Intraspecific variation in Aetosauroides scagliai Casamiquela (Archosauria: Aetosauria) from the Upper Triassic of Argentina and Brazil: evidence of sexual dimorphism?

TABORDA, JEREMÍAS R A; HECKERT, A B; DESOJO, JULIA B

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

Aetosaurs are quadrupedal pseudosuchian armoured archosaurs recovered from Upper Triassic strata across Pangaea. They are characterized by dorsal and ventral carapaces, and appendicular osteoderms, all of them ornamented. Aetosaurs have been proposed as index fossils, but this possibility is complicated due to questions about the taxonomic reliability of osteoderms, which are frequently used to identify taxa. Therefore, it is important to understand the intraspecific variation of osteoderms in aetosaurs. Recent studies of histological sections in osteoderms of Aetosauroides scagliai Casamiquela have provided novel information about the growth pattern of the paramedian osteoderms. Furthermore, the absence of remodeling of the osteoderm tissues allows estimation of the ontogenetic age by counting lines of arrested growth (LAGs). Here we describe three types of ornamentation in the dorsal armour of A. scagliai, particularly on the medial area of paramedial osteoderms. One of these consists of radial grooves and ridges, with small pits inside the grooves, named the ?radial pattern?; the other end member, called the ?anastomosing pattern?, is composed of anastomosing crests and proportionately larger, more irregular pits. The third pattern is an ?intermediate? or ?transitional pattern? between the ?radial? and ?anastomosing patterns?. The articulated dorsal armour preserved in specimens PVL 2059 and PVL 2073 exhibit all three patterns of ornamentation, but they differ in the position of each throughout the carapace. Because the specimens are uniformly well preserved, with no indication of damage or resorption of the external surface of osteoderms, and the histological sections also show no evidence of post-mortem damage in these structures, we reject the hypothesis of taphonomic explanations of ornamentation variation. Moreover, the holotype (PVL 2073) is slightly smaller (around 10%), but also ontogenetically younger (5 LAGs), than PVL 2059 (10 LAGs). Because both share other taxonomically informative features (e.g. presence of an oval fossae ventral to the neurocentral suture on the lateral sides of the centra), they appear to represent the same species, requiring an explanation for the disparity in size and details of ornamentation patterns. Combining this information with the evidence of ontogenetic state and general body size of all the analyzed specimens, and comparing with living crocodiles (where the male specimens are typically relatively larger than the females at the same age), we conclude that the intraspecific variation in A. scagliai is compatible with the hypothesis of sexual dimorphism. These results show the need to explore the sources of intraspecific variation in aetosaurs.