Alligator appendicular architecture across an ontogenetic niche shift

Abstract

A variety of species undergo ontogenetic niche shifts in either diet, habitat, or both. As a result, multiple ontogenetic stages are able to take advantage of different resources and live in sympatry without competing with one another. The American alligator (Alligator mississippiensis) begins to undergo an ontogenetic niche shift in both diet and habitat at a length of 1.2 m. They transition from a terrestrial wetland environment to a riverine environment and take advantage of different dietary resources. At 1.8 m, A. mississippiensis reaches sexual maturity. Ontogenetic shifts in habitat have the capacity to alter morphology, especially limb morphology, as different age classes traverse different ecological systems. We evaluated shape trends in the scapulae, humeri, ilia, and femora using geometric morphometrics to test whether there were punctuated changes in limb shape, shape disparity, and integration corresponding to either the ontogenetic habitat shift or onset of sexual maturity. We found size to strongly correlate with limb shape but found a continuous size gradient rather than punctuated changes in size. Furthermore, we found that adults (total length > 1.8 m) had significantly higher limb shape disparity than juveniles or subadults, likely related to ontogenetic decreases in limb use and a reduction in limb constraints. Finally, we found that the forelimb and hindlimb acted as a single integrated unit and that neither the forelimb nor hindlimb was significantly more integrated than the other. Therefore, the ontogenetic niche shift itself did not impact limb morphology in A. mississippiensis.

Femoral specializations to locomotor habits in early archosauriforms

“…However, we did observe a separation between juveniles and adult Crocodylus along the axes pertaining to femoral specialization to locomotor mode (as in PC2; Figure 3a; “Cro”). Thus, juvenile Crocodylus had straighter femora (i.e., lower anterior curvature) than the adult specimen, which is congruent with findings described under “femoral robusticity” in Alligator by Hedrick et al (2021). Morphological variation of the femur that seemed to indicate a shift of estimated locomotor mode from bipedal to quadrupedal across ontogeny was also observed in extant crocodylians by McPhee et al (2018; Caiman ) and Bishop et al (2020; Alligator ).…”

“…Femoral bowing is known to vary across all archosauriforms (Gauthier et al, 1988, more specifically with the origin of a more erect posture (Hutchinson, 2001) and body size variations (Biewener, 1983; Carrano, 2000). This feature is suggested to better predict mechanical bending stress related to a bipedal locomotor habit (Hutchinson, 2001), whereas a straightening of the shaft correlates with increased body mass in quadrupedal animals, except across crocodylian ontogeny (Biewener, 1983; Carrano, 2000, Hedrick et al, 2021; our results for Crocodylus ). However, large bipedal archosaurs, such as sauropodomorphs and theropods, retained an anteroposteriorly bowed femur (Hutchinson, 2001).…”

“…Femoral curvature is negatively correlated with increased body size in dinosaurs (Carrano, 2001), and we demonstrated that this applies more broadly to archosauriforms. However, we found the opposite trend in our limited sample of crocodylian ontogeny (Figure 3a, 3g; see also Hedrick et al, 2021). Femoral curvature was enhanced plesiomorphically in avemetatarsalians with the origin of an erect limb posture and perhaps bipedalism (Hutchinson, 2001).…”

“…We showed that this pattern does not result from the presence of juvenile crocodylians in the sample, because Revueltosaurus and Parringtonia also had low centroid sizes with highly robust femora (Figure 3a "Cro, Rev, Par," Figure 7a, Figure S6). In addition, Dodson (1975) and Hedrick et al (2021) demonstrated that the femoral robusticity in Alligator mississippiensis varied significantly along ontogeny, with the fourth trochanter migrating down the shaft toward the adult age, along with an increase of femoral disparity. This morphological variation is identical to the one we highlighted along the specialization to body size.…”

Architecture of the bronchial tree in Cuvier's dwarf caiman (Paleosuchus palpebrosus)

“…To date, the vast majority of anatomical work on crocodilians has been done on the American alligator (Allen et al, 2010; Crosby, 1917; Dodson, 1975; Hedrick et al, 2021; Holliday et al, 2013; Lessner & Holliday, 2020). Comparatively, the functional morphology of other crocodilian taxa has received considerably less attention with much of the research focusing instead on behavior, ecology, and systematics (Bittencourt et al, 2019; Campos et al, 2010; Campos et al, 2012).…”