764 



TITAXOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



THE SKULL OF PEIMITIVE PERISSODACTYLA 



As the vast diversity in the structure of the modern 

 perissodactyl skull, described in Chapter I, was evi- 

 dently acquired during the Tertiar.v period, the 

 uniformity of skull structure in lower Eocene time is 

 remarkable; horses, tapirs, phenacodonts, titanotheres 

 all look alike when seen from above or from the side. 

 (See figs. 694, 695.) As in the feet, this uniformity 

 points irresistibly to a common ancestral form, which 

 would be a composite of skulls A, C, D, E. The palatal 

 and occipital views of these skulls are also similar. 



Historical notes. — There is little literature on the 

 primitive perissodactyl skull, because the materials 

 have hitherto been so scanty. 



Deperet (1903.1) made a special study of the skull 

 of LopModon, based upon L. Jeptorhynchus, from the 

 gres de Cesseras deposits of the Lutetien stage, equiv- 

 alent to our lower Bridger middle Eocene time. 

 This skull, while retaining seven primitive charac- 

 ters, correctly enumerated by Deperet (op. cit., p. 28), 

 also contains several specialized characters, namely* 

 broadening of the zygomatic arches, abbreviation of 

 the face, and elongation of the cranium. In these 

 features it is a specialized form as compared with the 

 much more ancient lower Eocene Sparnacian skulls 

 here examined. Deperet concluded (op. cit., p. 49) 

 that the skull of the primitive Perissodactyla had 

 primitive features in common with those of the Hyra- 

 coidea, the Condylarthra, and especially the Ambly- 

 poda; but he traced the origin of the perissodactyl 

 skull back of these groups to common protungulate 

 ancestors in Cretaceous time. 



Gregory (1910.1, pp. 389-391) enumerated many 

 primitive characters of the perissodactyl skull in 

 addition to those enumerated by Deperet, all of which 

 are noted below, with the exception of the designa- 

 tion of the "muzzle" as heavy. After a close com- 

 parison with the skull of the Condylarthra, he con- 

 cludes (p. 391): "The preceding skull characters may 

 confidently be assigned to the stem perissodactyl and 

 are all inherited from an insectivore-creodont-condy- 

 larth plan." 



The primitive perissodactyl skull is certainly closer 

 to the condylarth plan than the primitive perissodactyl 

 foot. 



General cJiaracters. — As seen from above, the skulls 

 of certain Insectivora, of the Condylarthra, and of 

 most primitive Perissodactyla are strikingly uniform, 

 or analogous, namely: 



(1) Narrow, elongate proportions, constricted ante- 

 riorly; (2) small craniocerebral region; (3) capacious 

 temporal fossae; (4) limited zygomatic arches parallel 

 to sides of skull; (5) moderately broad orbital region, 

 orbits open posteriorly; (6) abruptly constricted pre- 

 orbital region of face, similar to that of Solenodon; 

 (7) occiput narrow and overhanging the condyles. 



Seen from the side, we observe a striking similarity 

 of proportion and in the contour of the cranial 

 profile, except in the specialized skull of Heptodon and 

 of Helaletes, namely, (8) superior cranial profile sim- 

 ple, arched, with sagittal crest; (9) orbit placed mid- 

 way between occiput and premaxillary symphysis; (10) 

 face sloping downward toward anterior nares; (11) 

 occiput slightly overhanging condyles; (12) temporal 

 fossae large, with slender zygomatic arches; (13) area 

 occupied by grinding teeth limited. 



As to proportions, the ancestral skull is dolicho- 

 cephalic — that is, the basilar length greatly exceeds 

 the zygomatic width — and it is orthocephalic — that 

 is, the palatal and basicranial regions are in parallel 

 horizontal planes, whereas among modernized Perisso- 

 dactyla many skulls are cyptocephalic — that is, the 

 face is upturned or downturned. Also the face 

 slightly or considerably exceeds the cranium in length, 

 a condition technically known as proopic dolicho- 

 cephaly, or dolichopy. 



Adaptation. — The constricted anterior portion of the 

 face, including the small terminal anterior nares and 

 premaxillaries uniting with the nasals, indicates the 

 presence of a long, narrow tongue, a short upper lip, 

 limited prehension by lips, small lateral-terminal 

 nostrils. This is in contrast to the specialized tapir- 

 like narial region of Heptodon and Helaletes (fig. 694, 

 B, G), in which the anterior nares are relatively more 

 widely open or receding, v/ith premaxillaries not reach- 

 ing the nasals, distinctly lateral nostrils, and a more or 

 less prehensile upper lip; obviously a specialized 

 condition. 



The skull suddenly expands (fig. 695, A, B, C, D, E) 

 opposite the orbits, which a,re usually large and widely 

 open, indicating alert visual powers. The senses were 

 probably keen in these animals, although no auditory 

 bullae or osseous tympanic tubes are observed until 

 the beginning of the upper Eocene {Eomoropus, 

 Triplopus). 



It does not appear that the narial chamber was 

 large, that the narial respiratory duct was very 

 capacious, or that the olfactory chamber was large. 



The sagittal crest was prominent, arched, forming 

 the highest point of the cranium, terminating in the 

 high, narrow occiput, which indicates that the tem- 

 poralis muscle constituted the chief musculature of the 

 jaw. The masseters, attached to the rather slender 

 zygomatic arches, were relatively feeble. The supe- 

 rior borders of the zygomatic arches do not so continu- 

 ously nor so distinctly connect with the lateral portions 

 of the occipital crest, as indicated in LopModon by 

 Deperet. 



The construction of the ear region is very primitive, 

 the external auditory meatus being widely open; the 

 osseous portion of the tympanic, if present, was loosely 

 attached and seldom preserved. The mastoid is 



