760 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Euprotogonia , besides its very numerous resem- 

 blances to what we may suppose to have been the 

 stem perissodactyl type exhibits the following dif- 

 ferences: (1) Humerus with large entepicondyle and 

 entepicond.ylar foramen; (2) carpus with lunar resting 

 on magnum and unciform; (3) astragalus with round 

 or convex navicular joint; (4) brain (by inference 

 from Plienacodus) extremely small and smooth, with 

 large olfactory lobes and small frontal lobes, or 

 cerebrum. 



There are thus strong differences between the most 

 primitive perissodactyl humerus, carpus, and tarsus 

 and those of Euprotogonia, Phenacodus, and Menis- 

 cotherium, which tend to support Osborn's view that 

 the Condylarthra represent a separate specialization 

 or radiation of their own, while the Perissodactyla 

 sprang from an independent primitive stock, Perisso- 

 dactyla primitiva, of unknown derivation but probably 

 related to the insectivore-creodont group. 



This opinion is further supported by many of the 

 observations presented by Gregory in his "Orders of 

 mammals," and especially by his demonstration 

 (op. cit., p. 393) that the carpus of the ancestral 

 Perissodactyla differs sharply from that of the loiown 

 Condylarthra, a point which is fully treated below. 



Figure 6S9. — Generalized carpus of 



insectivore-creodont type 



After Gregory, 1910.1. 



In the condjdarth the head of the astragalus 

 (navicular facet) is hemispherical and there is no facet 

 for the cuboid; the sustentacular facet is more cen- 

 tral in position; there is a definite constriction, or 

 neck, between the head and the proximal end; the 

 trochlea is wide, with low keels. In the primitive 

 perissodactyl, on the other hand, the head, or navicular 

 facet, is concave in the front view and convex in the 

 side view; the sustentacular facet is vertically ex- 

 tended and shifted toward the middle side; there is a 

 distinct facet for the cuboid, which meets the susten- 

 tacular facet at an open angle; the trochlea has high, 

 sharp keels. 



CONCLUSION 



The remote ancestry of the perissodactyl stem, 

 whether from "Condylarthra primitiva" or from an 



independent "Perissodactyla primitiva" placental 

 stock, must be left an unsettled problem awaiting 

 future discovery. 



Skull and dental indices nf certain ungulates 



Phenacodus wortmani._ 

 Systemodon priinaevus_ 



Eohippus venticolus 



Isectolophus 



Hyraohyus 



Triplopus 



Eotitanops 



Lophiodon, c? 



Lambdotherium 



Paloplotherium 



Zygomatic- 

 cephalio 

 (bueco- 

 cephalic) 



39? 



47 



41 



46? 



46 



50? 



52 



41 



Facio- 

 cephalic 



51 



52 



60 



56 



62 



53 



59 



49 



65? 



62 



Premolar- 

 molar- 

 cephalic 



26 



30 



30 



39? 



32 



30 



26 



30 



27? 



35 



Note.— Premolar-molar cephalic index obtained from p^-m'^^Iength, premaxil- 

 lary to condyle. 



PRINCIPAI CHARACTERS OF THE ANCESTRAL 

 PERISSODACTYLS 



The following features characterize the ancestral 

 perissodactyls: 



1. Ungulates of small size and cursorial locomotion, alert 

 and quick in movement. 



2. Elongate limbs, narrow and stilted feet, of perfect mech- 

 anism. 



3. Tetradactyl manus, tridactyl pes with reduced or vestigial 

 fifth digit, terminal phalanges narrow and cleft distally. 



4. SkuU dolichocephalic, with facial region longer than 

 cranial region. 



5. Brain, visual and auditory organs, and senses well de- 

 veloped. 



6. Eutherian dentition, |-:-j-;-4;|; teeth throughout similar to 

 those of Euprotogonia. 



7. Diet probably succulent, tender shrubs, herbage, berries, 

 tubers, etc. 



8. Limb skeleton, humerus, carpus, and astragalonavicular 

 joint all of perissodactyl type. 



Tetradactylism in the manus of Perissodactyla is 

 in contrast to the pentadactylism in the manus of the 

 Condylarthra. Not a single trace of the poUex has 

 been actually observed in any primitive Perissodac- 

 tyla. That the "Perissodactyla primitiva" once 

 possessed a pollex is attested by the presence of the 

 extensor ossis metacarpi pollicis muscle in the tapir 

 and horse. The homology of this muscle with that 

 in pentadactyl mammals can not be doubted. 



Tridactylism in the pes is accompanied by an 

 occasional vestige of Mtc V (Eohippus). 



For details of perissodactyl structures see descrip- 

 tion of skull and carpus below, and Gregory's outline 

 (1910.1, pp. 387-395) of the principal characters of 

 the stem Perissodactyla. 



