''Pachycostasaurus'' (meaning 'thick-ribbed lizard') is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

Pliosauroid Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive tooth ...

from the

Oxford Clay The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specifical ...

formation of

Peterborough Peterborough () is a cathedral city in Cambridgeshire, east of England. It is the largest part of the City of Peterborough unitary authority district (which covers a larger area than Peterborough itself). It was part of Northamptonshire until ...

, England.

History and naming

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

fossil of Pachycostasaurus was discovered by Alan Dawn, an amateur geologist and museum volunteer, in what is now a quarry in

. The fossil was described in 1997 by Palaeontologists Arthur Cruickshank, David Martill and Leslie Noè, due to its distinct set of features not present in contemporary pliosaurids ''

Liopleurodon ''Liopleurodon'' (; meaning 'smooth-sided teeth') is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. ''Liopleurodon'' lived from the Callovian Stage of the Mi ...

'' and '' Simolestes.''

Description

The type species, ''P. dawni'', is represented by a single near complete specimen (PETMG R338) which was approximately long and weighed . Pachycostasaurus recon

Perhaps the most defining feature of this genus is the large, pachyostic ribs and

gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these ...

described as ‘sausage-like’ in profile.

Pachyostosis Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification (osteosclerosis), reducing inner ca ...

is known from a number of Pliosaurid genera (for example ''

Monquirasaurus ''Monquirasaurus'' is a monotypic genus of giant pliosaurid from the Cretaceous Paja Formation in Colombia. It contains a single species, ''M. boyacensis'', previously known as ''Kronosaurus boyacensis''. History and naming The holotype and only ...

''), and is present in modern animals such as

Sirenian The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

s (

Dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...

and

Manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...

s). This robust, barrel shaped torso, coupled with a relatively short neck and small flippers, indicate that it was not an agile swimmer.A. R. I. Cruickshank, D. M. Martill, and L. F. Noe. 1996. A pliosaur (Reptilia, Sauropterygia) exhibiting pachyostosis from the Middle Jurassic of England. Journal of the Geological Society of London 153:873-879 Histological sections taken from these elements show that they are osteosclerotic as well (combined, this represents the condition

pachyosteosclerosis Pachyosteosclerosis is a combination of thickening (pachyostosis) and densification (osteosclerosis) of bones. It makes bones more heavy, but also more fragile. The condition often occurs in aquatic vertebrates, especially those living in shallow w ...

). The holotypic specimen possessed five stout and conical tooth pairs on the

mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral halves ...

(the length of which is a clear characteristic of the genus in comparison to other contemporary Pliosaurids '' Simolestes'' and ''

''). 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 a

Paedomorphic Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compared ...

trait.) 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. The

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and posterior Zygapophyses 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 of ''Pachycostasaurus'' are short, small compared to its body size, and lightly constructed. Pachycostasaurus_front

It has been suggested that even though the holotypic individual appears to be a juvenile or sub-adult, its age is probably not solely responsible for these unusual bone structures.Street, Hallie P. "A study of the morphology of Tatenectes laramiensis, a cryptocleidoid plesiosaur from the Sundance Formation (Wyoming, USA)." (2009).

Palaeobiology

Judging by its anatomy, ''Pachycostasaurus'' was most probably a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

predator. The relative size and shape of its teeth indicate a diet ranging from soft teuthoids, such as belemnites, to

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

, 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 if ''Pachycostasaurus'' was a twist feeder like other contemporary pliosaurids. The rarity of the animal in the formation may suggest that ''Pachycostasaurus'' was

allochthonous River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts.Angelier ...

Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

remains from the quarry were likely swept by freshwater in to the marine basin present, implicating that the holotype specimen may have been a

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

estuarine An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

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 that ''Pachycostasaurus'' was a

deep sea The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...

inhabitant from further offshore, and drifted or travelled to the shallow marine location before

fossilisation A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved i ...

References

{{Taxonbar, from=Q3888477 Jurassic plesiosaurs of Europe Oxford Clay Sauropterygian genera Fossil taxa described in 1997

History and naming

Description

Palaeobiology

See also

References