540 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



SEXUAL CHAEACTERS 



In the females of both series we observe a far more 

 marked distmction from the males than that observed 

 in the menodontine group. Not only are the canine 

 teeth smaller, but there is a pronounced diEference in 

 the proportions of the horns. There are certain other 

 detailed characters which so far as our observations 

 are valid seem to separate the females from the males. 

 A summary of these contrasting sexual characters in 

 Brontotherium is given below. 



Males 



Horns long; connecting crests 

 large and prominent; nasals 

 decidedly short. Canines 

 larger; incisors more con- 

 stant, formula f;! or f;-?-. 

 Buccal processes of zj'go- 

 mata greatly produced; occi- 

 put greatly extended behind 

 the line of the zygomata. 



Females 



Horns relatively short; con- 

 necting crests less promi- 

 nent. Nasals relatively long. 

 Canines smaller; incisors less 

 . persistent, sometimes want- 

 ing. Buccal processes of 

 zygomata less prominent; 

 occiput not so widely ex- 

 tended behind zygomata. 



From some specimens it would appear also as if the 

 premolars of brontotheres were somewhat less pro- 

 gressive in evolution in the females than in the males. 

 This would controvert the general principle observed 

 in Menodus that the grinding series of the teeth, 

 which are so essential to the nutrition of the females, 

 is the one character in which the sexes do not differ. 

 Specimens of the female sex are smaller in size through- 

 out, as seen in the detailed table of measurements. 



SUBFAMIIY MEGACEEOPINAE 



Relatively small, long-horned titanotheres, known 

 chiefly from the middle Titanotherium zone. Horns 

 precociously evolved, vertical in position, placed 

 above the orbits, with little or no connecting crest. 

 Incisor teeth much reduced or actually vestigial, 

 canines very small, placed close together, thus tend- 

 ing to contract the premaxUlaries. 



Geologic Jiorizon and geographic distribution. — So far 

 as known these animals are of medium size or rela- 

 tively small and are recorded chiefly from the middle 

 Titanotherium zone of Cedar Creek, Colorado, and 

 from the lower portion of the upper Titanotherium 

 zone of South Dakota, possibly also from Assiniboia, 

 Canada. 



Four skulls of Megacerops were found in lower 

 Oligocene (Chadron) deposits at the levels indicated 

 below, chiefly according to the records of J. B. 

 Hatcher: 



Level C: 



?M. acer (type). 



M. copei, Nat. Mus. 4711 (type;. 

 M. acer, Univ. Wyo. Mus. 

 Level B: 



M. bucco, Nat. Mus. 4705, 5 . (Level rather 

 doubtful.— J. B. Hatcher.) 



Distinguishing features. — The males develop long 

 horns, which, imlike those of the typical Brontotherium., 

 are placed only slightly in front of the orbits, so that, 

 as shown in Charles R. Knight's models and restora- 



tions (PI. XVIII, C; fig. 454), the eyes appear almost 

 directly below the horns. As in the brontotheres the 

 eyes were small. The horns are directed almost 

 vertically upward, with the long basal axis placed 

 obliquely, the basal section never tending to become 

 transversely oval, as ia Brontops roiustus and Bronto- 

 therium, or triangular, as in Menodus. The reason for 

 this is that the horns are supported or braced prin- 

 cipally in a fore and aft direction, instead of across 

 the skull by means of the connecting crest, indicating 

 that they were used largely in a vertical or tossing 

 motion of the head rather than in the lateral motion 

 characteristic of the brontotheres. 



The narrowness of the chin and premaxillary 

 region and the entire absence of incisor teeth indi- 

 cates that these animals were provided with a narrow 

 and pointed prehensile upper lip, contrasting with 

 the somewhat broader lips in the brontotheres. 



A decided and highly characteristic feature is the 

 marked abbreviation (brachyopy) of the facial por- 

 tion of the skull, correlated with the cyptocephaly 

 or upward flexure of the anterior grinding teeth. 



Materials. — Remains of Megacerops are compara- 

 tively rare. Their existence was first made known 

 by Leidy in 1871 (see p. 210) through the type spe- 

 cies Megacerops coloradensis ; secondly, by Cope in 

 1873 from the skulls from Colorado described as 

 Symborodon bucco, S. acer, S. altirostris. Cope's 

 specimens are in the American Museum of Natural 

 History. A lower jaw ia the American Museum 

 (No. 6364) appears to represent a very small new 

 species of this genus, known as M. riggsi. Two ex- 

 ceptionally complete skulls from South Dakota are 

 in the National Museum, one of which is referred to 

 a third species, M. copei, related to M. bucco. The 

 first (Nat. Mus. 4705, skull O') is small horned, ap- 

 parently belonging to a female of M. bucco, and is 

 recorded by Hatcher from the top of Chadron B ; the 

 second (Nat. Mus. 4711, skull V'), the type of M. 

 copei, apparently a male, is recorded by Hatcher from 

 the middle beds and was known by the collectors as 

 the "rabbit skull," because of the resemblance of the 

 flaring horns to the ears of the jack rabbit. 



The Megacerops phylum as represented in the Hatcher collection 

 of six skulls and lower jaws from the Chadron formation, in 

 the United States National Museum 



