| Pachycostasaurus Temporal range:Middle Jurassic | |
|---|---|
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| Holotype fossil,Peterborough Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Suborder: | †Pliosauroidea |
| Family: | †Pliosauridae |
| Genus: | †Pachycostasaurus Cruickshank et al., 1996 |
| Species: | †P. dawni |
| Binomial name | |
| †Pachycostasaurus dawni Cruickshank et al., 1996 | |
Pachycostasaurus (meaning 'thick-ribbed lizard') is anextinctPliosauroid from theOxford Clay formation ofPeterborough, England.
Theholotype fossil ofPachycostasaurus was discovered by Alan Dawn, an amateur geologist and museum volunteer, in what is now a quarry inPeterborough. The fossil was described in 1996 by Palaeontologists Arthur Cruickshank, David Martill and Leslie Noè, due to its distinct set of features not present in contemporary pliosauridsLiopleurodon andSimolestes.
The type species,P. dawni, is represented by a single near complete specimen (PETMG R338) which was approximately 3 metres (9.8 ft) long.[1]
Perhaps the most defining feature of this genus is the large, pachyostic ribs andgastralia described as ‘sausage-like’ in profile.Pachyostosis is known from a number of Pliosaurid genera (for exampleMonquirasaurus), and is present in modern animals such asSirenians (Dugong andManatees). This robust, barrel shaped torso, coupled with a relatively short neck and small flippers, indicate that it was not an agile swimmer.[1] Histological sections taken from these elements show that they areosteosclerotic as well (combined, this represents the conditionpachyosteosclerosis).
The holotypic specimen possessed five stout and conical tooth pairs on themandibular symphysis (the length of which is a clear characteristic of the genus in comparison to other contemporary PliosauridsSimolestes andLiopleurodon). Its head was about 20% of overall body length, typical for most Pliosaurids of its time. The specimen had a cervical vertebral count of at least 13, these vertebrae having shortened centra and an enlarged neural canal, which were not heavily ossified except in the region of the neural spine (the neural spines themselves were not fused to the centra, which suggests that the specimen was not fully grown; however, this could also be aPaedomorphic trait.[2]) The pectoral centra of the specimen are triangular in outline, and the dorsal vertebral centra are heavily ossified, with indistinct subcentral foramina and flat anterior and posterior faces. Theanterior andposteriorZygapophyses on the dorsal vertebrae are reduced, however the neural spines thickened, with midline anterior and posterior excavations. This is one of the other main characteristics of the genus. The humeri ofPachycostasaurus are short, small compared to its body size, and lightly constructed.[1]
It has been suggested that even though the holotypic individual appears to be ajuvenile orsub-adult, its age is probably not solely responsible for these unusual bone structures.[3]
Judging by its anatomy,Pachycostasaurus was most probably abenthic predator. The relative size and shape of its teeth indicate a diet ranging from soft teuthoids, such asbelemnites, tobony fish, and perhaps some of the larger reptiles.Pachycostasaurus has a lightly constructed skull, a short, rather weak jaw symphysis, and ventral ballast for stability which would have resisted roll. Thus it is doubtful ifPachycostasaurus was a twist feeder like other contemporary pliosaurids.[1]
The rarity of the animal in the formation may suggest thatPachycostasaurus wasallochthonous.Dinosaur remains from the quarry were likely swept by freshwater in to the marine basin present, implicating that the holotype specimen may have been afreshwater /estuarine inhabitant drifted in to the marine environment by rivers. However, more coastal and quasi-marine sediments of the formation have yet to yield any plesiosaur remains. Another, perhaps less likely explanation is thatPachycostasaurus was adeep sea inhabitant from further offshore, and drifted or travelled to the shallow marine location beforefossilisation.[1]
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