TURTLES OF THE JUDITH RIVER FORMATION AND COMPARISONS AMONG LATE CRETACEOUS FAUNAS IN WESTERN NORTH AMERICA

Turtles are among the most diverse aquatic vertebrate groups in the Late Cretaceous and are common in the fossil record of western North America. As such, they provide a unique and complementary study system for assessing hypotheses of biogeographic provinciality and climatically-controlled latitudinal variation based on other parts of the biota. Recent publications of diverse turtle faunas from the Campanian of Utah, Alberta, and New Mexico, in combination with new data from Montana, permit a new synthesis.

Here we report on the turtle assemblages of the Judith River Fm. from 80 localities in north-central Montana, primarily from the Kennedy Coulee sections for which radiometric dates have been previously published, and smaller samples from the Havre badlands and the more marine-influenced sections in Liberty County. For each locality, genus and family level presence-absence data were tabulated, and minimum lineage richness and relative abundance were calculated.

On a regional scale, the Judith River assemblages bear greatest resemblance to those of southern Alberta, especially in the absence of the pleurosternid Compsemys and kinosternoids (mud turtles). The Judith River Fm. and Albertan assemblages are notably distinct from those of New Mexico and further south, but they uniquely share some taxa (e.g., chelydrids or snapping turtles) with assemblages from the Kaiparowits Fm. of Utah. These patterns are generally consistent with some provinciality hypotheses and possible climatic controls on selected taxa. However, tabulating occurrences on a locality by locality basis to assess richness and abundance reveals subtle differences that are not readily apparent in formation-level summaries. Some taxa are restricted to certain localities and show clear habitat preferences (e.g., channel size or distance from salt water). Therefore, variation among formations or areas is also likely due to the variety and types of local environments being collected. These sampling differences across the paleo-landscape cannot be overlooked as an additional source of the observed geographic variation among Late Cretaceous faunas.