Dinosaur embryo, Museum of the Rockies, Bozeman, Montana
Much of what makes birds so different from other animals relates to the skull, specialized in birds to accommodate the visual and neuromuscular systems needed to coordinate flight and vision, and an ultra-adaptable beak. This comparison of skull morphology in birds with that of extinct theropod dinosaurs suggests that the characteristic features of the bird skull evolved in large part through paedomorphosis — the retention of juvenile features in the adult. With prominent eyes, larger brains and a short face, primitive stem-group birds resemble the embryos and juveniles of archosaurs, the non-avian branch of the theropod dinosaur line that includes the extant crocodylians.
https://youtu.be/jGvZUTUinpk Video Arhat Abzhanov, Harvard
Birds have paedomorphic dinosaur skulls
NATURE Volume:487,Pages:223–226Date published:(12 July 2012)
The interplay of evolution and development has been at the heart of evolutionary theory for more than a century1. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals2, including humans1. Birds are the most speciose land vertebrates, with more than 10,000 living species3 representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system4, 5 allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals6. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.