730 PERISSODACTYLS xxx. 5- 



small and with a complete tooth row, in which the molars already 

 show an ectoloph and the parallel transverse lophs characteristic of the 

 group. The *Hyracodonts were an Eocene and Oligocene line speci- 

 alized for swifter movement ('running rhinoceroses') with long legs 

 and three toes in each foot, much as in the earlier horses. The members 

 of another line, the *Amynodonts, were larger, probably semi-aquatic 

 forms. The true rhinoceroses appear in the Oligocene, already as 

 large creatures, fully terrestrial and hence with stout limbs and a good 

 grinding battery, with molarized premolars. *Baluchitherium became 

 of enormous size, as much as 18 ft high. Rhinoceroses became numer- 

 ous in the Miocene and Pliocene (*Aceratherium). Various types per- 

 sisted through the Pliocene and Pleistocene, and the modern single- 

 horned Rhinoceros of Asia and two-horned Dicerorhinns of the East 

 Indies and Diceros of Africa are derived from some of these. Extinct 

 types such as the woolly rhinoceros are known from Palaeolithic draw- 

 ings and from partly embalmed specimens. Modern rhinoceroses all 

 have a very thick, almost hairless skin, with characteristic folds. The 

 tendency to active keratin development also produces the horns, 

 either one, two, or occasionally three median outgrowths on the head, 

 often compared to clumped masses of hairs but essentially similar to 

 the horns of ruminants but without a bony core. 



6. *Brontotheres (*Titanotheres) 



These were early, heavily built ungulates, reaching large size in the 

 later Eocene and Oligocene, in fact preceding the rhinoceroses as 

 large herbivores. The fully developed forms, such as *Brontops (Fig. 

 479) of the Lower Oligocene, were of typical graviportal type, up to 

 8 ft high, with high thoracic spines, numerous ribs, vertical and 

 laterally expanded ilia, and rather short legs, with four digits in front 

 and three behind. The tooth row was complete and the molars large 

 but low-crowned, with a ridge along the outer side, but isolated cusps 

 on the inner (hence 'bunolophodont'). A single pair of large horns 

 was carried on the front of the skull. The brain was even smaller than 

 that of rhinoceroses. 



The earliest fossils that can be referred to this type, * Lambdotherium 

 of the Lower Eocene, were much smaller, and without horns; they 

 could well have been derived from *Hyracotherium. *Eotitanops from 

 the Middle Eocene was larger, but still hornless. From some such 

 stage numerous lines probably diverged, each becoming larger and 

 independently acquiring horns. For obvious reasons it is difficult to 

 obtain a proper idea of the evolution of such giants, as Simpson points 



