A high-latitude assemblage of fossil turles (Testudines) from the Late Campanian of Alberta, Canada

The diversity of present-day turtles is highest in tropical/subtropical latitudes, with a peak at approximately 25°N. Only six species extend their ranges north of the 50th parallel, and none are known to reach the 60thparallel. In comparison, Late Cretaceous terrestrial turtles are well-known above 50°N paleolatitude, but the northernmost limit of their range is thought to have been climatically restricted not far beyond the well-sampled assemblages of southern Alberta, with the exact biogeographic boundary depending on fluctuations in global temperature through time. To date, only a few shell fragments have been reported from northern Alberta during this time interval. Here, we report on a diverse high-latitude turtle assemblage from the Late Campanian Wapiti Formation in northern Alberta. Remains represent at least five contemporaneous species across three families including the first high-latitude cranial material of a baenid and a trionychid, elements and shell fragments from at least two species within each of those families, and the northernmost remains of a chelydrid. Specimens can be referred to four specific taxa: Plesiobaena antiqua, Aspideretoides foveatus, cf. Axestemys splendida, and cf. Boremys pulchra. Plesiobaena antiqua and B. pulchra were previously identified as occurring only in more southerly localities within North America. Interestingly, all recovered specimens would have had carapace lengths less than 400 mm; even the recovered skull of cf. Axestemys splendida is approximately half the size of typical southern specimens. Other larger taxa are missing entirely, suggesting that the northern Alberta assemblage is indeed near the climatically-controlled biogeographic limit of these species’ ranges. This assemblage provides the first detailed examination of Campanian turtles at their northernmost limit with implications for their biogeography within North America. The relative diversity and abundance, but potentially small body size, of a climate-sensitive vertebrate group in a near paleo-Arctic (close to 66°N) setting also raises new questions about the reconstruction of the Campanian terrestrial latitudinal temperature gradient and its climate-driven limitations on diversity during the latest Cretaceous. These discoveries underscore the paleobiological importance of the Wapiti Formation for understanding the Cretaceous terrestrial vertebrate assemblages of northern Alberta as well as North America in general.

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