10 



TITANOTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



which have a significant bearing on the kinds of animals 

 preserved. For example, violent fluviatile action 

 may preserve for us chiefly the river-border and aquatic 

 fauna; but remains of the animals of the surrounding 

 plains and of the distant forests may not have entered 

 the river-channel sandstones. Forest-living animals, 

 like the chalicotheres [Moroyus], are relatively rare; 

 and arboreal animals, like the lemurs (NotJiardus) , are 

 seldom preserved in channel sandstones. Certain 

 mammals apparently arriving as new immigrants, like 

 the giant uintatheres, which suddenly appear in 

 Bridger C, doubtless came from the surrounding plains 

 or mountain regions, where the conditions were un- 

 favorable for their entombment and fossilization. 



The threefold division of the Wasatch and Bridger 

 mammals by Matthew (1909.1) and Loomis (1907.1) 

 according to their habitats, into meadow, forest, and 



Figure 10.- 



- Restorations of Eotilanops borealis (A) and Brontotherium plaly- 

 ceras (B) , drawn to the same scale 



titanotheres has extended we invariably find more than 

 one of the branches of the titanotheres, as in Wind 

 River and early Bridger time, and in some areas as many 

 as five or six contemporaneous branches. Altogether 

 twenty branches of the great titanothere family tree 

 have thus far been discovered in Eocene and lower Oligo- 

 cene strata. This multiple branching, known as poly- 

 phyletic evolution, has made the study of the titano- 

 theres more difficult and at the same time more 

 fascinating than if these mammals presented only 

 a single line of descent, as in monophyletic evolution. 

 Some of the phyla of the titanotheres can be traced 

 through a long series of successive evolutionary stages, 

 such as Palaeosyops, Manteoceras, and Dolichorhinus 

 in the Eocene, Brontops, Menodus, and Brontotherium 

 in the Oligocene. Other phyla, such as the supposed 

 river-dwelling Eometarhinus and Metarhinus, appear 

 in two life zones only, in the middle Eocene, 

 Huerfano B, and the upper Eocene, Uinta 

 B 1, under fluviatile conditions of sedi- 

 mentation favorable to fossilization. 



Extremes of evolution. — Members of these 

 twenty branches wandered in and out of 

 the regions favorable to fossilization, and 

 consequently no single branch (phylum) can 

 be traced over the whole period of time. 

 Even if this period covered 600,000 years 

 (minimum estimate), or 11,000,000 years 

 (maxunum estimate), the descent of a gi- 

 gantic horned quadruped, such as Bronto- 

 therium platyceras, from a small and de- 

 fenseless animal akin to Eotitanops borealis 

 would appear almost incredible were it not 

 that unremitting exploration during the last 

 half century has unearthed many phyla of 



One of the earliest members of the titanothere family (£. borealis of the Wind River formation, lower 

 Eocene) and one of the latest and most formidable ( B. platyceras of the White River group, lower 

 Oligocene). Frommodelsinthe American Museum of Natural History made by Erwin S. Christman spCcicS that are mOre Or IcSS intermediate 

 imder the direction of the author and of William K. Gregory. 



river living groups, not only has important bearing on 

 the gaps in the fossil record and on the interpretation 

 of the evidence relating to immigration and emigration 

 but is in accord with the principle of local adaptive 

 radiation developed by Osborn, as fully set forth at the 

 end of Chapter II. 



PRINCIPIE OF LOCAL AND CONTINENTAL ADAPTIVE 

 RADIATION 



The changes in the climatic and physiographic 

 conditions during the Eocene epoch, which favored not 

 only the evolution but the fossilization of this or that 

 type of animal, supply the key to the divergence in 

 anatomical structure and to the presence in the 

 diversified Eocky Mountain region and adjacent 

 plains of a great variety of titanotheres, in a measure 

 comparable to the great variety of ruminants found 

 to-day in the plain and plateau regions of the continent 

 of Africa. 



Twenty branches of titanotheres. — In the eight life 

 zones through which the observed evolution of the 



between these two extremes. Although the 

 whole period of life of the titanotheres was relatively brief 

 as compared with that of the surviving horses, tapirs, 

 and rhinoceroses, yet within this period the titano- 

 theres became much more specialized than the modern 

 tapirs; in fact, although in lower Eocene time they 

 resembled superficially the existing tapir {Tapirus 

 terrestris), by middle Eocene time they had reached and 

 passed the tapir-like stage of evolution. As compared 

 also with the contemporary horses they were more 

 rapidly progressive; the difference between the lower 

 Oligocene Brontotherium and the lower Eocene Eoti- 

 tanops is vastly greater than that between the lower 

 Oligocene horse Mesohippus and the lower Eocene 

 Eohippus. The titanotheres evolved rapidly, partly 

 because the environment was peculiarly favorable 

 to their rapid evolution; partly because their internal 

 germinal hereditary conditions favored their rapid 

 evolution and differentiation. 



Competition oj the titanotheres with other ungulates. — 

 In the course of their evolution the titanotheres came 

 into competition as herbivorous quadrupeds with 

 members of four orders of hoofed mammals. They 



