A three-dimensional geometric morphometric analysis of the morphological transformation of Caiman lower jaw during post-hatching ontogeny

FERNANDEZ BLANCO MV; CASSINI GH; BONA P

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Año: 2023

Shape ontogenetic changes of the lower jaw in crocodilians are poorly understood. In order to answer some questions related to the inter- and intraspecific morphological variation of the mandible of two extant Caiman species, we performed a three-dimensional geometric morphometric approach. For this purpose, we used landmarks and semilandmarks on two ontogenetic mandibular series of 48 and 15 post-hatching specimens of C. yacare and C. latirostris, respectively. We have also examined the relationship between these anatomical transformations and ontogenetic shifts in diet. We have performed a Principal Component Analysis (PCA) for the two species, and Regression and Partial Least Squares (PLS) analyses for each species separately. As a result, PC1 and PC2 accounted for almost 49% of the variance, and species were segregated along the PC1 with specimens of C. yacare showing slender mandibles and specimens of C. latirostris with more robust morphotypes. The PC2 and regression analyses showed an age gradient and represented ontogenetic shape changes. Allometric scaling explained 19.01% of shape variation in C. yacare and 22.31% in C. latirostris. Adult caiman mandibles are higher and wider than juvenile ones and shape changes are more conspicuous in C. latirostris. The PLS analyses showed a significant relationship between shape and diet. The first pair of PLS explains about 98.41% and 94.85% of covariation in C. latirostris and C. yacare, respectively. Morphological changes of the PLS1 of block-1 match with those of the regression analysis for both species. We have detected morphological transformations in areas where the musculature in charge of mandibular movements is attached. Common morphological changes occurring during ontogeny seem to reflect the same mechanical properties required for crushing and killing in both species, driven by an ontogenetic shift in the diet from invertebrates to vertebrates. Additionally, interspecific differences (gracile and robust morphotypes) were also found to be correlated to ontogenetic changes in diet and could be related to dissimilar feeding mechanical requirements (stiffness and toughness of the item consumed) and to different habitat preferences. Robust mandibles would be more suitable for shallow and fully vegetated environments, as it can see in C. latirostris, whereas slender jaws seem to be more proper for aquatic species such as C. yacare.