Phytosaurs: The flesh-eating "plant saurians"

On the occasion of our third anniversary my wife bought me one of my most favorite works of paleo-art; a scene from the late Triassic set in what is now Arizona's Petrified Forest National Park by Douglas Henderson. The painting is dominated by the gavial-like phytosaur Rutiodon, and seeing the framed print up on the wall reminded me of a question I have wondered about for quite a while. Why were these sharp-toothed creatures that fed on other animals dubbed phytosaurs, or (roughly) "plant reptiles"?

As anyone familiar with paleontology knows, complete skeletons, or even complete bones, are rare things. For every complete bone there are innumerable bone fragments and broken teeth, and it is exceedingly difficult to reconstruct an entire animal (much less details of their biology) from a few scraps. This was the dilemma faced by the early 19th century German paleontologist George Jaeger in 1828.

Two years earlier, in 1826, peculiar fossils were discovered in Wurtemberg, Germany. The remains consisted of parts of the skull and jaws of some unknown kind of reptile, and in the jaws were casts of what appeared to be teeth. It would be these casts that would give the fossil its name.

While most people think of bones when they hear the word "fossil" there are many other kinds of fossils, including casts and molds. Perhaps the most common examples of these fossil types were made by shells. In such a case the shell was covered by sediment but the actual shell dissolved away after the sediment hardened. This left a mold preserving the shape of the shell, and if that mold was filled with new sediment then a cast could be formed. A cast thus takes the shape of the original organic material, and this is what happened with the "teeth" of the Wurtemberg creature.

The word "teeth" is placed in quotes because the casts were not really of the animal's teeth at all. Sediment had filled in the tooth sockets of the creature so the casts were of the sockets, not the actual teeth. Jaeger did not realize this and thought this creature had stumpy, irregularly-shaped teeth. Indeed, the tooth shapes seemed awfully strange for a reptile, and given such odd dental equipment Jaeger thought that these animals could have only fed on plants. Thus he named them Phytosaurus, and he used subtle differences in the "tooth" casts to identify two species. As their names suggest, Phytosaurus cylindricodon seemed to have cylindrical teeth while Phytosaurus cubiocodon appeared to have teeth that were square shaped!

Jaeger's discovery was soon commented on by another German paleontologist, Hermann von Meyer, who discussed Phytosaurus in his paper "On the Structure of Fossil Saurians." (The research was subsequently translated by G.F. Richardson and communicated to the Magazine of Natural History by Gideon Mantell in 1837.) Like Jaeger, von Meyer thought that Phytosaurus was gavial-like in appearance, but he also noted that the complete teeth of the animal were probably conical in shape. Several such teeth had already been found, von Meyer noted, thus making it apparent that Jaeger based his paleobiological interpretations on incomplete evidence. (While von Meyer did not say whether he considered Phytosaurus to be a plant-eater, he did state that the conical teeth appeared to show evidence of grinding.)

Then, in 1841, the English anatomist Richard Owen threw cold water on the idea that Phytosaurus was a valid genus at all. In a paper delivered before the Geological Society Owen asserted that the remains Jaeger had described truly belonged to Mastodonsaurus, now known to be a terrifyingly large Triassic amphibian. Yet this rearrangement caused further problems as the name Mastodonsaurus invoked the idea of the extinct proboscidean Mammut americanum, the American mastodon. To resolve this disorder Owen proposed that the remains attributed to both Phytosaurus and Mastodonsaurus be sunk under the name Labyrinthodon.

Unfortunately for Owen priority trumps sensitivities about accuracy when it comes to taxonomy. Mastodonsaurus presently is a valid genus. Whether Phytosaurus is, however, appears to be up for debate. Being that I have not seen illustrations of the fossils Jaeger described I cannot say whether Phytosaurus truly belongs with subsequently-discovered phytosaurs or not.

Taxonomic categories and species names are not just labels. They are embedded within scientific debates and have their own histories. In the case of the phytosaurs, the group's name comes from the misinterpretation of fossils that might not have actually belonged to the kind of animal that now bears the name "phytosaur." A re-examination of the fossils Jaeger described is needed to work this out, but this dilemma does drive home the fact that behind every taxon there is a story of discovery and debate.