Red Hilll humerus skeleton

Red Hill humerus (ANSP 21350)

An isolated, but well preserved tetrapod humerus (upper arm) was recovered from Red Hill in 2002. The specimen (ANSP 21350) was removed from the same lens as Hynerpeton bassetti and same stratigraphic layer as Densignathus rowei, but can't be confidently identified as either of these two tetrapods.

The detailed comparison of ANSP 21350 with the humeri of a tristocopterid lobe-fin (Eustenopteron), rhizodont lobe-fins, an elipistotegian lobe-fin (Panderichthys) and several stem tetrapods (Acanthostega, Ichthyostega and Tulerpeton) has revealed further diversity in tetrapod forelimb morphology. ANSP 21350 shares many of the derived features of the other stem tetrapod humeri. On the other hand, it apparently retained the pronounced ventral ridge that's found on the humeri of tristocopterids but is not evident on Panderichthys and other stem tetrapods. ANSP 21350 also generally exhibits more robust and elaborate sculpting, particularly on the ventral surface, and the articular surfaces for the radius and ulna larger are oriented much more perpendicularly than those in Acanthostega and Pandericthys (i.e., more permannet flexion of the elbow). The nature of these articulations and the more distal muscle attachment sites indicates that the ANSP 21350 tetrapod possessed greater distal (forearm) mobility than did Acanthostega.

This comparison has also provided some insights on the evolution of the forelimb. Essentially, a significant transformation of the humerus and shoulder girdle was initiated in a common ancestors of Panderichthys and the stem tetrapods and elaborated upon in the stem tetrapods. The humerus was flattened dorsal-ventrally and muscle attachment on the dorsal surface were enhanced as was the muscle attachment in the corresponding regions of the shoulder. The dorsal-ventral flattening suggests a spatial separation of the muscles raising or lowering the humerus. This, along with the horizontal elongation of the articulation between the humerus and the shoulder, probably reduced rotation of the humerus. In short, the humerus became less mobile forward and back, and extended horizontally away from the body. Instead of a fin used for maneuver and braking, the forelimb became better suited to bear weight. The animals could do push-ups.

The enhanced ability to bear weight has obvious implications for walking on land. However, the new humeri would probably provide very limited forward movement. Consequently, these animals would have been a very poor walkers. Instead, this new weight-bearing ability probably served to prop-up the trunk and head in shallow (and less buoyant) water. It may have also helped maintain the animal's position in the current.

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Magazine:
Magazines:National Geographic. April 2004. "Fins to Limbs: New Fossils Gives Evolution Insight. "
Scientific Papers
Clack, J.A. "Enhanced: From Fins to Fingers." Science 304:57-58.
Shubin, N.H., E.D. Daeschler, and M.I. Coates. 2004. "The Early Evolution of the Tetrapod Humerus." Science 304: 90-93.

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