TERRESTRIAL TURTLES FROM THE UPPER CRETACEOUS (CAMPANIAN) JUDITH RIVER FORMATION, NORTH-CENTRAL MONTANA

The Campanian Judith River Formation of north-central Montana preserves a multitude of vertebrate microfossil bonebeds that yield abundant turtle fossils, but to date relatively little has been published in relation to the formation’s turtle assemblage. Here we report new data from the type area that reveal key aspects of the Judith River turtle record. Turtle carapace fragments represent a sizeable and highly recognizable fraction of the bioclasts recovered from vertebrate microfossil bonebeds in the formation. To date we have documented 1576 carapace fragments from surface collected samples of 21 microfossil bonebed sites (each site preserves at least 10 specimens), 1346 of which are identifiable based on diagnostic shell morphology. With regard to representation in the identifiable sample, trionychids are most common (n=923, 68.57%), followed by baenids (n=258, 19.17%), Basilemys (n=122, 9.06%), and chelydrids (n=43, 3.19%). Though multiple genera of trionychids and baends are likely represented, further classification was inhibited due to the fragmentary nature of the sample and the paucity of associated diagnostic skeletal material (e.g., skulls, vertebrae, claws) for comparison.

Focusing on the four largest collections (n>100 identifiable specimens), the relative abundance pattern documented for the unit as a whole holds, with trionychids comprising 52%-82% of site-specific collections. This turtle sample is comparable in the abundance of trionychids and baenids and the rarity of Adocus to a similarly sized collection (1804 fragments) from the Dinosaur Park Formation in Dinosaur Provincial Park. In assemblages from geographically closer regions, the Manyberries/Onefour area in Alberta and the Kennedy Coulee area in Montana, Adocus is more abundant. The rarity of Adocus in the Judith River Formation type area is unexpected because this taxon is fairly abundant in other localities to the south of Dinosaur Provincical Park.