AFTER MAMMALS EMERGED FROM CYNODONTS, they coexisted with dinosaurs for about 150 million years during the Mesozoic. The Mesozoic radiation of mammals consisted largely of groups that became extinct by the end of that era, without direct descendants; but some of them have been identified as structural, if not actual, stages in the evolution of the therian mammals prevalent today. Therefore a review of mammalian evolution during the Mesozoic will help to place Early Cenozoic mammals in perspective. All three major groups of living mammals—monotremes, metatherians, and eutherians—had evolved by the end of the Early Cretaceous, but they were not yet diverse or abundant. The only other group from the Mesozoic that unquestionably survived for a significant period into the Cenozoic (but is now extinct) is the Multituberculata.
Until recently Mesozoic mammals were quite rare, and our knowledge of most species (with a few notable exceptions) was restricted to the dentition. Over the past decade or so, however, new data have been accumulating at an astonishing rate, promoting tremendous strides in our knowledge of this early phase of mammalian evolution. Nevertheless, many relationships remain contentious, and in the last several years the field has been in a constant state of flux (see Cifelli, 2001, for a particularly useful recent review, and Kielan-Jaworowska et al., 2004, for a comprehensive and authoritative account). This chapter provides a brief summary of the current state of knowledge. A current classification of Mesozoic mammals is shown in Table 4.1.
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