Hynerpeton bassetti (early tetrapod)

Fossil shoulder of Hynerpeton bassetti. Photo courtesy of Ted Daeschler, ANS.

Hynerpeton was the first Late Devonian tetrapod discovered in North America outside of Greenland. It's also one of the earliest known tetrapods found anywhere. Its name means creeping animal from Hyner (Hyner, Pennsylvania).

The fragmentary remains of Hynerpeton were recovered from shallow channel-margin facies at Red Hill. These include the left shoulder bones, a lower jaw, as well as some small skull fragments and belly scutes.

The shoulder bones (cleithrum and scapulacoracoid) of stem tetrapods are distinctive and informative. Those of Hynerpeton were more robust and allowed greater limb mobility than those of other Late Devonian species. Moreover, the anterior border of the Hynerpeton cleithrum lacks the postbranchial lamina found in gill-breathing bony fishes and the two best known stem tetrapods, Acanthostega and Ichthyostega. In other words, Hynerpeton appears to have lacked internal gills.

A tetrapod jawbone located within the same stratigraphic layer but about 50 m away from the shoulder fossil was originally suspected to have also belonged to Hynerpeton. However, another jawbone located only 30 cm from the shoulder was then discovered. This second jawbone was attributed to Hynerpeton, whereas the other specimen was designated as a new, separate tetrapod, Densignathus rowei. Comparisons of the two jaws indicate that Hynerpeton was a more gracile animal than Densignathus and that the two species probably occupied different feeding niches.

Fossil jaw attributed to Hynerpeton bassetti. Photo courtesy of Ted Daeschler, ANS.

The subsequent discovery of an isolated humerus in the same lens has called into question the attribution of the jaw (and an isolated skull fragment) with the Hynerpeton shoulders. These attributed elements agreed well in terms of predicted size, but the new humerus is at least 50% greater in size than would be predicted on the basis of the shoulders.

The discovery of Hynerpeton was 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.

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Scientific Papers:: Daeschler, E.B., N.H. Shubin, K.S. Thomson and W.W. Amaral. 1994. "A Devonian tetrapod from North America." Science 265: 639-642.; Daeschler, E.B. 2000. "Early tetrapod jaws from the Late Devonian of Pennsylvania, USA." J. Paleont. 74(2): 301-308.
Image Credits:: The reconstruction of Acanthostega is copyrighted ©. (See Terms of Use.) It was based on the work of Jennifer A. Clack.

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