CAUSES OF EXTINCTION 



The causes which produced the extinction of the Merycoidodontidx, so widespread, diversified, 

 and large a family of American Tertiary mammals, are not clearly apparent. In the main, extinction 

 probably was the result of physical and biotic, i.e., environmental changes whereby their particular 

 mechanism and manner of life became inferior, absolutely or relatively, to those of competing forms; 

 and secondarily, it was brought about by their failure to develop a successful adaptability to chang- 

 ing conditions. There may have been, however, certain inherent (inadaptive) factors which con- 

 tributed to the final extinction of the merycoidodonts. 



The elimination of genera was gradual, not cataclysmic, and appears to have been a process of 

 slowly decreasing numbers of individuals and of gradual extinction in local areas and in small herds 

 or groups. The cumulative effect of this numerical reduction finally resulted in complete extinction. 



We know the geologic conditions prevailing when these animals lived, but about the animals 

 themselves we know nothing other than what the skeletons tell us. For example, in the absence of 

 the soft parts, we do not know whether or not they were cudchewers (ruminants), although other 

 evidence indicates that they were not true ruminants. Whether or not they roamed the plains in 

 great herds is a mooted question. It is more probable that they lived in small groups. 



Reduction in numbers of a group, or even extinction, may conceivably be the result of a single 

 factor, but usually a combination of causes of varying potency is responsible for the final result. 

 These factors dovetail, and the interrelationship and interdependency of an environmental and 

 internal complex must be carefully analyzed. 



Since the various phases of environment seem to have been of paramount importance in the 

 elimination of the merycoidodonts, we will consider these first. In the physical environment are the 

 geologic, geographic, and climatic factors, some of which are changes in land areas, land bridges, 

 heat, cold, and moisture, all of which may not be directly potent factors in extermination but may be 

 so indirectly, through the biotic environment, i.e., the changes in insect, plant, and animal life which 

 they induce. In other words, the raising or lowering of the land masses, as well as their expansion 

 and contraction, produced heat or cold, moisture or aridity, with consequent changes in vegetation 

 and water supply, which in turn directly affected the animals. As an example, the progressive secular 

 aridity in the later Tertiary produced a condition fatal to most of the browsing animals. Often- 

 times these conditions are localized rather than extended simultaneously over a whole continent. 



In part because of the elevation of the land in the Miocene and Pliocene, a progressively cooler 

 climate resulted, with concomitant phenomena, such as deforestation, overcrowding, enforced migra- 

 tions, changes of food, and changes in fertility and rate of reproduction. Greater aridity prevailed, 

 with a reduction of the amount of water in the streams, and perhaps the stream courses themselves 

 were restricted in that the water did not flow so far from its source. These conditions naturally 

 produced a change in the amount of forest cover, increased the areas of meadow and savannah, and 

 altered the kind of vegetation. The progressively cooler climate and the increasing aridity favored 

 the development of harsher, more resistant vegetation, such as the grasses. They also caused deforesta- 

 tion, removed forest barriers, and admitted new faunal competitors. 



With progressively increasing length and severity of the dry seasons, beginning in the late 

 Oligocene, it became necessary for many animals to travel widely in search of food and water. This 

 tended to eliminate those individuals which for any reason could not cover long distances, while 

 others were obliged to depend on a limited water supply. This condition favored the cursorial 

 quadrupeds that had long-crowned teeth for grazing, while it was distinctly inimical to the more 

 slowly moving, brachyodont quadrupeds that were browsing and forest-dwelling. 



Any change of diet, brought about by abnormal life conditions, may have caused some of the 

 animals through hunger to eat poisonous plants in their attempt to adapt themselves, and a natural 

 consequence would have been a diminution in numbers of individuals in a group, without extinction. 



