reconstruction of Ichthyostega Reconstruction of Ichthyostega, a Late Devonian relative of Densignathus ©

Densignathus rowei (early tetrapod)

Lower jaw bones belonging to a tetrapod were recovered from Red Hill about two years after the discovery of shoulder bones of Hynerpeton bassetti. The jawbones were found at about the same stratigraphic level, but were seperated from the Hynerpeton location by 50 m.

Whether these jaws belonged to Hynerpeton or to a new tetrapod wasn't answered until fossil preparation of rocks collected only 30 cm from the original Hynerpeton material revealed another, distinct lower jaw that probably came from Hynerpeton. It became clear that Red Hill had at least two species of tetrapods. It was also likely that they were contemporaries.

The name Densignathus (=thick jaw) refers to the robust qualities of the lower jaw. It's a third again as long as the Hynerpeton jaw. It's also substantially more robust and more heavily-toothed. The differences in jaw morphology between these two tetrapods suggest some degree of specialization between them with respect to prey preference and capture methods.

fossil of densignathus jaw Fossil jaw of Densignathus. Photo courtesy of Ted Daeschler, ANS.

The discovery of Densignathus is one of several recent findings that has forced a major re-evaluation of how tetrapods evolved and colonized the land. The most important of these findings involve Acanthostega, an early tetrapod from Eastern Greenland. These findings have upset the old order. Two other tetrapods, Hynerpeton and the Red Hill humerus, have also been found at Red Hill.

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Scientific Papers:
Daeschler, E.B. 2000. "Early tetrapod jaws from the Late Devonian of Pennsylvania, USA." J. Paleont. 74(2): 301-308.
Image Credits:
The Ichthyostega reconstruction at the top is copyrighted, © Dennis C. Murphy, 2002. (See Terms of Use.) It's based on work by Jennifer A. Clack.

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