778 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



should probably be added the Chalicotheroidea as an 

 aberrant supei'faniily with molar teeth related to the 

 titanothere pattern and perissodactyl feet provided 

 secondarilj' with claws. This arrangement is sub- 

 stantially that which has been adopted in this mono- 

 graph — that is, they have been divided into 5 super- 

 families (-oidea), 9 families (-idae), 35 subfamilies 

 (-inae). A subfamily may embrace one or more gen- 

 era; it pursues an independent line of development 

 and either terminates in an extreme or gives off a gen- 

 eralized branch from which another subfamily may 

 arise. 



Subsequent discovery (Osborn, 1913.398) has 

 brought the chalicotheres more intimately within the 

 order Perissodactyla, so that they are with some 

 certainty now placed as a branch mtermediate between 

 the Titanotheroidea and the Hippoidea. Thus accord- 

 ing to our present knowledge (1918) the tabular 

 classification of the Perissodactyla is as follows: 



Classification of the order Perissodactyla 



A. Bunoselenodont suborder or branch: 



I. Superfamily Titanotheroidea (titanotheres) : 



1. Family Brontotheriidae : 



Subfamily (phylum) : 



1. Lambdotheriinae. 



2. Eotitanopinae. 



3. Palaeosj'opinae. 



4. Telmatheriinae. 



5. Manteoceratinae. 



6. Dolichorhininae. 



7. Rhadinorhininae. 



8. Diplaoodontinae. 



9. Brontopinae. 



10. Menodontinae. 



11. Megaceropinae. 



12. Brontotheriinae. 



II. Superfamily Chalicotheroidea (chalicotheres) : 



2. Family ChaUcotheriidae : 



Subfamily (phylum) : 



13. Moropinae (America). 



14. Chalicotheriinae (Europe). 

 III. Superfamil}^ Hippoidea (paleotheres, horses) : 



3. Family Palaeotheriidae : 



Subfamily (phylum) : 



15. Paloplotheriinae. 



16. Palaeotheriinae. 



4. Family Equidae: 



Subfamily (phylum) : 



17. Hyracotheriinae. 



18. Anchitheriinae. 



19. Protohippinae. 



20. Equinae. 



B. Lophodont suborder or branch: 



IV. Superfamily Tapiroidea (tapirs, helaletids, lophio- 

 donts) : 



5. Family Tapiridae: 



Subfamily (phylum) : 



21. Systemodontinae. 



22. Tapirinae. 



6. Family Lophiodontidae: 



Subfamily (phylum) : 



23. Helaletinae. 



24. Lophiodontinae. 



B. Lophodont suborder or branch — Continued. 



V. Superfamily Rhinocerotoidea (amynodonts, hyraco- 

 donts, rhinoceroses) : 



7. Familj' Amynodontidae: 



Subfamily (phylum) : 



25. Amynodontinae. 



8. Famiily Hyracodontidae: 



Subfamily (phylum) : 



26. Hyrachyinae. 



27. Triplopodinae. 



28. Hyracodontinae. 



9. Family Rhinocerotidae: 



Subfamily (phjdum) : 



29. Aceratheriinae. 



30. Diceratheriinae. 



31. Teleoceratinae. 



32. Dicerorhinae. 



33. Rhinooerotinae. 



34. Dicerinae. 



35. Elasmotheriinae. 



CONVERGENCE IN HABITAT AND HABIT 



It is vezy important to observe that although the 

 nine typical perissodactyl families as a whole diverge 

 and radiate from one another, the branches into which 

 some of them subdivide under the law of adaptive 

 radiation converge, because these seek similar habitats 

 and assume similar habits. 



Cursorial habit in the plains habitat. — Cursorial 

 habits were independently assumed in one branch of 

 the titanotheres, in all branches of the horses except 

 one subfamily, in two branches of the lophiodonts, 

 in three subfamily branches of the rhinoceroses. 



Forest-living habit. — The very reverse of the cur- 

 sorial plains habit was independently assumed by 

 the chalicotheres, by the tapirs, possibly by certain 

 branches of the titanotheres (Dolichorhininae), also 

 by certain branches of the rhinoceroses. 



Aquatic habit and river habitat. — The swamp-living 

 and aquatic habit appears to have been assumed 

 independently by one branch of the titanotheres 

 (Metarhinus) , by the amphibious rhinoceroses (Amy- 

 nodontidae), and possibly by certam of the true short- 

 footed rhinoceroses (Teleoceratinae). 



Mediportal habit, browsing type. — Mediportal brows- 

 ing forms are characteristic of all the middle stages of 

 titanothere evolution, of the early stages of the 

 lophiodonts and hyracodonts, and of all the early 

 stages of the rhinoceroses. 



Graviportal habit, browsing and grazing types. — The 

 graviportal habit is illustrated in certain stages of 

 the evolution of the titanotheres, in the true lophio- 

 donts, in most of the upper Miocene, Pliocene, and 

 Recent subfamilies of the rhinoceroses. 



Secondary and independent adoption of similar 

 habitats and habits in the different families of peris- 

 sodactyls has given rise to the analogous, parallel, 

 convergent, and homoplastic adaptations in skull, 

 limb, and foot structure described in this chapter and 

 in Chapter IX and has produced resemblances so close 



