The brachial plexus of the Sumatran Rhino (Dicerorhinus sumatrensis) and application of brachial plexus anatomy toward mammal phylogeny

The peripheral nervous system is a promising resource for testing phylogeny although the branching patterns of peripheral nerves are not well documented outside of Homo sapiens. Here we describe the brachial plexus of the rare Sumatran rhinoceros (Dicerorhinus sumatrensis). We compare its brachial plexus to that of another perissodactyl (Equus asinus), an artiodactyl (Odocoileus virginianus), two carnivorans (Felis catus and Neovison vison), and one primate (Homo sapiens) and examine the phylogenetic structure of the resulting data. Brachial plexuses exhibit high rates of intraspecific polymorphism, but polymorphisms cannot be recognized from one specimen. To address concerns of error due to polymorphism, we dissected 52 mink brachial plexuses and compared them to human brachial plexus variation. Both species have numerous types of brachial plexus polymorphisms. Although most individual polymorphisms occur infrequently and unilaterally, because there are numerous types of polymorphisms, most humans and mink exhibit at least one polymorphism per brachial plexus. Parsimony analysis of 15 characters compiled from the brachial plexus data produced a tree that positions Artiodactyla and Perissodactyla as sister taxa, a result consistent with other analyses. Despite a high rate of polymorphism, the peripheral nervous system seems to carry a phylogenetic signal consistent with other morphological data. With a higher rate of taxon sampling, we suggest the brachial plexus will contribute valuable data for phylogenetic testing.