Wen-Yu Wu

The Hamsters of Yushe Basin

Fossil hamsters are a minor but constant microfauna component in the succession of Yushe Basin deposits. They indicate considerable hamster diversity for North China during the late Neogene, with at least six genera and eight species recorded from late Miocene to early Pleistocene assemblages. Neocricetodon is a long-ranging late Neogene hamster, with closely related species widespread throughout Europe and Asia. Yushe Basin famously produced the type material of Neocricetodon grangeri, which we show to come from the late Miocene Mahui Formation south of the town of Yushe. Neocricetodon grangeri ranges from late Miocene (6.3 Ma) to early Pliocene (4.7 Ma) deposits. Later long-ranging Pliocene hamster lineages are Allocricetus and Cricetinus, for each of which we define a new species. Joining the latter two genera during the Late Pliocene, the living Cricetulus brings to three the number of coeval hamster genera. Finally, by the early Pleistocene, the modern genus Phodopus appears and coexists with Cricetulus in Yushe Basin.

Small Mammal Exploration in Yushe Basin, Shanxi Province

Yushe Basin is an intermontane basin at the eastern edge of the Loess Plateau in northern China. Its fluvial, lake and superposed loess deposits accumulated during the last 7 myr and contain many fossiliferous horizons. Small mammal fossils occur throughout, and some fossil horizons produce diverse assemblages that appear to faithfully represent the micromammal component of Yushe paleocommunities. Yushe Basin includes assemblages that represent the late Baodean age (latest Miocene) microfaunas of North China, and document contrasting younger assemblages that characterize a distinct Pliocene Yushe chronofauna. Pliocene fossils represent two successive microfaunas that distinguish the Gaozhuangian and Mazegouan land mammal stage/ages. These are in turn distinct from the Pleistocene assemblages of Yushe that correlate with Nihewan and later faunas. The Sino-American project, especially in the interval of 1987–1991, developed the small mammal biostratigraphy of the Yuncu subbasin of Yushe Basin. Small mammal fossils occur in every Yuncu formation, from the Baodean age Mahui Formation, through the Pliocene Gaozhuang and Mazegou formations, to the early Pleistocene Haiyan Formation. Important elements of later Pleistocene faunas occur in overlying loess. Field teams collected specimens exposed on eroded surfaces and followed indications of small mammal concentrations to excavate bulk samples and process them for fossils by wet screening. Wet screening revealed a rich component of micromammal diversity to complement the wealth of larger species that previously had been known as surface finds.

The Murine Rodents of Yushe Basin

We review and expand the systematics and biostratigraphic record of murine rodents in Yushe Basin. Several new taxa were established for the Pliocene fossil record of North China following our initial field work of the 1980s. The rodent collection was expanded and developed by complementary field work in 1991, and subsequent study. Herein we present a new Late Miocene genus and review all occurrences of Yushe Murinae. Relatively primitive taxa characterize the Late Miocene of Yushe, and these appear to be related to early lineages that diversified in the Indian Subcontinent. The interval of about 6–5 Ma in Yushe records several murines that occur at Ertemte, Inner Mongolia: Karnimatoides hipparionus, Apodemus orientalis, and Micromys chalceus. Chardinomys is an additional genus that distinguishes Yushe microfaunas from other Asian assemblages, and the common C. yusheensis ranges from latest Miocene through early Pliocene. With Chardinomys, the genus Huaxiamys characterizes Late Neogene Yushe murine assemblages. The Pliocene of Yushe Basin records successive species of Chardinomys, Micromys, Huaxiamys, and Apodemus. Derived species of Chardinomys, Micromys, and Apodemus persist into the Pleistocene.

Dynamic Small Mammal Assemblages of Yushe Basin

Targeted prospecting and screening for microfossils has revealed rich small mammal assemblages at many stratigraphic levels from late Miocene through Pliocene to early Pleistocene formations of the Yushe Basin. The recovered rodents, lagomorphs, bats, and insectivorans include species of small body size not previously known from Yushe. These assemblages represent North China small mammal communities from about 6.5 to 2 Ma and may be used to characterize four land mammal stage/ages: Baodean, Gaozhuangian, Mazegouan, and Nihewanian. The fossil succession establishes diversity and stability in Pliocene small mammal faunas for Yushe Basin before significant turnover at the beginning of the Pleistocene. The Yushe Basin habitat was relatively moist, probably without strong annual temperature extremes, supporting high species diversity.

