Taphonomy of Extant Desert Tortoise (Gopherus Agassizii) and Loggerhead Sea Turtle (Caretta Caretta) Nesting Sites: Implications for Interpreting the Fossil Record

Abstract

Dinosaur reproductive biology is often inferred from the biology of extant taxa; however, taphonomic studies of modern nest sites have focused exclusively on avian, rather than reptilian species. We documented eight Agassiz's desert tortoise (Gopherus agassizii) nests and ten loggerhead sea turtle (Caretta caretta) nests. Gopherus agassizii excavated burrows up to 70 cm long and laid rigid-shelled eggs 10-12 cm below the burrow floor. The 19 cm 3 12 cm depressions consisted of hard consolidated sand surrounded by a 3-4-cm-high rim and contained 2-5 hatched eggs in a single layer. These hatched egg bottoms represent , 25% of the original egg, and five of 27 contained fully developed dead neonates. Desiccated membrane separated from the egg interior forming pockets that filled with eggshell and sand. Of 106 and 79 eggshell fragments in the hatched egg and surrounding sand, 48% and 23% occurred concave up, respectively. However, the combined numbers of eggshell fragments inside the eggs and in the immediately surrounding sand approximates the 60:40 ratios at in situ avian nests. Therefore, this ratio may provide reliable evidence for hatching sites regardless of the incubation strategy employed by the adult. Caretta caretta nests differed from those of tortoises in their greater depth (, 50 cm) and occurrence in moist, cohesive sand. Clutches contained over 100 pliable-shelled eggs that tore and collapsed upon hatching, without brittle fracture. Failed eggs in two clutches showed five development stages, indicating that the deaths occurred over an extended time period. With the exception of predation, the G. agassizii and C. caretta nests showed no significant eggshell or hatched eggs above the egg chamber.

A mixed vertebrate eggshell assemblage from the Transylvanian Late Cretaceous

“…A second proportion test (ratio: 42:58, χ 2 = 1.6941, 1 d.f., p = 0.4235, 95% CI between 0.3185 and 0.5354) shows that this distribution of preserved shell orientations is similar to the eggshell ratios recorded at chick-trampled sites (ratio: 42:58; χ 2 = 0.29, 1 d.f., p = 0.58; not significantly different; χ 2 = 1.49, 1 d.f., p = 0.22). This result suggests that Od site shell orientations are consistent with interpretation as a chick-trampled, non-transported accumulation 40,41 . It is also noteworthy that the shell ratio at the Od site is significantly different from previously published fragment orientations, including samples buried within substrates (ratio: 38:62; χ 2 = 0.02, 1 d.f., p = 0.86) and those that have been transported under experimental and natural fluvial conditions (ratio: 18:85; χ 2 = 5.66, 1 d.f., p = 0.01) 39 .…”

“…Previous work has shown that eggshell orientation can be used to distinguish nesting and predation localities from transported assemblages in the fossil record 18,39–41,59 . Specifically, non-transported eggshell fragments at hatching and predation sites tend to rest CU rather than CD 39 .…”

Reconstructing the mass and thermal ecology of North American Pleistocene tortoises

“…To allow for continuous growth, the bones of the tortoise shell never fuse entirely, but are held together by fibrous tissue and crenulated sutures. If not buried promptly, the shell disarticulates into individual osteoderms (Jackson et al 2015).…”

A gravid fossil turtle from the Early Cretaceous reveals a different egg development strategy to that of extant marine turtles

“…; Jackson et al . ) and also comprised components of the palaeoecology of the environments inhabited by D. padillai during the Early Cretaceous.…”