CHAPTER’ XV 
MARSUPIALIA 
IN ouR collection, the marsupials are represented, un- 
fortunately, by but a few specimens; though this Deseado 
fauna included, as is shown by the fragmentary remains, 
a wide range of forms from Pilchenia, the size of a mouse, 
up to the bear-sized Proborhyaena. ‘The small forms were 
probably insectivorous, while the larger forms took the 
place of the carnivors, the absence of true Carnivora being 
one of the striking features of the fauna of South America 
during earlier Tertiary times. 
The treatment of these forms has been as varied as their 
sizes. Ameghino, with his idea that the Casamayor and 
Deseado beds were Cretaceous in age, groups the larger 
forms as a suborder, Sparassodonta, and considers them 
ancestral to the Creodonta; while the small forms make up 
his Sarcobora which he considered ancestral on one side to 
the rodents, on the other to the diprotodont marsupials. 
Sinclair, after showing the marked similarity of the Spar- 
assodonta to the polyprotodont marsupials, especially the 
genus Thylacynus, abandons that term and puts them in 
the family Thylacynidae along with the Australian forms; 
the Microbiotheridae he finds similar to opossums and 
puts in the family Didelphidae; while the remaining small 
diprotodont forms he associates with Caenolestes, and using 
Ameghino’s families as subfamilies makes three divisions 
of the family, Palaeothentinae, Garzoninae, and A bderitinae. 
Matthew finds the sparassodonts to be true marsupials, 
and without phylogenetic relationship with the creodonts. 
Gregory diagrams the sparassodonts as coming from gener- 
alized didelphids and derives them from the same line as 
the Australian polyprotodonts; while the small caenoles- 
