PHENACOCCELUS 1 8 1 



Phenacoccelus and Leptauchenia have no close relationship but show a certain amount of 

 parallelism, as Peterson has pointed out. He says that the former resembles the latter in several 

 ways: 



(1) In the broadening and shortening of the skull; (2) in the tendency of the orbit, to be upward directed; 

 (3) in the presence of fronto-nasal foramina or openings; (4) in having reduced premaxillaries; (5) in the 

 shortening of the facial region; (6) in the enlargement of the lachrymal pits and the presence of facial vacuities; 

 (7) in the hypsodont cheek-dentition and other similarities in the construction of these teeth; (8) in having the 

 tympanic bulla of very large size and extending much below the postglenoid process; (9) in having the infraorbital 

 foramen above P 3 ; (10) in having the same number of dorsal and lumbar vertebrae; (11) in having a large 

 thoracic cavity; (12) by having a similar broad and deep pelvic cavity; (13) by the general similarity of the 

 greater parts of the structure of the limbs and feet. 



The differences he enumerates as follows: 



Phenacoccelus differs from Leftauchenia (1) in having larger and more numerous incisors [both genera 

 have the same number, 3 — M.R.T.], with the upper canine less rounded in cross-section; (2) in a more feebly 

 developed sagittal crest, and a longer skull; (3) by having more overhanging nasals; (4) by having large and 

 deep lateral excavations on the occipital plate; (5) by a narrower and more backward projecting occiput, together 

 with less developed temporal crests; (6) by a relatively smaller external ear placed lower down; (7) by a 

 smaller development of the dorsal arch of the atlas, and the absence of the canal for the vertebral artery, which 

 perforates the base of the transverse process; (8) by relatively lighter cervical vertebrae, due to the relatively 

 smaller head; (9) by the relatively greater anteroposterior and smaller transverse diameter of the neural spines of 

 the anterior dorsal vertebrae; (10) by a much less developed tubercle for the attachment of the internal humeral 

 part of the triceps on the olecranon process of the ulna; (11) by a relatively shorter and heavier tibia, different 

 shape of its shaft; (12) by having a shorter and broader hind foot. 



Peterson concluded that the direct ancestral line to this genus was an independent one, parallel- 

 ing Merycoidodon and Limnenetes. I consider that Phenacoccelus is a derivative from Eporeodon, 

 which it resembles in many ways, not necessarily in a direct line but on a branch, the intermediate 

 members of which we have not yet discovered, but which led to this genus, to Merycoides, and later 

 into Ticholeptus. Merycoides and Paroreodon are rather more specialized than we should expect the 

 ancestors of Ticholeptus to have been and were probably side lines which became extinct in 

 themselves. 



Etymology: Phenacoccelus (false + hollow, in allusion to the openings in the skull). 



Species: 



P. munro'e'nsis Peterson 1928. 



P. typus Peterson 1906. Genotype. 



Phenacoccelus munroensis Peterson 1928 

 PI. XXXVII, fig. 9 



Original Reference: Osteology of Phenacoccelus typus Peterson. Mem. Carnegie Mus., XI, pp. 161- 

 162, pi. XVIII, figs. 1-9. 



Type Locality: Head of Warbonnet Creek, Sioux County, Nebraska. 



Geologic Horizon: Lower Miocene (upper Monroe Creek). 



Type: Peterson says that the holotype, Cat. No. 1288 CM., consists of the following parts: "Lower 

 jaws, fragments of the atlas and other cervicals, two anterior dorsals, a fragment of the sacrum, fragments of 

 ribs, portions of both radii, and the manus fairly well preserved, the right side of the pelvis nearly complete, and a 

 fragment of the left ilium, the right femur very nearly complete, and the head of the left, both tibia; nearly com- 

 plete, and the pes well represented." The specific name was based upon the geological horizon in which the 

 specimen was found. 



Specific Characters: The species is slightly larger than P. typus. 



