GEIST, N.R.; RUBEN, J.A.; Sonoma State Univ.; Oregon State Univ.: Evolution of the Hard-Shelled Egg: Clues to Mesozoic Dominance and Eventual Demise of the Archosaurs

The ability to produce an amniotic egg surrounded by a rigid, calcium-rich shell conferred several distinct advantages to crocodilians, dinosaurs, birds, and presumably, a variety of basal archosaurian taxa. Such eggs are more resistant to desiccation, microbial infection, and predation than the leathery, proteinaceous shells of most other reptiles. In addition, the relatively high albumen allotment of typical archosaurian eggs provides a rich supply of water for the growing embryo, while highly permeable eggs of other reptiles are dependent on environmental water sources for proper development. We suggest that the evolution of hard-shelled eggs by basal archosaurians (Late Permian, ~250mya) may have been a crucial preadaptation that facilitated their spectacular Middle-Late Triassic adaptive radiation, a time characterized by widespread global drying that would have favored the less water-dependant archosaurian egg. This adaptation likely canalized archosaurs to obligate ovipary. The early stages in the evolution of viviparity probably relied upon extended retention of eggs in utero, and embryonic survival would have required levels of oxygen conductance incompatible with the hard-shelled egg. Furthermore, nest structures and patterns of egg burial of most dinosaurs are consistent with crocodile-like temperature-dependent sex determination, a selectively advantageous strategy in thermally equable climates of the Late Jurassic-Late Cretaceous. Global climatic instability at the end of the Cretaceous may have critically skewed dinosaurian sex ratios, hastening their extinction. The evolution of avian endothermy, coupled with heterogametic sex determination and egg incubation would have favored survival of birds during extended periods of global cooling at the K-T boundary.