THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



779 



that it may be difficult to determine the family from 

 a single bone, and it therefore becomes necessary to 

 determine family affinity from the teeth. 



DESCENT OF THE BUNOSEIENODONT FAMILIES 



/, Family 1 (see table on p. 778). — When the titano- 

 theres emerge in North America in Wind River time 

 (lower Ypresian) they appear to have already separated 

 into two divisions, namely, the cursorial Lambdotherii- 

 nae and the subcursorial Eotitanopinae. The subfam- 

 ily Eotitanopinae is believed to have been the source of 

 the mediportal Limnohyops and the graviportal Palaeo- 

 syops of the middle Eocene. From an unknown direct 

 source arise the short-horned titanotheres (Man- 

 feocems), which are related to the great graviportal 

 short-horned titanotheres of the lower Oligocene 

 (Brontops). Related to this stem are the forest-, 

 swamp-, and river-dwelling Dolichorhininae. Not far 

 from this source are the ancestors of the long-horned 

 titanotheres {Brontotherium., Megacerops), which ter- 

 minate in the graviportal stage and become extinct 

 at the end of lower Oligocene time. 



//, Family 2. — The chalicotheres (Ancylopoda of 

 Cope) in skull and tooth structure appear to occupy 

 a position intermediate between the Titanotheroidea 

 and the Hippoidea (paleotheres and horses). At the 

 end of middle Eocene time (upper Bridger, Bartonian) 

 there occur the Pernatlierium (of France) and the 

 Eomoropus (of Wyoming), sharply distinguished in 

 foot structure by the possession of powerful claws, 

 which were probably used as weapons of defense and 

 for grasping tree branches, since the fore limbs are 

 not of the fossorial type. The skull and body are 

 proportioned like the forest-living okapi, and the great 

 rarity of remains of these animals in the plains and 

 fluviatile deposits between middle Eocene and lower 

 Pliocene time, comparable to the rarity of remains of 

 forest-dwelling tapirs, points to their adoption of the 

 forest habitat and forest-dwelling habit. 



///, Family 3. — The paleotheres were probably 

 closely related in very remote times to the horses, 

 although they first emerge geologically in Europe in 

 Lutetian (middle Eocene) time, their earlier history 

 being unknown. They divide into the browsing medi- 

 portal Palaeotheriinae and the slenderly built Paloplo- 

 theriinae, which imitate the true horses. 



Ill, Family 4- — The horses emerge as small, progres- 

 sive cursorial types in Europe and America in Sparna- 

 cian (middle Wasatch) time in the genera Hyraco- 

 therium and Eohippus. There is some evidence that 

 they subdivide in early Oligocene time into (a) a 

 purely cursorial grazing type, (b) subcursorial brows- 

 ing types, and (c) subcursorial forest-living types. 

 In the types last named tridactylism is persistent in 

 a phylum that leads into the forest-frequenting 

 Hypohippus, which has crested, short-crowned teeth, 

 functionally resembling those of the forest-living 



tapirs, and spreading, three-toed feet. The main line 

 of horse evolution was into the extreme plains-living, 

 grazing, cursorial types of hipparions, primitive and 

 progressive horses, zebras, and asses, the latter retain- 

 iug much of the browsing habit. 



DESCENT OF THE lOPHODONT FAMIIIES 



The lophodont families spring from two great 

 branches, the Tapiroidea (IV) and the Rhinocerotoidea 

 (V). The families of the Tapiroidea are less uniform 

 in the structure of their grinding teeth (Osborn, 

 1895.105, p. 359, fig. 6), but there is considerable 

 evidence from the transitions in the skull, tooth, 

 and foot structure that the Tapiridae, Helaletidae, 

 and Lophiodontidae sprang from a common stem, 

 the lophiodonts being closest in their grinding-tooth 

 structure to the rhinoceroses. 



IV, Family 5. — The tapirs emerge in the subcur- 

 sorial Systemodon of the lower Eocene of America. 

 Systemodon is very abundant only in the Systemodon 

 zone, for throughout their entire geologic history 

 tapiroid remains are most rare, and we assume that 

 these animals early adopted the forest-dwelling habit 

 and hence escaped fossilization. The American line 

 leads through Isectolophus of the middle and upper 

 Eocene, while the Protapirus of the lower Oligocene 

 also represents a side branch of the main family, not 

 typically tapiroid in its molar-tooth structure and 

 therefore classed by Peterson with the Pseudo- 

 tapirinae. The tapirs throughout their history have 

 preserved the mediportal size and the forest, swamp, 

 and river frequenting habitat, although high moun- 

 tain dwelling forms evolve in the Andes. 



IV, Family 6. — A true lophodont branch in its 

 molar-tooth structure, known as the Helaletinae, 

 emerges in the extremely cursorial Heptodon of the 

 Heptodon zone of the Wasatch, as a contemporary of 

 Systemodon and Eohippus. The skulls of Heptodon 

 and Helaletes, we have seen, closely parallel those of 

 the tapir in the recession of the nasal bones in adapta- 

 tion to the development of a long prehensile upper lip ; 

 but the grinding teeth are exactly like those of the 

 typical lophiodonts of the upper Eocene of Europe. 

 The American stages are Helaletes of Bridger B and 

 DesmatotTierium of Washakie B. Unlike that of the 

 tapirs the manus in the Oligocene stage (Colodon) be- 

 comes excessively elongate, slender, and highly curso- 

 rial, although persistently tridactyl. These cursorial 

 lophiodonts were probably represented also in Europe. 



The true lophiodonts (Lophiodontidae) are medi- 

 portal quadrupeds, emerging in the Sparnacian of 

 Europe and breaking up into four independent 

 phyletic lines (Deperet). These include the cursorial 

 slender-limbed chasmotherines and the heavy-limbed 

 true lophiodonts. The latter became extremely gravi- 

 portal, suddenly terminating in heavy-bodied, pon- 

 derous forms in the lower Oligocene of Europe. 



