We describe and illustrate new, middle Cenozoic fossils of dentally zalambdodont, North American placentals, including six relatively complete crania of Apternodus and two of Oligoryctes, as well as many partial skulls, mandibles, and teeth of these and other taxa. Several of the new Apternodus specimens are also associated with postcrania. We recognize seven species of Apternodus, three of which are new, formally propose the combination Oligoryctes altitalonidus, and recognize two other genera of small, North American, anatomically zalambdodont placentals, Parapternodus and Koniaryctes. We regard two other taxa previously associated with North American fossil zalambdodonts, one Bridgerian and the other Tiffanian, as valid but do not name them in this paper. In addition, we argue that dental zalambdodonty entails a primary occlusal relationship between the paracone and the ectoflexid, and the reduction or absence of the metacone and talonid basin.

A phylogenetic analysis of cranial, dental, and postcranial characters of 30 fossil and Recent taxa leads us to conclude that (1) the Apternodontidae as defined in previous literature is not monophyletic and should be restricted to seven species of Apternodus, (2) the genus Oligoryctes contains at least two species and has a considerably longer geologic record than Apternodus, (3) neither Micropternodus nor currently known Paleocene taxa are closely related to Apternodus or Oligoryctes, and (4) a case can be made for a close relationship among modern soricids, Parapternodus, Koniaryctes, Oligoryctes, and Apternodus to the exclusion of other insectivoran-grade taxa. With the use of ordered, multistate character transformations, Solenodon comprises the sister taxon to a soricid-fossil zalambdodont clade.