Ontogenetic variation in the skulls of Caiman: the case of Caiman latirostris and Caiman yacare (Alligatoridae, Caimaninae)

Caiman is one of the five extant genera of alligatorid crocodylians. While several quantitative and qualitative studies exist on morphological variation in the genus, little is known about ontogenetic effects. Here, we quantify ontogenetic variation in morphology for Caiman yacare and C. latirostris in a phylogenetic context. A linear regression analysis on twelve skull measurements of C. yacare and C. latirostris against a measure of size (the first axis of a PCA of all variables) showed high correlation coefficients (r(2)=0.89-0.99) and negative allometry. Eight allometric trajectories showed common slopes at different intercepts, reflecting a common ontogenetic pattern of morphological growth fixed early in ontogeny. The anterior width of the snout and the posterior width of the skull table are suitable to discriminate between the two species. The relationship between snout width and snout length is isometric in C. latirostris while it is negatively allometric in C. yacare. These results confirm that the snout shape is a distinctive feature between species established early in ontogeny. The narrowing and lengthening of the snout in C. yacare during ontogeny results in adult forms widely represented in other extant taxa within the genus. The broader and shorter snout in C. latirostris is probably an autapomorphic feature of this species within Caimaninae.