EARLY CENOZOIC PALEONTOLOGY AND STRATIGRAPHY OF THE BIGHORN BASIN, WYOMING PAPERS ON PALEONTOLOGY-RECENT NUMBERS 15. Cranial Anatomy and Evolution of Early Tertiary Plesiadapidae (Mammalia, Primates) by Philip D. Gingerich 16. Planning Photography of Microfossils by Robert V. Kesling 17. Devonian Strata of the Afton-Onaway Area, Michigan by R. V. Kesling, A. M. Johnson, and H. 0. Sorensen 18. Ostracods of the Middle Devonian Silica Formation (Volumes I and 11) by Robert V. Kesling and Ruth B. Chilman 19. Late Pleistocene Cold-blooded Vertebrate Faunas from the Mid-Continental United States. I. Reptilia; Testudines, Crocodilia. by Robert E. Preston 20. The Maple Block Knoll Reef in the Bush Bay Dolostone (Silurian, Engadine Group), Northern Peninsula of Michigan by Allan M. Johnson, Robert V. Kesling, Richard T. Lilienthal, and Harry 0. Sorensen 21. A Synopsis of Fossil Grasshopper Mice, Genus Onychomys, and their Relationships to Recent Species by Michael D. Carleton and Ralph E. Eshelman Museum of Paleontology The University of Michigan Ann Arbor, Michigan 48109 EARLY CENOZOIC PALEONTOLOGY AND STRATIGRAPHY OF THE BIGHORN BASIN Frontispiece: Sketch map of the Bighorn Basin, northwestern Wyoming, showing major physiographic features (from Bown, 1979) EARiY CENOZOIC PALEONTOLOGY AND STRATIGRAPHY OF THE BIGHORN BASIN, WYOMING Commemorating the 100th Anniversary of J. L. Wortman's Discovery of Fossil Mammals in the Bighorn Basin Edited by Philip D. Gingerich UNIVERSITY OF MICHIGAN PAPERS ON PALEONTOLOGY NO. 24 Papers on Paleontology No. 24 Museum of Paleontology The University of Michigan Ann Arbor, Michigan 48109 Gerald R. Smith, Director June 1, 1980 CONTENTS Preface and Acknowledgments .................................................... vi The Bighorn Basin-Why is it so Important? PHILIP D. GINGERICH ....................................................... 1 History of Early Cenozoic Vertebrate Paleontology in the Bighorn Basin PHILIP D. GINGERICH ..................................................... 7 Summary of Latest Cretaceous and Cenozoic Sedimentary, Tectonic, and Erosional Events, Bighorn Basin, Wyoming THOMAS M. BOWN ..................................................... 25 Paleocene Stratigraphy and Flora of the Clark's Fork Basin LEO J. HICKEY ........................................................... 33 Early Cenozoic Mammalian Faunas of the Clark's Fork Basin-Polecat Bench Area, Northwestern Wyoming PHILIP D. GINGERICH, KENNETH D. ROSE, and DAVID W. KRAUSE ..... 51 Early Tertiary Amphibians from the Bighorn Basin, Wyoming DAVID W. KRAUSE ........................................................ 69 Early Cenozoic Reptiles and Birds from the Bighorn Basin, Wyoming WILLIAM S. BARTELS ..................................................... 73 Taphonomy and Faunal Sampling in the Early Wasatchian of the Clark's Fork Basin DALE A. WINKLER ........................................................ 81 Genesis of a Fluvial Sheet Sandstone, Willwood Formation, Northwest Wyoming MARY J. KRAUS ........................................................... 87 Magnetic Polarity Stratigraphy and Paleocene-Eocene Biostratigraphy of Polecat Bench, Northwestern Wyoming ROBERT F. BUTLER, EVERETT H. LINDSAY, and PHILIP D. GINGERICH . 95 Faunal Zonation of the Willwood Formation in the Central Bighorn Basin, Wyoming DAVID M. SCHANKLER ................................................... 99 Turtle Stratigraphy of the Willwood Formation, Wyoming: Preliminary Results J. H. HUTCHISON ..........................................................115 Fossil Floras and Plant-bearing Beds of the Central Bighorn Basin SCOTT L. WING ............................................................ 119 The Willwood Formation (Lower Eocene) of the Southern Bighorn Basin, Wyoming, and its Mammalian Fauna THOMAS M. BOWN ........................................................ 127 Preliminary Report on Paleontological Exploration of the Southeastern Absaroka Range, Wyoming JEFFREY G. EATON ....................................................... 139 Remaining Evidence of Oligocene Sedimentary Rocks Previously Present Across the Bighorn Basin, Wyoming MALCOLM C. McKENNA ................................................... 