Tail anatomy of the Alvarezsauria (Theropoda, Coelurosauria), and its functional and behavioural implications

Abstract

The anatomy of the alvarezsaurian tail has received relatively little attention in the paleontological literature, even though it shows a peculiar combination of anatomical characteristics that are unique among theropod dinosaurs. Nearly complete, informative tails are known from early-branching, intermediate, and late-branching taxa, allowing for robust inferences about their evolution. The alvarezsaurian tail is notable in being the longest among maniraptoran theropods, both in number of caudal vertebrae and proportional length. We examined the comparative anatomy and myology of the tail, and performed a cladistic analysis on a tail-character-based data matrix that provided a general framework for reconstructing alvarezsaurian tail evolution. Our results show that caudal vertebrae of alvarezsaurians have a combination of derived osteological features, intervertebral joint morphology, and inferred musculature, which together suggest that the tail possessed a unique function among theropods. We interpret these features as indicators of an exceptional capacity to change rotational inertia. The form and function of the tail, in combination with the fossorial forelimb, suggests that alvarezsaurians had an ecological niche similar to today's aardvark, pangolins and anteaters.