A new species of Aetosauroides (Archosauria: Aetosauria) from southern Brazil

DESOJO, J.B.; KISCHLAT, E-E.

Rio de Janeiro, Brasil

Congreso; II Congreso Latino-Americano de Paleontología de Vertebrados; 2005

Museo Nacional/UFRJ

Aetosauria is a well-corroborated clade of quadrupedal armoured archosaurs that was a characteristic component of continental ecosystems during much of the Late Triassic in North America, Europe, North Africa, India, and South America. In the latter continent aetosaurian remains are known in the Carnian of Argentina and Brazil. In 1977 the dorsal and ventral amour of a small aetosaur (Museu de Ciências e Tecnología MCP 13a, 13b) was discovered in the Alemoa Member of the Santa Maria Formation. Susequently, in 1982 J.D. Zacarias described this material in her unpublished Master thesis and assigned it to a new species of Aetosauroides Casamiquela, a nominative genus represented in Argentina by a single species, A. scagliai. In 2001, Lucas and Heckert redescribed the material studied by Zacarias, together with other specimens (Municipal Collecion CPE2 168 and MCP 3450-PV) posteriorly collected from the same formation but assigned them to the nominative genus Stagonolepis Agassiz. One year later, da Rosa and Leal published additional aetosaur material (Universidade Federal de Santa Maria UFSM 11070), which consists of skeletal elements and osteoderms from the Santa Maria Formation at another locality. Despite the lack of cranial remains, fragmentary condition, and poor preservation of all these materials, the revision indicates that they represent a new species of Aetosauroides, as Zacarias preliminarly proposed. This conclusion is based on presacral neural spines lower than the height of the centra and transverse processes on presacral vertebrae less than twice the centrum length. In addition, the ornamentation of the dorsal and ventral osteoderms consists of a radial pattern of pits and grooves, and the dorsal protuberance generally is not in contact with the posterior margin of the paramedial osteoderms. However, it differs from A. scagliai in having cervical vertebrae without ventral keels and presacral centra lacking lateral depressions. Moreover, juvenile specimens of A. scagliai have osteoderms ornamented mostly with pits, whereas the osteoderms of those of Brazil have mostly grooves (e.g. MCP 13a); this suggests a different developmental pattern.