INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



35 



similar ancestral forms is shown in Figure 29. The 

 earliest members of every family had low-crowned 

 (brachyodont) molar teeth, of relatively simple 

 pattern, composed of six principal cusps ranged in 

 three pairs — an external pair, the paracone and meta- 

 cone; an intermediate pair, the protoconule and 

 metaconule; and an internal pair, the protocone 

 and hypocone. 



In the titanotheres, chalicotheres, paleotheres, 

 and horses the internal pair of cusps assume the 

 conical, rounded shape (bunoid), whereas the 

 two external cusps assume the double crescentic 

 shape (selenoid), together forming a W, hence 

 this type of tooth is termed bunoselenodont. 

 These bunoselenodonts apparently formed origi- 

 nally a natural group from which the horses 

 (Eohippus), the titanotheres (Eotitanops), and 

 the chalicotheres (Eomoropus) gradually diverged 

 very early in Eocene time. This is shown in 

 Figure 30. 



Another group of perissodactyls is the bunolo- 

 phodonts, which includes the tapirs and lophio- 

 donts, in which the internal and external pairs 

 of cusps alike assume an elongate, crested, or 

 lophoid pattern. This group has two main 

 branches, the tapirs and the lophiodonts. The 

 tapirs as forest-seeking animals escaped fossiliza- 

 tion and are rarely found; only isolated remains 

 of them have been found in Europe and America; 

 yet they constituted one of the most persistent 

 of all the perissodactyl phyla. The lophiodonts 

 were tapir-like animals, in which the posterior 

 outer molar cusps were flattened and thus are 

 intermediate in shape between the tapir tooth 

 and the rhinoceros tooth. These animals doubt- 

 less had a wide expansion in the luxuriant 

 forests of Eocene France, and they attained 

 very great size just before their extinction, 

 which occurred contemporaneously with the 

 extinction of the titanotheres in America — that 

 is, in lower Oligocene time. Only one branch of 

 the lophiodonts, the swift-footed Helaletinae, 

 reached North America in lower Eocene time, 

 soon after the arrival of the tapirs (Systemodon) 

 and the horses (EoTiippus). 



The grinding tooth of the rhinoceroses is lopho- 

 dont — that is, all the cusps are turned into elon- 

 gate crests, of lophoid type, and the posterior 

 outer cusps of the upper grinding teeth are 

 elongated as well as flattened, producing an asym- 

 metry of the cusps of the outer wall (ectoloph) of the 

 crown. A grinding tooth of this kind is far more 

 effective than that of the bunoselenodont titano- 

 theres or of the bunolophodont tapirs. Such a tooth 

 is a very efficient cutting instrument for an animal 

 of either the browsing or the grazing habit. It is 

 also capable of elongation (hypsodonty), and in 



two subfamilies of the rhinoceroses, the white rhinoc- 

 eroses and the elasmotheres, the grinding teeth 

 become hypsodont, greatly increasing the longevity 

 and consequent reproductive power of each indi- 

 vidual. 



Figure 27. — Restorations of nine species of titanotheres from the 



lower, middle, and upper Eocene and the lower Oligocene 



Drawn by Mrs. E. M. Fulda. About one-fiftietb natural size. 



The rhinoceroses gave off at least twelve distinct 

 branches (phyla) and were thus more plastic in adapta- 

 tion than the titanotheres. These branches became 

 adapted to every habitat, aquatic as well as terrestrial, 

 to every mode of locomotion — cursorial, mediportal, 

 and graviportal — and to every kind of feeding — brows- 

 ing and grazing. Like the titanotheres some of the 

 rhinoceroses passed from the mediportal to the gravi- 



