EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



563 



i ' may be an age character. The crowns are absolutely 

 smooth, oval, and without cingulum. This incisor 

 reduction may also indicate female sex; if not, it 

 would appear to place B. Tiypoceras out of direct 

 ancestry to the B. gigas-platyceras line. Since all 

 specimens of B. gigas show two well-developed incisors 

 the presence of but a single incisor appears to be a 

 distinctive and specific character in B. Jiypoceras. 

 The characters of the premolars and molars and es- 

 pecially the development of the tetartocones are 

 about the same as in the first skull, except that there 

 is a sessile crenulate cingulum on the inner sides of 

 P2, P4, and the tetartocone on p4 is somewhat smaller. 



Brontotherium hatcheri Osborn, 1908 



Plates XIX, XXXIX-XLII, CXXIV, CLXVII-CLXX, 

 CLXXII; text figures 202, 395, 399, 400, 404, 457, 458, 465- 

 468, 470, 630, 632, 639 



[For original description and type references see p. 235. For skeletal characters 

 see p. 695] 



Geologic Jiorizon. — Middle Titanotherium zone of 

 South Dakota. 



Specific characters. — If, P|. Nasals moderately 

 long (97 mm.), thin at the edges. Horns, 250 + 

 millimeters, two-thirds the length of the B. gigas 

 horns. Skull length (premaxiUaries to condyles) 

 710 millimeters (estimated), width (across zygomata) 

 530 (estimated). 



This species was named in honor of J. B. Hatcher, 

 paleontologist and explorer. 



Materials. — The species is represented by four speci- 

 mens, namely, the type skull (Nat. Mus. 1216, skull 

 a) and the referred material, including in the National 

 Museum a second skuU (No. 4255, skull Q), in the 

 American Museum the anterior portion of a skull, 

 No. 1070, associated with a lower jaw, and in the 

 Field Museum, Chicago, a complete uncrushed skull 

 (No. P 5926). 



The type is the large skull Nat. Mus. 1216, com- 

 plete except the premaxiUaries, recorded by Hatcher 

 from the uppermost levels of the middle Titano- 

 therium zone. Although a fully adult animal, in the 

 ninth stage of growth, as compared with B. gigas 

 it has shorter horns (250 mm.) of rounded section, a 

 relatively lower connecting crest, and longer nasals 

 (97 mm.). It appears to represent an early phase 

 of evolution of B. gigas, the type brontothere. The 

 horns are very round or convex in section and have a 

 well-defined external ridge on the lower outer portion ; 

 the connecting crest is relatively shallow, and the 

 nasals are thin; the zygomata are broad and flat. 

 The premolars are well advanced, the tetartocone of 

 p* being quite distinct. 



A skull in the National Museum (No. 8313) de- 

 scribed below under Brontotherium tichoceras should 

 also be compared with the type of this species but 

 differs in the absence of a connecting crest. 



A second skull of B. hatcheri (Nat. Mus. 4255), 

 equally primitive, is also recorded by Hatcher from 

 the uppermost level (Chadron C) of the middle Titano- 

 therium zone. This animal, in the seventh stage of 

 growth, exhibits intermediate horns (280 mm.) with a 

 still lower connecting crest than in the type of B. 

 hatcheri; the basal horn section has a more prominent 

 nasal angle than in the type. The nasals and horns 

 both in form and in section might be described as 

 intermediate between Brontops rohustus and Bronto- 

 therium gigas. 



In the American Museum is a skull (No. 1070) which 

 may represent either an earlier stage of evolution, such 

 as B. hatcheri, or a variety of B. gigas. This skull is that 

 of a bull in the seventh stage of growth and affords 

 valuable supplementary knowledge of the dental and 

 mandibular structure of an advanced B. hatcheri 

 (Pl.CLXIX). It proves that the formula is If, Pf. P' 

 may drop out in old ani- 

 mals, just as in B. gigas. 

 The inferior lateral inci- 

 sors are robust teeth with 

 a crenulate posterior 

 cingulum; the median 

 pair are smaller. 



Degeneration of the 

 cingulum. — The premo- 

 lars of this specimen 

 (Am. Mus. 1070) have 

 an especial interest as 

 showing several stages 

 in the decline of the 

 internal cingulum, 

 which, as we have ob- 

 served, is very feebly 

 developed in B. leidyi 

 and B. hypoceras. On 

 p^, right, the cingulum 

 is a blunt ridge slightly 

 crenulate; on p^, right, 

 the cingulum is in- 

 complete, the crenula- 

 tions being much more 

 prominent and more separate; on p', left, the 

 line of the cingulum is very faint, the crenula- 

 tions are very prominent, separate, and appar- 

 ently in process of being absorbed into the bases 

 of the internal cones. The occurrence of the cin- 

 gulum in this imdoubted bull coincides with other 

 evidence against the hypothesis that the cingulum is a 

 sexual character. 



A peculiar feature of the enamel of the lower and 

 upper teeth is the fine horizontal striation of structure, 

 which is also observed in succeeding members of this 

 series. 



In this animal the horns are rounder and shorter 

 than in the type of B. gigas, but the connecting crest 



FiGTJHE 465. — Sections and 

 contours of skull of Bron- 

 totherium hatcheri 



Nat. Mus. 1216 <type). One-seventh 

 natural size. 



