EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



599 



Figure 505. — Left astragalus 

 and calcaneum of Eoiita- 

 nops sp. 



FAMILY AND SUBFAMILY CHAEACTERS OF SKELETAL 

 PAKTS IN MIDDLE EOCENE TITANOTHERES 



VERTEBRAE, ATLAS 



Palaeosyopinae . — In the atlas of LimnoJiyops, as in 

 Palaeosyops, the spinal nerves (see fig. 508, A) issue 

 nearer the median line than in Telmaiherium (fig. 508, 

 B); the neural arch and the bridge over the vertebral 

 artery are narrow. The 

 axis is rather narrow, with 

 a slender odontoid process. 

 The postero-inferior tuber- 

 osity is moderately devel- 

 oped. 



In the atlas of Palaeo- 

 syops (Am. Mus. 1580, fig. 

 508, A), as in LimnoJiyops, 

 the spinal nerves issue 

 nearer the median line than 

 in Telmafherium. Unlike 

 LimnoJiyops the neural arch 

 is broad and powerful; the 

 inferior vertebrarterial 

 bridge is wide; the pleura- 

 pophyses expand broadly 



Am. Mus. 14895, Wind River Basin, • u„i-u j;..„„t;„„„ T^kn 

 Wyo.; Wind River formation, level '" both du-ections. The 



B. Facet for tibia (/ft). Two-tiiirds postcro-inferior tuberosity 

 °^'""' '"■'■ is moderately strong. 



TelmatJieriinae. — In the atlas of Telmatlierium ulti- 

 mum (Am. Mus. 2060; fig. 508, B) the spinal nerves 

 are well separated, the pleurapophyses expand lat- 

 erally but do not flare, the inferior vertebrarterial 

 bridge is broad, but the canal itself is contracted, fore- 

 shadowing the exclusion of the vertebral artery from 

 the canal, which is a very distinctive character of 

 some of the Oligocene titanotheres. The occipital 

 cotylus is broad with massive borders; the odontoid 

 facet is exceptionally broad and open (thus distin- 

 guished from that of Manteoceras) . The postero- 

 inferior tuberosity is moderately strong. 



Manteoceratinae. — The atlas of Manteoceras (Am. 

 Mus. 12204, fig. 508, E) has the characters we should 

 expect to find in a broad-skulled member of the 

 Manteoceratinae: the vertebrarterial opening is very 

 large posteriorly; the bridge is narrow (16 mm.) and 

 rounded; the articular facets for the axis form a widely 

 open angle; the postero-inferior tuberosity, which 

 underlies the axis, is stout and prominent, as in the 

 Oligocene titanotheres. 



Dolichorhininae. — In Mesatirhinus (Am. Mus. 1523. 

 fig. 508, C) the pleurapophyseal wings are not known; 

 the atlas presents an approximation to that of Doli- 

 chorMnus: (1) the centrum is relatively elongate, (2) 

 the spinous foramina are nearer together, (3) the ver- 

 tebrarterial canal is covered by a bridge of medium 

 length, (4) the facets for the axis are approximated and 

 obliquely face each other, (5) the cotyli for the occip- 



ital condyles are relatively deep, (6) there is a reduced 

 postero-inferior tuberosity. 



In DolicJiorhinus (fig. 508, D, Am. Mus. 1844, 13164 

 associated with skull), the body is relatively elongate, 

 the occipital cotyli are very broad and deep, the 

 spinous nerve notch is very deep and narrow, the 

 pleurapophyses flare widely, the vertebraterial canal 

 is more open than in Telmatlierium ultimum. As 

 in Mesatirhinus prominent bony processes connect the 

 transverse ligament above the odontoid process. 



Summary. — The atlas of Palaeosyops and Limno- 

 Jiyops conforms to the brachycephahc types of skull, 

 that of DolicJiorJiinus to the dolichocephalic type; 

 those of Telmatlierium and Manteoceras are interme- 

 diate between these extremes. 



From the limited materials in our possession {Palaeo- 

 syops, DolichorJiinus, MetarMnus) we observe that the 

 scapula of the Eocene titanotheres is subject to wide 

 adaptive range from the more elongate mediportal 

 type of the supposed MetarMnus to the broader sub- 

 graviportal type of Palaeosyops. The scapula is, how- 

 ever, quite distinctive in its family or syngenetic 

 form as compared with that of other Perissodactyla, 

 being characterized as follows: (1) a vertically elon- 

 gated supraspinous fossa, which is equally broad above 

 and below, giving a relatively straight anterior border; 

 (2) a relatively large infraspinous fossa of subtriangu- 

 lar form, in which the border rapidly contracts toward 



Figure 506. — Metatarsal and tibia of Eotitanops 

 major 



Am. iVIus. 14894 (type), Wind River Basin, Wyo.; level B. 

 Median metatarsal: Back (A>), front (A^), distal (A'), and 

 proximal (A') views. Distal end of left tibia: Front (B') and 

 distal (B-) views. One-iialf natural size. 



the neck — on this border is a distinct "angle" which 

 becomes sharply marked in later types; (3) a broad, 

 short neck, or collum scapulae. This is the graviportal 

 prototype from which the scapula of the heavy Oligo- 

 cene titanotheres is readily derived. On the whole, 

 this middle Eocene scapula is analogous to that of the 

 mediportal Tapirus, but it is distinctly tending toward 

 and even modified in the graviportal direction. 



