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Exquisitely preserved specimens of the Late Cretaceous eutherian Zalambdalestes recently collected from the Djadokhta Formation (Early Campanian) of the Gobi Desert by the Mongolian Academy of Sciences-American Museum of Natural History Expeditions are the centerpiece of a thorough redescription of this taxon’s craniodental morphology. Resolved and amended are uncertainties and errors in prior descriptions based on poorer preserved specimens collected by earlier expeditions to the Gobi. Preserved and described for the first time in Zalambdalestes is the basicranium, including an ectotympanic bone and portions of the hyoid arch.

Zalambdalestes with a skull length of nearly 50 mm is large compared with other Cretaceous eutherians. It is also highly specialized with a long, thin, tubular snout, large diastemata in the anterior upper dentition, and an elongated mesial lower incisor with restricted enamel. These specializations, though less extreme, are also present in the zalambdalestids Barunlestes from the slightly younger Barun Goyot Formation of the Gobi and Kulbeckia from the late Turonian and Coniacian of Uzbekistan and the Santonian of Tadjikistan.

No phylogenetic analysis published to date includes enough taxonomic and morphological breadth to evaluate the relationships of Zalambdalestes. Nevertheless, we investigate the impact of our observations on seven phylogenetic analyses published since 1993 that include Zalambdalestes. A comprehensive phylogenetic analysis testing the relationships of Zalambdalestes is not included here, but it is expected to result from our ongoing efforts to produce a phylogeny of basal tribosphenic and therian mammals.

Currently, zalambdalestids are viewed either as stem eutherians or as having affinities to Glires (lagomorphs and rodents). Our comparisons with other extinct and extant taxa support a position for Zalambdalestes within Eutheria but outside the crown-group Placentalia. Supporting this basal position for Zalambdalestes are such primitive features as the last upper incisor in the maxilla, nasals broadly expanded posteriorly to contact the lacrimals, pterygoids meeting on the midline, and the position of the glenoid fossa on the zygoma and not the braincase proper, in addition to the occurrence of epipubic bones reported previously. Zalambdalestes shares a number of apomorphies with Asioryctitheria, the clade including the Mongolian Late Cretaceous Asioryctes, Ukhaatherium, and Kennalestes. Among the unusual specializations supporting a zalambdalestid-asioryctithere clade are: the postglenoid foramen anterior rather than posterior to the postglenoid process; the postglenoid and entoglenoid processes of the squamosal continuous; a fusiform ectotympanic expanded laterally and contacting the entoglenoid process; a suprameatal foramen in the squamosal; a crista interfenestralis connecting from the petrosal promontorium to a fingerlike tympanic process behind the round window; a large piriform fenestra in the anterior roof of the tympanic cavity, which transmitted the ramus inferior of the stapedial artery endocranially to the orbit; a foramen ovate between the alisphenoid and squamosal; and a medially positioned internal carotid artery. All but the last two of these specializations are reminiscent of those occurring in various extant lipotyphlans, including taxa placed by recent DNA sequence analyses within Afrotheria and Eulipotyphla, and may provide a link between the Mongolian Cretaceous eutherians and lipotyphlans.

The available sample of Zalambdalestes exhibits a remarkable degree of individual variation, including the incidence of the upper maxillary incisor, the first upper premolar, and the second lower premolar. The possibility exists that more than a single species, Z. lechei, is represented.