Enigmatic Triassic Hellasaur Thursday — The mostest unkindestest cut

8 August 2008

Venom–toxic fluid injected to subdue prey or deter potential predators–is widespread in the animal kingdom, from jellyfish to scorpions to platypodes. A case could even be made that stinging nettle is an example of a venomous plant, since it injects its toxin into victims. However, most toxic plants, as well as toxic animals and fungi that rely on passive delivery of toxins (e.g. newts) are considered poisonous but not venomous.

Snakes are one of the most familiar groups of venomous animal although a majority of snakes lack venom. Most people are also aware of the venomous beaded lizards (or, “gila monsters”) in the genus Heloderma. Far less well known is that varanid monitor lizards and bearded dragon, Pogona, popular in the pet trade, also possess a mild venom. We’re talking real venom here, not the bacterial brew that produces the much discussed septic bite of some varanid lizards. In fact, the discovery that venom occurs in reptiles aside from snakes and Heloderma was made only a few years ago and has forced us to rethink the evolutionary origins of venom among squamates (Fry et al. 2006).

So, what does any of this have to do with enigmatic Triassic hellasaurs?

Uh, good question. Nothing really, except as an introduction to one of the most intriguing, and poorly known, ETHs of all.

Meet Uatchitodon… ah, the name just rolls right off the tongue doesn’t it? Named for the Egyptian cobra goddess Wadjet, Uatchitodon is known only from isolated, but distinctive teeth from upper Triassic sediments of Virginia (Sues 1991) and Arizona (Sues 1996).

The two forms vary a bit in their morphology. The Virginian type species, Uatchitodon kroehleri, possesses two deep, tapered grooves on the labial and lingual faces of the tooth. The slightly younger Arizonan fossils, from the enigmatic hellasuar lousy Chinle Formation, have entirely enclosed canals with oval exits near the tip of the tooth and likely represent a second, undescribed species (Sues 1996).

Figures 1 and 2, crudely adapted from Sues (1991, 1996), give a general sense of the tooth morphology of the two forms. They are presented neither to scale nor with any reasonable measure of anatomical accuracy.

Sues (1991) hypothesized that the unusual channels in the teeth were venom conduits analogous to those seen in the venom-conducting teeth of snakes and Heloderma. Of course, dental grooves do not an envenomator make, and claims of venomous fossil taxa are frequently met with substantial skepticism (Folinsbee et al. 2007). Still, the grooves seen in Uatchitodon are strikingly similar to those seen in venomous squamates and, to my knowledge, an alternate function for these teeth has not been advanced.

The most interesting feature of both forms is that they retain serrated cutting edges suggesting dual sectorial and envenomation function. This kind of exaptive multifunctionality illustrates precisely the kind of evolutionary transition that “half-a-wing” creationists swear is impossible. Turns out, they’re wrong. In fact, in living reptiles (and some venomous mammals like solenodons), venom is created by tricked out salivary glands and if Uatchitodon was venomous, we can reasonably assume that it did the same.

Venomous or not Uatchitodon is probably neither closely related to snakes nor Heloderma. True to enigmatic hellasaur form, no one is exactly sure what Uatchitodon was.  Archosauriform?  Maybe.   Reptilia incertae sedis? Sure, for now.  Totally metal hellasaur screaming through the black mists of time? Abso-effing-lutely.

And that’s about all there is to say about that.

REFS:

Fry, B. G. et al. (2006). “Early evolution of the venom system in lizards and snakes”. Nature 439:584–588.

Folinsbee K., Muller J., Reisz R. R. (2007). “Canine grooves: morphology, function, and relevance to venom Journal of Vertebrate Paleontology 27:547-551.

Sues, H. D. (1996). “A reptilian tooth with apparent venom canals from the Chinle Group (Upper Triassic) of Arizona.” Journal of Vertebrate Paleontology, 16:571-572.

Sues, H. D. (1991) “Venom-conducting teeth in a Triassic reptile” Nature 351:141–143.

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