The cervical centra lack a ventral longitudinal ridge, and the proximal articular surfaces of the radius and tibia are strongly convex.
Etymology
The generic name Pliosaurus is derived from the Greek pleion, meaning "more," and sauros, meaning "lizard."
The specific name carpenteri honors Simon Carpenter, who discovered and collected the holotype specimen, BRSMG Cd6172.
Biological description
Pliosaurus carpenteri is currently known from a single confirmed specimen, the holotype BRSMG Cd6172. The fossil was recovered from the Westbury Clay Pit in Wiltshire, England, from subdivision E4 of the Kimmeridge Clay Formation. It dates to the late Kimmeridgian of the Late Jurassic. The specimen was found approximately 7 m below the Crussoliceras Limestone. In 2013, Roger B. J. Benson and colleagues formally established the specimen as representing a new species.
The fossils of Pliosaurus carpenteri and Pliosaurus westburyensis were both recovered from the Westbury Clay Pit and occur at closely spaced stratigraphic levels. Their anatomical differences had previously been interpreted as possible intraspecific variation, and it was even suggested that the two specimens might represent sexual dimorphism. After comparing them with Pliosaurus specimens from other localities, Benson and colleagues concluded that the magnitude of the differences between the two Westbury specimens was sufficient to justify their recognition as separate species.
In the phylogenetic analysis, Pliosaurus carpenteri was consistently recovered as the sister taxon of Pliosaurus funkei from Svalbard. Another relatively stable sister-taxon pairing consisted of Pliosaurus kevani and the specimen CAMSM J.35990.
BRSMG Cd6172 represents a large individual, yet its vertebral neurocentral sutures remain unfused. In reptiles, the degree of neurocentral fusion is often used as an indicator of skeletal maturity. However, the same condition occurs in other very large thalassophonean pliosaurids. This suggests that neurocentral fusion may have occurred extremely late in development, or perhaps never became complete in these animals. An alternative possibility is that even very large individuals such as BRSMG Cd6172 had not yet reached full skeletal maturity. Because these possibilities cannot presently be distinguished, neurocentral sutural fusion cannot be regarded as a reliable indicator of ontogenetic stage in Thalassophonea.
Ecologically, Pliosaurus carpenteri belonged to a highly specialized assemblage of large marine predators during the Late Jurassic. Late Jurassic species of Pliosaurus are characterized by robust skulls, large body sizes, and trihedral teeth, features associated with macropredatory feeding habits. Most Late Jurassic Pliosaurus skulls fall within a length range of approximately 1.7–2.1 m, placing P. carpenteri firmly within this giant-bodied size class. The study also noted that adult specimens of large Late Jurassic pliosaurids suggest estimated total body lengths of approximately 10–12 m.
（Author: Shui-Ye You）
References
Benson RBJ et al. (2013). A Giant Pliosaurid Skull from the Late Jurassic of England. PLOS One.
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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