EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



521 



The Menodus pTiylum as represented by the Hatcher collection of 25 sTculls of species of Menodus from the Chadron 

 formation in the United States National Museum 



Species 



M. giganteus PomeL 



Do 



Do 



Do 



Do 



Do 



Do. 

 Do. 

 Do. 

 Do. 

 Do. 

 Do. 



M. (trigonooeras) giganteus 

 Pomel. 



M. trigonoceras (Cope) 



Do 



Do 



Do 



Do 



M.? trigonoceras (Cope) 



M. trigonoceras (Cope) 



Catalog No. 



M. proutii (Owen, Nor- 

 wood, and Evans) . 



Do 



Do . 



M. torvus (Cope) 



M. heloceras? (Cope) 



1220, cf 

 1212 



8745, & 

 8741, d' 

 8735, V 

 8761, cf 



8756 



8751 



8765, cf 



8781 



8778 



4745, 9 



8793 



1219, cf 



4291, cf 



4707 



8760, ? 9 



8768 



1246 



4257 



4701, ? 



8736 



113 



8799 



8744 



Material 



Skull. Old male, fuU size. Nasals typical; also horns. 



Posterior part of skull. 



Skull. Nasals and canines typical; also horns. Size medium (="B. ingens" stage). 



Skull. Measurements typical of smaller males; not so large as type. 



Skull. Slender canines. 



Anterior half of skull. Measurement of p'-p* agrees with that in smaller males of M. 



giganteus. Specific reference uncertain. 

 Skull. Poor. 



Anterior half of upper milk dentition. May be large M. trigonooeras. 

 SkuU. 



Complete left ramus and part of right. 



Last half of lower jaw and molars 1, 2, and 3 of upper right. 

 Lower jaw. 

 Upper milk dentition (dp'-dp^) and m', large animal. 



Anterior part of skull. An old male, approaching M. giganteus stage. 



Skull. Very fine progressive type; almost at M. giganteus stage. 



SkuU. 



Crushed skull. Inferior specimen. 



SkuU. Poorly preserved. 



Posterior parts of both rami. 



Skull from level B 3. Long nasals; vestigial incisor and measurements remove it from 



B. dispar. 

 SkuU from level C 1 and left fore foot. 



SkuU. 



Left ramus of jaw. Type. 

 Pair of lower jaws. 



Skull. Very young, m" just protruding; elongate proportions of Menodus. 

 specific reference. 



Doubtful 



Menodus trigonoceras (Cope), of Chadron B: It is 

 interesting to note that Cope's description of M. 

 trigonoceras immediately followed that by Marsh of 

 M. giganteus. M. trigonoceras, which is probably 

 characteristic of B, or the middle Titanotherium zone, 

 is distinguished from M. giganteus, first, by its inferior 

 dimensions throughout; second, by the shape of the 

 horns, the horn section being an equilateral triangle, 

 as the distance between the malar, nasal, and con- 

 necting ridges is practically equal. The skull of 

 M. trigonoceras is directly intermediate in size between 

 that of M. heloceras and M. giganteus. 



Menodus giganteus Pomel, of Chadron C: In 

 Menodus giganteus, a superb titanothere, specifically 

 equivalent to Menodus {" Brontotherium") ingens 

 Marsh, absolutely determined as characteristic of the 

 upper Titanotherium zone, and even of the uppermost 

 levels (Chadron C 3), we reach the climax of this 

 monophyletic series, which is distinguished by the 

 extreme development of all the distinctively phyletic 

 characters as compared with the inferior stages. Its 

 relative abundance indicates that it was capable of 

 holding its own in the struggle for existence between 

 the numerous phyla of Brontops and Brontotherium. 



Remains of species of Menodus have been found at 

 the stratigraphic levels indicated below: 



Upper Titanotherium zone: M. giganteus Pomel. Skulls large 

 (type 755 by 553 mm.). Buccal processes of zygomata | 



strongly developed. Tetartocones of p'-p* distinct. Hypo- 

 cone of m' usuaUy separated from cingulum. Horns large, 

 directed obliquely outward and upward. Connecting ridge 

 prominent. 



Middle Titanotherium zone: M. trigonoceras (Cope). Skulls 

 of medium size (type 678 by 490 mm.). Buccal processes of 

 zygomata moderately developed. Tetartocones of p'"^ 

 distinct. Hypocone of m^ sharp and distinct or a sharp 

 cingule. Horns of medium size; basal section equUateral; 

 connecting ridge. 



Lower Titanotherium zone: M. heloceras (Cope). Skulls small 

 (width of type across zygomata 392 mm.; nasals to occiput 

 (estimated) 545 mm.). Horns smaU, subtrihedral; basal 

 section subtriangular; internal angle rounded; no con- 

 necting ridge. 



Ohservations on the measurements of the Menodus 

 series. — The species of this genus constitute an as- 

 cending series extending from the lowest to the highest 

 levels of the Titanotherium zone. The range in the 

 chief measurements of the successive stages may be 

 epitomized as follows: 



Measurements of species of Menodus, in millimeters 



Pi-m3 



Pi-p* 



Mi-mS 



Pmx to condyles. 

 Zygomatic index. 

 Horn length 



385-465 

 141-176 

 246-285 

 760-825 

 62-70 

 150-290 



333-360 

 127-136 

 203-224 

 628-770 

 66-?79 

 132-214 



