Three New Sciuravids (rodentia) from the Latest-early to Middle Eocene of Wyoming: Mosaic Evolution and the Alpha Taxonomy of Sciuravus Nitidus

Sciuravids are a diverse group of early to middle Eocene rodents known from North America. The delineation among species of Sciuravus is challenging because of extensive intraspecific morphological variation. Previous studies have reviewed the type species of Sciuravus, S. nitidus, and have presented descriptions of several other species of the genus, but no comprehensive comparisons have been made to assess the validity of these species. In this study, a sample of sciuravids collected from two distinct biochronologic zones in the middle Eocene of the Green River Basin, Wyoming were compared to known species of Sciuravus from a variety of time intervals and geographic regions, making it possible to recognize the limits of intraand interspecific variation. Several specimens fit into previously described species, but others did not. Discovery of specimens that exceeded the variation within known species categories led to the recognition of three new species of Sciuravus: S. metalinguas, S. inclinatus, and S. nexus. Description and analysis of the new species effectively clarifies the alpha taxonomy of S. nitidus and reveals a pattern of mosaic evolution—each species shares some crown pattern and/or zygomatic arch features with another species or two, some ancestral, some derived. In addition, I describe the upper dentition of S. wilsoni, a species that is significant to resolving questions about sciuravid origins and evolutionary relationships. These sciuravid species evolved during the early to middle Eocene, a time period known to be associated with significant global climate change. The results reported here further support the growing evidence that changing climates directly influenced Eocene mammalian diversity and geographic distribution patterns. Contributions from the Museum of Paleontology, University of Michigan 1St. Norbert College, 100 Grant Street, Street, De Pere, WI 54115 ([email protected]) INTRODUCTION Sciuravids are a diverse group of rodents found in the early to middle Eocene (Wasatchian–Uintan) sediments of North America (Wilson, 1949). Their abundance peaked during the Bridgerian and Uintan (latest-early to middle Eocene), time periods marked by significant climatic shifts and major faunal turnovers (Walton and Porter, 2008; Woodburne et al., 2009a, b). Traditionally the term “sciuravids” has been used to refer collectively to three lineages: species of Sciuravus, species of Knightomys and Prolapsus, and species of Pauromys (Walton and Porter, 2008). Recent work has restricted the family name Sciuravidae to only one of these three lineages, species of Sciuravus, which are thought to be part of a stem group of Myomorpha, a diverse group of mouselike rodents (Wood, 1955; Gazin, 1961; Dawson, 1966; Wang and Dawson, 1994; Porter, 2001). Although the origin of the Sciuravidae remains unclear (Dawson, 2015), its most likely sister taxon is Microparamys, an ischyromyid (Ivy, 1990; Rose, 2006) that also has a protrogomorphous skull structure, sciurognathous jaw, retains two upper premolars, and has brachydont cheek teeth (Wilson, 1938, 1949; Dawson, 1977; Walton and Porter, 2008). Sciuravids differ from