A densely sampled, diverse new fauna from the uppermost Cedar Mountain Formation, Utah, indicates that the basic pattern of faunal composition for the Late Cretaceous of North America was already established by the Albian-Cenomanian boundary. Multiple, concordant 40 Ar͞ 39 Ar determinations from a volcanic ash associated with the fauna have an average age of 98.39 ؎ 0.07 million years. The fauna of the Cedar Mountain Formation records the first global appearance of hadrosaurid dinosaurs, advanced lizard (e.g., Helodermatidae), and mammal (e.g., Marsupialia) groups, and the first North American appearance of other taxa such as tyrannosaurids, pachycephalosaurs, and snakes. Although the origin of many groups is unclear, combined biostratigraphic and phylogenetic evidence suggests an Old World, specifically Asian, origin for some of the taxa, an hypothesis that is consistent with existing evidence from tectonics and marine invertebrates. Large-bodied herbivores are mainly represented by low-level browsers, ornithopod dinosaurs, whose radiations have been hypothesized to be related to the initial diversification of angiosperm plants. Diversity at the largest body sizes (>10 6 g) is low, in contrast to both preceding and succeeding faunas; sauropods, which underwent demise in the Northern hemisphere coincident with the radiation of angiosperms, apparently went temporarily unreplaced by other megaherbivores. Morphologic and taxonomic diversity among small, omnivorous mammals, multituberculates, is also low. A later apparent increase in diversity occurred during the Campanian, coincident with the appearance of major fruit types among angiosperms, suggesting the possibility of adaptive response to new resources.

We present new geochronologic (magnetostratigraphy, fission-track and 40 Ar/ 39 Ar radioisotopic dates, biostratigraphy) data constraining the age of three separate sequences and a composite section from Guanajuato, Mexico. Those data make this one of the most complete and precisely age-calibrated sequences in North America spanning the Hemphillian/Blancan North American Land Mammal "Age" (NALMA) boundary interval, and the data further constrain the timing and pattern of the Great American Biotic Interchange (GABI). In total, 196 samples (77 sites) were used to construct the magnetic polarity stratigraphies, with eight fission-track analyses and four new 40 Ar/ 39 Ar radioisotopic dates. The sections sampled are possibly latest Miocene to Late Pliocene in age, spanning about 2.7 m.yr. (∼5.5-2.7 Ma), from Chrons C3n.4n or C3r to C2An.1n. The radioisotopic dates and magnetostratigraphic correlations indicate the Hemphillian/Blancan NALMA boundary at Guanajuato in central Mexico is ∼4.7-4.8 Ma, and it probably lies within Chron C3n.2r, consistent with most prior age estimates and correlations. Our analyses indicate that a variety of "invaders" (taxa with South American ancestry, including Megalonyx, Glossotherium, and Plaina) clearly were present by at least 4.7-4.8 Ma and therefore much earlier in lower latitude Middle America than in more temperate parts of North America. Others (e.g., Neochoerus, Glyptotherium) were present shortly thereafter but still much earlier than in higher latitudes. Thus, the first appearances of these five immigrant taxa can no longer be used to define the beginning of the late Blancan. This timing significantly predates the earliest documented major influxes of North American forms into South America, is at least 1.5-2 m.yr. earlier than the beginning of the GABI previously recorded in North America (beginning of the Blancan [Bl 2], ∼2.7 Ma), and suggests faunal provinciality and more staggered arrival and dispersal of South American immigrants than indicated in temperate sequences.