Saturday, November 20, 2010

First Fangs

Uatchitodon kroehleri
Jonathan Mitchell, from the University of Chicago, and colleagues (2010) have described what appear to be the oldest known fangs for venom delivery in a tetrapod. The fossilized fangs have been found at three sites in the USA: Tomahawk in Virginia, the oldest of the three sites; Moncure in North Carolina; and the Placerias Quarry in Arizona. Mitchell et al. analyzed teeth from the three locations and found the fossils from Tomahawk had grooves instead of fully developed enclosed canals. The venom conducting canal extended from the base of the tooth to about a quarter of the way to the tip. Slightly older teeth from the site had a longer, deeper groove. Specimens from Moncure and the Placerias Quarry had the groove completely enclosed and there was a seam suggesting the tooth may have been folded to form the canal. Fourteen grooved Uatchitodon teeth were recovered from Tomahawk and another 26 were found at Moncure and Placerias with fully developed canals. The teeth are about 10 mm long. The authors suggest two species are represented by these teeth. Hans-Dieter Sues, one of the authors on this paper had previously named the grooved- tooth species Uatchitodon kroehleri in 1991 and the species with the canalized teeth is named Uatchitodon schneiderii in this paper. Uatchitodon teeth are quite distinctive in that they possess compound serrations and two venom conducting channels on each tooth. Uatchitodon kroehleri has one groove on the labial (outer) surface and another on lingual (inner) surface of the tooth. The grooves form deep invaginations constricting the tooth’s pulp cavity. So what kind of animal was Uatachitodon? Clearly it was not a snake. It was most likely carnivorous, and the authors suggest it is an archosauromorph. Archosauriformes are thought to have evolved as semi-aquatic predators (Family Proterosuchidae) in Pangea during the late Permian. After the Permian extinction event which killed off 95% of all life, about 251 million years ago the large, dominant therapsid reptiles disappeared and allowed the proterosuchids to radiate into top carnivores. Within five million years, in the Triassic the proterosuchids had evolved into a wide variety of terrestrial and semi-aquatic carnivores, niches previously held by the therapsids. Since Uatachitodon is known only from its teeth, placement in the Archosauriformes is quite speculative.However, the parallel between Uatachitodon fang evolution and snake fang evolution is striking. Grooved teeth in an older species and canalized teeth in a later species. Snakes can have no fangs, solid rear fangs, grooved rear fangs, front fangs with canals, and solid front fangs, and in at least one species grooved rear fangs and solid front fangs. Thus, snakes and Uatachitodon appear to have evolved fangs in the same way, solid teeth become grooved, grooved teeth become folded and form a central canal for conducting venom to the venom aperture under high pressure so that venom can be injected into prey.

Mitchell, J. S., A. B. Heckert, and H.-D. Sues. 2010. Grooves to tubes: evolution of the venom delivery system in a Late Triassic “reptile”. Naturwissenschaften DOI 10.1007/s00114-010-0729-0

Sues, H.-D. 1991. Venom-conducting teeth in a Triassic reptile. Nature 351: 141–143. doi:10.1038/351141a0

No comments:

Post a Comment