Discussion

Phylogenetic Hypotheses

McKenna and Bell (1997) included the North American Paleocene genera Goniacodon, Eoconodon, Triisodon, and Stelocyon, as well as the Asian Eocene Andrewsarchus in the Family Triisodontidae. In subsequent phylogenetic analyses "triisodontids" often were represented by only two genera, Eoconodon and Andrewsarchus (e.g., Geisler and McKenna 2007). In some analyses, these two genera were found to be relatively closely related, but inclusion of Andrewsarchus in a monophyletic Triisodontidae was frequently challenged (see O'Leary et al. 2003 and references cited therein). Similarly, interpretations of the phylogenetic relationships of Eoconodon have varied widely. In some studies, the genus was interpreted as very distantly related to the Cetartiodactyla and allied with, for example, the archaic ungulate Arctocyon or the late Eocene to late Oligocene leptictid Leptictis (e.g., O'Leary and Geisler 1999).

Williamson and Carr (2007) undertook an analysis of the phylogenetic relationships of basal ungulates and basal mesonychians directed toward testing hypotheses of the phylogenetic interrelationships of "triisodontids" (Eoconodon, Goniacodon, and Tricentes), Oxyclaenus, Microclaenodon, and mesonychians. In this study they considered only species of Eoconodon (E. coryphaeus, E. gaudrianus, and E. ginibitohia), Goniacodon (G. levisanus, G. crassicuspis, and G. hiawathae), and Tricentes (T. quiverensis) known from the San Juan Basin and other southern localities. The northern species of Eoconodon (E. copanus, E. nidhoggi, and E. hutchisoni) were not considered. Their analysis, which is followed here, demonstrated that "triisodontids" are a paraphyletic group. It was inconclusive in clarifying the interrelationships of the species of "triisodontids" yielding a polytomy including the three southern species of Eoconodon, a lineage composed of three species of Goniacodon and the Triisodon quiverensis lineage.