Field of Science

Aetobarbakinoides brasiliensis, a New Aetosaur from the Late Triassic of Brazil

This is an interesting new specimen from the Santa Maria Formation of Brazil. I've had the chance to personally study this material and although poorly preserved and despite possesses a radial patterning of the dorsal osteoderms it clearly does not belong to the South American genera Aetosauroides or Neoaetosauroides mainly because of characteristics of the vertebrae. In fact, the vertebrae with their well developed accessory processes and lack of ventral keels strongly resemble those of desmatosuchines. This is supported by the phylogenetic analysis.

Notably this is the first aetosaur taxon to be diagnosed using postcranial characters rather than those of the osteoderms. Indeed only a few poorly preserved osteoderms are present in the specimen. I've argued in the past that despite the long use of armor ornamentation to diagnose aetosaur species, new specimens are demonstrating that these characters are highly convergent between hypothesized main aetosaur clades and caution must be used.

This paper also finds Aetosaurinae (sensu Parker, 2007) to be paraphyletic. Again this is not surprising given the poor support for the clade in the original analysis, the fact that Aetobarbakinoides possesses "Aetosaurinae"-like armor with desmatosuchine-like vertebrae, and the fact that lateral armor is lacking in this new taxon whereas lateral armor characters strongly affect the topology of Parker (2007). This is not surprising given that the analysis of Parker (2007) was explicitly testing the phylogenetic signal of lateral osteoderms in aetosaurs.

This analysis also recovers Aetosauroides outside of Stagonolepididiae (sensu Heckert and Lucas, 2000), which demonstrates the presence of non-stagonolepidid aetosaurs. Thus the names Stagonolepididae and Aetosauria cannot be used interchangeably (as they commonly are) as I cautioned in 2007.

There is much more work today with the phylogeny of the Aetosauria and many new undescribed specimens.  I am focusing on a lot of these in my ongoing PhD work.

Desojo, J. B., Ezcurra, M. D., and E. E. Kischlat. 2012. A new aetosaur genus (Archosauria: Pseudosuchia) from the early Late Triassic of southern Brazil. Zootaxa 3166:1-33.

Abstract - We describe the new aetosaur Aetobarbakinoides brasiliensis gen. et sp. nov. from the early Late Triassic (late Carnian early Norian) Brazilian Santa Maria Formation. The holotype is composed of a partial postcranium including several cervical and dorsal vertebrae and ribs, one anterior caudal vertebra, right scapula, right humerus, right tibia, partial right pes, and anterior and mid-dorsal paramedian osteoderms. Aetobarbakinoides is differentiated from other aetosaurs by the presence of cervical vertebrae with widely laterally extended prezygapophyses, mid-cervical vertebrae with anterior articular facet width more than 1.2 times wider than the posterior one, anterior caudal vertebrae with extremely anteroposteriorly short prezygapophyses, elongated humerus and tibia in relation to the axial skeleton, and paramedian osteoderms with a weakly raised anterior bar. A cladistic analysis recovered the new species as more derived than the South American genera Aetosauroides (late Carnian-early Norian) and Neoaetosauroides (late Norian-Rhaetian), and it is nested as the sister-taxon of an unnamed clade, composed of Typothoracisinae and Desmatosuchinae, due to the absence of a ventral keel in the cervical vertebrae. Aetobarbakinoides presents a skeletal anatomy previously unknown among South American aetosaurs, with the combination of presacral vertebrae with hyposphene, anteroposteriorly short and unkeeled cervical vertebrae, gracile limbs, and paramedian osteoderms with a weakly raised anterior bar. Aetobarbakinoides is among the oldest known aetosaurs together with Aetosauroides from Argentina and Brazil and Stagonolepis robertsoni from Scotland, indicating Aetobarbakinoides, which is one of the oldest known aetosaurs, is in agreement with an older origin for the group, as it is expected by the extensive ghost lineages at the base of the main pseudosuchian clades.

5 comments:

  1. "This paper also finds Aetosaurinae (sensu Parker, 2007) to be paraphyletic. Again this is not surprising given the poor support for the clade in the original analysis."

    What does that mean? A clade can't be found to be paraphyletic. Do you mean that the clade is found to include an OTU (Aetobarbakinoides?) that was expected to be found outside? If so, that just means that Aetobarbakinoides is an aetosaurine.

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  2. one comment: Neoaetosaroides and Aetosauroides are diagnosed by several cranial and postcranial characters (see Desojo & Baez, 2005, 2007; Desojo & Ezcurra 2011). So, this is not the first aetosaur taxon to be diagnosed using postcranial characters.

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  3. @Mike: I don't quite understand your point. A hypothesized named clade can be found to be paraphyletic in which case it ceases to be the clade in the sense of the defining author. So in this specific case Aetosaurinae (all taxa closer to Aetosaurus ferratus than to Desmatosuchus spurensis) now includes only Aetosaurus ferratus and no longer includes all of the taxa considered by Parker (2007) (e.g. Typothoracisinae).

    @Julia: It's true that you have rediagnosed these taxa using more non-armor material, but in papers such as Heckert and Lucas (2000) they were diagnosed mainly on armor characters.

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  4. Maybe it is better to say that Aetosauriane (sensu Parker 2007) was not recovered in this analysis :)

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  5. Bill,

    In your 2007 paper (p58), you accept the definition of Aetosaurinae provided by Heckert and Lucus (2002): "a stem-based taxon containing all taxa more closely related to Aetosaurus than to the last common ancestor of Aetosaurus and Desmatosuchus." That is a clade, always, whatever topology you are working with: only the content changes. So it's always monophyletic and it's always "recovered", but it may or may not contain the animals you expected.

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