Christopher T. Griffin and Sterling J. Nesbitt (2016)
Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds.
Proceedings of the National Academy of Sciences (advance online publication)
Birds—the fastest growing terrestrial vertebrates—develop unlike all other living reptiles. As part of this postnatal developmental mode, birds possess a low amount of intraspecific variation, and the timing of the origin of this low variation is poorly constrained. By studying well-sampled growth series of nonavian dinosaurs and their closest relatives, we were able to identify this transition within Mesozoic theropod dinosaurs. Surprisingly, the earliest dinosaurs and their close relatives possessed an extremely high amount of variation, higher than either crocodylians or birds. This high variation is the ancestral dinosaurian scheme and was lost in more derived nonavian theropods, including Allosaurus and Tyrannosaurus. High variation could have contributed to the rise of dinosaurian dominance during the Triassic–Jurassic mass extinction.
Compared with all other living reptiles, birds grow extremely fast and possess unusually low levels of intraspecific variation during postnatal development. It is now clear that birds inherited their high rates of growth from their dinosaurian ancestors, but the origin of the avian condition of low variation during development is poorly constrained. The most well-understood growth trajectories of later Mesozoic theropods (e.g., Tyrannosaurus, Allosaurus) show similarly low variation to birds, contrasting with higher variation in extant crocodylians. Here, we show that deep within Dinosauria, among the earliest-diverging dinosaurs, anomalously high intraspecific variation is widespread but then is lost in more derived theropods. This style of development is ancestral for dinosaurs and their closest relatives, and, surprisingly, this level of variation is far higher than in living crocodylians. Among early dinosaurs, this variation is widespread across Pangaea in the Triassic and Early Jurassic, and among early-diverging theropods (ceratosaurs), this variation is maintained for 165 million years to the end of the Cretaceous. Because the Late Triassic environment across Pangaea was volatile and heterogeneous, this variation may have contributed to the rise of dinosaurian dominance through the end of the Triassic Period.