EVOLUTLON 
Little is known of the affinities of the superfamily Aplodontoidea 
and its monotypic representative, Aplodontia rufa, among the Rodentia. 
Taylor (1918) wrote that the family Aplodontidae is exclusively North 
American in origin, development and present distribution. The mountain 
beaver is the only living member of the family and is generally con- 
sidered to be the most primitive of living rodents. 

Wilson (1949) pointed out that the known geologic history of the 
Aplodontidae is from the Upper Eocene to Recent. Taylor (1918) remarked 
that the former range of the Aplodontidae was greater than that at 
present, but conceded that little is known of the relationships of the 
early aplodontiids to the present species. He stated that its ancestry 
dates to the earliest aplodontiid genus Allomys which was discovered in 
the Oligocene deposits of the Middle John Day Region in Oregon, Matthew 
(1904) recovered an undetermined species of Allomys from the Lower 
Miocene of South Dakota. 
From mandibular and dental characters, Schlosser (1924) described 
Aplodontia asiatica which he believed to be an authentic record of a 
Tertiary aplodontiid from Mongolia. However, Miller (1927) doubted the 
affinity of Schlosser's Mongolian species to the aplodontiid stock. He 
compared characters of the ramus in Liodontia and Aplodontia, and found 
that the animal represented a member of the dipodine group. Miller 
ascribed Schlosser's animal to the genus Pseudoplodon. Gazin (1932) 
compared specimens of Liodontia with Schlosser's species and suggested 
that the latter had separated from the aplodontiid stock at a time when 
development had reached a stage between Meniscomys and Liodontia (fig.1). 
McGrew (1941) pointed out that there were no characters separating 
Aplodontidae from Ischyromyoidae. He recognized two phylogenetic lines 
from Eohaplomys to Aplodontia and Mylagaulus. McGrew stated that the 
direct line from Eohaplomys to Aplodontia represented the basic stock 
and was of family rank, whereas the offshoot from Promylagaulus to 
Mylagaulus represented extreme divergence and adaptive specialization 
and should be ranked in the family Mylagaulidae. 
Figure 1 illustrates McGrew's systematic arrangement of the 
primitive aplodontiid stock, 
