A recent online paper by Nesbitt et al. 2013 introduces us to a new Middle Triassic East African archosauriform, Asperoris mnyama (NHMUK PV R36615), known from several 3D skull pieces (Fig. 1).
Figure 1. Asperoris skull reconstruction. Turns out it’s just an ordinary rauisuchid linking Vjushkovia to more derived forms. Tooth length is pure guesswork.
Nesbitt et al. was unable to resolve what Asperoris was (Fig. 2). So they considered it a “non-archosaurian archosauriform” (incerta sedis). Unfortunately they included rauisuchids within the Archosauria in their analysis AND they included several unrelated taxa: a lepidosauriform (Mesosuchus), a thalattosaur (Vancleavea) and two pararchosauriform supragenera (Proterochampsidae and Phytosauria). That didn’t leave too many related taxa to compare Asperoris to. And they missed the most obvious candidates, like Vjushkovia (Fig. 3). That’s why we turn to the large reptile tree (Fig. 2, subset here) to find out where Asperoris might nest.
Figure 2. The confused nesting of Asperoris according to Nesbitt et al. 2013 (above) and the exact nesting according to the large reptile tree (below) as as sister to Vjushkovia.
The large reptile tree data analysis started with a reconstruction of Asperoris (Fig.1), which enabled the employment of several traits not readily apparent from the separated bones. The results nested Asperoris within the Rauisuchidae as the sister to Vjushkovia, a taxon not included in the Nesbitt (2011) study on archosaurs.
The Nesbitt et al. 21013 definition of Archosauria is not confirmed here.
Nesbitt et al. report, “Archosauria, the crown clade that includes living birds and crocodilians as well as extinct dinosaurs, pterosaurs and pseudosuchians (stem-crocodilians), is one of the most successful evolutionary radiations in the history of vertebrate life on land.” In the large reptile tree, Archosauria includes just crocs and dinos (including birds). Pterosaurs nest with tritosaur lizards. Pseudosuchians, it turns out, are diphyletic with parasuchians and proterochampsids nesting with choristoderes, on a separate branch from the rest of the traditional archosauriforms.
Not far from Vjushkovia
Asperoris shared many skull traits with Vjushkovia and was about the same size. Like Vjushkovia, Asperoris also lacked an antorbital fossa on the maxilla. According to Nesbitt et al. “Asperoris mnyama differs from all known archosauriforms in having highly sculptured cranial elements including the premaxilla, maxilla, nasal, prefrontal, frontal, postfrontal, and parietal, and in having a highly sculptured, dorsoventrally deep orbital margin of the frontal.” This may be a distinct trait indeed.
Figure 3. Vjushkovia had 3 premaxillary teeth and a descending jugal, but otherwise would have been similar to Asperoris in shape and size.
So no great shakes.
Asperoris was about the size of Vjushkovia and Batrachotomus, so smaller than some rauisuchids. It’s not a key taxon providing new insight into relationships.
having access to data that showed pterosaurs and Vancleavea should not be included in archosauriform studies was not followed by any testing of these oddly nested taxa. Rather Nesbitt et al. (2013) held on to their old traditions — which is a major problem with their study.
Once again, I have not seen the original material (nothing here changes in interpretation anyway), but I do have a larger gamut study from which more accurate and confident nestings can be made. This is available to anyone who wishes to use it. Moreover, the taxonomic clades that result can provide guidance for future inclusion sets. At least they should be tested.
Nesbitt SJ 2011. The early evolution of archosaurs: Relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Nesbitt SJ, Butler RJ, Gower DJ 2013. A New Archosauriform (Reptilia: Diapsida) from the Manda Beds (Middle Triassic) of Southwestern Tanzania. PLoS ONE 8(9): e72753. doi:10.1371/journal.pone.0072753
NHMUK, The Natural History Museum, London, United Kingdom;