268 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



5. The secondary accessory folds, known as "styles," 

 or pillars, and "lophs," or crests, as well as the 

 "fossettes," or pits, may also be homologized by 

 comparing the superior and inferior molars of the 



ECTOLOPH 

 MESOSTYLE'' 



PARASTYLE 



PROTOCONULE- 



-METACONULE 



-HYPOCONE 



PROTOCONW 



PROTOLOPHID 



PARACONID- 



-HYPOCONID 



METALOPHW 



ENTOCONID 



METACONID \ METACRJSTID 

 METASTYUD 



FiGUEE 221. — Upper (A) and lower (B) molars of bunosele- 



nodont pattern 

 Molars of Lambdotherium, a lower Eocene titanothere. Enlarged. The worn 

 enamel surface is cross hatched; the exposed dentine is shown in dense black. 



titanothere (TelmatJierium) and of equine forms 

 {AncMtherium) with those of the rhinoceros. 



6. Arrested or retro- 

 gressive development is 

 the chief characteristic 

 of the titanothere molar 

 evolution- — that is, 

 parts are arrested or 

 vestigial in titanotheres 

 that evolve rapidly and 

 strongly in paleotheres, 

 chalicotheres, and 

 equine s. Thus the 

 titanothere molar be- 

 gins its evolution in the 

 form of the Lamhdothe- 

 rium molar (figs. 221, 

 235) or of the Eofitanops 

 molar (figs. 229, 249), in which 32 primary and 

 secondary elements may be more or less clearly dis- 

 tinguished in the typical upper and lower grinders. 



In the course of evolution these grinders, through 

 arrested development, lose six or more of these ele- 

 ments. Thus the grinding teeth are impoverished as 

 compared with those of the other bimoselenodonts 

 (fig. 223). 



The parts that gradually become vestigial or dis- 

 appear in titanotheres are the following: 



Protoconule, anterior intermediate cusp of superior molars; 

 degenerates. 



Metaconule, posterior intermediate cusp of superior molars; 

 degenerates. 



Protoloph, anterior transverse crest of superior molars, formed 

 of protocone, protoconule, paracone (inner base) ; disappears. 



Metaloph, posterior transverse crest of superior molars, 

 formed of hypocone, metaconule, metaoone (inner base) ; dis- 

 appears. 



Paraconid, antero-internal cusp of infe- 

 rior molars, reduced or vestigial in all Peris- 

 sodactyla; disappears. 



Hypoconulid, posteromedian cusp of in- 

 ferior molars; abortive except in third 

 inferior molar. 



7. All stages in the reduction and 

 disappearance of these six or more 

 elements in the upper and lower 

 grinding teeth are observed among the 

 Eocene titanotheres (Pis. LIV-LXV), 

 whereas the lower Oligocene titano- 

 theres exhibit grinding teeth (fig. 381) 

 in which all these parts have totally 

 disappeared and certain new secondary 

 rectigradations, such as the "fossette" 

 and crochet, have appeared. 



8. The appearance or disappearance 

 of these single elements is generally 

 gradual or continuous; yet it is much 

 more rapid in certain phyla than in others. The 

 variation in the rate of degeneration distinguishes 

 the phyla from one another and thus becomes a char- 



B 



Figure 2 2 2. — 

 Upper (A) and 

 lower (B) molar 

 patterns of 

 Hyracotherium, 

 a primitive 

 Eocene equine 

 perissodactyl 

 (ancestor o f 

 the horse) 

 Enlarged. 



a 



Figure 223. — Bunoselenodont patterns of upper and lower molars in Tertiary perissodactyls 

 Telmatherium vltimum, an upper Eocene titanothere, upper molar; a, Telmatherium cuUridens, a middle Eocene titanothere, lower 



molar; B, b, Moropus sp. 

 Miocene hippoid. 



a Miocene chalicothere; C, c, Palaeotheriuvi sp., an Eocene paleothere; D, d, Atichitherium sp., a 



acter of generic value. The numerical gain or loss of 

 one of these elements is of specific value and marks off 

 the subspecific stages or mutations. 



