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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.
Literature
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