CLIMATE GRADIENTS AND FILTER CORRIDORS: FREQUENT ASIA-LARAMIDIA BIOTIC INTERCHANGE AND THE COMPLEXITY OF LARAMIDIA’S DINOSAUR “PROVINCIALITY”

The steady pace of new fossil discoveries over the past two decades coupled with recently refined geochronology across many non-marine, upper Cretaceous (Campanian-Maastrichtian) horizons in western North America has permitted unprecedented clarity regarding the distribution and evolution of many dinosaur clades during this interval. Previous studies regarding these patterns centered around three primary hypotheses: (1) Campanian Laramidia was divided into distinct ‘northern’ and ‘southern’ provinces, with limited overlap among dinosaur taxa between regions; (2) Campanian Laramidia was divided along an ecological latitudinal gradient from southern tropical and subtropical to northern temperate assemblages; and (3) apparent differences between regions were the result of temporally incongruous assemblages and/or over-splitting of closely related taxa. All three hypotheses generally accept periodic but limited interchange with Asian faunas via sporadic Beringian connections to explain the appearance of novel or shared taxonomic groups. Recent discoveries, primarily of new tyrannosaurids, hadrosaurids, ceratopsids, and ankylosaurians from ‘southern’ Laramidia localities in Utah, New Mexico, Texas, and Coahuila have provided crucial new information to this debate. Emerging patterns indicate a more complex biogeographic interpretation of Campanian-Maastrichtian assemblages, with distinct histories for each major dinosaurian clade and with each tightly linked to climatic regimes. Currently, both centrosaurine and chasmosaurine ceratopsid distributions continue to follow the pattern of distinct northern and southern radiations during the Campanian. Conversely, hadrosaurids reflect a general southward range contraction of several subclades (parasaurolophins, kritosaurins) as the climate cooled, tracking warmer climatic conditions. These retreating hadrosaurids were replaced in ‘northern’ and ‘central’ assemblages by radiations of taxa closely linked with Campanian faunas in Asia. Indeed, new phylogenetic results from tyrannosaurids suggests that the northern corridor connecting Asia and Laramidia may have been physically “open” for much of the Campanian but dispersal was limited by climatic and physical barriers (e.g., light, topography) as a polar filter corridor. Bi-directional dispersal of temperate-adapted clades (e.g., titanosaurians, tyrannosaurines, ankylosaurines, edmontosaurins) was favored over dispersal of presumably cool-intolerant clades (e.g., chasmosaurines, panoplosaurins, kritosaurins, alioramines). This led to the establishment of a shared Asia-Laramidia temperate fauna in the north for much of the Campanian and early Maastrichtian, with distinct and separate southern assemblages in both North America and Asia. By the middle to late Maastrichtian, climatic cooling and reconnection of Laramidia to the eastern landmass of Appalachia coincided with expansion of this temperate-adapted dinosaur fauna across most of western North America, supplemented by dispersals from the east, though some variation in the distribution of this temperate fauna is evident. These emerging biogeographic patterns can only be tested with additional field-based discoveries, continued efforts to refine geochronology in Asia and North America, and updated phylogenetic datasets and techniques.