“Macrobaenidae” (Testudines: Eucryptodira) from the Late Paleocene (Clarkforkian) of Montana and the Taxonomic Treatment of “Clemmys”backmani

Abstract

Two partial “macrobaenid” shells are described from a single Late Paleocene (Clarkforkian 1) quarry at Burns Mine near Washoe, Carbon County, Montana, USA, and are referred to “Clemmys” backmani, a taxon typified in the Paleocene Ravenscrag Formation of Saskatchewan, Canada. Though tentative, morphological, temporal and spatial arguments highlight similarities between “Clemmys” backmani and the Campanian taxon Judithemys sukhanovi, and we therefore suggest the new combination Judithemys backmani. Judithemys sukhanovi and Judithemys backmani both originate from the eastern margins of the western landmass Laramidia and exhibit a pronounced ridge along the visceral side of the first costal and C-shaped bridge peripherals into which the flat thoracic ribs insert superficially, characters that must be interpreted as symplesiomorphies. By contrast, the coeval taxon Osteopygis emarginatus originates from the eastern shores of the eastern landmass Appalachia, lacks the visceral ridge of the first costal, and possesses massive, triangular peripherals into which the peglike thoracic ribs insert deeply.