768 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The constricted antorbital region of tlie face and 

 terminal anterior nares is also distinctive, being ob- 

 served in Plienacodus, Systemodon, LamMotlierium, 

 Eohippus, LopModon, Isectolophus. This is a primitive 

 character. The proboscis-bearing, receding nares of the 

 specialized lophiodonts, Heptodon and Helaletes, are 

 secondary analogies with the tapirs, closely similar to 

 the condition observed in Profapirus of the lower 

 Oligocene. 



This primitive perissodactyl skull is closely analo- 

 gous or is actually related to the skull of the 

 Condylarthra. In his detailed enumeration of the 

 cranial characters of the primitive Perissodactyla 

 Gregory points out (1910.1, p. 390) that this skull 

 is of the insectivore-creodont-condylarth plan. 



FiGUHE 696. — Skulls of Lophiodon leptorhynchus 

 A, Side view; 23, top view (of a different individujil). After Deperet. One-fomth natural s 



SECTION 2. ORIGIN AND PHYLETIC RADIATIONS 

 OF THE TITANOTHERES AND OTHER PERISSO- 

 DACTYLA 



The comparison of perissodactyls here made con- 

 tinues that begun in Chapter I with a preliminary 

 description of the relations of the titanotheres to the 

 chalicotheres, paleotheres, and horses in the bunosele- 

 nodont structure of their grinding teeth, as composing 

 a great bunoselenodont branch in contrast to the great 

 lophodont branch, which includes the tapirs, lophio- 

 donts, and rhinoceroses, as presented in the accom- 

 panying diagrams (figs. 697, 698), showing the general 

 and detailed phylogeny of the Perissodactyla. 



SKULL OF THE PEIMITIVE TITANOTHERE 



Comparison of the skull of the primitive titano- 

 theres {Larnbdotherium, Eotitanops) with that of the 

 Condylarthra {Phenacodus) and, on the other hand, 

 with that of all the primitive Eocene perissodactyls 



in which the skull is known, as shown above, reveals 

 a striking general resemblance to the skulls of Eocene 

 horses, paleotheres, tapirs, lophiodonts, the primitive 

 rhinoceroses, and the chaHcotheres, which is attribut- 

 able to two causes, namely, similar ancestry and 

 similar habits. 



First, none of these skulls has diverged very far 

 from the common ancestral forms; the orbit is 

 situated midway of the head, and the face and the 

 cranium are equal in length; the grinding teeth are 

 short-crowned, brachyodont, and no special provision 

 of the face is necessary for the accommodation of 

 elongated teeth; the incisors and canines are relatively 

 uniform in size, presenting no very striking enlarge- 

 ment or reduction. Second, the similarity of the skulls 

 is due to similarity of function and adaptation. 

 These skulls are all adapted to the browsing 

 habit; they are the type belonging to animals 

 of cursorial, subcursorial, and mediportal gait, 

 the muscles of mastication occupying similar 

 areas on the zygomatic arches and on either side 

 of the brain case, as exhibited both in the lateral 

 and superior views of the skulls. 



In brief, the similarity in the skulls points 

 both to similar ancestry and to analogy in 

 habit. Nevertheless, we detect in the lower 

 Eocene titanotheres evidences of incipient diver- 

 gence, especially in the adaptation through 

 greater or less I'ecession of the nasal bones for 

 the prehensile function of the upper lip. While 

 the nasal bones are full and elongate in the 

 titanotheres (Eotitanops) and in the larger 

 of the lophiodonts {Lophiodon) they recede both 

 in the smaller cursorial lophiodonts (Helaletes) 

 and in the cursorial paleotheres (Anoploihe- 

 rium). The purpose of these adaptations for 

 the prehensile function of the upper lip, so 

 characteristic of the modern tapirs, is to subserve 

 the browsing function, as described in the com- 

 parison of the skull and mouth parts of the rhinoc- 

 eroses in Chapter I (p. 32). This is the open-nostril 

 type. The closed-nostril type is well illustrated in the 

 skull of LopModon leptorhynchus, figured above, an 

 appropriately named species, in which the nostrils 

 are terminal and there is no room for the retractor 

 muscles of the prehensile upper lip. In general the 

 titanotheres conform to the latter type, with terminal 

 nares and long nasals, but certain upper Eocene 

 titanotheres (RhadinorJiinus) evolve the open nasal 

 structure. In the two lower Eocene forms known, the 

 cursorial Larnbdotherium and subcursorial Eotitanops, 

 the skull is of the prevailing primitive perissodactyl 

 type, similar in proportions to that of the horse 

 Eohippus, yet it forms a starting point of evolution 

 which finally results in a type fundamentally different 

 from that of the horse in all its proportions. 



