584 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



1. Primitive ambulatory (plantigrade), such as 

 Pantolamhda (amblypod) ; also the opposite extreme. 



2. Cursorial unguligrade, with swift type of limb 

 and secondarily elongated feet, such as NeoJiipparion, 

 Equus (horse), Gazella (gazelle), all light, long-footed 



Figure 482. — Evolution of the skeleton in titanotheres 



A, First stage (subcursorial): EotitaTiops borealis^ lower Eocene. B, Middle stage 



(mediportal): Palaeosyops Uidyi; middle Eocene. C, Final stage (graviportal); 



iJroniops ro6«siw5; lower Oligocene. Scales approximate; A too large. CSeefig.483. 



types, in which the limbs terminate in single hoofs and 

 the pads beneath the phalanges are reduced or wanting. 

 3. Rectigrade-graviportal, straight-limbed, weight- 

 bearing type, with special pillar-like or vertical 

 weight-bearing disposition of the limbs and short 

 gravigrade feet, such as ElejpJias, Mastodon (elephants) ; 



also such clumsy, slow-moving forms as Coryphodon 

 and Uintatherium (amblypods), with gigantic or heavy 

 bodies supported on straight or column-like limbs in 

 which the terminal phalanges and hoofs are reduced 

 and the limb is supported on a heavy pad. 



In general, the stages 

 represented by these 

 types indicate that two 

 main divergent direc- 

 tions were taken in 

 limb adaptation — 

 namely, into cursorial 

 or speed types or into 

 graviportal or weight 

 types. Bridging over these extremes are types 

 that combine speed and weight. 



In the titanotheres there is an evolution of 

 constantly changing proportion both in the mass 

 or weight of the muscles and bones and in the 

 length of the limb segments, adapted to con- 

 stantly changing habit of speed and weight, 

 passing through the subcursorial, mediportal, 

 subgraviportal, and graviportal types and ap- 

 proaching a rectigrade-graviportal type in the 

 huid limbs. Thus, while the bones of the head 

 are constantly changing, every muscle and bone in 

 the skeleton is also constantly changing. 



SIZE AND PROPORTIONS OF EOCENE TITANOTHERES 



As shown in Figures 483 and 661, the members of 

 the six or seven phyla of middle Eocene titanotheres 

 differ much less in the height of the shoulder and of 

 the hip than in weight and speed. 



The height of a quadruped depends upon the total 

 combined length of the segments — the scapula, hu- 

 merus, radius, manus — modified, by the flexure or 

 angidation at the four joints — shoulder, elbow, wrist, 

 and phalangeal joints. 



Quadrupeds differ widely in respect to angulation : 

 heavy animals have straighter limbs than light, 

 swift-moving animals. The normal angles at the 

 shoulder, the elbow, and the phalangeal joints may 

 be determined precisely by a study of the planes of 

 the articular facets (figs. 518, 664, 666, 667). 



Naturally the best method of ascertaining the height 

 of the animal is to make a restoration (Palaeosyops, 

 fig. 536), if a sufficient number of parts are available, 

 laying out the limbs in their proper angulation, as 

 indicated by the planes of the articular facets and 

 measuring the net height directly. 



It is seldom that the material is so complete that 

 all the limb segments and articular facets can be 

 measured and determined for the purpose of estimat- 

 ing height, so that we may be forced to make estimates 

 based on the ratios of the length of the parts preserved 

 to those of similar animals of loiown height. 



