274 THE MERYCOIDODONTID^E 



been two- fold: first, large numbers of animals perished soon after each storm because their lungs 

 became saturated with dust; second, all of the grassy areas were obliterated and the remaining 

 animals survived for a time on weeds, chief of which was the Russian thistle. Those domestic animals 

 which survived through artificial feeding were shipped out of the area as soon as possible, while the 

 hardier members of the wild fauna migrated and the majority of the smaller and weaker members 

 perished. 



During the Oligocene and Miocene loess was formed and dust storms, comparable to those of 

 modern times, must have occurred, with the resultant loss of life, both animal and vegetal. 



Quicksands, although very rarely found in such deposits, may have trapped some of the 

 younger and older individuals, as a result of lack of experience or of strength to extricate themselves, 

 or because the older, heavier animals broke through and became inextricably mired. 



Volcanic activity was a fairly common phenomenon throughout Oligocene-Miocene time in 

 western North America. Volcanic ash is present in nearly all of the strata of these two periods, and 

 in some formations it is the dominant constituent. In fact, Matthew expressed the belief that volcanic 

 ash is "almost the principal source of terrestrial sediments in the Tertiary formations of the West." 



As a result of his studies on the 1912 Mount Katmai eruption, one of about a dozen really great 

 eruptions of historic times, Griggs estimates that 6% cubic miles of material were ejected in a week. 

 In contrast, the White Ash layer in the Great Plains upper Oligocene represents, according to 

 Wanless (1923), an ash fall of more than 100 cubic miles, or 16 times the amount from Mount 

 Katmai. We must bear in mind, moreover, that the White Ash layer is but one of many formations 

 bearing large amounts of volcanic debris. The John Day formation is another example, several 

 thousands of feet thick and composed almost wholly of ash or volcanic conglomerates. 



What was the effect of these great ash falls on the plants and animals? Primarily it seems 

 reasonable to suppose that they would bury and destroy much vegetation over considerable areas, 

 as in fact they actually did in the John Day Basin. This would set up other complexes of conditions, 

 such as restricted areas, overcrowding, and migrations, with their resultant effects. Again turning 

 for analogy to the Katmai area, we find that the ash blanket killed all the weaker plants, and the 

 territory was desert-like beyond the forested area for more than a year after the eruption. It is true 

 that the vegetative cover returns all the more luxuriant, for the ash has minerals needed as plant 

 food and is also porous and therefore holds water. The animals, however, need food during the 

 year or two elapsing before the vegetation recovers. Of the large numbers of moose and caribou in 

 the Katmai region before the eruption, Griggs says: ". . . After the eruption these herbivorous 

 animals were very seriously affected by the coating of sharp abrasive dust on the vegetation, which 

 quickly wore out their teeth. Large numbers of them are said to have perished through inability to 

 feed properly." Nearly all the animals migrated or perished, and the fauna returned only after the 

 vegetative cover had been restored. 



The early and middle merycoidodont genera had short-crowned teeth which would wear away 

 rapidly in the presence of such abrasive dust. It is quite possible that the dust and sulphur fumes 

 may have choked to death some of these shorter-legged forms. There have been ash falls several 

 feet deep within historic times, and there is evidence in Patagonia that many animals were over- 

 whelmed and buried in volcanic dust. The four specimens of Promerycochosrus, found huddled 

 together by Peterson some years ago in Nebraska, seem to have met death in this manner. Another 

 point to be taken into consideration is that these animals were all of short stature and that 

 perhaps many, in a region of so much vulcanism, might have been overcome in crossing gas-filled 

 depressions. 



I do not mean to imply that vulcanism caused any spectacular overwhelming of whole races of 

 animals by ash falls or by gases, but I believe that these phenomena could have produced, and prob- 

 ably did produce in local areas, a numerical diminution of individuals or possibly extinction of small 

 herds, if the opportunity and means to migrate were denied them. However, nearly all of the 

 fossils in the ash beds are not in pure ash but in reworked volcanic debris. 



There are certain internal factors which perhaps operated in causing the extinction of some of 

 the genera but which are not applicable to the race as a whole. For example, the teeth are short- 



