Cfjarles! B. anb iHarp *^aux Siialcott aaegearci) Jfunb 



EARLY CENOZOIC VERTEBRATES IN THE RED 

 CONGLOMERATE AT GUANAJUATO, MEXICO ' 



By carl fries, JR. 

 f/. 6". Geological Survey 



CLAUDE W. HIBBARD 



University of Michigan 



AND 



DAVID H. DUNKLE 

 U. S. National Museum 



INTRODUCTION 



Most of Mexico's mineral deposits are scattered over the high 

 central plateau country that extends from the Sierre Madre Oriental 

 westward to the Sierra Madre Occidental and from the United States 

 border southward to the Sierra Madre del Sur. The principal ore 

 bodies in this region are in either Mesozoic rocks, nonmarine Cenozoic 

 rocks, or both, and are apparently of Cenozoic age. One of the great 

 problems in studying these ore deposits is the lack of any Cenozoic 

 time marker earlier than middle Pliocene for the nonmarine deposits 

 between the United States border and the northern part of South 

 America, with the consequent difficulty in correlating Cenozoic rocks 



1 The authors deeply appreciate the cooperative assistance received during 

 the course of the project here reported from the administrations of the Insti- 

 tute Nacional para la Investigacion de Recursos Minerales and the Institute 

 de Geologia of Mexico. They are grateful also to Drs. Doris M. Cochran and 

 C. Lewis Gazin, U. S. National Museum; Dr. Joseph T. Gregory, Peabody 

 Museum, Yale University ; Dr. George Gaylord Simpson, American Museum 

 of Natural History; and Dr. Edward H. Taylor, University of Kansas, for 

 generous permission to study specimens under their care. Dr. Robert W. Wilson, 

 University of Kansas Museum, has given numerous especially pertinent sug- 

 gestions in the study of the rodent and has kindly read that part of the manu- 

 script pertaining to it. Thanks are also extended to Drs. E. H. Colbert, 

 John A. Dorr, Jr., C. L. Gazin, George Haas, G. E. Lewis, E. H. Taylor, and 

 T. E. White for various helpful criticisms. 



The illustrations of the rodent were made possible by the financial support 

 accorded to Hibbard by the Board of Governors of the Horace H. Rackham 

 School of Graduate Study of the University of Michigan. 



SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 123 NO. 7 



