Mylagaulidae Aplodontidae 
Aplodontia 


Mylagaulus 
‘ 
\ 
Mesogaulus 
\ 
Mylagaulodon 
‘ 
Promylagaulus 
~~ 

Liodontia 
pecan Meniscomys 
ana 
we 
pia ore, Allomys 
or 
EohapLlomys 
Prociurus 



Ischyromyoidea 
Figure 1. Relationships of the primitive aplodontiids (after McGrew) 
DISTRIBUTION 
The known past and present distribution of the mountain beaver is 
unique in that its range is limited to a small strip of heavy rainfall 
along the Pacific Coast from British Columbia to central California 
(fig. 2). Not only is its range limited but so far as is known, every 
fossil form which might be ancestral to it also came from the same 
general area, 
Matthew (1915) stated that such distribution cannot be explained by 
the principles of mammalian dispersal, where mammalian evolution and 
migration are stimulated primarily by climatic and environmental changes. 
He wrote: 
"A species should be at first most progressive at its point of 
original dispersal, and continue this progress at that point in 
response to whatever stimulus originally caused it and spread 
out in successive waves of migration, each wave a stage higher 
than the previous one at any one time, therefore, the most ad- 
vanced stages should be nearest the center of dispersal, the 
most conservative stage farthest from it." 
