Plagiomenidae and Mixodectidae

INTRODUCTION Plagiomenids and mixodectids are small early Tertiary insectivore-like mammals that are known mainly from their teeth and jaws. Their broader relationships among Eutheria have been difficult to work out because of the very fragmentary nature of known fossil remains. In the case of plagiomenids, peculiarities of the molar cusp patterns and of the incisor crowns are reminiscent of the teeth of living flying lemurs; consequently plagiomenids have long been considered to be related to or members of the Dermoptera (Matthew, 1918; Simpson, 1945; Jepsen, 1962). Only Plagiomene is known from cranial material other than jaws, and these skulls have proven critical in reassessing the affinities of the genus (Dawson et al., 1986; MacPhee, Cartmill, and Rose, 1989). However, no postcrania have been described for any plagiomenid. Mixodectids have been recognized for decades as possible close relatives of plagiomenids, based on dental similarities (e.g., Simpson, 1936; Sloan, 1969). A couple of poorly preserved skull fragments of Mixodectes are the only reported cranial remains for the family. More informative are several postcranial elements recently attributed to Mixodectes by Szalay and Lucas (1996), which suggest arboreal habits and archontan relationships. In their recent classification of mammals, McKenna and Bell (1997) included both Plagiomenidae and Mixodectidae in the Dermoptera, Grandorder Archonta. Both plagiomenids and mixodectids seem to have been exclusively North American clades. (Early Eocene Placentidens from France, once considered a plagiomenid [Russell, Louis, and Savage, 1973], is regarded as an erinaceomorph lipotyphlan by McKenna and Bell [1997].