Titanosaur reproductive biology : comparison of the Auca Mahuevo Titanosaur nesting locality (Argentina), to the Pinyes Megaloolithus nesting locality (Spain)

Abstract

Taphonomic and microstructural studies at the Late Cretaceous Auca Mahuevo titanosaur nesting site (Argentina) reveal significant differences in reproductive attributes compared to alleged sauropods producing Megaloolithus eggs at the Pinyes locality (Spain). Auca Mahuevo clutches contain 15-40 M. patagonicus eggs; many of the 12-14 cm eggs contain titanosaur remains. Six clutches include both normal and abnormal eggs exhibiting three types of abnormal morphology: Type I displays two normal, superimposed eggshells, while Type II and III exhibit a normal inner eggshell, with one and three overlying eggshells, respectively. Previous studies that endeavor to link egg abnormalities to dinosaur extinction lack taphonomic and rigorous statistical methods. The Pinyes locality occurs in the Tremp Formation, exposed in the Spanish Pyrenees. The overbank deposits contain clutches with 4-12 eggs; none of the 16-24 cm M. Siruguei eggs contain embryos. Although often assigned to sauropod dinosaurs, M. siruguei differs from M. patagonicus in clutch size, egg volume, shell thickness, pore density, and incubation mode; thus, taxonomic assignment to sauropods seems questionable.

The water vapor conductance rates (ĠH2O) of the Auca Mahuevo and Pinyes eggs are 341 and 3979 mg H2O/(dayTorr), respectively. These values support previous interpretations of egg burial for Pinyes clutches and open incubation (substantiated by trace fossil nests) for the much larger Auca Mahuevo titanosaur clutches. In addition, the potential ĠH2O of the titanosaur egg resembles that of some Late Cretaceous theropod eggs that are partially buried in sediment. The ĠH2O of Auca Mahuevo egg is 2.7 times greater than an avian egg of comparable size and the microenvironment of the nest remains unclear. Comparison of the fossil eggs to those of modern reptiles is difficult, due to the paucity of studies and broader range of values reported for reptile eggs. Detailed sedimentological studies are essential in order to distinguish biological features from those resulting from taphonomic or geologic phenomena. The taphonomic context and spatial association of fossil eggs provide an essential framework for comparisons of the reproductive biology of different dinosaur species, time periods, and paleogeographic regions.