Asian Eocene lagomorphs are mainly known from the Irdinmanhan, Sharamurunian, Ulangochuan, and Ergilian faunas that range from the middle Eocene to late Eocene. Nine genera of lagomorphs are known from these Eocene faunas, including Lushilagus, Shamolagus, Gobiolagus, Hypsimylus, Strenulagus, Dituberolagus, Valerilagus, Annalagus and Desmatolagus. Since the report on Paleogene lagomorphs of Matthew and Granger (1923), many studies have documented the rich fossil records from Asia (Teilhard de Chardin 1926; Burke 1941; Bohlin 1942; Li 1965; Sych 1975; de Muizon 1977; Huang 1986, 1987; Tong and Lei 1987; Qi 1988; Tong 1997; Erbajeva 1999; Meng et al. 1999; Meng and Hu 2004). These forms represent the earliest radiation of the lagomorphs and are the stem taxa of the crown clade Lagomorpha (McKenna 1982). These species are also useful for biostratigraphic division and correlation of early Tertiary beds of the region because of their distinctive morphologies, restricted geological distributions, and numerical abundance (Huang 1987; Tong et al. 1995; Wang 1997a, 1997b; Erbajeva and Tyutkova 1997).
Among the collections of fossil mammals made by the Central Asiatic Expedition by the American Museum of Natural History in 1925, there are 21 fragmentary specimens of lagomorphs from the Shara Murun Formation at the Ula Usu locality, Shara Murun region, Inner Mongolia. These specimens bear the field number 507, noted as miscellaneous from the “main Ula Usu pocket” in the field notebook (Granger 1925) and have been lumped in one catalog number, AMNH 21750. Several species belonging to Gobiolagus, Desmatolagus, and Shamolagus were discovered from localities, Ula Usu included, in the Shara Murun region of Inner Mongolia (Burke 1941; Li 1965; Qi 1988). The AMNH lagomorph specimens collected from Ula Usu belong to two species of Gobiolagus: G. tolmachovi and a new species. These specimens provide additional knowledge of the morphology of Gobiolagus and add to the diversity of Eocene lagomorphs of the region. In this study, we describe these specimens that have been in the collections for 80 years.
In addition to description of the new specimens from Ula Usu, we provide a brief review of the Eocene lagomorphs from Asia with focus on diagnoses of the genera and comments on the localities where the specimens came from. With discoveries of Eocene lagomorph species during the last few decades, a comparative study of these taxa appears needed. We also include photographs of the holotypes housed in the AMNH and believe these photographs will provide more detailed morphologies of the species than previously illustrated. Most of the previous studies provided only line drawings of the specimens, which do not display the detailed morphologies critical in identifying the species. For instance, the line drawing of the holotype of G. andrewsi was somewhat idealized and did not display detailed structures on the p3 and worn condition of other cheek teeth that are critical for the establishment of the species.
In the description, we follow Wood (1940) for most of the terminology of the lagomorph dental morphology, and note the inconsistency in terminology used for cheek teeth of lagomorphs. Taxonomy of lagomorphs follows McKenna and Bell (1997). Measurements of teeth were made using an Ultra-Call Mark III digital caliper. Abbreviations: AMNH, American Museum of Natural History; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing.