Range of motion and myology support a digging function for the forelimbs of alvarezsauroid dinosaurs

Numerous lineages of theropod dinosaurs display notable modification of the forelimb, particularly reduction in size and number of digits. Alvarezsauroids are one of the most striking examples of this, exhibiting extreme shortening and increased robusticity of forelimb elements, with a functionally monodactylous manus in late-diverging taxa. These features are generally interpreted as adaptations for digging, possibly as part of a myrmecophagous ecology. Here, we test this hypothesis, using computational range of motion analysis of the shoulder and elbow joints to demonstrate the feasibility of digging behaviours in Mononykus olecranus, a highly specialized alvarezsauroid, and the less specialized Bannykus wulatensis. We find that Bannykus has the capacity for various digging styles and generalized forelimb function, while Mononykus has more restricted motion and may have employed a highly specialized digging style. We also identify similarities in forelimb muscle moment arms between alvarezsaurs and specialized mammalian diggers, supporting adaptation for digging. These findings are consistent with interpretations of insectivory in alvarezsauroids, and suggest increasing specialization to myrmecophagy throughout their evolutionary history, shedding new light on the evolution of this enigmatic clade and the ecological diversity of non-avian theropod dinosaurs.