Field of Science

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).


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.


  1. It sounds etymologically weird and a bit funny: Crocodylia, the "true crocs" being part of Pseudosuchia "false crocs".

  2. Wow, I missed that news about phytosaurs! Very interesting.

    "Pseudosuchia" does seem pretty prime for replacement by "Pan-Crocodylia". (With "Pan-Aves" for the sister group, including dinosaurs.)

  3. Sereno has proposed a different, unproblematic, stem-based definition for Crurotarsi on his Taxonsearch website. Given that no rules of priority for phylogenetic definitions exist, I personally see no problem with using Crurotarsi but using this more recent definition. We're currently doing this for other clades for which the earliest definitions provided are extremely problematic. The dinosaur clade Ornithopoda is a great example: as originally defined as a node-based clade it would incorporate essentially all ornithischians under the 'heterodontosaurids are basal ornithischians' hypothesis. Rather than use such a radically different taxonomic content or abandon the name, we've just changed the definition that we're using.

  4. I suppose the fact that there are no rules of priority 'enforced' yet for phylogenetic names that kind of puts us in this quandry, although I do note that most workers try to use the names with priority anyhow. The Pseudosuchia vs. Crurotarsi debate is an exception where it appears that workers simply use the name they prefer.

    It is true that Sereno (2005)has proposed a 'correction' for Crurotarsi; however, most users cite his earlier work in regards to meaning (sensu Sereno and Arcucci, 1990, etc.). Moreover, he provides new definitions for many different archosaurian clades and in a sense is 'cornering the market' on taxonomic names. How broadly used are his 2005 definitions and how acceptable is it in our field to not use priority of names regardless of the lack of existing rules to enforce this?

    Now Nesbitt (2011) provides another definition revision for Crurotarsi, giving us at least three options for that clade definition; whereas Pseudosuchia has priority and one unambiguous phylogenetic definition. Sure it is an old taxonomic name and 'etymologically wierd' but many taxonomic names have these problems (e.g., Phytosauria), and we are dealing with the converted clade name, which is not at all confusing.

    I find it telling that every analysis of proper clade names for Archosauriformes (e.g., Padian and May, 1993; Brochu, 1997; Senter, 2005)has determined that Pseudosuchia is the proper name for the croc-line clade, yet most workers still use Crurotarsi, suggesting to me a preference for the name rather than the definition or priority.

    Sadly, the ending of this debate is going to come down to who writes the definitions for croc-line archosaurs for Phylonyms or who rushes out the first set of definitions after Phylocode is enacted. I'm sure that some people already have a manuscript ready to go so they can get their name all over the taxonomy.

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  6. Id say that the less ambiguous a taxonomic name is, the better that taxonomic name is
    I think that Pseudosuchia isnt just simply "weird", its ACTUALLY MISLEADING
    Otherwise, divulgation of science can be very hard and would have some problems

    I dont think Phytosauria can be compared to Pseudosuchia: in the latter, its implicated that crocodylians and other suchians originated from (and therefore are) pseudosuchians

    Anyway, what are the new definitions of Crurotarsi and Pseudosuchia, according to Nesbitt (2011)?

  7. I guess I'm just a stickler for name priority, whether the rules currently exist or not (e.g., PhyloCode). Without priority what's to stop everyone from erecting new names for every clade whenever they see fit to ignore the older names.

    Pseudosuchia is only troublesome if you think about the etymology too much. But misleading etymologies are commonplace in paleontology and should not be grounds for abandonment of taxonomic priority.

    The aetosaurian clade name "Stagonolepididae" suggests that they are fish,or at least scaled, but no one seems to mind.

  8. Etymologically suspect names are commonplace, including the spectacularly misnamed phytosaurs, therapods, proceratosaurs, Basilosaurus and more. Possibly Pseudosuchia goes to a new level of absurd in calling the group that includes "true crocodiles" the "false crocodiles", but since no one seems to mind the other absurd misnamed groups out there, I don't know why Pseudosuchia should be so picked on.

  9. Also; I'm certainly curious how this new cladogram will show the relationship between phytosaurs and proterochampsids, which most of the recent cladograms (including Nesbitt's 2007 one) show them as the sister group to archosaurs.

  10. According to the cladogram the next successive outgroups are Euparkeria and then Proterochampsidae.

  11. I'm going to have to convert. Finally read your post on Pseudosuchia vs. Crurotarsi and your reasoning is very sound. With this new information popping up, Pseudosuchia seems to fit even better, despite being somewhat misleading (not that this has been a problem for other names, as some have already pointed out). Can't wait to get the full story when I read Nesbitt's new archosaur phylogeny


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