In Pursuit of Early Mammals

Foreword

ix F Foreword Since the demise of non-avian dinosaurs some 65 million years ago, modern mammals—that is to say, marsupials, placentals, and monotremes—have become conspicuous, diverse elements of Earth’s biota. But the family tree that includes mammals and their nearest relatives (mammaliaforms, in technical parlance) is far more deeply rooted in time, extending back to perhaps 220 million years ago. Clearly, then, the varieties known from the “age of mammals” that followed dinosaur extinctions represent only the uppermost boughs of what is increasingly being revealed as a bushy, complex tree. Knowledge of early mammalian history—the lower part of that tree—has long been limited by a fossil record that is disappointingly sparse, even to those paleontologists who set a low bar when it comes to basic data. Fossils are generally tiny, fragmentary (most species are known by isolated teeth or jaw fragments), and incredibly hard to find. The dawn of discovery took place in the late nineteenth century. Synthetic study of these fossils, almost all from Britain and the western United States, was completed in the 1920s by the great evolutionary biologist G. G. Simpson. It was also at this time that the first mammalian skulls of Mesozoic age were discovered in Cretaceous rocks of Mongolia. Simpson recognized the existence of various now-extinct Mesozoic mammal lineages, as well as representatives (or, it now appears, relatives) of modern groups. But this early period of discovery and study raised fundamental questions that could not be addressed with the existing record. Did the egg-laying platypus and echidnas of Australia and New Guinea, for example, independently evolve from “reptilian” precursors? Meaningful biological interpretation also remained beyond reach: about all one could say is that early mammals were generally small, shrew-like creatures that probably preferred an insect-based diet. (The stereotype has persisted: as this book shows, it is erroneous.) Early mammal history had emerged from total darkness, but remained in the shadows. The situation has changed profoundly in recent decades. A new period of fossil discovery began in the mid-twentieth century and its pace has quickened to a dizzying rate. A number of species are now known by breathtakingly complete specimens, some even preserving remnants of fur. Long-term, dedicated effort has resulted in the collection of large samples and comprehensive assemblages for some areas and time intervals. Informative fossils have been recovered from most major landmasses, and though substantial gaps in the record persist, much of the Mesozoic time scale has been sampled. And, of course, many x Foreword new species and some major groups have been discovered, revealing previously unimagined diversity among early mammals: some hopped; others climbed, burrowed, swam, and even glided; at least one species fed on young dinosaurs. Amazingly, a venom-delivery system (in the form of an ankle spur as seen in living monotremes) seems to have been widespread among early groups. This explosive growth in basic data— the fossils themselves—has been coupled with important conceptual, technological, and methodological advances. Ultrahigh-resolution X-ray computed tomography, for example, has made accessible a wealth of new anatomical information. Comparative studies of new fossils show that early mammals played a broad array of biological roles. Key evolutionary transformations, such as the appearance of the multi-function molar, expansion of the brain, and the development of the characteristic three-boned middle ear, are now relatively well understood. Notably, it appears that each of these transformations took place more than once in mammalian history. The fossil record now provides reasonable time constraints on the divergence of modern lineages—constraints that are consistent with estimates derived from molecular studies based on living species. Perhaps most significantly, new means of reconstructing genealogy have provided the outlines and many details of the mammalian tree mentioned earlier, and there is now general consensus about placement of many of the major branches on that tree. Now, finally, we know enough about Mesozoic mammals that a general book on the subject is not only possible, but urgently needed. This volume serves that purpose. The author, Zofia Kielan-Jaworowska, has been at the vanguard of the field for half a century. In the following pages she recounts the origin, evolutionary history, relationships, and biology of early mammals—largely from firsthand experience in both field and lab. More than that, she tells us about the people who have discovered and studied the fossils unearthed during the modern era. No field program has contributed more to—or symbolizes better—this modern...