Introduction

The last (and only) general book on Mesozoic mammals, edited by Jason A. Lillegraven, Zofia Kielan-Jaworowska, and William A. Clemens, appeared in 1979. The need for an updated compendium on the subject has been clear for some time. Our basic source of information—a fossil record comprised of unbelievably tiny, delicate, and elusive fossils—has expanded by orders of magnitude over the past quarter-century. New specimens, some of them breathtakingly complete, have poured in from almost all parts of the globe. These discoveries, together with the application of new scientific approaches and techniques, have led to profound changes in our interpretation of early mammalian history—changes that are coming about at a greater pace today than ever before. Primary interest in the subject itself has grown considerably. Perhaps more significantly, however, Mesozoic mammals have come into their own as an important, rich source of information for evolutionary biology in general. Their record of episodic, successive radiations speaks to the pace and mode of evolution. Early mammals were small, but they provide key information on the morphological transformations that led to modern mammals, including our own lineage of Placentalia. Early mammals were significant and often rapidly evolving elements of the terrestrial biota for much of the Mesozoic—biota that are playing an increasingly important role in studies of paleoecology, faunal turnover, and historical biogeogra-phy. Finally, of course, the record of early mammals occupies center stage when it comes to testing molecular evolutionary hypotheses on the timing and sequence of mammalian radiations.

Given this proliferation of new information and multi-disciplinary interest in Mesozoic mammals, it is not surprising that compilation of this volume was both an exciting and a humbling experience for us. We quickly found that we had given ourselves far too much credit when it came to knowledge of the subject matter. Along the way, we encountered some critical topics where additional first-hand experience was needed; in other cases, we ran into unresolved problems that would have to be faced if the book was to be completed. This led us to undertake several collateral studies, the most important of which were published as Kielan-Jaworowska et al. (1998), Kielan-Jaworowska and Hurum (2001), Luo, Cifelli, and Kielan-Jaworowska (2001), Kielan-Jaworowska, Cifelli, and Luo (2002), and Luo et al. (2002). Work on these projects inevitably delayed completion of the book, as did the pace of discovery and conceptual change itself. No sooner had a chapter been drafted, for example, when a new discovery or phylogenetic analysis would require its complete revision. But change is a measure of vitality, and we are convinced that the study of mammalian evolution is not only vitally alive, but that it is poised at the edge of paradigm shift. This is, perhaps, the most exhilarating time in history for students of early mammals, and there is no doubt that monumental changes will await a future volume that succeeds this one. In the meantime, we hope that our book will stimulate and foster those changes and that it might ultimately help sow the seeds of its own obsolescence.

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