Life in the Oxford Clay sea

Following the metriorhynchid post a few people have asked for more details on the Oxford Clay fauna. Luckily the accompanying image shows you all you need to know :) [for the same image at much better resolution visit the relevant page at my flickr site]. Note that it’s a CARTOON (in fact it was deliberately made to go on the front of a t-shirt). The term Oxford Clay refers to a series of mudrocks, formally grouped together as the Oxford Clay Formation, laid down during the Callovian and early Oxfordian of the Middle Jurassic, a time when shallow tropical seas covered much of Europe. Layers of Oxford Clay several hundred metres thick were deposited over Britain and northern France. Judging by the huge number of fossils discovered in the unit, organic productivity was extremely high and rich sources of terrestrially-derived nutrients must have been discharged into the sea. Pretty much everything you’d want to know about the Oxford Clay Formation is included in Martill & Hudson (1991).

The most abundant macrofossils of the Oxford Clay Formation are ammonites and belemnites. Fishes were also diverse and abundant – there were a number of sharks and other chondrichthyans but also bony fishes, and among them is probably the biggest bony fish of them all: Leedsichthys. In the picture here it looks like an immense tuna with a sort of little bony cap on its head, and this is because I followed the very odd life restoration produced by Martill (1985). How big Leedsichthys was has been controversial, but last I heard it was definitely over 10 m long.

Enough of the non-tetrapods. Marine reptiles are what make the Oxford Clay really interesting, and they’re represented by three groups: thalattosuchian crocodilians, plesiosaurs and ichthyosaurs. Thalattosuchians – the ‘sea crocodiles’ – are represented by both of their sub-groups, the amphibious teleosaurids and the fully aquatic metriorhynchids. Unlike metriorhynchids, teleosaurids possessed dorsal osteoderms. Their limbs, though proportionally small, were apparently not unlike those of extant crocodilians so they could likely still use them to move around on land. Two metriorhynchids (both coloured black) are in the scene, while the teleosaurid Steneosaurus (the long-snouted gharial-like crocodilian) swims at centre-left.

Plesiosaurs included both long-necked and short-necked forms. The latter, generally known as pliosaurs, include the huge scary macropredator Liopleurodon, shown at top right biting an ichthyosaur to death. Its immense long-jawed skull and huge, subconical caniniform teeth were well suited for predation on other marine reptiles, and numerous Oxford Clay reptiles exhibit bite marks that match Liopleurodon teeth (Anderson 2005). As in the case of Leedsichthys, the total length reached by Liopleurodon has been controversial. It definitely got to 6 m (Noè et al. 2003), and perhaps to 10 m, with some unpublished bits and pieces hinting at lengths of 15 m or so (McHenry et al. 1996, Naish et al. 2001).

A smallish, gracile-snouted pliosaurid, Peloneustes, is shown at far left in the scene. There are some indications that Peloneustes had particularly big wing-like paddles (Bakker 1993), but further study is needed to confirm this. Its slim snout suggests that it wasn’t a macropredator. The small, short-necked plesiosaur near the sea floor is the pachyostotic pliosaurid Pachycostasaurus, described by Cruickshank et al. (1996). Only known from one juvenile specimen that would have been about 3 m long, it’s poorly known but seems to have been a specialised bottom-cruising form. It might then have preyed on benthic animals, such as burrowing shrimps. The adjacent image of a Peloneustes skeleton is borrowed from Palaeos.com.

Long-necked plesiosaurs are represented in the Oxford Clay by Cryptoclidus, Muraenosaurus and a few others. Cryptoclidus had numerous gracile teeth and has usually been interpreted as a predator of small prey like little fish or crustaceans and is relatively well known thanks to recent redescriptions (Brown 1981, Brown & Cruickshank 1994). Views on Muraenosaurus have changed recently. Conventionally imagined as a dainty-headed predator of small prey, and an early elasmosaurid, new specimens show that it was quite robust-skulled with features suggesting that its head was well suited for handling fairly large prey (M. Evans, data presented at SVPCA). Rather than being an elasmosaurid, it may in fact be a close relative of the cryptoclidids (O’Keefe 2001).

Finally, ichthyosaurs. Only thunniform Ophthalmosaurus – named for its immense eyes – is known from the Oxford Clay, though jaw fragments sporting big teeth have been suggested to belong to a second genus by some workers. However, these fragments might belong to Ophthalmosaurus as, while usually characterised as toothless, there are now indications that this wasn’t so. The huge eyes suggest that Ophthalmosaurus was a deep-diver, perhaps hunting well offshore for deep-sea cephalopods. The adjacent image is taken from the Saurier Museum Rundgang site.

And I could say more but I tried to be as brief as possible. Coming next: Why putting your hand in a peccary’s mouth is a really bad idea. For the latest news on Tetrapod Zoology do go here.

Refs - -

Anderson, K. 2005. A new system of classifying bite marks on marine reptile bones from the Oxford Clay, Peterborough. The Quarterly Journal of the Dinosaur Society 4 (3), 12-15, 28.

Bakker, R. T. 1993. Plesiosaur extinction cycles - events that mark the beginning, middle and end of the Cretaceous. In Caldwell, W. G. E. & Kauffman, E. G. (eds) Evolution of the Western Interior Basin: Geological Association of Canada, Special Paper 39, 641-664.

Brown, D. S. 1981. The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria. Bulletin of the British Museum of Natural History (Geology Series) 35, 253-347.

- . & Cruickshank, A. R. I. 1994. The skull of the Callovian plesiosaur Cryptoclidus eurymerus, and the sauropterygian cheek. Palaeontology 37, 941-953.

Martill, D. M. 1985. The world’s largest fish. Geology Today 2, 61-63.

- . & Hudson, J. D. 1991. Fossils of the Oxford Clay. The Palaeontological Association, London.

McHenry, C., Martill, D., Noè, L. & Cruickshank, A. 1996. Just when you thought it was safe to go back to the water - the biggest pliosaur yet. Palaeontology Newsletter 32, 22.

Naish, D., Noè, L. F. & Martill, D. M. 2001. Giant pliosaurs and the mysterious ‘Megapleurodon’. Dino Press 4, 98-103.

Noè, L., Liston, J. & Evans, M. 2003. The first relatively complete exoccipital-opisthotic from the braincase of the Callovian pliosaur, Liopleurodon. Geological Magazine 140, 479-486.

O’Keefe, F. R. 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213, 1-63.