Poposaurs – Topology Shift

Yes, I was wrong. And it’s time to man-up.
Earlier, based on available data, the large reptile tree nested poposaurids with phytodinosaurs and attributed the appearance and growth of the calcaneal tuber in certain poposaurs to convergence with the Crocodylomorpha. So the earlier data recovered poposaurs as dinosaurs with an odd ankle, not crocs with a “massive convergence” with dinosaurs, which is still the widespread hypothesis (see Nesbitt 2011 and others).

That seemed to make sense — except some poposaurs, like Lotosaurus and Asilisaurus (Fig. 1), appeared a little too early in the Triassic. They seemed to be anachronistic, and that can be a red flag.

So going back to the phylogenetic analysis,
I reexamined certain specimens, discovered a few items not originally presented (I trusted original tracings instead of making my own from in situ photos) and I found several bad scores. The newly recovered tree finds poposaurids derived from Turfanosuchus, a taxon that earlier stood alone at the base of the Archosauria (basal to crocs and dinos). Now things seem to make more sense, phylogenetically, chronologically (Fig.1) and morphologically. See if you agree…

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos. Among these, only Silesaurus and Asilisaurus lost the calcaneal tuber.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Now the odd thing is: 
Poposaurs appear to provide a sort of preview to what would eventually evolve in the Dinosauria itself, likely filling similar niches in earlier strata.

Turfanosuchus and Poposaurus (Fig. 1) were convergent with theropods. The larger Asilisaurus was convergent with sauropodomorphs. The remainder were convergent with various ornithischians, even down to the toothless predentary they shared by convergence. Lotosaurus was a stegosaur mimic. Shuvosaurus was a Dryosaurus mimic. Silesaurus was a Camptosaurus mimic, down to losing the calcaneal tuber. Sacisaurus was a little Agilisaurus mimic. Effigia was still the oddball with those vestigial hands and back-sloped braincase.

So poposaurids are not dinosaurs. They are also not basal to rauisuchidae (contra Nesbitt 2011), but were derived from basal rauisuchia like Decuriasuchus and Vjushkovia. They are the most basal archosaurs. Basal poposaurs were the last common ancestors of crocs and birds. From their basalmost taxon, a sister to little Turfanosuchus, both tiny basal bipedal crocs and tiny bipedal basal dinos evolved.

Poposaurs, in the form of Nyasasaurus, Asilisaurus and Lotosaurus (Fig. 1), were the first archosaurs to evolve substantial size in the Middle Triassic. Crocs and dinos remained small until the late Triassic (mid-Triassic for the basal Herrerasaurus) when they had their great radiation and poposaurs began to fade. This is an unrecognized faunal turnover.

Discovering and correcting errors is what scientists do. 
And I was happy that these new insights appeared.

What took so long?
Inattention to red flags. We should all look more closely at problems. They lead to new insights.

M.M. I hope this helps the cause. And yes, I have made and will make changes to earlier posts on this subject.

Brusatte SL, Benton MJ, Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
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, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.