598 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Measurements of Eoiitanops major and other species, in 

 millimeters 



bones of the carpus and tarsus, or in the shape of the 

 small bones on the sides of the manus, such as the tra- 

 pezium and pisiform, and of the pes, such as the 

 entocuneiform. There are also certain proportions in 

 the respective width or depth of the elements of the 

 carpus and tarsus which remain highly distinctive. 



Every segment of each limb develops as a unit 

 independent^ of every other segment. Allometric 

 adaptation of each segment follows the general or 

 uniform laws that are observed in other quadrupeds. 

 These laws bring about the convergence or parallelism. 



Eokzppiis B^eptodon. -LamMd/}- 



N 



-EotiioTiops 



Figure 503. — Astragalus and calcaneum of cursorial and submediportal lower Eocene Perissodact^la 



Front and distal views. A, Eohippus sp., with narrow astragalus (in distal view the astragalus barely touches the cuboid); B, 

 Heptodon calcicidus; C, Lambdotkeriiim popoagicum; D, Eotiianops gregoryi; E, Eoiitanops borealis; F, Eoiitanops sp. The 

 cuboid facet (cb) on the astragalus is better developed in the later stages, though it is still much narrower than in middle 

 Eocene titanotheres. Two-thirds natural size. 



SECTION 3. MIDDLE EOCENE GROUPS AND PHYLA 



The titanotheres of middle Eocene time may be 

 grouped as follows: 



Palaeosyops (subgraviportal and brachj'podal) . 

 Limnohyops (mediportal and mesatipodal) . 

 Manteoceras and Dolichorhinus (subgraviportal and 



brachypodal) . 

 Mesatirhinus (mediportal and mesatipodal) . 



DOUBLE PARALLELISM IN THE PALAEOSYOPINE AND 

 MANTE0CERAS-D0LICH0RHINU3 GROUPS 



The double parallelism of graviportal and medi- 

 portal proportions is so strong that it may again be 

 said that truly ancestral (paleotelic) characters prove 

 to be less conspicuous, obvious, or apparent than 

 adaptive (cenotelic) characters. We must search 

 for real subfamily relationships in inconspicuous parts 

 of the limbs, as in the form of the facets between the 



Allometric change may progress entirely independ- 

 ently of affiliation to remote ancestral stock or 

 syngenesis; descendants of primitive mesatipodal 

 forms may remain mesatipodal or may become either 

 brachypodal or dolichopodal. 



Figure 504. — Astragalocalcaneal facets in lower 

 Eocene Perissodactyla 



A, Eohippus sp.; B, Heptodon calciculus; C, Systemodon primac- 

 vus; D, Lambdoiherium popoagicum. Two-thirds natural size. 



Like brachycephaly and dolichocephaly in the skull, 

 an allometric tendency toward brachypody [or 

 dolichopody generally proceeds to an extreme, '.but 

 not invariably. 



