Phylogenetic systematics of basal perissodactyls

ABSTRACT

The relationships among basal perissodactyls and among those taxa historically included in “Hyracotherium” are complicated. These taxa are morphologically similar, possessing few of the character states that diagnose the crown groups. To understand better these relationships, cladistic techniques were used to generate a matrix of 41 taxa and 125 characters including five non-perissodactyl outgroup taxa, representative basal tapiroids, brontotheres, chalicotheres, palaeotheres, and equids. A maximum parsimony analysis of this matrix using PAUP 3.0s generated two most parsimonious trees (length 627, CI 0.32). This analysis supports a monophyletic perissodactyl clade composed of the Tapiromorpha and the Hippomorpha. The tapiromorph clade consists of a number of basal “tapiroids”, a paraphyletic “isectolophid” group, the ceratomorphs (tapirs and rhinoceroses and extinct relatives), and the ancylopods (chalicotheres and relatives). The basal tapiromorphs are distributed holarctically, indicating divergence and dispersal by the latest Paleocene. The hippomorph clade is composed of two distinct clades, the European palaeothere clade and the equids. The brontotheres are tentatively derived from within the palaeothere clade. Hallensia is identified as the most basal palaeothere, sister taxon to Hyracotheriwn leporinum and all more derived palaeotheres. The equid clade is composed of a stratigraphically consistent sequence of paraphyletic equid taxa, and a clade composed of “Hyracotheriwn” craspedotum and Xenicohippus. The majority of the pectinately arranged equid taxa in the analysis lack autapomorphies, which suggests that these taxa may represent lineage segments from a single anagenetically evolving lineage. The results also suggest that “Hyracotherium” is not representative of the basal morphology of the perissodactyls, and no currently identified fossil provides a good candidate for that morphology.