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.

Why I Use Pseudosuchia Instead of Crurotarsi for the Crocodile-line Archosaurs

This issue was briefly mentioned on the Vertebrate Paleontology List-server about a week ago and I feel like discussing this a bit further. A quick perusal of archosaurian literature will show that there is a dichotomy in the nomenclature when referring to the lineage of Archosauria that leads to the crocodylians. Some workers will refer to this clade as Pseudosuchia and others as Crurotarsi, with the latter seeming to be the most popular. However, I along with some of my closest Triassic collaborators prefer Pseudosuchia over Crurotarsi, and often when I review papers I will suggest that workers using Crurotarsi switch. Why? Well for me it comes down to two issues, priority and stability.

This problem has been reviewed by several workers, such as Brochu (1997) and most recently Senter (2005). Gauthier and Padian (1985) defined Pseudosuchia as "crocodiles and all archosaurs closer to crocodiles than to birds"; however, Pseudosuchia has a much longer history dating back to the late 1800s, and although it has always had the intent of containing non-dinosaurian archosaurs, the membership of this clade has changed through the years and at some points even included some of what are now considered to be non-dinosaurian ornithodirans (the bird-line clade).

Several authors disliked Gauthier and Padian's (1985) redefining of what they considered to be an "ill-defined and misused" name. Furthermore, the name is in a sense contradictory as Pseudosuchia means "false-crocodiles" yet includes crocodiles as members. Accordingly Benton and Clark (1988) suggested a new name, "Crocodylotarsi", and although they did not explicitly define this clade they inferred that it was the same as Pseudosuchia. Sereno and Arcucci (1990) proposed a third name, Crurotarsi, which Sereno (1991) defined as "Parasuchia, Ornithosuchidae, Prestosuchus, Suchia, and all decendents of their common ancestor".

However, just because a group once contained members that have since been recognized as belonging to other groups does not warrant abandonment. Indeed if this were the case very few names would be valid, including Dinosauria. Likewise, contradictory names also are not grounds for dismissal, for example the name phytosaur means 'plant-reptile' although they surely ate everything but. Despite this we are still stuck with the contradictory name. Furthermore, there is no confusion among modern workers as to the meaning of Pseudosuchia, so statements to the contrary are moot. When I say something is a pseudosuchian, those familiar with archosaurs clearly understand what I mean.

Crocodylotarsi has been used by some workers but has since fallen out of usage with most modern workers using either Pseudosuchia or Crurotarsi. As all three groups currently have the same membership it has been argued by some that Pseudosuchia should be used as it has precedence. I agree in principle that the first defined name should have priority and this is the reason that I use Pseudosuchia instead of Crocodylotarsi. However, as noted by Brochu (1997) and Senter (2005) the definitions of Pseudosuchia and Crurotarsi are not the same.

Pseudosuchia is stem-based and thus is flexible to future changes, as any archosaur that is not an ornithodiran is included in this group. Crurotarsi, however, is defined as a node-based taxon and thus has an explicit membership, most notably phytosaurs (parasuchians), ornithosuchids, Prestosuchus, aetosaurs, "rauisuchians", and crocodylomorphs. This definition is much less flexible. In fact let's just suppose that the basal most group of crurotarsans, the phytosaurs, fell outside of the crown-clade Archosauria, and were instead considered to be derived archosauriforms. What would happen to Pseudosuchia and Crurotarsi? The content of Pseudosuchia would be pretty much the same except that phytosaurs would no longer be constituents. In contrast, because the base of Crurotarsi is specified by phytosaurs, Crurotarsi would now include phytosaurs plus all of Archosauria. Thus dinosaurs (including birds) would be Crurotarsans by definition. As you can see this definition is much less stable, another reason why I prefer and highly recommend that all workers use Pseudosuchia over Crurotarsi. Admit it, having to say "non-phytosaurian and non-ornithiodiran crurotarsan" is pretty clunky!

Actually, if this ever did happen ;), in my eyes Crurotarsi might actually now be a useful name when discussing phytosaurs, as you could now simply say that phytosaurs are the basalmost crurotarsans and still be correct. This is probably just slightly more explicit than simply saying they are derived archosauriforms.

For much more detail on this issue read Brochu (1997) and Senter (2005). You can also check out this page for a different opinion.

REFERENCES

Benton, M.J., and J.M. Clark. 1988. Archosaur phylogeny and the relationships of the Crocodylia. Pp. 295–338 in M.J. Benton (ed.). The Phylogeny and Classifi cation of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press, Oxford.

Brochu, C.J. 1997. Synonymy, Redundancy, and the name of the crocodile stem group. Journal of Vertebrate Paleontology 17:448-449.

Gauthier, J., and K. Padian. 1985. Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight. Pp. 185–197 in M.K. Hecht, J.H. Ostrom, G. Viohl, and P.
Wellnhofer (eds.). The Beginnings of Birds. Freunde des Jura- Museums, Eichstätt.

Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios 25:1–7.
Sereno, P.C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir 2:1–53.

Sereno, P.C., and A.B. Arcucci. 1990. The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen
180:21–52.