A FOSSIL DEER FROM ARGENTINA 



209 



peratures even in tlie tropics, due to the on- 

 coming glacial period. It resulted not only 

 in a great invasion of the northern fauna 

 into South America, but also in a not incon- 

 siderable invasion of the native South Amer- 

 ican fauna into Central and North America. 

 In the final outcome the invaders from the 

 south were unable to maintain their footing 

 and mostly became extinct, while the north- 

 ern colonists in South America eventually 

 supplanted the native fauna in great part. 

 To-day the only remains of the southern 

 mammal fauna are a few edentates (sloths, 

 anteaters and armadillos), a large minority 

 of the rodents, and perhaps some or all of 

 the monkeys, bats and opossums. The rest 

 of the mammals are animals whose Tertiary 

 ancestors lived in North America but not in 

 South America. 



Among these last must certainly be listed 

 the deer. Yet the distribution of these rumi- 

 nants is a curious one, not easily explained. 

 In the northern half of North America 

 range the largest and most progressive of 

 the deer tribe, the wapiti, caribou, and 

 moose, closely related to species of the 

 northern Old World. Overlapping their 

 range and stretching to the southward as far 

 as the highlands of Central America and 

 Colombia is the Odoeoileus or Virginia deer 

 group of species, with forked antlers of 

 three or more tines, distinguished from most 

 of their Old World relatives by the absence 

 of a brow tine. The place of the brow tine 

 is partly supplied by another tine known as 

 the sub-basal snag; but its relations to the 

 beam are different from those of a true 

 brow tine. 



Throughout tropical South America and 

 Central America the only deer are the brock- 

 ets, little fellows much smaller than the Vir- 

 ginia deer and with simple spikelike horns. 

 Far to the southward we again meet with 

 larger deer with complex antlers, inhabiting 

 parts of southern Brazil and Bolivia, Chile, 

 and Argentina south to the Strait of Magel- 

 lan, These deer lack the brow tine, but un- 

 like Odoeoileus they also lack the sub-basal 

 snag. In the guemals of the mountain dis- 

 tricts the antler is simply forked; in the 

 pampas deer the hinder fork is again di- 

 vided, and in the marsh deer it is a little- 

 further complicated. 



All these southern deer look a good deal 

 like the Virginia deer group of the north, 

 and they are in fact related to them more 



nearly than to most of the Old World deer. 

 The relationship is not so close, however, as 

 it at first seems to be, and it has been found 

 necessary to separate them as distinct gen- 

 era, the guemals under the very unsuitable 

 name of Hippocamelus (for they have noth- 

 ing to do with either horse or camel), the 

 pampas and marsh deer under the name of 

 Blastocerus. These two genera might very 

 well be united into one, but we will follow 

 the customary usage. Their affinities to the 

 Virginia deer are thought to be due to com- 

 mon descent from some smaller simpler type, 

 pretty closely represented by the brockets 

 (Pudu). (The name brocket, belonging 

 originally to a young animal of the red deer, 

 is applied by naturalists to these small 

 forms which never get beyond the spike- 

 horned stage.) 



This distribution is not very easy to ac- 

 count for. We might suppose that the brock- 

 ets first penetrated into South America and 

 that in the far south they evolved into gue- 

 mals and pampas deer, while in temperate 

 North America they evolved into the true 

 Virginia deer. But the palseontologic rec- 

 ord does not seem to support this view. The 

 earliest fossil deer of the Argentine found 

 in the Monte Hermoso formation (late Plio- 

 cene) were already moderately large and 

 had complex antlers, while in the Pampean 

 (Pleistocene) they were fully as large and 

 the antlers fully as complex as any modern 

 types. Lydekker suggested that the brock- 

 ets are degenerate descendants of Pleisto- 

 cene invaders with complex antlers which 

 were the original invaders of South Amer- 

 ica; but it is difficult to see any reason for 

 such degeneration. Probably a better un- 

 derstanding of the affinities of the later Ter- 

 tiary deer of North America would afford 

 some clues, but it must be admitted that as 

 yet these are very imperfectly understood. 

 We do not know for certain whether the 

 Virginia deer group arose in North America 

 or immigrated from the Old World; it has 

 not been positively recognized in formations 

 earlier than the Pleistocene. 



Until better evidence is obtained from the 

 fossil record, the best clues to the derivation 

 of these South American deer are perhaps 

 to be found in Dr. Frank M. Chapman's re- 

 searches upon the geographic and zonal dis- 

 tribution of the fauna, especially the bird 

 fauna, of Colombia. These point apparently 

 to geographic or climatic conditions during 



