CENOGRAMS OF FOSSIL MAMMALS INDICATED LATE CENOZOIC ENVIRONMENTAL CHANGES IN THE LINXIA BASIN (GANSU, CHINA)

　  The evolutionary history of mammalian communities is significant for reconstructing past environments and climate. A cenogram is a rank-ordered body mass distribution of non-predatory terrestrial mammal species within a fauna. Based on comparisons with modern faunas, cenograms of fossil faunas have been used for inferring environments and their changes through geological time.

　  Dr. Tao Deng, Institute of Vertebrate Paleontology and Paleoanthropology, ChineseAcademy of Sciences, uses the cenogram method to study paleoecological conditions for the abundant fossils of the Late Cenozoic mammalian faunas from the LinxiaBasin. The time span in this study ranges from the Oligocene to the Pleistocene. His analyses are compared to other paleoenvironmental evidences from fossil morphology, lithology, paleoclimatology, and stable isotopes in order to examine the accuracy of paleoecological reconstruction based on cenograms.

　  In his analyses, body sizes for fossil taxa were estimated using regressions of body weight based on the area of the first lower molar for most species and on other teeth or limb bones for a few species. Most measurements for the body estimations are from the Linxia fossils, while a few are from the literature. Cenogram statistics are calculated for seven fossil faunas, allowing paleoenvironmental interpretations to be made. These analyses reveal open conditions during the Late Miocene, Early Pliocene and Early Pleistocene; less open conditions during the Late Oligocene but a closed environment during the Middle Miocene; arid conditions during the Late Oligocene and earliest Late Miocene; less arid during the Late Miocene, Early Pliocene, and Early Pleistocene but humid during the Middle Miocene.

　  A well-developed successive sedimentary sequence ranging from the Oligocene to the Pleistocene is exposed in the LinxiaBasin in Gansu, northwestern China, and contains a large number of mammal fossils. The LinxiaBasin is located in the transitional zone between the Tibetan and Loess plateaus, so it provides a very good site for studying the uplift history of the Tibetan Plateau and its influence on the environment. Mammals are very sensitive to environmental changes. The strong uplift of the Tibetan Plateau during the Late Cenozoic may have dramatically affected the environment, which must be reflected in the evolution of mammalian faunas.

　  Valverde (1964) firstly showed vertebrate body sizes within a fauna in a univariate plot according to a decreasing rank order, and he called this method as cenogram. Valverde (1964, 1967) studied predator-prey body size relationships in modern faunas based on cenograms. As a result, he considered that cenogram curves of terrestrial vertebrates in different habitats had different shapes. The cenogram method for mammalian faunas was used in paleoecological studies by some authors (Legendre, 1986, 1989; Montuire and Desclaux, 1997; Montuire, 1999).

　  In the last decade, numerous paleoclimatic interpretations have been proposed based on the rich Late Cenozoic fossils and sedimentary records in China (Qiu et al., 1999; Qiu, 2003; Qiu and Li, 2003, 2005; Deng and Downs, 2002; Guo et al., 2002; Wang and Deng, 2005), but most of the methods used have been based on more or less restricted taxonomical subsets of the total fossil fauna (e. g., rodents, large mammals, etc. ). In this sense, it is important to point out that different mammal groups may offer different paleoecological information. Particularly, large and small mammals usually provide slightly different paleoenvironmental reconstructions due to disparities in scale of landscape perception and physiological constraints (Hernández Fernández et al., 2006).

　  Legendre (1986) used cenograms to study the paleoecological features of fossil mammals. Other authors studied more Cenozoic mammalian faunas (Gingerich, 1989; de Bonis et al., 1992; Gunnell, 1994, 1997; Montuire and Desclaux, 1997; Wilf et al., 1998; Croft, 2001; Montuire and Marcolini, 2002). However, few of these studies have dealt with Asian localities, and none have included fossil faunas from China.