Figure 1. Azendohsaurus skull reconstructed with two premaxillary teeth, not four.
The new paper
on Azendohsaurus (Dutuit J-M 1972, Middle to Upper Triassic, Figs. 1-5) post-crania (Nesbitt et al. 2015, Fig. 2) has been eagerly awaited. The skull was published five years ago (Flynn et al. 2010). The description of the post-cranial bones is excellent, as was the earlier description of the cranial bones.
The is definitely a different sort of reptile…at first glance.
The big phylogenetic question is…
is Azendohsaurus closer to Pamelaria (the protorosaur archosauromorph)? Or closer to Trilophosaurus (the rhynchocephalian lepidosaur)? Or closer to Sapheosaurus and Noteosuchus (two rhynchocephalian lepidosaurs)? Or closer to Eohyosaurus or Mesosauchus (yet other rhynchocephalians)?
Let’s test one analysis against another.
Figure 2. Reconstruction of Azendohsaurus from Nesbitt et al. 2015. Length: a bit over one meter. Manus on left. Pes on right. Click to enlarge. No matter which clade this nests in, it is an oddity. But at present it is less odd nesting with Trilophosaurus and Sapheosaurus according to the input traits.
Unfortunately too few taxa were tested in the Nesbitt et al. study.
Nesbitt et al. tested 29 taxa and found that Azendohsaurus nested between Teraterpeton + Trilophosaurus and Pamelaria at the base of their Archosauromorpha (Fig. 3). Several mismatches occur here with great phylogenetic distances between purported sister taxa. The tree also mixes lepidosauromorphs with archosauromorphs, once again demonstrating convergence within the two major clades of reptiles.
Figure 3. The Nesbitt et al. 2015 cladogram nesting Azendohsaurus between Trilophosaurus + Teraterpeton and Pamelaria. This is not supported by the large reptile tree. Here green taxa are lepidosauromorphs. Black taxa are archosauromorphs. The proposed Allokotosauria is diphyletic. So is the proposed Azendohsauridae according to the large reptile tree. The black taxa are improperly included in this too small study on Azendohsaurus relationships.
the large reptile tree tested 610 taxa and maintained a long-standing and fully tested sisterhood between Trilophosaurus and Azendohsaurus. Furthermore, these two nested within the Rhynchocephalia at the transition to Mesosuchus, Eohyosaurus and the rhynchosaurs on the lepidosauromorph branch of the reptile (amniote) tree. Ancestral sisters include Sapheosaurus, Noteosuchus and tiny Leptosaurus (Fig. 6), taxa not listed in the Nesbitt et al. tree. Pamelaria continues to nest as a derived protorosaur on the archosauromorph branch of the reptiles.
Azendohsaurus + Trilophosaurus + the rhynchosaurs and kin over to Pamelaria raises the MPTs from 9109 to 9127, a pretty small bump considering the great phylogenetic distance. The number drops to 9124 nesting this clade with Protorosaurus or the BPI 375 specimen of Youngina. Such similarity is due to convergence.
Pameleria does not have two parallel rows of large teeth as in rhynchocephalians including Azendohsaurus and rhynchosaurs. Trilophosaurus does something odd with not two, but three laterally aligned cusps.
Figure 4. Rhynchocephalian subset of the large reptile tree with Azendohsaurus highlighted.
The nesting of rhynchosaurs and Trilophosaurus as basal archosauromorphs close to or within the protorosaurs has been a long standing problem in paleontology. They do indeed evolve away from the basic rhynchocephalian bauplan seen in Sphenodon.
Reducing the large reptile tree taxon list to that of Nesbitt et al. 2015
recovers a tree (Fig. 5) that again splits up archosauromorphs (inverted type) and lepidosauromorphs (black type).
Figure 5 A subset of the large reptile tree matched to the Nesbitt et al. 2015 taxon list. Here the tree mixes taxa from the two major branches demonstrating the convergence of the derived taxa and the importance of including a large gamut (610) of tested and verified taxa rather than fewer than 30 favorites or traditional guesses. Inverted taxa are archosauromorphs. Others are lepidosauromorphs in the large reptile tree. Note that both Pamelaria and Terterpeton are only one node away from the Azendohsaurus clade here.
