Author Summary
Animals display huge morphological and ecological diversity.
One possible explanation of how this diversity evolved
is the "niche filling" model of adaptive radiation—under
which evolutionary rates are highest early in the evolution
of a group, as lineages diversify to fill disparate ecological
niches. We studied patterns of body size evolution in
dinosaurs and birds to test this model, and to explore the
links between modern day diversity and major extinct
radiations. We found rapid evolutionary rates in early
dinosaur evolution, beginning more than 200 million years
ago, as dinosaur body sizes diversified rapidly to fill new
ecological niches, including herbivory. High rates were
maintained only on the evolutionary line leading to birds,
which continued to produce new ecological diversity not
seen in other dinosaurs. Small body size might have been
key to maintaining evolutionary potential (evolvability) in
birds, which broke the lower body size limit of about 1 kg
seen in other dinosaurs. Our results suggest that the
maintenance of evolvability in only some lineages explains
the unbalanced distribution of morphological and ecological
diversity seen among groups of animals, both extinct
and extant. Important living groups such as birds might
therefore result from sustained, rapid evolutionary rates
over timescales of hundreds of millions of years.