Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. They are members of the theropod dinosaur subgroup Coelurosauria, a diverse clade that includes tyrannosauroids and dromaeosaurids, among others. Features like “hollow” bones and postcranial skeletal pneumaticity, feathers, a unique forelimb digit formula, endothermy, and rapid growth rate arose in non-avian dinosaurs in a gradual process occurring over tens of millions of years.
In contrast with all other living reptiles, birds grow extremely fast and possess unusually low levels of intraspecific variation during postnatal development, suggesting that this avian style of development must have evolved after its most recent common ancestor with crocodylians but before the origin of Aves. Most studies indicates that the low levels of variation that characterize avian ontogeny were present in close non-avian relatives as well.
Compared with birds, the theropod Coelophysis bauri possess a large amount of intraspecific variation. Coelophysis bauri is the type species of the genus Coelophysis, a group of small, slenderly-built, ground-dwelling, bipedal carnivores, that lived approximately 203 million years ago during the latter part of the Triassic Period in what is now the southwestern United States. Using this taxon to interpret development among early dinosaurs, geoscientists Christopher Griffin and Sterling Nesbitt discovered that the earliest dinosaurs had a far higher level of variation in growth patterns between individuals than crocodiles and birds. The presence of scars on the bones left from muscle attachment and marks where bones had fused together helped the researchers assess how mature the animals were compared with their size.
Body size and extinction risk have been found to be related in various vertebrate groups, therefore a high level of variation within a species may be advantageous in an ecologically unstable environment and may have contributed to the early success of dinosaurs relative to many pseudosuchian clades in the latest Triassic and through the End-Triassic Mass Extinction into the Early Jurassic.
Christopher T. Griffin and Sterling J. Nesbitt, Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds. PNAS 2016 : 1613813113v1-201613813.
Brusatte SL, Lloyd GT, Wang SC, Norell MA (2014) Gradual assembly of avian body plan culminated in rapid rates of evolution across the dinosaur-bird transition. Curr Biol 24(20):2386–2392
Puttick, M. N., Thomas, G. H. and Benton, M. J. (2014), HIGH RATES OF EVOLUTION PRECEDED THE ORIGIN OF BIRDS. Evolution, 68: 1497–1510. doi: 10.1111/evo.12363 A.