Aetosaurs have always sort of stood alone.
Paleo-workers have been looking for a proximal outgroup to this well-defined clade for a long time. In October 2011 the LRT nested Ticinosuchus (Fig. 2) as the proximal outgroup to the Aetosauria. That same year Nesbitt 2011 nested Revueltosaurus (Fig. 1) as the proximal outgroup to the Aetosauria and Ticinosuchus nested nearby.
Both outgroup candidates are less than completely armored in scutes.
Only one has the distinctive sharp premaxilla of an aetosaur, a bone that was misidentified as a tentative lacrimal by Nesbitt 2011 (Fig. 3). Revueltosaurus has distinctive symmetrical teeth with large serrations that were originally considered basal ornithischian dinosaur teeth. Some aetosaurs have symmetrical teeth, but without serrations. Others have recurved teeth. All LRT sisters to Revueltosaurus have recurved teeth. Teeth do not tell the whole tale, as we’ve seen many times before.
While Revueltosaurus has been published (Parker et al. 2005) and videos have been made of it, a more comprehensive paper that more precisely describes three species is in the pipeline as of this date.
Today
I hope that putting LRT sister taxa and Nesbitt 2011 sister taxa on display will help make the case that with taxon inclusion and correct identification of a certain premaxilla (see above), the LRT’s case is stronger and more parsimonious than Nesbitt 2011 and Wikipedia’s current entry on Revueltosaurus that follows Nesbitt 2011.
In the LRT
Revueltosaurus nests closer to the base of the Euarchosauriformes (= descendants of Euparkeria, Fig. 1) distinct from descendants of Proterosuchus (= Choristodera + Chañaresuchus + Proterochampsa and kin).
Figure 1. How the LRT nests Revueltosaurus, as a basal euparkeriid, between Tasmaniosaurus and Fugusuchus. Overall they all resemble one another. Note the gradual accumulation of derived traits.
According to Wikipedia,
“Revueltosaurus was placed at the base of the clade Suchia as the sister taxon to the armored and herbivorous aetosaurs. However, Revueltosaurus itself is not an aetosaur, since Aetosauria was redefined to exclude it. The analysis found Gracilisuchus, Turfanosuchus and the Revueltosaurus+Aetosauria clade to nest in a large polytomy with Ticinosuchus+Paracrocodylomorpha.”
We talked about several problems with Nesbitt 2011
earlier here, here, here and here (3 parts of a 9-part series and 1 part of another series looking at Nesbitt characters ). Nesbitt did not nest crocs with dinos as the LRT does, so the membership of Nesbitt’s Archosauria is much larger than in the LRT.
Figure 2. How the LRT nests three tested aetosaurs, arising from Ticinosuchus. Nesbitt xxxx wondered if the distinctive triangular premaxilla was a lacrimal. It is difficult to imagine Revueltosaurus nesting closer to aetosaurs than it does in the LRT. Note the gradual accumulation of derived traits.
Revueltosaurus callenderi (Hunt 1989) Late Carnian, Late Triassic ~210mya, was first known from its teeth, which reminded Hunt (1989) of ornithischian dinosaurs. Parker et al. (2005) reidentified the remains as crocodylomorph. Here Revueltosaurus was derived from a sister to Osmolskina and Euparkeria. Fugusuchus and Tasmaniosaurus (Fig. 1) are sister taxa.
Ticinosuchus ferox (Krebs 1965) Middle Triassic, ~ 230 mya, ~3 m in length, was derived from a sister to Vjushkovia and Arizonasaurus and was nested at the base of the aetosaurs, Stagonolepis, Aetosauroides and Aetosaurus (Fig. 2).
Figure 3. Nesbitt (2011) reassembled the in situ skull, but misidentified the premaxilla as a lacrimal.
Figure 4. Aetosaurus, Stagonlepis and Ticinosuchus shown together to scale. Ticinosuchus is the basalmost taxon in this clade, unrecognized by other cladograms. Perhaps this is due to differences in skull reconstructions.
Note that Nesbitt et al. 2017
did not include Fugusuchus and Tasmaniosaurus in their taxon list, but did include two pterosaurs, which the wider gamut LRT nests within Lepidosauria. They did not include Younginoides or any younginid, so they do not have validated outgroup taxa for their study. The Nesbitt team omitted Junggarsuchus and Pseudhesperosuchus, so they omitted key dinosaur outgroup taxa. The Nesbitt team included the lepidosaur, Mesosuchus and the thallatosaur, Vancleavea… so it’s no surprise that their tree is resolved only in parts.
Figure 5. Revueltosaurus compared to its big sister, Fugusuchus, a erythrosuchid mimic.
Ladies and gentlemen, colleagues and fellow enthusiasts,
taxon exclusion is the number one cladogram killer. Number two is including taxa that do not belong in a focused study. Don’t keep making the same mistakes that Nesbitt and his team keep making. Take a look at the LRT before you send off your cladograms to the editors, referees and publisher to make sure you have not made these mistakes.
References
Hunt AP 1989. A new ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. In Lucas, S. G. and A. P. Hunt (Eds.), Dawn of the age of dinosaurs in the American Southwest 355–358.
Nesbitt SJ et al. (10 co-authors) 2017. The earliest bird-line archosaurs and the assembly of the dinosaur body plan. Nature doi:10.1038/nature22037. (online pdf)
Parker WG., et al. 2005. The Pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. In Proceedings of the Royal Society London B 272(1566):963–969.
Thulborn, RA 1986. The Australian Triassic reptile Tasmaniosaurus triassicus (Thecodontia: Proterosuchia). Journal of Vertebrate Paleontology 6(2):123–142.
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