The Bamboo Rats and Porcupines of Yushe Basin

Surface finds of small mammal fossils tend to be large body-size species; small species are retrieved more readily by applying special collecting techniques. The bias against small fossil recovery from Yushe Basin was greatly diminished by screen washing, beginning in 1987. Prior to that time, the historical monographic study by Teilhard de Chardin on the rodents of Yushe Basin focused on relatively large body-size species, especially beavers and zokors. Two other large kinds of rodents, bamboo rats and porcupines, were known to Teilhard and are reviewed here. Of these, the bamboo rats include Brachyrhizomys shansius Teilhard de Chardin, 1942 named for fossils from the Pliocene of Yushe, and a smaller late Miocene bamboo rat recovered by the Sino-American collaborative field team in 1991. This species is the oldest rhizomyine in North China, and the oldest species assignable to the extant Rhizomys group. Specimens representing the Old World porcupine Hystrix have stratigraphic importance, with a higher crowned species replacing Hystrix gansuensis by the early Pleistocene.

The Lagomorphs (Ochotonidae, Leporidae) of Yushe Basin

Of historical collections of small mammals recovered from the Yushe Basin, fossil lagomorphs comprise a significant proportion because they are relatively large fossils and dentaries with teeth occur on the surfaces of exposures. Key fossils recovered prior to 1940 from various parts of North China, including the Yushe Basin, formed the basis for early systematic studies of both ochotonids and leporids. Several Yushe Basin specimens were important in that early phase of research, and continue to be important in current revisions of Lagomorpha. This systematic treatment of the Yushe Lagomorpha embraces collections made by modern teams and classical collections of the Tianjin Natural History Museum, the American Museum of Natural History, and the Institute of Vertebrate Paleontology and Paleoanthropology. For Yushe our study recognizes an array of Ochotona (pikas) and species of the larger genus Ochotonoides . Leporids are diverse for the late Neogene deposits of Yushe Basin, and included the genera Alilepus, Hypolagus, Trischizolagus , and Sericolagus ; overlying Pleistocene loess yielded Lepus as well. These findings, including four new species, are important for lagomorph systematics, but also for the chronology of lagomorph evolution, because the Yushe sequence improves understanding of the relative ages of isolated Late Neogene deposits in China, and more broadly across Asia.

The Lipotyphla of Yushe Basin

Insectivoran mammals have been recovered by Sino-American field teams from various localities in each formation of the Yuncu and Tancun subbasins. They were an important component of late Miocene through Pliocene faunas of North China. One hedgehog has been found in late Pliocene and Pleistocene localities. Moles, however, are diverse and occur throughout the Miocene and Pliocene section, with one early Pliocene record of the water mole Desmana . The Yushe collection includes the Pliocene talpine Scaptochirus , which is at present the oldest fossil of this genus. Shrews are also diverse, including multiple representatives of the tribes Soricini, Nectogalini, and Beremendiini. Presence of a blarinine suggests immigration of this element from North America by 6 Ma, and morphological similarity with beremendiines suggests that recognition of these groups should be reviewed. The early Pleistocene Haiyan Formation has yielded only Sorex, and to date only Crocidura has emerged from the Pleistocene loess. Declining insectivoran diversity corresponds with a hypothetical decline in mean annual temperature. The insectivoran component of Yushe shows affinity with fossil and living faunas of North China, and a majority of elements are shared broadly across Eurasia. Two taxa at least ( Yunoscaptor and Soriculus) also occur in South China, indicating former wide distribution of these genera, and suggesting southward retraction of their preferred paleohabitat from Shanxi Province since the Pliocene.

The Shanxi Gerbils

Despite the wealth of small mammals in the Late Neogene of Yushe Basin, gerbils are not an abundant or diverse element. A single genus, Pseudomeriones Schaub, 1934, is known in the Miocene-Pliocene deposits of Yushe, although the Pleistocene introduces a new fauna containing Meriones Illiger, 1811. The well-known species Pseudomeriones abbreviatus (Teilhard de Chardin, 1926) is recorded in the Pliocene deposits of Zhangcun subbasin, south of Yushe. Whereas this species remains undocumented for Yuncu subbasin, Miocene age sediments have produced a lower crowned member of the genus, P. latidens Sen, 2001. We describe these remains and summarize their stratigraphic occurrence. The low crowned Miocene Yushe material suggests that the Pseudomeriones lineage involves increasing hypsodonty, with latest Miocene and Pliocene Pseudomeriones abbreviatus showing greater crown height than older late Miocene P. latidens.

Yushe Basin Prometheomyini (Arvicolinae, Rodentia)

Absent from Miocene and Pleistocene assemblages of Yushe Basin, prometheomyine Arvicolinae are a common and characteristic element of Pliocene faunas. Two time-successive species of the genus Germanomys are well represented in the Gaozhuang and Mazegou formations and serve as index fossils for Gaozhuangian and Mazegouan faunas. The genus Germanomys is well known in the Pliocene of Europe, but the oldest Yushe Germanomys (at about 4.7 Ma) is appreciably older than the European records, yet relatively derived in stage of evolution of crown height and sinuousity of the base of the enamel on molars. The younger Germanomys species is late Pliocene in age, higher crowned, and shows a more derived sinuous line. In addition to the Germanomys lineage that is well represented in the Yushe Pliocene sequence, we find evidence of the distinct genus Stachomys and perhaps another prometheomyine in Yushe Basin.