780 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



T', Family 7. — The amynodonts, a ven" distinct 

 aquatic branch of the Rhinocerotoidea, are first 

 Icnown at the base of the upper Eocene in America 

 and Europe, their distinctive features being very 

 large upper and lower canine tusks (which are lost 

 in the true rhinoceroses), broad, flat skulls, very promi- 

 nent orbits, and broad, spreading, tetradactyl fore feet. 

 The pi'eservation of these animals in the river-channel 

 •sandstones {Metamynodon zone) of the lower Oligocene, 

 their protruding orbit structure, and their spreading 

 fore feet suggest amphibious habits. They are more 

 .aquatic than any other perissodactyls except possibly 

 Metarhinus of the titanothere family. 



T^, Family S. — The hja-acodonts divide into three 

 subfamilies, all of cursorial and subcursorial habits 

 and of the plains and meadow habitat. The light- 

 limbed Triplopodinae of the American upper Eocene 

 are subcursorial. The Hyracodontinae of the lower 

 and middle Oligocene are the most highly cursorial of 

 all the known rhinoceroses; they are, in fact, small- 

 headed, tridactyl, swift-running rhinoceroses. The 

 Hyrachyinae are more conservative mediportal forms, 

 resembling the tapir in proportions, characteristic of 

 the middle and upper Eocene of America. They 

 stand very close to the origin of the true rhinoceroses, 

 if not directly ancestral to them. Apparently none of 

 the hyracodonts found their way into Europe, though 

 certain fossil remains of middle Europe have been 

 referred to Hyracodon. 



V, Family 9. — The true rhinoceroses suddenly 

 emerge in two subfamilies of the lower Oligocene, 

 namely, the pair-horned diceratheres {Diceratherium) , 

 of the Sannoisian and White River, of Europe and 

 America respectively, and the hornless Aceratheriinae, 

 animals having the same geographic range, which are 

 distinguished by the retarded development of the 

 median horn. These mediportal animals expand in 

 number and variety immediately after the extinction 

 of the titanotheres in lower Oligocene time and be- 

 come the dominant quadrupeds of middle and upper 

 Oligocene time both in Europe and America. The 

 diceratheres became extinct in middle Oligocene time, 

 but the aceratheres developed a middle horn on the 

 top of the skull (Osborn, 1899.166), and it is possible 

 that the AceratJierium incisivum of the lower Pliocene 

 may have given rise to the gigantic single-horned 

 ElasmotJierium of the Pleistocene; but this descent 

 is highly conjectural. In lower Miocene time the 

 short-footed, graviportal Teleoceratinae emerge in 

 Europe and Asia and migrate to America. In body 

 and limb structure they even present analogies to 

 the hippopotami but exhibit no aquatic adaptations 

 in the skull. 



The Dicerorhinae also appear in the lower Miocene 

 of Europe and become the dominant European 

 rhinoceroses in Pliocene and Pleistocene time, a 

 .branch surviving to-day in the existing Sumatran 



rhinoceros, a forest-frequenting, browsing animal, 

 relatively primitive in cranial and dental structure, 

 mediportal in proportions. 



In the lower Pliocene of Eui'ope also appear the 

 Dicerinae, related to the existing African white 

 rhinoceros Ceratotherium simum, which is distinctly a 

 grazing, hypsodont, extremely graviportal type. 



In the meantime the true Rhinocerotinae appear in 

 Asia in the lower Miocene. 



In the Pleistocene of Eurasia occur the steppe-fre- 

 quenting, grazing, hypsodont Elasmotheriinae, gigan- 

 tic animals of graviportal type, which may represent 

 a branch from the original Aceratheriinae stock 

 (Osborn, 1900.192). 



STJEVIVAI AND EXTINCTION OF THE PERISSODACTYLA 



The theoretic causes of the extinction of so many 

 branches of the Perissodactyla in early Oligocene time 

 and the survival to the present time of only five of 

 these branches — the Equinae, Tapiridae, Dicerorhinae 

 (Sumatran), Dicerinae (African), and Rhinocerotinae 

 (Indian) are discussed in Chapter XI, section 2. 



PHYLETIC BRANCHING OF THE TITANOTHERES 



As compared with other perissodactyls the adaptive 

 radiation and phyletic evolution of the titanotheres is 

 seen to be limited by their conservative grinding tooth 

 structure and their closely correlated foot structure. 

 The grinding tooth of the titanotheres is mechanically 

 incapable of transformation into the grazing type; 

 the feet do not evolve in a cursorial direction after 

 the first essaying of cursorial structure in Latnhdo- 

 therium. Consequently the evolution and specializa- 

 tion of the titanotheres took place principally within 

 the browsing habitats of meadows, the borders of for- 

 ests, and the borders of streams and rivers, the last 

 affording amphibious and possibly aquatic habitats. 

 In respect both to hypsodonty of the teeth, which is 

 invariably a grazing adaptation, and to elongation of 

 the feet for the seasonal migrations connected with 

 grazing habits, the titanotheres are greatly inferior in 

 plasticity to the rhinoceroses, which independently de- 

 velop an extreme cursorial type (Hyracodon), an extreme 

 aquatic type {Amynodon), also two extreme grazing 

 types {Elasmotherium and Ceratotherium simum). 



The phyletic branching of the titanotheres is sum- 

 marized in Figure 697. 



SECTION 3. SUMMARY OF THE CRANIAL AND SKEL- 

 ETAL EVOLUTION OF THE TITANOTHERES 



GENERAL CONCLUSIONS REACHED 



Epitome of the evolution. — An epitome of the entire 

 evolution of the skull, teeth, and skeleton of the titano- 

 theres brings out three principal conclusions: 



First, the evolution of the feet "and limbs, as already 

 described in detail, from the mediportal to the gravi- 

 portal condition, closely parallels similar stages of 



