<italic>Basilemys morrinensis</italic>, a new species of nanhsiungchelyid turtle from the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta, Canada.

We describe a new species of nanhsiungchelyid turtle, <italic>Basilemys morrinensis</italic>, based on a nearly complete shell from the Horsethief Member (lower Maastrichtian) of the Horseshoe Canyon Formation of Alberta. The species is intermediate in age between the Campanian forms <italic>B. variolosa</italic> and <italic>B. gaffneyi</italic> and the upper Maastrichtian forms <italic>B. sinuosa</italic> and <italic>B. praeclara</italic>. It is also intermediate in its morphology, possessing a unique suite of both plesiomorphic (e.g., divided extragulars) and derived (e.g., square epiplastral beak, pygal wider than long) traits. It is further characterized by an autapomorphic square cervical scale. Phylogenetic analysis assuming parsimony recovers <italic>B. morrinensis</italic> in a polytomy with <italic>B. variolosa</italic> and <italic>B. gaffneyi</italic>, outside the clade formed by the upper Maastrichtian forms <italic>B. sinuosa</italic> and <italic>B. praeclara</italic>. The holotype of <italic>Basilemys morrinensis</italic> provides the first evidence that this genus reached large size (∼1 m long) in the Horseshoe Canyon Formation and was not diminutive as previously thought based on less complete shell material. Although <italic>Basilemys</italic> is usually regarded as terrestrial in habit based on its skull and limb morphology, we note that the low profile of its shell is a derived feature usually indicative of an aquatic mode of life. This suggests that there is yet much to learn about the life habits of this interesting turtle. https://zoobank.org/urn:lsid:zoobank.org:pub:F1F61F25-C441-4D7D-99B0-D460ABCC7918 SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at <ext-link>www.tandfonline.com/UJVP</ext-link> Citation for this article: Mallon, J. C., and D. B. Brinkman. 2018. <italic>Basilemys morrinensis</italic>, a new species of nanhsiungchelyid turtle from the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta, Canada. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2018.1431922. [ABSTRACT FROM AUTHOR]