Field of Science

Aetosaurs: New Phylogenetic Analysis, New Taxon; and New Technique to Analyze Incongruent Character Datasets

My new paper in PeerJ features a new phylogenetic analysis of the Aetosauria (Archosauria: Pseudosuchia). I've added many new characters and feature all known valid aetosaur taxa. In the Supplemental Materials, each character is described and figured to clarify them for future use.

 

This paper also introduces a new aetosaur, Scutarx deltatylus, from the Chinle Formation of Arizona. To date I've collected four partial Scutarx skeletons, the first in 2002, from Petrified Forest National Park. The holotype specimen includes a partial skull, the first good aetosaur skull material collected from Arizona since the 1930s. In previous papers I have assigned all of these specimens to Calyptosuchus (Stagonolepis) wellesi; however, when I undertook a revision of that taxon I noted that my Petrified Forest specimens (all from the Sonsela Member of the Chinle Formation) all possessed raised triangular bosses on the posteromedial corner of the paramdian osteoderms.  These are not present in the type specimen from Texas, or from the large amount of material from the Placerias Quarry of Arizona. Interestingly the boss-bearing and non-boss-bearing specimens were separated stratigraphically. Presently the specimens from the Blue Mesa Member of Arizona and the Tecovas Formation of Texas are those that lack the boss (Calyptosuchus wellesi), and those from the Sonsela Member, and middle Cooper Canyon Formation are those that possess the boss (Scutarx deltatylus). The skull and ilium of Scutarx are also autapomorphic as discussed in the paper.


Finally I adapted the method to compute Partioned Bremer Support to look at the possibility of character conflict between anatomical partitions in aetosaurs. Essentially does the armor have a different phylogenetic signal than the rest of the skeleton. Bremer Support is used in all phylogenetic analyses to show branch support; however, what is generally not realized is that the individual support values of characters from each anatomical partition combine to total this number. But if these character sets actually support a different tree than the one recovered in the total character analysis, they will reduce the Bremer Support value (negative Bremer Support). Some character sets are ambivalent and actually provide no support to the branches.  This analytic tool allows you to look at your tree support node by node and see what character datasets support the topology.  This technique has been used since the late 1990s to compare molecular vs. morphological character sets, but this is the first time it has been employed to look at anatomical partitions in a purely morphological study.  


That said, I am a little disappointed with my final discussion section in the paper.  While I am excited about the potential for Partitioned Bremer Support, I still am learning all that it demonstrates, thus my discussion is not as strong as I would have liked. I hope that others will find it of interest and utility and that we can see if if it works for other taxonomic groups. 


Parker, W.G. 2016. Revised phylogenetic analysis of the Aetosauria (Archosauria: Pseudosuchia); assessing the effects of incongruent morphological character sets. PeerJ 4:e1583 https://doi.org/10.7717/peerj.1583

Abstract- Aetosauria is an early-diverging clade of pseudosuchians (crocodile-line archosaurs) that had a global distribution and high species diversity as a key component of various Late Triassic terrestrial faunas. It is one of only two Late Triassic clades of large herbivorous archosaurs, and thus served a critical ecological role. Nonetheless, aetosaur phylogenetic relationships are still poorly understood, owing to an overreliance on osteoderm characters, which are often poorly constructed and suspected to be highly homoplastic. A new phylogenetic analysis of the Aetosauria, comprising 27 taxa and 83 characters, includes more than 40 new characters that focus on better sampling the cranial and endoskeletal regions, and represents the most comprenhensive phylogeny of the clade to date. Parsimony analysis recovered three most parsimonious trees; the strict consensus of these trees finds an Aetosauria that is divided into two main clades: Desmatosuchia, which includes the Desmatosuchinae and the Stagonolepidinae, and Aetosaurinae, which includes the Typothoracinae. As defined Desmatosuchinae now contains Neoaetosauroides engaeus and several taxa that were previously referred to the genus Stagonolepis, and a new clade, Desmatosuchini, is erected for taxa more closely related to Desmatosuchus. Overall support for some clades is still weak, and Partitioned Bremer Support (PBS) is applied for the first time to a strictly morphological dataset demonstrating that this weak support is in part because of conflict in the phylogenetic signals of cranial versus postcranial characters. PBS helps identify homoplasy among characters from various body regions, presumably the result of convergent evolution within discrete anatomical modules. It is likely that at least some of this character conflict results from different body regions evolving at different rates, which may have been under different selective pressures.

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