Aquatic Predatory Tetrapod: Crassigyrinus scoticus

A large aquatic predatory tetrapod from the Early Carboniferous of Scotland
UMZC 2011.9.1, lower jaw portion of cf. Crassigyrinus.

© University Museum of Zoology, Cambridge 2013

© J.A. Clack 2013

Professor Jenny Clack, curator of Vertebrate Palaeontology at the museum, writes about a finding fossils in Scotland:

Crassigyrinus scoticus, whose skull is illustrated at the bottom, has been known from fossils since the early 20th century. It has been found at a few localities in Scotland from rocks about 320 million years old, a time known as the late Visean and early Namurian stages of the Carboniferous period. The animal was over two metres long and its jaws were lined with many large sharp teeth adapted to produce a powerful snap-trap bite.

Last year, my husband and I went to explore some newly discovered fossil localities yielding specimens from an earlier time, about 350 million years ago. In one layer of rock we noticed a blade-like bone exposed at the edge of the rock in part of the cliff-face. The cliff formed the edge of a farmer’s field, and faced the sea, so the rocks were wet, and this particular layer was very crumbly. Very carefully, we picked out what we could of the bone (top image). In the lab, I slowly and with much frustration, consolidated the bone and tried to reassemble the pieces. They would often fall apart in new places as I stuck them together, but in the end, we could recognise the bone, in fact a series of bones. They formed the lower part of the lower jaw of an animal very like known specimens of Crassigyrinus, but 30 million years older (middle two images). Regrettably, the tooth-bearing bones were missing.

This was a very exciting find for a number of reasons. First, it showed that relatives of Crassigyrinus scoticus existed long before we thought they did. This will give us new information about the evolution of the group to which it belongs. Secondly, and more importantly, it forms part of a growing collection of fossils from the same age: the Tournaisian stage of the Carboniferous. Formerly hardly represented by fossils of vertebrates or invertebrates, this time-period has been known as ‘Romer’s Gap’. This was very unfortunate because it was during this time that terrestrial ecosystems were rebuilt following a mass extinction at the end of the Devonian period, and the time when tetrapods became more fully adapted to life on land. Thus this previously poorly known period set the stage for modern ecosystems to diversify.

We now have a large NERC-funded consortium project to study this period and these events intensively for the first time, to find out what happened at the end of the Devonian – why was there a mass extinction? – and how did terrestrial life recover from it? This fossil was one of the initial finds that helped us secure funding for the project. TW:eed project website

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