METHODS PAL.EONTOLOGICAL 33 



As the animals were thus entombed ahve, it would be ex- 

 pected that a large number of complete skeletons would be 

 preserved, but this is not the case: ''connected skeletons are 

 not common." This scattering and mingling of the bones 

 were due partly to the trampling of the heavier animals in 

 their struggles to escape, but, in more important degree, to 

 the movements within the tar and asphalt. 



In arid and semi-arid regions great quantities of sand and 

 dust are transported by the wind and deposited where the 

 winds fail, or where vegetation entangles and holds the dust. 

 Any bones, skeletons or carcasses which are lying on the 

 surface will thus be buried, and even living animals may be 

 suffocated and buried by the clouds of dust. An example of 

 such wind-made accumulations is the Sheridan formation 

 (Equus Beds, see p. 131), which covers vast areas of the Great 

 Plains from Nebraska to Mexico and contains innumerable 

 bones, especially of horses. In this formation in northwestern 

 Kansas, Professor Williston found nine skeletons of the large 

 peccary ('\Platygonus leptorhinus) , lying huddled together, with 

 their heads all pointing in the same direction, and in the upper 

 Miocene (p. 121) of South Dakota Mr. Gidley discovered six 

 skeletons of three-toed horses {'\N eohipparion whitneyi) crowded 

 together, killed and buried probably by a sandstorm. Similar 

 illustrations might be gathered from many other parts of the 

 world. 



Swamps and bogs may, especially under certain conditions, 

 become the burial places of great numbers of animals, which 

 venture into them, become buried and are unable to extricate 

 themselves. Especially is this true in times of great drought, 

 when animals are not only crazed with thirst, but very much 

 weakened as well, and so unable to climb out of the clinging 

 mud. In an oft-quoted passage, Darwin gives a vivid 

 description of the effects of a long drought in Argentina be- 

 tween the years 1827 and 1830. ''During this time so little 



D t Extinct. 



