Azendohsaurus postcrania – svp abstracts 2013

From the abstract:
Nesbitt et al. 2013 wrote, “During the Triassic, a number of highly disparate archosauromorphs populated both terrestrial (e.g., Trilophosaurus, rhynchosaurs) and marine ecosystems (e.g., tanystropheids) across Pangea. Unfortunately, the unique and sometimes utterly bizarre body plans of these reptiles (e.g., specialized feeding adaptations) create a major challenge in understanding early archosauromorph relationships and patterns of diversification, as teasing apart homology from homoplasy has been difficult with the current sample of taxa.

“Here we present the postcranial anatomy of Azendohsaurus madagaskarensis, an early archosauromorph from the Middle-Late Triassic of Madagascar. Azendohsaurus madagaskarensis comes from a monotypic bone bed containing an ontogenetically variable sample, with preservation ranging from whole, disarticulated bones, to articulated partial skeletons. From this bonebed, the entire anatomy of the taxon is represented. Azendohsaurus madagaskarensis possessed an elongated neck, short tail, and stocky limbs. The manus and pes have unexpectedly short digits, terminating in large, recurved ungual phalanges. Together with the skull, knowledge of the postcranial skeleton elevates A. madagaskarensis to another highly apomorphic and bizarre Triassic archosauromorph.

“Even so, recovery, description and analysis of the full anatomy of A. madagaskarensis provides clues to understanding the relationships of this species and other problematic and anatomically specialized taxa, including the North American Late Triassic archosauromorphs Trilophosaurus and Teraterpeton. For example, A. madagaskarensis, Trilophosaurus, and Teraterpeton share a dorsally hooked quadrate and enlarged, trenchant unguals, whereas Trilophosaurus and Teraterpeton alone share a number of other character states (e.g., restricted scapular blade, premaxillary beak). We tested these observations in a newly constructed phylogenetic analysis centered on Triassic archosauromorphs and archosauriforms. We find that A. madagaskarensis, Trilophosaurus, Spinosuchus, and Teraterpeton form a clade within Archosauromorpha, but the relationships of this clade to other groups of Triassic archosauromorphs (e.g., archosauriforms, rhynchosaurs, tanystropheids) remains poorly supported. The newly recognized clade containing A. madagaskarensis, Trilophosaurus, and Teraterpeton demonstrates high disparity of feeding adaptations even within a closely related group of basal archosauromorphs.”

First of all,
its great to hear the postcrania of Azendohsaurus is finally on the table, soon to be published, I presume. The skull nests it as a sister to Trilophosaurus, so the long neck is no surprise and Sapheosaurus. The short tail, short digits and stocky limbs of Azendohsaurus are found phylogenetically nearby in the rhynchosaur Hyperodapedon and the proto-rhynchosaur, Mesosuchus. So, again, no surprise there. This clade has surprising diversity.

Second of all,
Nesbitt et al. 2013 think these taxa are bizarre only because they are under the presumption that they are all archosauromorphs. They are not, as documented by the large reptile tree. It’s no surprise then that Nesbitt et al. report, “relationships remains poorly supported.” Evidently Nesbitt et al. 2013 don’t have a large enough gamut in their reptile tree to recovers their featured taxa as lepidosauromorphs, closer to lepidosaurs than to archosaurs.

When Nesbitt et al. finally do figure out how to nest those taxa, they’ll also find out that their new sister taxa provide a gradual accumulation of traits that lead to all their oddball traits. It’s the same problem Nesbitt 2011 made nesting pterosaurs with archosaurs. In reality, when you don’t exclude the better candidates, tritosaur lepidosaurs provide the gradual accumulation of pterosaurian traits.

Nesbitt 2011 already made the big mistake of nesting Mesosuchus as a basal archosauromorph when it is way closer to sphenodontids. Youngina would have been a better choice as a basal archosauromorph. It’s way more plesiomorphic with regards to proterosuchids, erythrosuchids and choristorderes. 

The exception

Teraterpeton is the exception, a pararchosauriform archosaurormorph nesting between Chanaresuchus and Tropidosuchus in the large reptile tree. Earlier we looked at Teraterpeton here. Nesbitt et al. found that Teraterpeton, Trilophosaurus and Azendohsaurus shared a dorsally hooked quadrate and enlarged trenchant unguals. Unfortunately, more that 30 steps are added when Teraterpeton shifts to nest with the other two and the unguals are not really that big in Teraterpeton. They’re actually bigger in the sister taxon, Chanaresuchus. The quadrate often hooks in herbivores. That they have in common.

Gosh,
I hope the Nesbitt team goes to a larger gamut tree and tests their taxa against other candidates before they publish another problem-filled paper like Nesbitt (2011) with strange bedfellows (sisters that don’t share very many traits) all over the place. The nestings of these featured taxa in the large reptile tree are strongly supported.

References
Nesbitt, S, Flynn J, Ranivohrimanina L, Pritchard A and Wyss A 2013. Relationships among the bizarre: the anatomy of Azendohsaurus madagaskarensis and its implications for resolving early archosauromroph phylogeny. Journal of Vertebrate Paleontology abstracts 2013.