Sexual Dimorphism Discovered in Upper Triassic Aetosaur Scagliali casamiquela?

INTRASPECIFIC VARIATION IN AETOSAUROIDES SCAGLIAI CASAMIQUELA (ARCHOSAURIA: AETOSAURIA) FROM THE UPPER TRIASSIC OF ARGENTINA AND BRAZIL: AN EXAMPLE OF SEXUAL DIMORPHISM?

Authors:

Taborda et al

Abstract:

Aetosaurs are a group of quadrupedal, armoured pseudosuchian archosaurs from the Upper Triassic. They are characterized by dorsal and ventral carapaces, and appendicular osteoderms, all of them ornamented. Aetosaurs have been proposed as index fossils based in large part on the distinctiveness of some osteoderms; therefore, it is important to understand the intraspecific variation of these elements in the clade. In the present contribution, we describe three types of ornamentation on the medial area of paramedian osteoderms in the dorsal armour of Aetosauroides scagliai: the “radial pattern” consists of radial grooves and ridges, with small pits inside the grooves; the “anastomosing pattern” is composed of anastomosing crests and proportionately larger, more irregular pits; and an “intermediate” or “transitional pattern” between the “radial” and “anastomosing patterns”. The articulated dorsal armour preserved in specimens PVL 2059 and PVL 2073 possess all three patterns of ornamentation, but they differ in the position of each throughout the carapace. Recent studies of histological thin-sections in osteoderms of A. scagliai show the absence of remodeling of osteoderm tissues, allowing the estimation of age by counting LAGs (lines of arrested growth). PVL 2073 is slightly longer (~10% longer centra) but also ontogenetically younger (5 LAGs) than PVL 2059 (10 LAGs; slightly wider osteoderms). Combining this information and comparing it with that of living crocodiles (where the male specimens are typically relatively larger than the females of the same age), we conclude that the intraspecific variation observed 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.