Embryonic growth patterns in ostrich and other ratite skeletons

Heterochrony, in particular neoteny, is commonly invoked to describe the evolution of the ostrich and other ratites. Paedomorphic features include large eyes, down-like plumage, and unfused cranial sutures in the adult animal. Also, the large size of most ratites and the flightlessness of all modern members of the group have been attributed to neoteny. A formal analysis of embryo growth patterns was undertaken to test this hypothesis.; A post-hatch ontogenetic series of ostrich skulls disproves that cranial sutures remain open and unfused throughout ontogeny. In sexually mature individuals, skull sutures are no longer visible, thus invalidating one of the claimed neotenic features.; To obtain controlled growth series, pre-hatch embryos of the elegant-crested tinamou, chicken, emu and ostrich were processed to stain cartilaginous tissues blue, calcified skeletal tissues red, and surrounding soft tissues transparent. Measurements were then made of the body, wing, leg, and skull lengths, and diameter of the eye, and plotted against the known ages. Within a phylogenetic context the growth trajectories of the four species reveal that none of the patterns observed in the ostrich conform to the classical definition of paedomorphosis but, rather correspond to peramorphosis. Certain emu growth patterns show a neotenic mode of evolution.; These results challenge the historical conclusion that all ratites evolved via paedomorphosis. Adult morphology does not necessarily reflect evolutionary processes. Different morphogenetic pathways may lead to similar morphological patterns, even in closely related taxa.