Dinosaurs Are Crurotarsans

There is much confusion surrounding the taxonomic name given to the crocodylian branch of Archosauria. Two names are often given for this clade, Pseudosuchia and Crurotarsi, and despite detailed discussion by Brochu (1997) and others, most recently Senter (2005), these names are often used interchangeably. However, this general usage is not correct because both taxonomic names have been explicitly defined cladistically and their definitions vary. As I have stated in an earlier post, Pseudosuchia is a stem-based (branch-based) definition that includes all archosaurs closer to crocodylians than to avians; whereas Crurotarsi has a node-based definition delimited by the inclusion of specific taxa, including parasuchians or phytosaurs. It has long been recognized that the definition of Crurotarsi is unstable (e.g., Brochu, 1997), because if one of the specifier taxa used to define this clade is found to lie outside of Archosauria then the taxonomic composition of Crurotarsi would change, possibly to something not actually meant when one uses the name or Crurotarsi could actually end up being a synonym of another node-based clade such as Archosauria. On the other hand the content of Pseudosuchia would not change, and thus that definition is more stable. Nonetheless to date this has not really been a problem because as defined Crurotarsi and Pseudosuchia currently comprise the same taxa. The former name is used much more frequently by workers presumably because of a preference for the name itself rather than a dislike for its definition (Brochu, 1997).

Sterling Nesbitt’s upcoming detailed phylogenetic analysis of the Archosauria (Nesbitt, in press), which is previewed in the recent paper on Poposaurus by Gauthier et al. (2011), recovers phytosaurs as the sister taxon to Archosauria. This placement is extremely well-supported in his analysis and actually makes a lot of sense if you spend a lot of time working with this group and with pseudosuchians. As I noted earlier the recovery of Phytosauria outside of Archosauria changes the definition of Crurotarsi quite significantly, with Crurotarsi now the name of the clade Phytosauridae + Archosauria. This means that all ornithodirans including dinosaurs are now crurotarsans. Clearly this is not exactly what is meant when workers utilize this name.

This is also fairly significant in the evolutionary sense because it means that phytosaurs are ancestral to dinosaurs and other ornithodirans. Gauthier et al. (2011) discuss this ancestry in the sense of the functional ankle. They note that phytosaurs possess a primitive form of a crurotarsal joint that is quite different from that in suchians, and also that the ankle joint in the earliest ornithodiran, Lagosuchus, also utilizes crurotarsal motion that is lost in later ornithodirans with the development of the hinge-like ankle joint characteristic of that clade.

Overall the placement of phytosaurs outside of Archosauria is very well supported and may not be overturned. Thus, following Brochu (1997) I advocate the use of Pseudosuchia for the crocodylian branch of Archosauria to promote taxonomic stability. Furthermore, IMHO it is much easier and proper to use than non-ornithodiran crurotarsan, although I expect that more people will start to use Pan-Crocodylia for the clade because of the general dislike of the name Pseudosuchia.

“Careful attention to their [Crurotarsi and Pseudosuchia] ultimate distinctness can be a source of stability for future phylogenetic work. The definitions will remain stable, and we have a nomenclatural framework within which new fossils can be placed. Taxa more closely related to crocodiles than to birds, but not descended from the last common ancestor of parasuchians, ornithosuchids, prestosuchids, and suchians, will still be pseudosuchians. We fully expect diagnoses, group memberships, and minimum divergence times to change as new fossils or data sets are analyzed, and the parameters of Pseudosuchia and Crurotarsi will diverge as more basal pseudosuchians are found” (Brochu, 1997:448).

REFERENCES

Brochu, C. A. 1997. Synonymy, Redundancy, and the Name of the Crocodile Stem-Group. Journal of Vertebrate Paleontology 17:448-449.

Gauthier, J. A., Nesbitt, S. J., Schachner, E. R., Bever, G. S., and W. G. Joyce. 2011. The bipedal stem-crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.

Nesbitt, S. J. in press. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352:1-292.

Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios 25:1–7.