The order Rodentia is the most abundant and successful group of mammals, and it has been a focal point of attention for compar ative and evolutionary biologists for many years. In addition, rodents are the most commonly used experimental mammals for bio medical research, and they have played a central role in investi gations of the genetic and molecular mechanisms of speciation in mammals. During recent decades, a tremendous amount of new data from various aspects of the biology of living and fossil rodents has been accumulated by specialists from different disciplines, ranging from molecular biology to paleontology. Paradoxically, our understanding of the possible evolutionary relationships among different rodent families, as well as the possible affinities of rodents with other eutherian mammals, has not kept pace with this information "explosion. " This abundance of new biological data has not been incorporated into a broad synthesis of rodent phylo geny, in part because of the difficulty for any single student of rodent evolution to evaluate the phylogenetic significance of new findings from such diverse disciplines as paleontology, embryology, comparative anatomy, molecular biology, and cytogenetics. The origin and subsequent radiation of the order Rodentia were based primarily on the acquisition of a key character complex: specializations of the incisors, cheek teeth, and associated mus culoskeletal features of the jaws and skull for gnawing and chewing."

The order Rodentia is the most abundant and successful group of mammals, and it has been a focal point of attention for compar ative and evolutionary biologists for many years. In addition, rodents are the most commonly used experimental mammals for bio medical research, and they have played a central role in investi gations of the genetic and molecular mechanisms of speciation in mammals. During recent decades, a tremendous amount of new data from various aspects of the biology of living and fossil rodents has been accumulated by specialists from different disciplines, ranging from molecular biology to paleontology. Paradoxically, our understanding of the possible evolutionary relationships among different rodent families, as well as the possible affinities of rodents with other eutherian mammals, has not kept pace with this information "explosion. " This abundance of new biological data has not been incorporated into a broad synthesis of rodent phylo geny, in part because of the difficulty for any single student of rodent evolution to evaluate the phylogenetic significance of new findings from such diverse disciplines as paleontology, embryology, comparative anatomy, molecular biology, and cytogenetics. The origin and subsequent radiation of the order Rodentia were based primarily on the acquisition of a key character complex: specializations of the incisors, cheek teeth, and associated mus culoskeletal features of the jaws and skull for gnawing and chewing."

Tree shrews are small-bodied, scansorial, squirrel-like mammals that occupy a wide range of arboreal, semi-arboreal, and forest floor niches in Southeast Asia and adjacent islands. Comparative aspects of tree shrew biology have been the subject of extensive investigations during the past two decades. These studies were initiated in part because of the widely accepted belief that tupaiids are primitive primates, and, as such, might provide valuable insight into the evolutionary origin of complex patterns of primate behavior, locomotion, neurobiology, and reproduction. During the same period, there has been a renewed interest in the methodology of phylogenetic reconstruction and in the use of data from a variety of biological disciplines to test or formulate hypotheses of evolutionary relationships. In particular, interest in the com parative and systematic biology of mammals has focused on analysis of phy logenetic relationships among Primates and a search for their closest relatives. Assessment of the possible primate affinities of tree shrews has comprised an important part of these studies, and a considerable amount of dental, cranio skeletal, neuroanatomical, reproductive, developmental, and molecular evi dence has been marshalled to either corroborate or refute hypotheses of a special tupaiid-primate relationship. These contrasting viewpoints have re sulted from differing interpretations of the basic data, as well as alternative approaches to the evolutionary analysis of data."