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

FORTH, C.; BONA, P; DESOJO, J. B.

Congreso; 10th International Congress of vertebrate morphology; 2013

Melanosuchus niger is a caimanine alligatorid, which is widely distributed in the north of South America. This species has been the focus of a number of ecological, genetic and some morphological studies, but investigations on intraspecific variation such as ontogenetic variation and sexual dimorphism are rare. Here we present the first assessment of intraspecific variation in the skull of M. niger using a two-dimensional geometric morphometric approach. The crania of 52 sexed individuals of different sizes were analyzed to quantify shape variation and to assign observed shape changes to ontogenetic variation, sexual dimorphism and bite force performance. Most of the variation in this species affects relative snout length, skull depth, orbit size and the width of the postorbital region, which is strongly correlated with the ontogenetic variation. During ontogeny the snout becomes longer, narrower (in dorsal view) and deeper (in lateral view), and the snout tip becomes blunter. A subnarial gap develops between the premaxilla and maxilla due to ventral expansion of the anterior part of the maxilla. The postrostrum becomes flattened and relative orbit size decreases, whereas the jugal region becomes broader and deeper. The postorbital region becomes shorter, but expands postero-laterally and the jaw joint moves substantially posterior to the posterior end of the skull roof. Comparing ontogenetic changes in M. niger with those of other crocodylian taxa indicates that ontogenetic trajectories are highly constrained within Crocodylia. Statistical support for cranial sexual dimorphism in M. niger is ambiguous, but allometric skull shape variation is strongly correlated with bite force performance. This correlation between shape and function indicates that the observed ontogenetic variation could be related to the generation of higher bite forces and a decrease in mechanical stress along with a change in diet.