Phylogenetic systematics of the Brontotheriidae (Mammalia, Perissodactyla)

The Brontotheriidae is an extinct family of mostly large-bodied, horned perissodactyls whose temporal range is restricted to the Eocene. Brontothere fossils occur throughout North America, Asia, and, rarely, in Eastern Europe. Serious research on brontothere systematics or other aspects of brontothere paleobiology has been minimal since the cladistic revolution, despite the fact that earlier works on brontothere taxonomy and systematics are widely recognized as being flawed due to outdated methodologies and the reliance on the discredited evolutionary theory of orthogenesis. The present study reevaluates the taxonomy of most species of the family Brontotheriidae, based on skulls, jaws, and teeth. A phylogenetic species concept, where species are defined as the smallest diagnosable clusters of specimens, was adopted for this study. Monospecific quarry samples (mass death assemblages) that are known for a few species were used to assess the level of variability within brontothere populations. Out of the 97 previously named species that were considered for taxonomic revision, 34 species were found to be valid, 34 species were found to be invalid junior synonyms, and 29 species were found to be nomina dubia or otherwise problematical. Additionally, two new species were named, and five other unnamed but probably valid species (known from very poor fossil material) are recognized. Phylogenetic analysis of 40 brontothere species was undertaken with 213 character states distributed among 82 characters. The analysis of both ordered and unordered character data yielded excessive numbers of most parsimonious trees and poorly resolved strict consensus trees, largely due to highly fragmentary taxa. Safe taxonomic deletion and strict reduced consensus procedures were used to identify and prune wildcard taxa, thus reducing the number of trees. The results of the phylogenetic analysis differ substantially from prior orthogenetic hypotheses of brontothere phylogeny. All brontotheres except Eotitanops and Palaeosyops form a clade supported primarily by numerous molar apomorphies. The subfamily Brontotheriinae is expanded to include all brontotheres with derived molars. Many of the previously named subfamilies refer to subclades of the Brontotheriinae. A greatly simplified classification consisting of five nested clades is proposed. The resulting brontothere cladograms imply frequent intercontinental dispersals between North America and Asia during the middle Eocene, suggesting rampant biotic interchange between those two continents during that time.