612 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Contrast in features of tJie pes in the middle Eocene groups 



I. Palacosyopine group: Palaeosyops, Limnohyops 



1. Tarsus broad, composed of anteroposteriorly deep elements, 



correlated with more or less spreading digits. 



2. Calcaneum with somewhat rounded or oval shaft of tuber 



calcis moderately expanded at the summit. 



3. Cuboid relatively broad. 



4. Proximal facets on the metatarsals deeply extended antero- 



posteriorlv. 



II. Manteoceras-Dolichorhinus group: Manteoceras, Mesatirhinus, Dolichorhinus 



1. Tarsus more shallow anteroposteriorly; more elevated in all 



its elements. Digits less spreading distally. 



2. Calcaneum with laterally compressed and distally truncate 



tuber calcis. 



3. Cuboid high, relatively narrow. 



4. Metatarsals with proximal facets relatively shallow. Distal 



facets more flattened. 



PES OF THE PALAEOSYOPINE GEOTJP 



Limnohyops (figs. 520, 530). — The adaptive charac- 

 ters of the pes of this animal, like those of the manus, 

 are primitive, mediportal, and mesatipodal. Digits 

 broader than in Mesatirhinus and stouter and heavier 

 than in Tapirus — that is, of less swift type. Astraga- 

 lus with elongate neck and vertically elongate sus- 

 tentacular facet, unlike the rounded facet of Palaeo- 

 syops. Calcaneum with tuber deep and expanding 

 at summit. Cuboid deep (shallow in Palaeosyops). 

 Entocuneiform very large. Mesocuneiform very small. 

 Ectocuneiform very large, quadrilateral (not vertically 

 extended) as in Mesatirhinus. Metatarsals of medium 

 length; Mts III abutting against cuboid, Mts II 

 abutting against ectocuneiform. Terminal phalanges 

 cleft and expanding distally somewhat as in Mesati- 

 rhinus but more robust. 



Palaeosyops. — The most robust and graviportal pes 

 (figs. 520, 540) known among Eocene titanotheres. 

 Digits stout and widely spreading; prominent muscu- 

 lar rugosities indicating powerful flexor and extensor 

 attachments; astragalus readily distinguished by its 

 abbreviated neck, broad, shallow tibial trochlea, and 

 especially by the large and oval sustentacular facet for 

 the calcaneum, as well as by the broad cuboid facet; 

 calcaneum with obliquely placed tuber calcis and ex- 

 panded summit. Cuboid in two specimens very 

 short, thus Mts IV articulating with ectocuneiform; 

 the broad, abbreviated cuboid apparently a constant 

 character. Other 'elements of tarsus relatively broad 

 and low, deep anteroposteriorly; Metatarsals broad 

 and stout, broadening at the lower extremities; 

 ectocuneiform large and triangular; mesocuneiform 

 very small, with rounded edges; entocuneiform large, 

 of irregular shape. Terminal phalanges obtusely 

 rounded, very distinctive. 



In the passage from Limnohyops to Palaeosyops we 

 again note the transition from a rather heavy medi- 

 portal type, of the proportions of the tapir, to a much 

 heavier (subgraviportal) type with broad, spreading 

 feet adapted to supporting the animal in swampy 

 ground; the reduction of the terminal phalanges points 

 to reduced hoofs, a possible amphibious adaptation. 



PES OF THE MANTEOCEEAS-DOIICHOKHINlrS GROUP 



As in the manus, we may distinguish the pes by the 

 relatively high and narrow proportions of the podial 

 elements. The mediportal Mesatirhinus presents a 

 wide contrast with the subgraviportal Manteoceras. 



Mesatirhinus. — This mediportal pes (figs. 520, 569, 

 572) is readily distinguished by its relatively high and 

 narrow proportions. The angles and facets are all 

 sharply defined and sculptured. The bones of the 

 second row of tarsals and proximal facets of meta- 

 tarsals deep anteroposteriorly. Astragalus with elon- 

 gate neck, narrow, vertically produced sustentacular 

 facet for calcaneum; a narrow cuboidal facet. Cal- 

 caneum very distinctive, with its deep, laterally 

 compressed tuber and truncate distal extremity. 

 Navicular relatively deep and narrow. Cuboid, also 

 high, narrow and compressed in median line. Meso- 

 cuneiform and ectocuneiform distinguished by their 

 elevated and sharply quadrate form. 



Manteoceras (fig. 557). — Astragalus only known. 

 Tarsus apparently of the same type as in Mesatirhinus 

 but relatively broader, judging from the astragalus. 

 Astragalus with a sharply defined tibial trochlea, 

 subelongate neck, vertically elongate, straight-sided 

 sustentacular facet for the calcaneum, of same type 

 as in Mesatirhinus but relatively broader; a broad 

 cuboidal facet (unlike Mesatirhinus). 



Dolichorhinus (PI. XXXI). — Mesatipodal — that is, 

 metapodials relatively abbreviated, as in Manteoceras. 

 A full description appears upon a subsequent page. 



Comparing these three forms with respect to the' 

 elongation of the metapodials, Mesatirhinus is mesati- 

 podal, Dolichorhinus is stiU mesatipodal but transi- 

 tional, whde Manteoceras approaches the brachypodal 

 condition. Another type of more elongated manteo- 

 ceratine foot (Am. Mus. 2352) has been discovered and 

 is described in full below. It is omitted here because 

 its generic association is somewhat doubtful. 



SYSTEMATIC DESCRIPTIONS OF MIDDLE EOCENE 

 TITANOTHERES 



SUBFAMILY PALAEOSYOPINAE 



Mediportal and progressively graviportal titano- 

 theres of the lower and upper Bridger deposits and the 

 lower deposits of the Washakie Basin. Feet mesa- 

 tipodal to brachypodal. Ungual phalanges truncate 

 to rounded. Astragalus progressively widening. 



Limnohyops 



The skeleton of Limnohyops, so far as laiown, is 

 readily distinguished in all its parts from that of 

 Palaeosyops by its mediportal type — that is, by its 

 lighter construction. Yet the body was heavier and 



