A grant has been awarded to Dr. Novacek at the American Museum of Natural History, Dr. Rougier at the University of Louisville, and Dr. Wible at the Carnegie Museum of Natural History for a comprehensive study of the Cretaceous (145 to 65 million-years-ago) radiation of therian mammals. Therians include living marsupials and placentals plus their extinct relatives. Therians are represented today by more than 4600 species accounting for nearly all the diversity of living mammals, the exception being the three species of egg-laying monotremes. The study of the evolutionary relationships among therians has been addressed with data from molecular biology (the structure of DNA) and comparative anatomy (the structure of living and extinct forms). The family trees (phylogenies) that result from these two approaches are not entirely congruent nor do they suggest the same time of origin of the major lineages of living therians. However, both molecular and anatomical workers agree that insectivore-like forms are at the base of the therian radiation. Available to Drs. Novacek, Rougier, and Wible as the core study group for their project are exquisitely preserved, 80 million-year-old insectivore-like fossils, some related to placentals and others to marsupials, collected from the famed dinosaur and fossil mammal beds of Mongolia's Gobi Desert by recent joint expeditions from the American Museum and the Mongolian Academy of Sciences. Unlike the vast majority of Cretaceous therian fossils, which are merely isolated jaws and teeth, these are nearly complete skeletons. They will provide the most comprehensive anatomical data base to date for studying the evolutionary relationships among Cretaceous therians and their living descendants, and evaluating the controversial difference between DNA and anatomical results. Most of the Cretaceous therians to be studied are shrew-sized and, therefore, cannot effectively be investigated with the naked eye alone. Two tools are needed for the anatomical study of these small forms: standard stereoscopic microscopy and high-resolution computer tomographic imaging (CT-scanning). Both are available, the latter at a National Science Foundation supported facility at Pennsylvania State University. Drs. Novacek, Rougier, and Wible will use their expertise in the anatomy of living and extinct mammals, and the collections at the American Museum, Carnegie Museum, and other institutions to build an anatomical data base of comparative information. Family trees will be generated from this data base by various available computer programs and compared with those previously published based on other information. These exquisite Mongolian fossils are a unique resource that ultimately will be returned to their country of origin. Careful study and extensive documentation of these fossils now is critical in order to make them widely accessible for scientific advancement. A by-product of this study will be a standardized anatomical terminology for the therian skeleton; currently, researchers studying particular therian lineages employ different terms that are often incongruent. This study also affords the opportunity for educating and training of new scientists at both the pre- and postdoctoral level. Finally, the valuable scientific knowledge contained in these fossils will be translated for the public through the exhibits and educational programs of the American Museum and Carnegie Museum.
