An UNDERTAKING OF THIS SORT could not be accomplished without the support, assistance, and input of many people. First, I thank my colleagues in the Center for Functional Anatomy and Evolution (FAE), Valerie De Leon, Chris Ruff, Mark Teaford, and Dave Weishampel, for their encouragement, illuminating discussions, sharing of knowledge, and numerous other favors. I also thank the FAE graduate students, past and present, who have helped to inspire this book. Many of them have read and corrected chapters, offered helpful insights, or provided information or other assistance, which is much appreciated. Jay Mussell and Mary Silcox deserve special mention for many stimulating discussions and enlightening me on several topics. I am especially grateful to Shawn Zack, who has freely shared his broad knowledge of mammalian fossils and the literature, and who has helped with countless tasks in the preparation of this book. The assistance of Arlene Daniel in the FAE administrative office has been much appreciated during all stages of the project.

I am enormously indebted to Anne Marie Boustani, who undertook the formidable task of scanning, resizing, arranging, and labeling all of the figures in the book, in some cases multiple times, to achieve the best possible result. In so doing she has made a huge and fundamental contribution to this work. She also drafted all the cladograms and a number of original figures. Her dedication, perseverance, and cheerfulness throughout this painstaking process are very much appreciated.

Part of the preparation of this volume was undertaken during my tenure of an Alexander von Humboldt Award in the Institut für Paläontologie at the University of Bonn, Germany, in 2003-2004, for which I thank the A. von Humboldt Foundation. The faculty, staff, and students at the Institut für Paläontologie, especially Prof. Wighart xiv Acknowledgments von Koenigswald, created an ideal environment for this work, and I am very grateful for their support.

Throughout preparation of this book, I have consulted with numerous colleagues about their areas of expertise. Their generosity in providing advice, information, casts or images, permission to reproduce illustrations, and other assistance has been overwhelming and has been instrumental in completion of the work. I extend my gratitude to them all. Almost half of those listed sent original photographs, slides, drawings, or electronic images, which often required considerable time and effort on their part. I have attempted to acknowledge here all those who have contributed; nevertheless, as this project has been a decade in development, inadvertent omissions are likely, and I ask the indulgence of anyone overlooked. My appreciation goes to David Archibald, Rob Asher, Chris Beard, Lüian Bergqvist, Jon Bloch, José Bonaparte, Louis de Bonis, Tom Bown, Percy Butler, Rich Cifelli, Russ Ciochon, Bill Clemens, Jean-Yves Crochet, Fuzz Crompton, Demberelyin Dashzeveg, Mary Dawson, Daryl Domning, Stéphane Ducrocq, Bob Emry, Burkart Engesser, Jörg Erfurt, John Fleagle, Ewan Fordyce, Dick Fox, Jens Franzen, Eberhard Frey, Emmanuel Gheerbrant, Philip Gingerich, Marc Godinot, Gabriele Gruber, Gregg Gunnell, Jörg Habersetzer, Gerhard Hahn, Sue Hand, Jean-Louis Hartenberger, Ron Heinrich, Jerry Hooker, Jim Hopson, Yaoming Hu, Bob Hunt, Jean-Jacques Jaeger, Christine Janis, Farish Jenkins, Dany Kalthoff, Zofia Kielan-Jaworowska, Wighart von Koenigswald, Bill Korth, Dave Krause, Conny Kurz, Brigitte Lange-Badré, Chuankuei Li, Jay Lillegraven, Alexey Lopatin, Spencer Lucas, Zhexi Luo, Bruce Mac-Fadden, Thomas Martin, Malcolm McKenna, Jim Mellett, Jin Meng, Michael Morlo, Christian de Muizon, Xijun Ni, Mike Novacek, Rosendo Pascual, Hans-Ulrich Pfretzschner, Don Prothero, Rajendra Rana, Tab Rasmussen, John Rens-berger, Guillermo Rougier, Don Russell, Bob Schoch, Erik Seiffert, Bernard Sigé, Denise Sigogneau-Russell, Elwyn Simons, Gerhard Storch, Jean Sudre, Hans-Dieter Sues, Fred Szalay, Hans Thewissen, Suyin Ting, Yuki Tomida, Yongsheng Tong, Bill Turnbull, Mark Uhen, Banyue Wang, Xaioming Wang, John Wible, Jack Wilson, and Shawn Zack.

Wherever possible, illustrators have been acknowledged as well (see the last printed pages of the book). Special thanks are due the following scientific illustrators for allowing reproduction, and in many cases providing images, of their work: Doug Boyer, Bonnie Dalzell, Utako Kikutani, John Klausmeyer, Mark Klingler, Karen Klitz,Jay Matternes, Bonnie Miljour, Mary Parrish, and especially Elaine Kasmer, my illustrator for many years.

I have also benefited from the experience and wisdom of friends and esteemed colleagues who reviewed sections of the text for accuracy, including Rich Cifelli, Mary Dawson, Daryl Domning, John Fleagle, John Flynn, Ewan Fordyce, Jerry Hooker, Christine Janis, Zofia Kielan-Jaworowska, Wighart von Koenigswald, Zhexi Luo, Thierry Smith, Scott Wing, and Shawn Zack. I am grateful to all of them for numerous corrections and clarifications, which improved the text substantially. I am especially indebted to Bill Clemens and Malcolm McKenna, whose critical reading of the entire text and sage advice has been invaluable. Although I have relied on the counsel of these distinguished authorities to avoid errors, omissions, and ambiguities, I did not always follow their suggestions, and any shortcomings that remain are, of course, my own responsibility.

