Morphology variations in the skull of marine turtles in the Gulf of Venezuela: Identification of multiple ecotypes.

Marine turtle skull morphology undergoes significant ontogenetic shifts influenced by diet, area of distribution, population of origin, age class and species. This study evaluates the morphological differences between Chelonia mydas and Eretmochelys imbricata according to the geographic origins of their distribution in the Gulf of Venezuela (Upper Guajira and lower Guajira). Using geometric morphometrics models, were analyzed 83 skulls (69 C. mydas; 14 E. imbricata). The photographs obtained were digitized and landmarks were established for the dorsal, ventral, and lateral views using TPS Dig2 program. Subsequently, a Procrustes superimposition was applied to standardize the dataset. Geometric morphometry analysis was performed with MOG_27 and PAST-Paleontological Statistic software. Shape variations and size-class differences were assessed through Principal Component Analysis (PCA) and Thin Plate Splines (TPS). The results reveal variations in all three views for C. mydas, primarily linked to the ontogenetic dietary shift from immature to adult stages. Positive allometry was identified in the parietal bones (dorsal view) for both species. In the ventral view, the greatest development was concentrated in the vomer and the opening of adductor fossa, while the lateral view showed positive allometry in the postorbital, squamosal, and quadratojugal bones. Notably, E. imbricata exhibited negative allometry in the beak. These morphological changes correlate with the development of the adductor muscles, compression and bite force required for adult diets. Furthermore, shape similarities between specific groups of C. mydas and E. imbricata of different sizes suggest the potential presence of distinct ecotypes in the region.