Phylogeny, life history and the timing of molar crown formation in two archaic ungulates, Meniscotherium and Phenacodus (Mammalia, “Condylarthra”)

Dirks, W; Anemone, RL; Holroyd, PA; Reid, DJ; Walton, P

doi:10.1159/000242381

Phylogeny, life history and the timing of molar crown formation in two archaic ungulates, Meniscotherium and Phenacodus (Mammalia, “Condylarthra”)

Lookup NU author(s): Dr Wendy Dirks, Dr Don Reid, Pamela Walton

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Abstract

The condylarths, or archaic ungulates, are a paraphyletic mammalian group including a number of fossil taxa whose relationships are unresolved. Included are two genera from the Paleocene and Eocene of North America, Meniscotherium and Phenacodus. Some workers place both genera in the family Phenacodontidae, while others exclude the highly dentally derived Meniscotherium. In this study, we use growth increments in histological thin sections to examine the timing of crown formation in five molars of Meniscotherium and one each of Phenacodus intermedius and Phenacodus trilobatus. We also use perikymata counts on an additional six molars of Meniscotherium. Although estimated body mass and molar dimensions in Meniscotherium are smaller than in either species of Phenacodus, molar formation times are longer, ranging from 0.71 to 1.44 years. Both Phenacodus molars take less than a year to form. Crown extension rates, the rate at which the crown grows in height, are as low as 3-15µm per day in Meniscotherium, but range from 13-54µm per day in Phenacodus. Although striae periodicities and daily enamel secretion rate are similar in both genera, the differences in the crown extension rate and overall timing of crown formation suggest differences in life histories and raise questions about the phylogenetic relationship of the two genera.