Spalacotheriid symmetrodonts (Mammalia) from the medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local fauna, Cedar Mountain Formation, Utah, USA

Symmetrodont mammals, generally rare and poorly represented in the fossil record, are exceptionally abundant in the Mussentuchit local fauna of the upper Cedar Mountain Formation (upper Albian or lower Cenomanian), Emery County, Utah, USA. Herein we describe three new species of symmetrodonts (four or more are present in the fauna); one is referable to Spalacotheridium, otherwise known from the Turonian (Late Cretaceous), and the other two are referred to a new genus. With the possible exception of Mictodon, all North American Cretaceous symmetrodonts are referable to the Spalacotheriidae. Spalacotheriids are distinctly different from more primitive symmetrodonts such as Kuehneotherium in jaw structure (e.g., detachment ofpostdentary elements, presence of pterygoid crest) and molar morphology and function (e.g., development of continuous mesial and distal shearing surfaces). To this extent, they are more clearly similar to tribosphenic therians than are archaic symmetrodonts, although they are uniquely specialized. Some features of advanced spalacotheres, such as the loss of the coronoid and meckelian groove, developed convergently in tribosphenidans and many other groups, and hence represent iterative themes in the evolution of Mesozoic mammals. Features of the molars and dentary suggest that the Family Spalacotheriidae is a monophyletic group, with the European Spalacotherium and Chinese Zhangheotherium forming successive outgroups to remaining genera. Within the family, North American taxa appear to form a monophyletic clade, culminating in the highly specialized Symmetrodontoides of the Late Cretaceous; Microderson, known only by a single upper molar from the Cretaceous of Morocco, is of enigmatic affinities. Spalacotheriids were clearly present in North America by the Aptian-Albian and, assuming that North American taxa form an endemic and exclusive monophyletic group, their presence on the continent cannot be attributed to an hypothesized mid-Cretaceous interchange with Asia. Instead, phylogenetic data suggest their origin from western Europe sometime in the Early Cretaceous, supporting the hypothesis that there was some degree of faunal continuity at that time between the two landmasses, based initially on similarities of the dinosaur assemblages.