Tail anatomy of alvarezsauroids (Theropoda, Coelurosauria).

MESO, JORGE; CANALE, JUAN IGNACIO; SALGADO LEONARDO

Neuquén

Jornada; II Jornadas de Paleovertebrados de la Cuenca Neuquina; 2019

The anatomy of the alvarezsauroid tail has been poorly described in the literature, although it shows a peculiar combination of characters that make it unique among theropods. To this respect, the most complete and informative alvarezsauroid taxa are Alvarezsaurus calvoi Bonaparte, 1991, Shuvuuia deserti Chiappe et al., 1998, Haplocheirus sollers Choiniere et al., 2010, and Linhenykus monodactylus Xu et al., 2011. The alvarezsauroid tail is characterized for being the longest among coelurosaurs in both number of vertebrae and length. The vertebrae have procoelous and mediolaterally compressed centra, subtriangular and laterodistally directed transverse processes, short pre and postzygapophyses, and the neural spines are low, posteriorly inclined, and dorsoventrally taller than anteroposteriorly long. The most proximal centra are ventrally sharp. These osteological traits, the morphology of intervertebral joint and inferred muscular reconstruction, suggest a specific function of the tail, with an exceptional capacity of rotational inertia, and a high degree of development of M. spinalis and M. caudofemoralis, comparable to some extent with extant anteaters and pangolins. This provides for the first time new details of the tail anatomy and function among alvarezsaurian coelurosaurs. The preliminary results of this study support previous ideas, based on the morphology of the alvarezsaurid forelimb and skull, which suggest that this group occupied an ecological niche equivalent to that of anteaters or pangolins. Nevertheless, this hypothesis should be tested further under more complex biomechanical analyses.