Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds

Abstract

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.I n comparison with other reptiles, avian biology is highly unusual, characterized by "hollow" bones and postcranial skeletal pneumaticity, feathers, a unique forelimb digit formula, endothermy, and rapid growth rates. However, these peculiarities initially arose in nonavian dinosaurs (1-7) in a gradual process occurring over tens of millions of years (8,9). In addition to their extremely rapid rates of growth, avian ontogeny possesses a characteristically low level of morphological variation within a species relative to that of other reptiles. The majority of individuals in a given avian species undergoes the same morphological changes during ontogeny in the same order at similar body sizes (10, 11), whereas their closest living relatives, crocodylians, possess higher variation (10,(12)(13)(14). How and when this feature of avian biology evolved is poorly constrained.This avian style of development must have evolved after its most recent common ancestor with crocodylians but before the origin of Aves. Most studies of the ontogeny of more-derived theropods (15-17) suggest that the low levels of variation that characterize avian ontogeny were present in close nonavian relatives as well. Closer to the origin of Dinosauria, morphological variation within species is widespread (18-26), but whether this variation is the result of taxonomic diversity, ontogeny, sexual dimorphism, or simple individual variation is not clear (22,23,27). Furthermore, close dinosaurian relatives or "dinosaur precursors" (e.g., Silesaurus, Asilisaurus) possess high intraspecific variation in growth sequences [i.e., sequence polymorphism (28)], suggesting that this condition could be ancestral for Dinosauria (21).In contrast to ...

Histological skeletochronology indicates developmental plasticity in the early Permian stem lissamphibian Doleserpeton annectens

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Protracted growth impedes the detection of sexual dimorphism in non‐avian dinosaurs

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Developmental patterns and variation among early theropods

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“…In this study, I describe postcranial variation in the early neotheropod dinosaurs Coelophysis bauri and Megapnosaurus rhodesiensis in detail, and place this variation in the context of the evolution of ontogenetic change in early theropods. Coelophysis bauri and M. rhodesiensis provide excellent study taxa to study morphological changes in ontogeny in early theropods and other dinosaurs because: (i) they have been reported to possess a high amount of variation in the presence of bone scars and co‐ossifications (= bone ‘fusions’; Raath, , ; Colbert, , ; Genin, ; Griffin & Nesbitt, ; Barta et al. ), similar to non‐dinosaurian dinosauriforms (Griffin & Nesbitt, ,b); (ii) they are both early‐diverging neotheropods, and therefore in a close phylogenetic position to the common dinosaurian ancestor, possessing many character states in common with this ancestor (Nesbitt et al.…”