60 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



BASAL EOCENE TIME IN MONTANA AND NEW MEXICO 



Fori Union formation of Montana. — The typical Fort 

 Union formation of Hay den (Meek and Hayden, 1862.1, 

 p. 433), at the junction of Yellowstone and Missouri 

 Rivers, lies east of the center of an ancient forested 

 swamp in which was laid down the upper part of 

 Hayden's "Great Lignite Group." One of the most 

 interesting results of discoveries made in 1901 (Doug- 

 lass, 1902.1) is revealed in an exposure of the Fort 

 Union in Sweet Grass County, Mont., near the head- 

 waters of the Musselshell, containing a rich fauna of 

 the archaic species of basal Eocene animals, some of 

 which are identical with those found on the head- 

 waters of San Juan River, in northern New Mexico, 

 a thousand miles to the south. Both lie near the 

 one hundred and seventh meridian. The presence in 

 large numbers of animals belonging to similar species 

 shows that uniform climatic and physiographic con- 

 ditions existed in this great mountain-basin region 

 for a very long time, because similar generic fonns of 

 life (Olaenodon, Pantolambda) persist through 3,000 

 feet of Fort Union sediments. The remains of the 

 oldest of these mammals are found immediately above 

 the dinosaur-bearing beds at a level which is here 

 identical with that of the Lance formation; and the 

 present opinion is that sedimentation may have been 

 continuous throughout Upper Cretaceous and basal 

 Eocene time in this region in Montana. 



The mode in which these Fort Union beds were 

 formed has not yet been positively determined, but 

 the masses of fresh-water shells which they contain in 

 certain localities indicate that they were in part laid 

 down in shallow lagoons and swamps, which were in 

 some places crossed by river channels. At some 

 places the beds contain multitudes of leaves, which 

 give us a complete record of the forest life of the time. 

 Vast areas of warm-temperate and more hardy trees 

 were interspersed with areas where swamp vegetation 

 accumulated rapidly enough to form great beds of 

 lignite. Amid the glades of these forests there wan- 

 dered swamp turtles, alligators, and especially the 

 choristoderan reptiles of the characteristic genus 

 Champsosaurus. 



Puerco and Torrejon formations of New Mexico. — 

 A southern center of this archaic mammal life is the 

 type locality of the Puerco formation, on the divide 

 between the Rio Grande and the San Juan, in north- 

 western New Mexico, a formation described by Cope 

 (1875.1) as the "Puerco marls." Cope listed the 

 first mammalian fauna from those beds in 1881, 

 opening a new epoch in mammalian paleontology. In 

 1885 he assigned to the formation a thickness of 850 

 feet and distinguished it from the underlying beds, 

 which he supposed to be Laramie but which have 

 since been divided into the Qjo Alamo sandstone, the 

 Kirtland shale, and the Fruitland formation, all 

 probably of Montana age, older than Laramie. The 



Puerco of Cope appears to be a single formation geo- 

 logically, deposited with apparent conformity between 

 the upper and lower divisions, but it is sharply divided 

 faunistically into two main life zones, a lower, which 

 retains the name Puerco, and an upper, to which the 

 name Torrejon was given by Wortman in 1895 

 (Osborn and Earle, 1895.95, pp. 1-3A). In 1910 

 Gardner (1910.1) applied the name Nacimiento 

 group to both divisions. In 1897 Matthew (1897.2) 

 separated the mammal fauna of the two levels, and 

 in 1912 and 1913 Sinclair and Granger (1914.1) estab- 

 lished in this group no less than four faunistic levels, 

 which are shown in the accompanying section (fig. 

 43). Two faunistic levels were observed by Wortman 

 in the Puerco, and two distinct faunistic levels are dis- 

 tinguished by Granger, Sinclair, and Matthew in the 

 Torrejon. 



These four successive changes in the archaic fauna 

 occurred during a period of continuous sedimentation, 

 for no unconformity has been observed between the 

 Puerco and Torrejon. The rate of deposition of the 

 800 feet of Puerco and Torrejon sediments was rela- 

 tively slow as compared with that of the deposition 

 of the 6,000 feet of the corresponding Fort Union sedi- 

 ments to the north. As the mammals distributed 

 through 4,000 feet of the northern part of the Fort 

 Union deposits correspond chiefly with those of the 

 Torrejon, it appears possible that the underlying 

 Puerco fauna may belong in part in upper Lance time. 

 We observe that the Fort Union was deposited upon 

 the Lance continuously, without recognized notable 

 unconformity, whereas the Puerco lies upon the eroded 

 surface of the Ojo Alamo, which, because of its 

 dinosaur fauna, is considered of probable Judith River 

 and Belly River age. 



The close resemblance of the crestless trachodont 

 dinosaur, Kritosaurus navajovius, from the Ojo Alamo, 

 to a corresponding form from the Belly River forma- 

 tion of Alberta also suggests a close correlation in 

 time.* 



In 1912 and 1913 Sinclair and Granger thoroughly 

 explored the basal Eocene deposits of the San Juan 

 Basin, with the results enumerated above. 



SUMMARY OF FAUNAE EVENTS OF BASAL EOCENE TIME 



In addition to the four fossiliferous zones observed 

 tn the Puerco and Torrejon formations, all distinc- 

 tively basal Eocene, there is an overlying zone in the 

 "Tiffany beds," beyond the border of Colorado, deter- 

 mined by Gidley (1909) and Granger (1916). These 

 beds contain a fifth fauna, which is strictly interme- 

 diate between basal Eocene and lower Eocene. This 

 transitional basal-lower Eocene zone is described on 

 pages 64-65. The basal Eocene mammalian life 



> See Parks, W. A., The osteology of the trachodont dinosaur Kritosaurus incur- 

 limanus: Univ. Toronto Studies, Qeol. series, 1920. 



