When the orbit is on top of the skull… Hypsognathus

Earlier we looked at possible eyeball orientations for a basal tetrapod (amphibian) with orbits on top of its skull. The procolophonid, Hypsognathus (Fig. 1) is similar, only the orbit is so large that the eyeball shares the space with bulging temporal muscles.

Figure 1. Hypsognathus restored with an eyeball and jaw muscle filling the orbit.

Figure 1. Hypsognathus restored with an eyeball and jaw muscle filling the orbit. Rather than pointing skyward, the eyeballs probably were still oriented laterally, scanning the horizon, like crocs and frogs. Scale bar = 3 cm.

Even though the skull is quite odd, the eyeballs probably were oriented laterally, to scan the horizon, not the sky. When the orbit and jaw muscles fill the same space, as they also do in basal mammals, every time the jaw muscle moves it changes ever slightly the eyeball shape, either pushing it or relaxing it. Evidently this was not such a big problem for Hypsognathus or basal mammals, because Nature did not correct it. However, when you get to primates, the plate at the back of the primate orbit prevents this interaction because sharp eyesight is paramount for these tree dwellers with binocular vision.

Procolophonids descend from basal diadectomorphs, plant eaters that had to watch out for predators. They produced no descendants.

References
Gilmore CW 1928. New Fossil Reptile from the Triassic of New Jersey. Proceedings of the United States National Museum 73(7):1-8
Owen R 1876. Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum. London, British Museum (Natural History).
Sues H-D, Olsen PE, Scott DM and Spencer PS 2000. Cranial Osteology of Hypsognathus fenneri, a Latest Triassic Procolophonid Reptile from the Newark Supergroup of Eastern North America. Journal of Vertebrate Paleontology, 20(2):275-284.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s