The South American Aetosaur Aetosauroides is Not Referable to Stagonolepis

In a series of papers Heckert and Lucas (1999, 2000, 2002) and Lucas and Heckert (2001) proposed that the American aetosaur taxon Aetosauroides was a junior synonym of Stagonolepis, and that as a result the Ischigualasto (Argentina) and the Santa Maria (Brazil) formations were Adamanian (latest Carnian) in age based upon vertebrate biostratigraphy and correlable with other Adamanian strata such as the lower part of the Chinle Formation in western North America. However, these synonomies were based on very superficial resemblances and not on apomorphy based comparisons (Parker, 2008).

The new issue of the Journal of Vertebrate Paleontology has a paper by Julia Desojo and Martin Ezcurra redescribing some of the Aetosauroides material and refuting the synonomy with Stagonolepis using a detailed comparison of characters. Especially important are characters of the skull, which clearly demonstrates strong differences between the two taxa.

As a result Stagonolepis is not known from South America and therefore the proposed biostratigraphic correlations between South America, North America, and Europe using aetosaurs is not supported by this study. This is also supported by radiometric dates which show a 10 million year difference between the main fossil horizon in ht eIschigualasto Formation and the base of the main fossil bearing beds in the Chinle Formation (Irmis and Mundil, 2008).

I have suggested elsewhere (Parker, 2008) that Stagonolepis does not occur in North America either and thus aetosaur taxa appear much more endemic than argued by Heckert and Lucas and thus of limited (only regional) biostratigraphic utility.

Biostratigraphy and taxonomy aside, this redescription highlights the plesiomorphic nature of Aetosauroides, especially regarding features of the skull including a gracile, fully-toothed dentary, different from the "slipper-shaped", anteriorly edentulous mandibles of more derived aetosaurs, and the meeting of the premaxilla and nasal to exclude the maxilla from the margin of the external naris. Thus, Aetosauroides is an interesting and clearly plesiomorphic form with an armor ornamentation convergent with more derived forms such as Stagonolepis and Calyptosuchus.

Desojo, J. B., and M. D. Ezcurra. 2011. A reappraisal of the taxonomic status of Aetosauroides (Archosauria, Aetosauria) specimens from the Late Triassic of South America and their proposed synonymy with Stagonolepis. Journal of Vertebrate Paleontology 31:596-609. DOI: 10.1080/02724634.2011.572936

Abstract - The South American record of early Late Triassic aetosaurs is composed of two species: Aetosauroides scagliai and “Aetosauroides subsulcatus.” Previously undescribed materials belonging to “Aetosauroides subsulcatus” allow us to reassess its taxonomy, leading us to consider it a junior synonym of Aetosauroides scagliai. Based on the emended diagnosis of the species provided here, we recognize that specimens assignable to Aetosauroides scagliai are less common than thought previously and several of them are not diagnostic beyond indeterminate non-typothorasicine aetosaurins. Previous assignments of Aetosauroides as a junior synonym of Stagonolepis are not followed because the South American taxon is distinct due to the presence of a maxilla excluded from the external narial margin, tooth crowns with a straight distal margin and without a constriction between the root and crown, a gradually convex ventral margin of the dentary, oval fossae ventral to the neurocentral suture on the lateral sides of the centra, and a ratio between the length and the width between the distal-most tips of the postzygapophyses equal to or lower than 0.75. The evidence provided here bolsters the validity of Aetosauroides and extends the distribution of Aetosauroides scagliai into southern Brazil. Although Stagonolepis was employed as an index taxon for the Adamanian LVF, this genus is currently restricted to Europe and North America. Thus, no overlapping genera or species of aetosaur are shared between South America and other landmasses. Accordingly, the record of aetosaurs is not useful for providing biostratigraphical correlations between Late Triassic South American beds and those in other regions.

REFERENCES

Heckert, A. B., and S. G. Lucas. 1999. A new aetosaur from the Upper Triassic of Texas and the phylogeny of aetosaurs. Journal of Vertebrate Paleontology 19:50–68.

Heckert, A. B., and S. G. Lucas. 2000. Taxonomy, phylogeny, biostratigraphy, biochronology, paleobiogeography, and evolution of the Late Triassic Aetosauria (Archosauria: Crurotarsi). Zentralblatt fur Geologie und Palaontologie, 1998, Teil I, Heft 11–12:1539–1587.

Heckert, A. B., and S. G. Lucas. 2002. South American occurrences of the Adamanian (Late Triassic, Latest Carnian) index taxon Stagonolepis (Archosauria: Aetosauria) and their biochronological significance. Journal of Paleontology 76:852–863.

Irmis, R. B., and R. Mundil. 2008. New age constraints from the Chinle Formation revise global comparisons of Late Triassic vertebrate assemblages. Journal of Vertebrate Paleontology 28:95A.

Lucas, S. G., and A. B. Heckert. 2001. The aetosaur Stagonolepis from the Upper Triassic of Brazil and its biochronological significance. Neues Jahrebuch fur Geologie und Palaontologie, Monatshefte 2001:719–732.
Parker, W. G. 2008. How many valid aetosaur taxa are there? Reviewing the alpha-taxonomy of the Aetosauria (Archosauria: Pseudosuchia) and its implications for Late Triassic global biostratigraphy. Journal of Vertebrate Paleontology 28:125A.