143 PREFACE AND ACKNOWLEDGMENTS This volume was originally conceived as a guidebook for a field conference commemorating 100 years of Cenozoic vertebrate paleontology in the Bighorn Basin. The field conference itself, sponsored jointly by the Society of Vertebrate Paleontology and the Department of Geological Sciences and Museum of Paleontology at the University of Michigan, is scheduled for the first four days of August, 1980. The results of paleontological inquiry, like those of any scientific endeavor, are cumulative. As this book began to take shape, it became increasingly clear that there has been a tremendous increase in our understanding of early Cenozoic paleontology in the Bighorn Basin in the past twenty years, not only in vertebrate paleontology but also in the related fields of paleobotany, fluvial sedimentology, taphonomy, and paleomagnetic stratigraphy. The present volume represents an attempt to bring together summaries of results achieved to date by each of the principal research groups working on early Cenozoic and related problems in the Bighorn Basin. A guidebook format has been retained only in the three appendices giving road logs to be followed during the field conference. I am confident that the papers presented here will have lasting value in promoting an integrated approach to paleontological field work in the Bighorn Basin. The kinds of evolutionary questions that are interesting in paleontology today require a holistic approach integrating faunas, floras, paleoenvironments, and refined geochronology. There is not a single paper on the Paleocene or early Eocene in this volume that does not relate in an important way to all of the other papers. Eaton's brief contribution on middle-late Eocene faunas and McKenna's paper on the Oligocene fit less obviously into the whole, but nonetheless contribute to understanding the later geological and faunal history of the Bighorn Basin. Many people have helped to make this compilation a reality. Our present state of knowledge, as reflected in the following papers, is literally the result of a century of field work in the Bighorn Basin. Each of the contributions presented here builds on what Wortman, Cope, Osborn, Sinclair, Granger, Stein, Simpson, Jepsen, and others learned before us. Our association with the Churchill family of Powell, Wyoming, does not go back quite so far in time, but it is fitting to record here as well the debt of gratitude we owe three generations of Churchills for the many summers of support they have generously given to paleontologists working in the Bighorn Basin. At the University of Michigan, Gerald R. Smith, Evelyn Peer, Donna Robbins, Karna Steelquist, David Krause, William Bartels, Gregg Gunnell, and Amanda Alexander all helped in preparing manuscripts for the printer. Mrs. Peer and Mr. Krause, in particular, did far more than their share of the work to make this volume a reality. Publication has been partially supported by grants from the Scott Turner Fund of the Department of Geological Sciences at the University of Michigan and from the National Science Foundation (DEB 77-13465). PDG THE BIGHORN BASIN-WHY IS IT SO IMPORTANT? Philip D. Gingerich Abstract.- The Bighorn Basin of Wyoming is important paleontologically for two reasons. First, it is the only place in the world where the early Cenozoic diversification of mammalian life is recorded through such a long sequence of continental sediments representing virtually continuous deposition. The major groups of modern mammals appeared during this interval, and the Bighorn Basin is the best place to study how this happened. Secondly, many areas of the Bighorn Basin are richly fossiliferous, making it possible to trace evolutionary changes through time in much more detail than is possible elsewhere. Such studies promise to contribute in an important way to our understanding of the evolutionary process itself, both as it affects individual species and as it influences whole faunas. A better knowledge of species-level evolution contributes in turn to biostratigraphy and the refined biochronology necessary to understand the timing of mountain building and other events in the geological history of western North America. Thus the paleontological resources of the Bighorn Basin are of exceptional scientific interest and importance. INTRODUCTION history of the Bighorn Basin in technical terms. In this introductory chapter I want to outline some of the One hundred years ago, in 1880, the paleontologist background knowledge necessary to put these in a Jacob L. Wortman first crossed the Bridger Moun- broader geological and biological perspective. tains and entered the Bighorn Basin of northwestern Wyoming. He obtained a small collection of fossil mammals, and then returned in 188 1 to make a much EARTH HISTORY larger collection. The extraordinary richness of this Earth history is divided into several major inter- new fossil field has since become world renowned. vals, based in part on the forms of life present in each Indeed it is unique, no other place has been dis- interval. The earth itself is thought to have originated covered anywhere that has such a thick, continuous, about 5 billion years ago, judging from the study of richly fossiliferous sequence of sedimentary rocks radioactive isotopes of chemical elements