Eight taxa of therian mammals are herein recorded from the Smoky Hollow Member of the Straight Cliffs Formation, southcentral Utah. The assemblage is of middle to late Turonian age (early Late Cretaceous), a time period for which mammals are poorly represented worldwide in the fossil record. Two symmetrodonts, family Spalacotheriidae, are described as new; one is referred to the genus Symmetrodontoides, otherwise known from the early Campanian, and the other is placed in a new genus. Primitive tribosphenic taxa include two archaic forms characterized by single rank molar shearing; one is referred to the Deltatheridiidae and the other, of uncertain affinities, is similar to Picopsis of the early Campanian. A new species is tentatively referred to Anchistodelphys. This genus is marsupial-like in having an anteroposteriorly expanded protocone and, especially, in features of the lower molars (lingually placed paraconid and hypoconulid, reduced height differential between trigonid and talonid, presence of labial postcingulid) but lacks specializations of the stylar shelf thought to characterize marsupials, suggesting that acquisition of stylar cusps C and D occurred later in the evolution of marsupials than did the above-mentioned characters. Three unidentified and unnamed marsupial taxa are also present in the fauna. Evidence of some marsupial diversification in North America by the Turonian, the presence on the continent of primitive marsupial and marsupial-like taxa (depending upon how the group is defined), and the hypothesized derivation of South American and Australian taxa from an Alphadon-like form, suggest that marsupial origination in North America, followed by dispersal to South America, may be the simplest explanation for the temporal and geographic distribution of the group.