Evolution and diversity of Oligo-Miocene Mongolian rodents

The Oligocene/Miocene transition (c. 23 Ma) is a critical period in the evolutionary history of Eurasian mammals. Indeed most of Palaeogene groups of mammals disappear at the end of the Oligocene or along the course of the Early Miocene and are progressively replaced by the modern groups of mammals that compose the extant biodiversity. The climatic changes that encompass the Oligocene/Miocene transition might play a major role on the diversification and dispersal processes of modern mammal across Eurasia. The project aims at better understanding the impact of climatic and environmental changes on the diversity and evolution of rodents around the Oligocene- Miocene transition. This problem is actually two-folded because before addressing mammalian evolutionary issues, it is necessary to better characterize the evolution of terrestrial environments. To tackle these questions the study of the fossil material from the Palaeogene/Neogene sequence from the Valley of Lakes in Central Mongolia will be the base of integrated geological, palaeoecological and palaeoenvironmental studies in order to answer this problematic. The project will be carried out following three main approaches: (1) a detailed study and description of the fossil specimens associated with cladistic analyses in order to interpret the phylogenetic relationships of the Mongolian species with other Eurasian species, (2) palaeoenvironmentals reconstruction based on geological, palaeoecological and geochemical analyses, and (3) quantification of the palaeobiodiversity of Mongolian assemblages and other regions in order to follow its evolution, especially through the Oligocene/Miocene transition. This research project will carried out in the scope of the project "Impact of Oligo-Miocene climate changes on Mongolian mammals" (FWF-Project: P-23061-N19), lead by Gudrun Daxner-Höck and the teams of the Natural History Museum of Vienna. All the members of the team of the Natural History Museum of Vienna plus international collaborators gathered for the FWF-Project are specialists of various disciplines in geology, palaeontology and geochemistry giving the opportunity to develop the integrated multidisciplinary project presented above.

Resulting from a collaboration between the Science Academy of Mongolia and the Naturhistorisches Museum Wien, paleontological and geological investigations have been conducted for almost 20 year in the so called "Valley of lakes" of central Mongolia, focusing on the Oligocene and Miocene fossil record (from 34 to 7 Million years ago). There, about 40 localities have been discovered and 123 fossiliferous layers have been sampled, yielding more than 10 thousand specimens of mammalian remains. The present project focused on fossil rodents including systematic and paleoecologic analyses of the remains. The studies revealed an unexpected richness of the rodent faunas with more than a hundred identified species altogether. Some family of rodents such as the Dipodidae (jerboa and jumping mice) and Cricetidae (hamsters) are especially diversified and abundant and compose the majority of the remains. The investigations revealed also the occurrence of some extinct groups of rodents, generally rare in the fossil record of central Asia, such as Eomyidae (an extinct family of rodent closely related to extant pocket gophers), including a new species, and Tachyoryctoidinae (an extinct fossorial group related to extant mice and hamsters). Some newly discovered remains also provide interesting insights on the passed environments of central Mongolia. For the first time a few teeth have been identified as belonging to a flying squirrel, thus demonstrating the presence of significant forest environments 20 million years ago as opposed to the semi-arid environments today in central Mongolia. The Oligocene-Miocene transition (which is marked by a world-wide short glaciations event) is a critical time for all these rodents as many of them disappear at the end of the Oligocene and are replaced by new ones at the beginning of the Miocene. More generally the associations of fossil rodents allow us to draw a more detailed picture of the passed landscapes, mainly composed of opened prairies crossed by large rivers, and with the presence of some isolated forest patches. The high diversity of small mammals indicates abundant and diverse vegetations in accordance with the presence of several large herbivorous mammals at the time (such as rhinoceros and elephant-like animals). All these observations proof that the desertification of central Mongolia (and more generally central Asia) occurred quite recently, within the last 5 million years. This is later than expected considering the ongoing climatic changes triggered by the elevation of the Himalayan mountain range already since the Late Oligocene, around 25 Million years ago