Few paleontologists have ventured to guess, or determine through analysis, from whence arrived the aetosaurs. They don’t look much like any other archosauriforms. They seem to appear as sideshows in various analyses. Notably the latest analyses find no consensus. Nesbitt (2011) nested aetosaurs with Revueltosaurus. Outgroup taxa included Turfanosuchus and Gracilisuchus. Brusatte et al. (2010) nested Aetosauria with Gracilisuchus, Erpetosuchus and Crocodylomorpha. The Phytosauria was the outgroup.
“From whence arrived the praying mantis?” — Ogden Nash
Here aetosaurs nested with Ticinosuchus, a basal rauisuchian with a small head, short rostrum, a reduced lateral temporal fenestra, a large mandibular fenestra, an upturned toothless dentary tip, a toothless premaxilla, a smaller pectoral girdle and scutes both above and below its tail. The hands and feet are also close matches. Ticinosuchus was also a sister to Qianosuchus and Yarasuchus, the long-necked rauisuchians sharing a dorsal naris with the basal rauisuchian, Vjushkovia and aetosaurs. It helped, of course, to actually reconstruct the skull of Ticinosuchus. It’s more aetosaur-like than previously thought. The size reduction between Ticinosuchus and Aetosaurus, the most primitive aetosaur, parallels other size reductions prior to major morphological changes in basal reptiles, mammals and birds. Chronologically the Late Triassic aetosaurs succeeded the Middle Triassic Ticinosuchus.
As we’re finding over and over again, whenever a major clade is introduced, its basal member is small. Aetosaurus is less than a third the size of its phylogenetic predecessor, Ticinosuchus, but the skull length is more than half that of Ticinosuchus. The development of more extensive armor and an herbivorous dentition coincides with this size reduction. The only catch is, Aetosaurus is not the earliest known aetosaur. Perhaps it was a late survivor. All other aetosaurs, including earlier specimens, were larger with a more extensive armor coating and an expanded gut for plant digestion.
Aetosauroides scagliai (Casamiquela 1960) Late Triassic (~210 mya) was a transitional taxon between Aetosaurus and Ticinosuchus. It had unconstricted tooth crowns, postnarial contact between the premaxilla and nasal, and a ventral margin of the dentary without a sharp inflexion. The teeth were primitive. I do not know the size of the skull. It was described as “large.”
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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Fraas O 1877. Aetosaurus ferratus Fr. Die gepanzerte Vogel-Echse aus dem Stubensandstein bei Stuttgar. Festshrift zur Feier des vierhundertjährigen Jubiläums der Eberhard-Karls-Universät zu Tübingen, Wurttembergische naturwissenschaftliche jahreshefte 33 (3): 1–22.
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Walker AD 1961. Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus and their allies. Philosophical Transactions of the Royal Society B 244:103-204.