Alligators Provide Evidence for the Evolution of an Archosaurian Mode of Oviparity1

The female reproductive tract of birds is different from that of other oviparous amniotes in that the eggshell membranes and calcareous layer are formed in separate regions of the uterus; the isthmus and shell gland, respectively. Phylogenetically, birds are included among the archosaurs, along with crocodilians and dinosaurs. Many dinosaurs were oviparous, producing hard-shelled eggs, yet the reproductive system of dinosaurs has proven difficult to investigate, due to poor preservation of soft anatomy. In this study, we examined functional morphology and eggshell formation in a reptilian archosaur, the American alligator, and demonstrated that the crocodilian reproductive tract has separate uterine regions for formation of the eggshell membranes and calcareous layer. These uterine regions are ultrastructurally comparable to the isthmus and shell gland of birds, and may be homologous. This similarity of reproductive functional morphology between crocodilians and birds may implicate the evolution of an archosaurian mode of oviparity that may shed light on dinosaur reproduction.