Orphodon Ameghino

The teeth of this type are a mass of dentine, each surrounded by a thin layer of cement, and each tooth subcylindrical in section, with the crown worn to two apposed oblique planes. The genus resembles Ortotherium.

Fig. 137. Type—natural size,
after Ameghino.

Orphodon hapaloides Ameghino

In addition to the above, Ameghino gives a figure, here reproduced, and the following measurements: Tooth, greatest diam. 12 mm., lesser diam., 10 mm.

CHAPTER XV
Marsupialia

In our collection, the marsupials are represented, unfortunately, 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 carnivores, 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 Sparassodonta 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 Abderitinae. Matthew finds the sparassodonts to be true marsupials, and without phylogenetic relationship with the creodonts. Gregory diagrams the sparassodonts as coming from generalized didelphids and derives them from the same line as the Australian polyprotodonts; while the small caenolestoids represent a line of descent from some still earlier generalized polyprotodonts and a separate stem from the Australian diprotodonts.

Sinclair has had the most complete material on which to work, and with his general grouping I have come to agree. This recognizes three divisions of South American Marsupials, the Didelphidae, representatives of which have not yet been found in the Deseado, though occurring in both the earlier and later formations; the Caenolestidae represented today by Caenolestes, the only survivor of the South American diprotodonts; and the Borhyaenidae (= Thylacynidae of Sinclair this name having been used to indicate a much nearer relationship to the Australian Thylacynus than I feel is warranted), which includes a large range of medium to large sized animals ranging from the Casamayor formation throughout the Santa Cruz beds.

The locality from which these marsupials emigrated to South America and the time of their arrival is not yet agreed upon, and can not be settled until much more complete material is discovered in the Casamayor formation. I feel, however, that the three groups were separate when they entered South America.