420 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



representing an animal widely distinct in aU its characters 

 from the contemporary P. foniinalis and more closely related 

 to the other group of middle Eocene titanotheres to which 

 Metarhinus, Mesaiirhinus, and Dolichorhinus belong. The most 

 surprising character in an animal of this geologic antiquity is 

 the very rudimentary osseous horns at the junction of the 

 nasals and frontals, indicating the horn rudiment, a very ancient 

 character in this phylum. The nasals are long, arched, de- 

 curved, and revolute on lateral borders, thus resembling the 

 rhadinorhinal type in the metarhine group. The malars below 

 the orbits are prominent. There was probably no infraorbital 

 shelf, as in Rhadinorhinus. The comparative measurements of 

 Eometarhinus, of Mesaiirhinus megarhinus, and of the contem- 

 porary Palaeosyops foniinalis are shown below. 



Eometarhinus huerfanensis Osborn 



Text figures 156, 355 

 [For original description and type reference see p. 200] 



Type locality and geologic horizon. — The type speci- 

 men was found near the Huerfano-Muddy divide, 3 

 miles west of Gardner, in the Huerfano Basin, Colo., 

 205 feet below the top of the Huerfano formation, in 

 the EometarJiinus-Palaeosyops foniinalis zone (Huer- 

 fano B). 



Specific characters. — Inferior in all measurements to 

 M. megarhinus. Premolars with small deuterocone. 

 Pl-m^ 124 miUimeters; p^-p*, 53; ml-m^ 72. 



Materials. — This species is known from the type 

 skuU (Am. Mus. 17412) and from two referred 

 specimens — a fragment of a lower jaw (Am. Mus. 

 17013) with the first and second molars preserved 

 (Osborn, 1919.494, fig. 7, B), and the right and left 

 fourth lower premolars with a fragment of a canine 

 (Am. Mus. 17416). Both of these specimens are from 

 approximately the same level as the type and from 

 the same general locality. Doubtfully referred to this 

 species are three fragmentary upper molars (Am. 

 Mus. 17415), found 3 miles north of Gardner, on the 

 lowest level of the upper Huerfano (Huerfano B), or 

 200 to 300 feet below the level of the type. 



General characters. — The principal skull characters 

 are noted above under the generic description. The, 

 dental formiila is normal. The premolars are small, 

 apparently very simple in pattern. The antero- 

 posterior diameters of the molars appear to exceed 

 the transverse; as they are in fractured condition, no 

 accurate measurements can be taken. Apparently a 

 hypocone on m^ 



Measurements of Eometarhinus, Mesaiirhinus, and Palaeosyops, 

 in millimeters 



A tibia, found in association with the type skull, 



measures 275 millimeters (estimated) in extreme 



length. 



Metarhinus Osborn 



Plates LII, LXXI, LXXIV, LXXVIII-LXXX; text figures 

 123, 124, 138, 139, 219, 302, 323, 324, 341, 356-361, 404, 407, 

 509, 521, 522, 573-578, 647, 745. 



[For original description and type references see p. 183] 



Animals of medium or small size, divergent in 

 structure, and probably different in habit and habitat 

 from members of the Mesatirhinus-DolicJiorhinus series, 

 hence the name Metarhinus; perhaps of fluviatile or 

 semiaquatic habit; skull with rudimentary horns, 

 elongate expanding nasals, orbits prominent; opposite 

 sides of the upper jaw firmly united, proportions 

 mesaticephalic; persistent sagittal crest and narrow 

 occipital condyles. 



Geologic horizon. — This group of small animals is at 

 present found only on one geologic level — namely, Uinta 

 B 1 and Washakie B 1, which may be Icnown as the 

 Metarhinus zone. It originally sprang from the same 

 ancestral stock as Mesatirhinus megarhinus, but in 

 these "metarhines" dolichocephaly was arrested and 

 mesaticephaly persisted. The four or five known 

 species exhibit a considerable number of characters in 

 common which afford ground for regarding them as a 

 related natural group of the smallest titanotheres of 

 the period; they are truly dwarfed forms. The skull 

 is very broad across the orbits, which are notably 

 prominent; immediately in front of the orbits the face 

 contracts; the nasals are long and expand distally. 

 There is some evidence that the phylum divides into 

 two lines, consisting of broad-headed and narrow- 

 headed forms that run parallel throughout the period 

 represented by Uinta B 1. 



The remains of these animals are very abundant, 

 and the genera Metarhinus and Dolichorhinus are 

 equally well represented (Riggs, 1912.1, p. 24) in 

 Uinta B 1. The remains are usually associated in 

 the same ledges and were apparently deposited under 

 the same conditions. In one ledge of sandstone a 

 large part of an articulated skeleton of D. longiceps 

 was so mingled with the skeleton of a young Meta- 

 rhinus that it was at first mistaken for a young animal 

 of that genus. In other ledges, however, Metarhinus 

 is very abundant and Dolichorhinus is absent. Other 

 genera occurring in the same life zone are the more 

 rare Rhadinorhinus, the giant amblypod Eohasileus, 

 the small hyracodont Triplopus, and two large creo- 

 donts, Mesonyx and Earpagolestes. Occasionally 

 crocodiles and numerous turtles are found, also beds 

 of fresh-water clams. Reeds, leaves, and branches of 

 trees are abundant in the upper sandstones of the 

 Metarhinus zone. 



Geologic occurrence in channels. — It is evident that 

 our knowledge of this Metarhinus zone fauna is con- 

 fined to that of the intrusive sandstone ledges of stream 



