Reproductive Traits of Non-avian Theropods

The reproductive biology of non-avian theropods, primarily maniraptorans, is revisited using recent fossil discoveries in the framework of the extant phylogenetic bracket. It is apparent that non-avian theropods shared some reproductive characters with crocodilians, some with birds, but also possessed their own unique features. Whereas the non-avian theropod reproductive system retained primitive archosaurian traits (two functional oviducts, hyper-ellipsoidal eggs), it also possessed derived bird-like characteristics (released one ovum per oviduct, complex eggshell ultrastructures, asymmetrical eggs). Behaviourally, non-avian theropods were more derived than crocodilians in some respects: they laid orderly clutches and the adults were in direct contact with the eggs during the incubation period. However, non-avian theropods and enantiornithine birds, like crocodilians, did not rotate their eggs during the incubation period, which suggests that such a behaviour evolved in birds more derived than enantiornithines.