Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals

Extant eutherian mammals and their most recent common ancestor constitute the crown group Placentalia. This taxon, plus all extinct taxa that share a more recent common ancestor with placentals than they do with Metatheria (including marsupials), constitute Eutheria(1). The oldest well documented eutherian-dominated fauna in the world is Dzharakuduk, Uzbekistan(2). Among eutherians that it yields is Kulbeckia, an 85-90-Myr-old member of Zalambdalestidae (a family of Late Cretaceous Asian eutherians)(3). This extends Zalambdalestidae back by some 10 million years from sites in the Gobi Desert, Mongolia(4). A phylogenetic analysis of well described Late Cretaceous eutherians strongly supports Zalambdalestidae, less strongly supports 'Zhelestidae' (a Late Cretaceous clade related to Tertiary ungulates), but does not support Asioryctitheria (a group of Late Cretaceous Asian eutherians). A second analysis incorporating placentals from clades that include rodents (Tribosphenomys), lagomorphs (Mimotona) and archaic ungulates (Protungulatum and Oxyprimus) strongly supports Zalambdalestidae in a clade with Glires (rabbits, rodents and extinct relatives) and less strongly 'Zhelestidae' within a clade that includes archaic ungulates ('condylarths'). This argues that some Late Cretaceous eutherians belong within the crown group Placentalia. The ages of these taxa are in line with molecularly based estimates of 64-104 Myr ago (median 84 Myr ago) for the superordinal diversification of some placentals(5), but provide no support for a Late Cretaceous diversification of extant placental orders.