212 



INTRODUCTION TO EVOLUTION 



anything found in modern elephants but anticipate hiter developments in 

 some of the other proboscideans. Moeritherium had slight formation of 

 air cells, diploe, in the posterior region of the skull. The molars were low- 

 crowned, possessed two transverse crests each, and occurred in rows as in 

 most mammals. The longitudinal method of molar replacement came 

 much later. 



Phiomia and Paleomastodon 



The relationships of the various prehistoric proboscideans to each other 

 are not entirely certain. In broad outline the arrangement shown in Fig. 

 10.11 is substantially correct. The Oligocene proboscideans, Phiomia and 

 Paleomastodon, were larger than Moeritherium and had attained limb 



structure quite like that of 

 modern elephants. The lower 

 jaw had become considerably 

 elongated (Fig. 10.13), and 

 the second upper incisors 

 formed downwardly curving 

 tusks. Recession of the nasal 

 openings indicates that a short 

 proboscis was present (Fig. 

 10.11). Lull suggests that the 

 trunk developed originally for 

 the purpose of reaching be- 

 yond the elongated lower jaw. 

 The molar teeth were more 

 complicated in structure than were those of Moeritherium, having on their 

 surfaces three transverse crests, with some accessory cusps. The posterior 

 portion of the skull was heightened by the presence of air cells in the bone. 

 Lack of space prohibits mention of all the varied and interesting probos- 

 cideans that roamed the earth in Cenozoic times. In the main we shall con- 

 tent ourselves with pointing out three principal evolutionary lines. One side 

 line is of sufficient interest to deserve passing attention, however. That 

 line, not indicated in Fig. 10.1 1, culminated in Dinotherium, a proboscidean 

 lacking upper tusks and having lower tusks curved downward and back- 

 ward in most unusual fashion (Fig. 10.14). The molar teeth, low-crowned 

 with but two or three cross ridges (Fig. 10.14), were relatively unspecial- 

 ized. A diet of soft, succulent vegetation seems indicated. Some specimens 

 exceeded modern elephants in size. Dinotherium ranged widely over 



FIG. 10.12. Skull of Moeriiherium. (After An- 

 drews; from Romer, Verfebrafe Paleontology, 

 University of Chicago Press, 1945, p. 409.) 



