New evidence concerning relationships within Artiodactyla, Mammalia: radicular morphology of the dp/4

Abstract

A synapomorphy of Artiodactyla is the three-lophed lower fourth deciduous molar (dp/4). Only a few non-artiodactyl mammals possess three lobes in the dp/4, among which are Macroscelididae and Deinotheriidae (both Afrotheria). Despite the general acceptance of this character for diagnosing the order Artiodactyla, there has been a curious lack of discussion about variability in root morphology of the tooth. This paper establishes the presence of three subgroups of Artiodactyla on the basis of root morphology of the dp/4. The first has a root under the protoconid (buccal side of the middle lophid) coalescent with the one under the paraconid (Cebochoeridae and some Choeropotamidae), the second group has a buccal root beneath the protoconid, well separated from the paraconid root (selenodont artiodactyls (ruminants and anthracotheres) and some Suiformes (suids, schizoporcids, sanitheres)), and the third group has no root associated with the protoconid (other Suiformes : Hippopotamidae, Doliochoeridae and Siderochoeridae, and non-suiform Camelidae and Diacodexidae (Diacodexis)). Further research is needed on the families Choeropotamidae and Anthracotheriidae, both of which appear to contain taxa with two kinds of root systems in the dp/4. The evidence of the radicular system of the dp/4 weakens the hypothesis that Anthracotheriidae represent the sister taxon of Hippopotamidae.