From eggs to hatchlings; nest site taphonomy of American crocodile (Crocodylus acutus) and broad-snouted caiman (Caiman latirostris)

Nesting behaviors of extant vertebrates can serve as taphonomic models for interpreting extinct archosaurian reproduction. Past studies have examined birds with open nests and nest-bound young and tortoises with buried nests and precocial young. Here we taphonomically describe nesting sites of two crocodylians, American crocodile (Crocodylus acutus) at Turkey Point, Florida and broad-snouted caiman (Caiman latirostris) from Santa Fe and Chaco provinces, Argentina. Surveys focused on eggshell abundance, orientation, and distribution and nest modification of successfully hatched nests. American crocodiles excavate triangular or semi-circular depressions into their nest during hatching. Maximum depths of these parent-assisted hatching traces ranged from 20-45 cm, with a breadth of 50-80 cm. Eggshell orientations outside these excavated pits favored concave down (53.1-80.0%). Broad-snouted caiman constructed mound nests of predominantly plant debris in forested areas with organic rich soil or on vegetation islands. Nests ranged in diameter from 1.2-1.6 m with a height of 0.3-0.6 m. Eggshell orientations within opened egg chambers favored concave up (61.8%), whereas fragments outside the chamber were nearly evenly distributed (51.8% concave-up). Eggshell distribution and orientation at these nesting sites result from adult females assisting and transporting eggs and young during hatching. Observed eggshell orientations in and around the egg chamber in caiman nests are similar to the 60:40 up:down hatching ratio reported in both bird and tortoise nests, whereas crocodile nests are more similar to a 40:60 ratio of trampled shell. Documentation of these nest characteristics and eggshell orientations may facilitate interpretations of parental assistance in the fossil record.