EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



313 



was by far the most abundant of the Bridger Basin 

 quadrupeds. The identified materials in the American 

 Museum from the Bridger Basin embrace parts of more 

 than 60 individuals, including 12 more or less com- 

 plete skulls. 



Materials. — Altogether, more than 70 specimens 

 have been examined for this monograph, including 

 those in the collections in the American, Yale, Na- 

 tional, and Philadelphia Museums. Most parts of the 



skeletons are dissociated from skulls; even the jaws 

 are rarely found with skulls; in only three of the speci- 

 mens of Palaeosyops studied are the jaws associated 

 with the skull. 



The following table shows the length of the lower 

 grinding series, the length of the upper grinding 

 series, the length along the bottom line of the skull, 

 and the breadth across the zygomatic arches : 



Comparative measurements oj species of Palaeosyops, in millimeters 



Bridger D 3 P. copei Osborn 



Bridger D ' P. robustus (Marsh) . 



Bridger C 2-4 _ 

 Bridger C 1 _ _ . 

 Bridger B 2-4. 

 Bridger B l-2_ 



Bridger B 



Bridger A 



Huerfano B 



P. leidyi Osborn 



P. grangeri Osborn.. 



P. major Leidy 



P. paludosus Leidy _. 

 P. longirostris Earle. 



P. fontinalis Cope 



P. fontinalis Cope__. 



(?) 



164 

 151 

 143 



170 

 180 

 174 

 180 

 155 

 137? 



153 

 170 

 158 

 165 

 147 

 124? 



(?) 



(?) 

 (?) 



(?) 

 (?) 



(?) 



'•440 

 415 

 (?) 

 389-435 

 (?) 

 (?) 

 (?) 



146 



(?) 

 '•340 

 310 

 (?) 

 ' 290-335 

 (?) 

 (?) 

 (?) 

 (?) 



There are steady increases in every dimension 

 measured until we reach the single specimen known 

 of the ultimate species, P. copei, from Bridger D 3, in 

 which a sudden falling off in size is observed. (See 

 also detailed table on p. 316.) 



Palaeosyops in general steadily evolves from a 

 smaller although very robust animal into a larger, ex- 

 tremely broad-skulled, or brachycephalic animal. 

 Certain characters are persistent in all the species. 

 There are added through orthogenesis other new char- 

 acters, or "rectigradations," which distinguish the 

 more advanced stages from those found in the lower 

 levels. The chief rectigradations and allometrons are 

 the following: (1) Increase in size and brachycephaly 

 of the skull; (2) steady increase in all dimensions of 

 the grinding teeth; (3) certain cusps in the lower pre- 

 molars (p2, Ps) analogous to the paraconid, metaconid, 

 and hypoconid of the true molars ; (4) the second cusps 

 (or tritocones) on the outer wall of the second upper 

 premolars (p^), cusps foreshadowed in the ancestral 

 species; (5) the median ridges (or mesostyles) added 

 on the outer walls of the two posterior upper premolars 

 (p^, p*), which happen to be quite distinctive because 

 these ridges do not arise in all titanotheres as they 

 do in all horses; (6) the second postero-internal cusp, 

 or tetartocone, on the superior premolar teeth is late in 

 evolution in all titanotheres and only appears in very 

 rudimentary form in P. copei; (7) the horns, or naso- 

 frontal bosses, are rectigradations, which appear to 

 be more retarded in evolution in Palaeosyops than in 

 several other phyla. 



Summarized, these seven principal cranial and den- 

 tal changes are shown below. 



I. Allometrons (proportional, or metatrophic) : 



1. Increase of the skull in size and in brachycephaly. 



2. Steady increase in all dimensions of the grinding 



teeth (see table on p. 316). 



3. Cusps on the lower premolars P2, Pa analogous to the 



paraconid, metaconid, and hypoconid of the true 

 molars; foreshadowed in P. paludosus and in- 

 creasing progressively in the higher stages. 

 II. Rectigradations (numerical, or the addition of new elements): 



4. Tritocone on p^ first occurring in P. major and in- 



creasing in all higher stages. 



5. Mesostyles added to p', p*, first occurring in P. 



leidyi and characterizing all higher stages. 



6. Tetartooones first appearing on superior premolars 



in P. copei (no higher stages at present known). 



7. Osseous frontonasal horns first appearing in P. 



leidyi and increasing in all higher stages. 

 Incisors. — The superior incisors exhibit low crowns 

 with irregularly folded posterior cingula; the opposite 

 series are usually separated by a median diastema and 

 from the canines by lateral diastemata. The incisors 

 increase regularly in size from i' to i'; the crowns are 

 convex anteriorly, with a basal subcingulate expan- 

 sion; the posterior faces slope gradually into an 

 irregularly folded basal cingulum; the lateral incisors 

 i' are much the largest and decidedly subcaniniform, 

 with obliquely sloping internal cingulum; i' is more 

 symmetrical, with short, compressed root; i^ is less 

 symmetrical, with longer root; i' is very asymmetrical, 

 with a very long, large root. The inferior incisors 

 are readily distinguished by their narrow crown, 

 sharply convex in front, and sloping posterior face 

 divided by a median ridge. As shown in P. leidyi 

 the crowns increase slightly but regularly in height 

 from ii to is. In the lower incisors the opposite ij 

 have very long roots nearly meeting in the midline 



