Paleontologists are still mulling over
the origin of aetosaurs. They also keep missing the premaxillae of Ticinosuchus (Fig. 1, PIZ T2817). Long time readers know these two factoids are related.
Here (Fig. 1) again, are the short, sharp, triangular premaxillae of Ticinosuchus. This is one of many key traits that nest Ticinosuchus with aetosaurs like these (Fig. 2).
Unfortunately this relationship between Ticinosuchus and aetosaurs continues to be ignored, largely because the the skull bones of Ticinosuchus are in such disarray that workers (Lautenschlager and Desojo 2011, Nesbitt 2011) have not had much success segregating and identifying them. Here (Fig. 1) DGS (digital graphic segregation), comes to the rescue once again.
Nesbitt (2011) wrote: “As a result, the phylogenetic position is poorly supported in the few analyses in which it has been included (e.g., Parrish, 1993; Benton, 2004). I examined the skull region very carefully and have a few comments. Unfortunately, the blocks with skull elements appear to have been reassembled incorrectly.”
Nesbitt has great insight.
Unfortunately Nesbitt (2011) identified one premaxilla as a tentative right lacrimal. Even so, and to his credit, Nesbitt nested Ticinosuchus very close to the aetosaurs, Unfortunately he nested the basal erythrosuchid, Revueltosaurus, at the base of the aetosaurs. Perhaps the misidentified premaxilla is the one key that would have helped Nesbitt nest Ticinosuchus a little closer to aetosaurs. Revueltosaurus does not have a triangular premaxilla. It nests with the very similar but much larger Fugusuchus.
Brusatte et al. 2010
nested parasuchians basal to aetosaurs. Gracilisuchus, Erpetosuchus and crocodylomorpha nested as sister clades, even though they share very few traits with aetosaurs. In Brusatte et al. Ticinosuchus nested with Stagonosuchus (a rauisuchian), Fasolasuchus (a giant rauisuchian known from a maxilla and premaxilla in the skull ) and Arganasuchus (rauisuchian bones recovered include part of the upper jaws, neck vertebrae, and hip bones). Send pdfs of these taxa if you have them and I can comment with more authority, but my guess is that none of these had a triangular premaxilla.
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Schoch R 2007. Osteology of the small archosaur Aetosaurus from the Upper Triassic of Germany. Neues Jahrbuch für Geologie und Paläontologie – Abhandlung. 246/1:.1–35. DOI: 10.1127/0077-7749/2007/0246-0001
Walker AD 1961. Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus and their allies. Philosophical Transactions of the Royal Society B 244:103-204.