I take this opportunity to acknowledge the encouragement and guidance of several people who fostered my interest in paleontology during my student years (listed more or less chronologically): Dave Stager, Margaret Thomas, Bob Salkin, Don Baird, Nick Hotton, Clayton Ray, Glenn Jepsen, Elwyn Simons, George Gaylord Simpson, Bryan Patterson, and Philip Gingerich. Without their support, particularly at pivotal periods in my life, I would not be a vertebrate paleontologist today

This project would never have made it to fruition without the able guidance of my editor at the Johns Hopkins University Press, Vincent Burke. To him, as well as to Wendy Harris, Linda Forlifer, Martha Sewall, and Carol Eckhart at the press, and to Peter Strupp, Cyd Westmoreland, and the staff at Princeton Editorial Associates, I extend my sincere thanks for seeing this volume through.

Last but not least, I am most grateful to my family— my wife Jennie and daughters Katie and Chelsea—for their unwavering faith in me and their steadfast encouragement throughout the long gestation of this project, especially when it seemed unachievable. They are to be credited with its completion.

The Beginning of the Age of Mammals

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MAMMALS ARE AMONG THE MOST successful animals on earth. They occupy every major habitat from the equator to the poles, on land, underground, in the trees, in the air, and in both fresh and marine waters. They have invaded diverse locomotor and dietary niches, and range in size from no larger than a bumblebee (the bumblebee bat Craseonycteris: body length 3 cm, weight 2 g) to the largest animal that ever evolved (the blue whale Balaenoptera: body length 30 m, weight > 100,000 kg). Just over a decade ago, the principal references recognized 4,327 or 4,629 extant mammal species in 21-26 orders (Corbet and Hill, 1991; Wilson and Reeder, 1993), the discrepancy mainly in marsupial orders. The most recent account now recognizes 29 orders of living mammals (the increase mainly reflecting the breakup of Insectivora), with more than 5,400 species in 1,229 genera (Wilson and Reeder, 2005). But many times those numbers of genera and species are extinct. Indeed, McKenna and Bell (1997) recognized more than 4,000 extinct mammal genera, many of which belong to remarkable clades that left no living descendants. The great majority of extinct taxa are from the Cenozoic, the last one-third of mammalian history. What were these extinct forms like? What made them successful, and what led to their eventual demise? How were they related to extant mammals? When, where, and how did the ancestors of modern mammals evolve, and what factors contributed to the survival of their clades?

This book addresses those questions by focusing on the mammalian radiation during the Paleocene and Eocene epochs, essentially the first half of the Cenozoic Era. Although this radiation has attracted far less popular interest than that of dinosaurs, it was a pivotal interval in the history of vertebrates, which set the stage for the present-day mammalian fauna, as well as our own evolution. At its start, the end of the Cretaceous Period, the last nonavian dinosaurs disappeared, leaving a vast, uninhabited ecospace. Mammals quickly moved in, partitioning this landscape in new ways. They were not, however, the first mammals.

Mammals evolved from their synapsid ancestors around the end of the Triassic Period, more than 200 million years ago, and coexisted with dinosaurs, other archosaurs, and various reptiles (among other creatures) for at least 140 million years during the Mesozoic Era. But during that first two-thirds of mammalian history, innovation was seemingly stifled—at least, in comparison to what followed in the early Cenozoic. It is fair to say that mammals survived during the Mesozoic but, with a few notable exceptions, rarely flourished. The biggest mammals during that era were little larger than a beaver, and only a few reached that size. Most Mesozoic mammals were relatively generalized compared to the mammals that evolved within the first 10-15 million years of the Cenozoic—although recent discoveries hint at greater diversity than was previously known. Kielan-Jaworowska et al. (2004) present a thorough, current account of mammalian evolution during the Mesozoic.

Like most clades, mammals were severely affected by the terminal Cretaceous mass extinctions. Most Mesozoic mammal radiations became extinct without issue. Indeed, two-thirds of the 35 families of Late Cretaceous mammals listed by McKenna and Bell (1997) disappeared at the end of the Cretaceous. In the northern Western Interior of North America, mammalian extinctions were even more severe, affecting 80-90% of lineages (Clemens, 2002). A small number of clades crossed the Cretaceous/Tertiary (K/T) boundary, most notably, several lineages of multitubercu-lates, eutherians, and marsupials; the latter two groups quickly dominated the vertebrate fauna on land. (Multi-tuberculates are an extinct group of small, herbivorous mammals that were the most successful Mesozoic mammals; see Chapter 4.) Those few lineages that survived the K/T extinctions are the mammals that ultimately gave rise to the diversity of Cenozoic mammals.

It is notable that all three of these groups had existed for at least as long before the K/T boundary as after it, yet the fossil evidence suggests that only the multituberculates radiated widely during the Mesozoic. The Mesozoic was the heyday of multituberculates. They shared the Earth with dinosaurs for 90 million years or more, becoming diverse and abundant in many northern faunas, only to be out-competed by other mammals before the end of the Eocene. Even those other mammals—metatherians and eutherians (often grouped as therians, or crown therians)—had diverged from a common stem by 125 million years ago. But this divergence occurred well after the multituberculate radiation was under way. Perhaps competition from multituber-culates and other archaic mammals—as well as archosaurs— prevented metatherians and eutherians from undergoing major adaptive radiations during the Mesozoic. Whatever the reason, during the Cretaceous, these groups failed to attain anything close to the morphological or taxonomic diversity they would achieve in the first 10-15 million years of the Cenozoic.

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