NO. 7 EARLY CENOZOIC VERTEBRATES — FRIES ET AL. 7 



a nearby small fault. Edwards and Ortiz pointed out the locality to 

 Fries late that same month, and by several hours of further examina- 

 tion Fries found some rostral parts of a tiny rodent and a fragment 

 of jaw bone with two minute cheek teeth. In transporting the ma- 

 terials and cleaning them for transmittal for study, the fragment 

 with the cheek teeth was unfortunately destroyed. The rest of the 

 material was examined in the winter of 1950-51 but was not found 

 to be diagnostic enough for an age assignment. 



The urgent need for assigning an age to the conglomerate stimu- 

 lated a continued search of the beds for additional fossil materials. 

 Fries revisited Guanajuato in the spring of 1951 without success. Ed- 

 wards returned to the locality in October 1951 to finish up some 

 mapping details and to look for more fossils but found nothing of 

 diagnostic value. Another trip to the area was made by Fries in 

 March 1952. Accompanied by Kenneth Segerstrom, of the Geological 

 Survey, he was this time fortunate in finding other parts of skulls 

 and jaws of a rodent with cheek teeth. This new material was picked 

 up at a point some 15 meters from the first find in beds perhaps 3 

 meters higher in the section. Finally, Mr. Segerstrom and several 

 associates guided the junior author Dunkle to the locality in October 

 1952, on which occasion the lizard specimens herein described and 

 some few additional rodent bones were obtained at widely scattered 

 and undetermined levels in the exposures. 



The rodent material recovered by Fries in 1952 was submitted to 

 Hibbard for study, and at about the same time the Instituto de 

 Geologia sent the original Edwards specimen to R. A. Stirton, of 

 the University of California, for an independent opinion as to its 

 identity. Stirton indicated that the remains probably belonged to an 

 artiodactyl of the family Merycoidodontidae, suggesting an early 

 Cenozoic age. The establishment of the early Cenozoic age of the 

 beds, within relatively narrow limits, has been based primarily on 

 Hibbard's study of the rodent remains. The lizard specimens have 

 been studied by Dunkle but, because of the meager fossil record of 

 the Lacertilia, have proved to be of slight use as time indicators. A 

 brief note on the discovery of the fossils and their significances was 

 pubHshed by Arellano (1952) in Mexico. 



DESCRIPTION OF THE FOSSIL-BEARING BEDS 



The beds in which the fossil remains have been found are shown 

 in the photograph in plate i. They are composed of reddish-brown 

 mudstone consisting mainly of silt with some sand ; they are generally 



