Abstract

New Palatobaena material from the Hell Creek Formation (Maastrichtian), including the first skull and shell association, from southwestern North Dakota represents a new species named herein Palatobaena cohen. The materialconsists of 4 skulls, 2 lower jaws, and 2 shells and represents a true biological population (spatially and temporally restricted), which provides unprecedented access to ontogenetic and other intraspecific variation found in this taxon. Theskull’s round shape and lack of a lingual ridge on the greatly expanded triturating surface indicate its Palatobaena affinities, but it differs from both previously existing Palatobaena taxa in a number of features. The addition of shell characters to the most inclusive baenid phylogenetic analyses (Maximum parsimony and Bayesian) to date indicate tha tPa. cohen is sister taxon to the other Palatobaena taxa. Notably, both the maximum parsimony analysis and Bayesiananalysis provide strong support for Plesiobaena antiqua as sister to the Palatobaena clade. In addition, both analyses provide strong support for Stygiochelys estesi as sister to the Eocene clade of Baena arenosa and Chisternon undatum,which significantly reduces this clades’ ghost lineage. The baenid topology reveals a demonstrably homoplastic trend towards the reduction of the temporal emargination and unique thickening of the posterior portion of the parietals that corresponds with the K/T boundary and is hypothesized to have provided limited protection from increasingly effective mammalian predators.