OSTEOLOGICAL AND ONTOGENETIC VARIATION IN CAIMAN CROCODILUS

The anatomy of the spectacled caiman, Caiman crocodilus, has been studied for over 150 years, but this work has failed to resolve its systematic position. Although it is known that caimans lie within the group Crocodylia and the more inclusive Alligatoridae, neither the relationships between extant caimans nor those within the extensive fossil stem-lineage Caimaninae are yet determined. Specifically, C. crocodilus may be the sister taxon of either C. latirostris or C. yacare, and may include three to six subspecies, none of which are analyzed phylogenetically. This uncertainty stems partly from the extensive interspecific, ontogenetic, and even inter-organismal variability of caimans, and of C. crocodilus in particular.

Morphologic descriptions of C. crocodilus tend to focus on the external characteristics of adult specimens only, thus excluding possible characters from ontogenetic sequences or internal structures such as the eustachian and pneumatic passages. Both ontogeny and internal skull morphology have been described in crocodiles, but not in C. crocodilus and not for use in systematic character coding. Descriptions of both these data sources, once compared with other caiman and crocodylid taxa, will thus add important new sources of data to future systematic analyses of caimans and will reduce the amount of unknown variation in the C. crocodilus complex.

This study describes internal osteological variation with the use of high-resolution computed tomography (CT) scans. CT can provide digital images of the internal cavities and structures of objects such as skulls without requiring destructive serial sectioning. The study results in the recognition of a wide range of variation in both the eustachian and pneumatic passages. The ontogenetic trajectory of C. crocodilus is described from a growth series of the species and shows a number of variations in systematic characters currently in use as well as many that may form potential systematic characters. Finally, these potential characters are proposed using descriptions from both the osteological and the ontogenetic data sets.