Abstract
BackgroundThe origin of birds is marked by a significant decrease in body size along with an increase in relative forelimb size. However, before the evolution of flight, both traits may have already been related: It has been proposed that an evolutionary trend of negative forelimb allometry existed in non-avian Theropoda, such that larger species often have relatively shorter forelimbs. Nevertheless, several exceptions exist, calling for rigorous phylogenetic statistical testing.ResultsHere, we re-assessed allometric patterns in the evolution of non-avian theropods, for the first time taking into account the non-independence among related species due to shared evolutionary history.We confirmed a main evolutionary trend of negative forelimb allometry for non-avian Theropoda, but also found support that some specific subclades (Coelophysoidea, Ornithomimosauria, and Oviraptorosauria) exhibit allometric trends that are closer to isometry, losing the ancestral negative forelimb allometry present in Theropoda as a whole.ConclusionsExplanations for negative forelimb allometry in the evolution of non-avian theropods have not been discussed, yet evolutionary allometric trends often reflect ontogenetic allometries, which suggests negative allometry of the forelimb in the ontogeny of most non-avian theropods. In modern birds, allometric growth of the limbs is related to locomotor and behavioral changes along ontogeny. After reviewing the evidence for such changes during the ontogeny of non-avian dinosaurs, we propose that proportionally longer arms of juveniles became adult traits in the small-sized and paedomorphic Aves.
Highlights
The origin of birds is marked by a significant decrease in body size along with an increase in relative forelimb size
The estimated parameters for the reduced dataset showed negative forelimb allometry that is significantly different from isometry (BM: 0.901, 95% CI [0.843, 0.959], λ = 0.954; OU: 0.906, 95% CI [0.848, 0.965], α = 0.013)
Our results confirm that negative forelimb allometry is the main evolutionary trend for non-avian theropods
Summary
The origin of birds is marked by a significant decrease in body size along with an increase in relative forelimb size. Before the evolution of flight, both traits may have already been related: It has been proposed that an evolutionary trend of negative forelimb allometry existed in non-avian Theropoda, such that larger species often have relatively shorter forelimbs. Two key morphological changes required for flight were the reduction of body size [6, 7] and the increase in relative forelimb size [8, 9] Both of these changes had already begun in non-avian Theropoda, long before the origin of avian flight. They were likely related to each other: An evolutionary trend of negative allometry appears to have existed in non-avian theropods, such that species with a larger body size often have proportionally. We review the evidence for forelimb function and allometry in the ontogeny of non-avian dinosaurs, and the potential relation of body size reduction, paedomorphosis, and forelimb size near the origin of birds
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