TAPHONOMY OF DESERT TORTOISE (GOPHERUS AGASSIZII) NESTING SITES FOR COMPARISON TO THE FOSSIL RECORD

We documented 8 nests of the Mojave desert tortoise (Gopherus agassizii) in order to investigate attributes of buried clutches for comparison to the fossil record. The shallow, irregularly shaped egg chambers occurred 10-15 cm below the floors of excavated burrows and measured 15-19 cm long and 8-12 cm wide. A compact rim encircled 4-5 hard-shelled eggs arranged in a single layer. Of 31 eggs, 22 hatched successfully, 4 hatchlings were missing, and 5 full-term deceased neonates remained within hatched hard-shelled egg bottoms. Limbs extending beyond the carapace retained desiccated integument, whereas skeletal elements within the carapace lacked soft tissue. Hatched eggs preserved only the lower 25% of the egg and exhibited sharp, broken edges, rather than the perforated margins expected of a taxon possessing an egg tooth. This likely reflects use of forelimbs in hatching and the unique tortoise bauplan. Desiccated membrane inside eggs separated from the interior of the egg, forming “pockets” that filled with sand. Eggshell fragments within this sand retained vertical orientations similar to eggshell preserved in some dinosaur eggs. Hatched eggs contained 106 eggshell fragments, whereas 68 occurred in the surrounding sand. Combined eggshell fragments from all 8 clutches yielded a ratio of concave-up to concave-down fragments of 57:43, comparing favorably with those documented on avian colonies. However, values varied widely from one clutch to the next, emphasizing the need for adequate sample sizes in making this assessment. Nevertheless, these ratios may be useful for distinguishing in situ from transported fragments at fossil localities, regardless of whether the eggs were buried underground or incubated at the ground surface. Studies of dinosaur localities often include generalized reviews of extant reptilian or avian nesting strategies; however, we advocate discussion of specific taxa. Attributes such as substrate composition, degree of eggshell mineralization, and adult and hatchling behavior vary widely among amniotes, and these characteristics influence potential preservation and orientation of biological materials.