Cranial Anatomy and Functional Morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Upper Jurassic of Westbury, Wiltshire

Michael A. Taylor , A. R. I. Cruickshank

Abstract

An exceptionally complete skull, mandible and other bones of Pliosaurus brachyspondylus were collected from the Kimmeridge Clay of Westbury, Wiltshire, in 1980. The recovery and preparation of this large specimen required special techniques. The specimen is apparently part of a more complete skeleton, mostly destroyed before discovery. The decayed carcass was apparently disrupted so that the skull finally lay upside down over many of the teeth, which had fallen out, while the mandible lay several metres away. The reasons for this are unclear. The skull does not differ markedly from the usual pliosauroid pattern, being long and low, with a wide gape, narrow snout, and high temporal region. There are no nasals. The mandible cannot be satisfactorily reconstructed due to crushing but does not appear to deviate from the usual pliosauroid pattern. The dentition is robust and caniniform anteriorly, presumably to penetrate, hold and kill large prey. The posterior teeth are hook-shaped posteriorly to act as ratchets, helping to move large prey items back into the gullet. The jaw musculature is reconstructed as a dual-function system, the pterygoideus musculature being specialized to close the open jaws rapidly against inertia and drag, and the main adductor mass being specialized to clamp the jaws tightly onto prey. The cranial skeleton is well adapted to resist bending stresses induced when the animal bit onto prey. However, there is no evidence for any adaptation to torsional resistance, such as a pterygoid flange-mandible contact, as would be useful in twist-feeding to dismember large prey. Pliosaurus, at about 10 m overall length, may have been large enough to swallow most potential prey without being particularly specialized to dismember it. Its wide gape would help it swallow large prey. However, the comparatively narrow anterior snout, and evidence from gut contents in other specimens, suggest that it was an opportunistic feeder on a wide variety of prey of different sizes, including cephalopods and presumably fish and other reptiles. Large orbits and the lack of acoustically isolated ears indicate that it was primarily a visual hunter. The nares seem too small to be used in respiration, and may instead have been used in underwater olfaction.