Tail Function in Oviraptorosaur Dinosaurs: Insights from a 3D Tail Model of Citipati Osmolskae (Theropoda: Oviraptorosauria)

Abstract

The oviraptorid dinosaur Citipati osmolskae (Theropoda: Oviraptorosauria) is known from brooding specimens and from MPC 100/978, an excellently preserved specimen with bones that are fully prepared from its host matrix. We present a detailed photogrammetric 3D model of the tail of MPC100/978 which was reconstructed from high-detail digital photographs using the software Agisoft Photoscan Pro 1.0. On the 3D model, the range of motion of the joints was modeled in Rhinoceros 5.0 by digitally manipulating the individual bone models with respect to each other. This was done both with and without intervertebral and zygapophyseal soft tissues which were inferred from the tail joints of modern crocodiles and birds. These data allowed us to quantitatively constrain tail mobility in Citipati for the first time. Using the Extant Phylogenetic Bracket approach we also produced detailed 3D models of the tail muscles including several variants of the M. caudofemoralis. Additionally, we determined the passive flexural resistance (stiffness) of the intervertebral tail joints using a Newtonian physics model based on lever and beam mechanics. As the most detailed model of an oviraptorosaur tail to date, our Citipati model can serve as a standard for unraveling tail function within this clade. Oviraptorosaur tails are suggested to have had a high degree of tail flexibility per unit of absolute tail length, based on their craniocaudally short and laterally broad prezygapophyses as well as their craniocaudally short centra, and large muscle volumes that presumably helped to actively stiffen or move them. Oviraptorosaur tails also had relatively high stiffness in their joints that passively supported it and enabled a range of muscular force vectors to be produced. These characteristics support the capability of the tail for bird-like display behaviors which involve the movement of the distal tail frond, as in Caudipteryx and Similicaudipteryx. In the context of this existing knowledge, our new tail model provides functional and morphological tail parameters, like mobility and muscle volume, that when compared to modern bird tails with observed display behaviors

allow us to appraise the plausibility of bird-like display behaviors in Citipati. Our methodological framework therefore has exciting potential applications in assessing tail function within theropods (including early birds) and other dinosaurs and vertebrates.