Friday, 23 December 2011

Crocs at Christmas

It occurred to me the other day that despite my extensive coverage of the Oxford Clay Formation and the abundant vertebrate fauna that inhabited these warm shallow tropical seas, I have actually not devoted any reasonable coverage to the marine crocodiles that shared the environment with the better known ichthyosaurs, plesiosaurs and pliosaurs.

There are currently two genera represented in the clay and these are Steneosaurus (the superb specimen above is from Holzmaden) and Metriorhynchus. These are both thalattosuchians that share their ancestry with the crocodiles and alligators of today and so belong to an ancient and very successful group whose origins can be traced back to the Latest Triassic – around 200 million years ago.
The early crocodiles were small and essentially land living animals. Eventually they evolved into water dwelling genera, the mesosuchians, which spread rapidly and became very successful, filling a variety of niches. These mesosuchians eventually gave way to more derived forms and some of these returned to the land while others went to sea.
Steneosaurus is a very familiar looking animal and could easily be mistaken for a gavial of today and it seems likely that it would have shared a very similar mode of life to its extant cousin. Approaching 4 metres in length, Steneosaurus was well adapted to a marine life. The skull was lightly constructed and the rostrum was elongate, narrow and there were over 40 teeth in both the upper and lower jaws. These teeth were conical, sharp and were ideal for catching fish and invertebrates that shared the environment although some specimens also display blunter teeth that may have been suited to dealing with more robust prey.
The limbs of Steneosaurus were typically crocodilian, as was the upper torso of the animal, which was covered with a layer of ornate protective osteoderms.  The tail, again, was extremely crocodilian, being long and powerful, and was clearly a significant propulsion unit for swimming.
Two species of Steneosaurus are generally accepted to have been part of the Oxford Clay fauna and these are S. leedsi and S. durobrivensis. The fact that their fossils are found in these marine sediments tends to confirm a gavial-like existence and Steneosaurus probably inhabited the coastal shores and the mouths of rivers that emptied into the sea.

The other crocodile that is found in the Oxford Clay is Metriorhynchus and this genus is much more robust than Steneosaurus, although not as long, and is fully adapted for life in the sea.  About 2 to 3 metres in length, the limbs of Metriorhynchus were fully developed flippers and were unsuitable for life on land. Of course, it would seem logical to assume that Metriorhynchus would have returned to shore to lay eggs but now, with evidence that plesiosaurs gave birth to live young, perhaps we should not so readily make such an assumption.

The tail is a little similar to an ichthyosaur tail in as much as the caudals bend downward to create a proper fish-like tail and this animal was obviously a very efficient swimmer. Metriorhynchus completely lacks the dermal ossifications of Steneosaurus and this is always the best identifier when a crocodile skeleton is first revealed in the clay. The skull is still typically long but the rostrum is much broader than in Steneosaurus, and looks more crocodilian than gavialan, and is also much more rugose.
a) Metriorhynchus b) Steneosaurus
From Martill & Hudson 1991
Metriorhynchus was obviously a highly developed marine hunter that probably preyed on fish, belemnites and any other creature it could adequately despatch. Some specimens display contents in the stomach region and these often include belemnite hooklets and also, in one exceptional specimen, the remains of a pterosaur have been identified, probably a carcass that was floating on the sea that was snapped up by the passing croc. Two species of Metriorhynchus are currently recognised from the clay and these are M. brachyrhynchus and M. superciliosus.
Marine crocodiles are often passed over in favour of the more famous reptilian inhabitants of the Callovian Sea but they are as equally fascinating and not to be underrated. Indeed, I have just been very privileged to see two skulls from these animals undergoing conservation at the Natural History Museum in London – both from the Leeds collection and what big skulls they are! Steneosaurus may have been gavial-like in existence but this was a serious hunter and could obviously take quite large prey. Some of its teeth are much bigger than the ones I’ve seen from clay and this was an eye opener for me. The skull of Metriorhynchus is massively constructed and obviously powerful – indeed I was surprised how massive it was – and it appears that it too would be very capable of tackling large prey.
This is a very brief introduction to these superbly adapted animals and I would like to think that we will be able to extract another specimen of these superb crocodilians from the Oxford Clay at some point in the future.
Season’s Greetings!
I would just like to take the opportunity to wish all my friends, colleagues and all of you who have taken the time to read my blog this year a very happy Christmas and hope you all enjoy a wonderful festive season.
Martill, D.M & Hudson, J.D 1991. Fossils of the Oxford Clay. The Palaeontological Association, London, UK


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