Note added after publication: For those interested the comments section sheds new light.
Mickey Mortimer, blogger of the Theropod database, put a lot of work into rescoring the dinosaur portion of the large reptile tree. With those changes, here is the recovered tree (taxa abbreviated but those in the know will know).
You’ll note that Saurischia is recovered with [Sauropodomorpha + Theropoda]. Panphagia + Pampadromaeus nest basal to Theropoda, and the [poposaurids + Arizonasaurus] nest close to the basal archosaur/crocodylomorph, Gracilisuchus. Ornithischians are in the middle.
These are different nestings from the large reptile tree (Fig. 1). Basically the Dinosauria has been flipped top to bottom.
Well, then, let’s test the Mortimer tree
You’ll recall when I removed taxa, clades, or large numbers of clades from the large reptile tree, the rest of the topology did not change. When I tested only skulls or only post-crania, the tree did not change. These are signs of stability and strength.
when I removed the [poposaurs + Arizonasaurus] from the Mortimer tree, the topology reverted to that of the large reptile tree. That’s a big change. And there’s more:
In the Mortimer tree theropods nest as the most derived clade of dinos, preceded by several clades of herbivores in this pattern:
Quoting from my reply to MM: “Turfanosuchus and Gracilisuchus (carnivores) at the base giving rise to Poposaurids (herbivores), then Silesaurids (herbivores), then Marasuchus (carnivore), then Pisanosaurus (herbivore), then a split between Ornithischians (herbivores) and Saurischians led by Sauropodomorphs on one branch (herbivores) and Daemonosaurus (?-vore) at the base of Panphagia + Pampadromaeus (herbivores) and theropods (carnivores). As you can see this is a varied mix of herbies and carnies, which is not the case in the Large Reptile Tree”
In the LRT the theropods nested as basal dinos, giving rise via Panphagia, Pampadromaeus and Daemonosaurus to herbivorous sauropodomorphs, poposaurids and ornithischia. Basically the same tree, just flipped top to bottom.
Not sure yet what ordering, scoring, what-have-you turned the Mortimer tree upside-down, but having theropods as derived and widely separating Gracilisuchus from Herrerasaurus raises red flags. It just ain’t right. Isn’t it more tenable that certain theropods gave rise to herbivores, first with saurischian pelves and later with ornithischian pelves?
The unstable topology is also bothersome. Removing taxa should not upset the topology. Hopefully we’ll come to a resolution on this. The devil is probably in the details many or all of which can be seen here at M. Mortimer’s blog post.
Arizonasaurus does not belong here. It nests with rauisuchians and shares few to no synapomorphic traits with Gracilisuchus, a basal crocodylomorph.
I appreciate the work by MM and, perhaps, some of the rescoring is justified. I don’t know. In any case, the results do not appear to be tenable.
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.