Evolutionary derivation inferences of the intrinsic shoulder and brachial muscles in crab-eating raccoon (Procyon cancrivorus, Caniformia, Carnivora) based on the topology, innervation, and anatomical variants

The crab-eating raccoon ( Procyon cancrivorus ) is a carnivoran of the family Procyonidae geographically distributed in Central and South America. It is a scansorial species with more terrestrial than arboreal abilities. Previous studies have described the intrinsic shoulder and brachial muscles in this species; however, the terminology and some muscle attachments differ among them. Besides, these studies did not consider the innervation to infer the evolutionary derivation of the muscles, and did not address the arterial supply. The present study aimed to analyze the anatomical arrangement of the intrinsic shoulder and brachial muscles in six Procyon cancrivorus specimens fixed with 10% formaldehyde. The shape, origin, insertion, arterial supply, and variations were described. Furthermore, the innervation previously reported was reviewed again in detail to infer the evolutionary derivation of these muscles. Differences were found with previously reported findings in the same species and other procyonids. Some intraspecific anatomical variants were discovered, such as an accessory head in the biceps brachii muscle bilaterally; a biceps brachii muscle joined to the brachialis muscle unilaterally; and a fusion of the lateral and accessory heads of the triceps brachii muscle. Tensor fasciae antebrachii muscle is divided into two parts in most cases, which are innervated by the radial nerve. The anconeus medialis muscle is independent of the triceps brachii muscle and is innervated by the ulnar nerve. In conclusion, these muscles in P. cancrivorus potentially conserve the evolutionary derivation of the last common ancestor of mammals based on the topology, anatomical variations, and innervation.