With an odd reptile like Azendohsaurus
it is necessary to use a large gamut cladogram, like the large reptile tree, to test all the possibilities and relationships, leaving out virtually no possibilities.
Figure 6. Azendohsaurus compared to sister taxa and putative sister taxa including Trllophosaurus, Pamelaria, Teraterpeton, Sapheosaurus and Leptosaurus. Diandongosaurus is ghosted and not related. Azendohsaurus nests with Trilophosaurus in both studies. Even so it is quite distinct.
Along the way
I learned more about Trilophosaurus (Fig. 7) by going to photographs of the material after trusting published reconstructions that combined the anterior skull specimen with a mismatched posterior skull specimen.What we’ve gotten used to seeing is a chimaera.
Figure 7. Trilophosaurus skulls, right side flipped. Note the deep jugal on two and the shallow jugal on the third. Also note the postjugal bone (deep blue), filling in the posterior jugal notch. This is a novel ossification. The asymmetry between the left and right quadrate/jugal suture appears to be natural. The palatine teeth align with the maxillary teeth, a unique trait.
I also learned
that the published Azendohsaurus premaxilla has a bit of maxilla on it (Fig. 1), reducing the premaxillary tooth count from four to two. The original researchers considered the crack the suture. No sister taxa have four teeth in the premaxilla. All have two teeth including the shallow jugal specimen of Trilophosaurus with two vestigial teeth in the premaxilla.
Figure 8. Eohyosaurus nests as a sister to the Trilophosaurus-Azendohsaurus clade.
have teeth ankylosed (fused) to the bone. In some cases the teeth are bone. Apparently when rhynchocephalians became phylogenetically miniaturized in tiny Leptosaurus, neotony re-produced regular socketed teeth of the sort one also sees in Eohyosaurus, Mesosuchus, Sapheosaurus and Azendohsaurus.
Nesbitt et al. report
“Teasing apart homology from homoplasy of anatomical characters in this broad suite of body types remains an enormous challenge with the current sample of taxa.”
Indeed that sample of taxa has to be greatly increased.
610 taxa demonstrate this amply. 29 is just too few. Too many actual sister taxa were overlooked and excluded in the Nesbitt et al. analysis. They relied on tradition rather than testing when oddly matched sister taxa nested with one another on their cladogram.
as can be seen by the reconstructions (Fig. 5), there is still a great deal of phylogenetic distance between tested sisters. The large reptile tree minimizes this, but the distances still remain great. New discoveries will help fill these gaps, but the correct inclusion group must be used. The tree subset that includes protorosaurs and basal archosauriforms (Fig. 9) does not include Azendohsaurus, Trilophosaurus, rhynchosaurs or tanystropheids (which are all lepidosauromorphs).
Figure 8. Sapheosaurus GIF animation. This smaller ancestral sister to Azendohsaurus was overlooked and excluded by the Nesbitt et al. study.
Nesbitt et al. created two suprageneric clades.
Unfortunately the proposed clade Allokotosauria is diphyletic. So is the proposed clade Azendohsauridae according to the large reptile tree.
Figure 9. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and basal Archosauriformes.
Click to enlarge. Azendohsaurus, Trilophosaurus, tanystropheids and rhynchosaurs do not nest here as they do in the smaller gamut Nesbitt et al. cladogram, which improperly included them.
Dutuit J-M 1972. Découverte d’un Dinosaure ornithischien dans le Trias supérieur de l’zhtlas occidental marocain. Comptes Rendus de l’Académie des Sciences à Paris, Série D 275:2841-2844.
Flynn JJ, Nesbitt, SJ, Parrish JM, Ranivoharimanana L and Wyss AR 2010. A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible. Palaeontology 53 (3): 669–688. doi:10.1111/j.1475-4983.2010.00954.x .
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.
Nesbitt SJ, Flynn JJ, Pritchard AC, Parrish JM, Ranivoharimanana L and Wyss AR 2015. Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. Bulletin of the American Museum of Natural History 398: 1-126.