EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



317 



Palaeosyops fontinalis (Cope) 



Test figures 97, 269-271 

 [For original description ^nd type references see p. 165] 



Type locality and geologic horizon. — Bridger Basin, 

 Wyo.; Bridger formation, level not clearly recorded 

 but probably EometarTiinus-Palaeosyops fontinalis zone 

 (Bridger A), as the type skull is recorded from Green 

 River. Also recorded from Huerfano Park, Colo., in 

 Bridger formation, horizon Huerfano B ( = Bridger A). 

 The six specimens from Huerfano B (see below) range 

 from 250 feet to 500 feet below the top of the Huerfano 

 formation. 



Specific characters .—01 relatively 

 small size. Superior molars with 

 sharply defined crescents and cusps; 

 m' extremely small (ap. 22 mm., tr. 25), 

 with protoconule and no metaconule. 



This little-known animal resembles 

 Palaeosyops in its cranial structure but 

 differs quite widely from any known 

 species in the form of the cusps of its 

 grinding teeth. It is the smallest, 

 probably the most primitive, and 

 certainly the oldest Bridger titanothere 

 known, and these facts in connection 

 with the very low geologic level give it 

 great importance. 



Materials. — This species is repre- 

 sented in Bridger (A?) merely by the 

 very immature skull (Am. Mus. 5107) 

 collected on Green River near Big 

 Sandy Creek. In Huerfano B it is 

 represented by six specimens in the 

 American Museum of Natural History, 

 as follows : 



17411. Superior dentition and portion of 

 palate (fig. 271, A), Huerfano-Muddy divide, 



2 miles west of Gardner, Colo., 414 feet 

 below the top of tlie Huerfano formation. 



17413. Two upper molars and incisor (fig. 

 271, C), 3 miles north of Gardner, 400 to 500 

 feet below the top of the Huerfano formation. 



17414. Three superior molars, fragmentary, 



3 miles north of Gardner, 400 to 500 feet 

 below the top of the Huerfano formation. 



17417. Ml, p', and milk teeth (fig. 271, B), 2 miles north 

 of Gardner, 400 to 500 feet below the top of the Huerfano 

 formation. 



17425. Series of right upper grinders, p'-m^ (fig. 271, D), 2 

 miles north of Gardner, 400 to 500 feet below the top of the 

 Huerfano formation. 



17450. Lower canine and fragment of ma, Huerfano-Muddy 

 divide, 2 miles west of Gardner, about 250 feet below the top 

 of the Huerfano formation. 



The immaturity of the type specimen is determined 

 by the fact that only one true upper molar (m') has 

 come into use, the second molar (m^) being still 

 deeply embedded in the jaw. In superior view 

 (fig. 269, A3) the cranium is valuable as exhibiting 

 the suture between the supraoccipital and the parietals, 

 a suture which closes very early in Palaeosyops. 

 Similarly in the lateral view (fig. 269, Ai) the maxilla 

 sends out a broad, spurlike process directly on the 

 outer side of the malar as in Palaeosyops, instead of 

 on the under side as in Limnohyops. The infraorbital 

 bridge is rounded as in Palaeosyops, rather than 



FiGiTEE 269. — Young skull of Palaeosyops fontinalis 



One-third natural size. Am. Mus. 5107 (type). 

 Basin, Wyo.; Bridger formation, level A?. 



Green River near mouth of Big Sandy Creek, Bridger 

 Li, Side view reversed: A2, palatal view; A3, top view. 



The geologic horizons of these specimens range 

 rom 250 to 500 feet below the top of the Huerfano 

 formation. 



angulate as in Limnohyops. The depressed or sessile 

 character of the supratemporal crest is probably due 

 to immaturity. Behind the post-tympanic process 

 the mastoid is exposed as a narrow strip. 



The specimen thus probably pertains to the genus 

 Palaeosyops, though its grinding teeth are not entirely 

 of the typical Palaeosyops form ; its specific distinction 

 from the larger and more robust forms is readily 

 determined from the teeth. 



Dentition. — The part of the crown of dp* that is 

 preserved exhibits the protocone more internally 



