It has long been recognized that the Edentata occupy a very isolated position among the placental mammals; their relationships to other orders and their point of departure from the main stem are unsolved problems. The South American fossils have so far thrown little light into these dark places, but they bear very cogent witness to the unity of origin of the five suborders, which were most probably all derived from a single early Eocene or Paleocene group.
In the Paleocene and through most of the Eocene of North America there lived an order of mammals called the †Tæniodontia (or †Ganodonta) which many of the foremost palæontologists regard as an ancestral type of the Edentata, and Dr. Schlosser actually includes them in that order. That the †tæniodonts had certain striking resemblances to the edentates, especially to the †ground-sloths, is not to be denied, but the interpretation of these resemblances is a very complex and difficult question. Unfortunately, no member of the order is known from an even approximately complete skeleton, and therefore a discussion of the matter here would be unprofitable. My own conclusion, however, may be stated, to the effect that the supposed relationship of the †tæniodonts to the edentates is illusory and not real. Definite decision must await the finding of more complete material both of the †tæniodonts and the most ancient South American edentates.
CHAPTER XVII
HISTORY OF THE MARSUPIALIA
The marsupials are a group of more primitive structure and greater antiquity than any which we have yet considered, so primitive, indeed, that they are referred to a separate infraclass, the Didelphia or Metatheria. The order is one of very great variety in size, form, appearance, diet and habits, and mimics several of the higher orders in quite remarkable fashion. Herbivorous, insectivorous and carnivorous forms are all numerous, as well as arboreal, terrestrial, cursorial, leaping and burrowing genera. Some are like hoofed mammals in appearance and the Rodentia, Carnivora and Insectivora are also closely imitated in externals. With all this diversity, most unusual within the limits of a single order, there is such a unity of structure, that a division of the group into two or more orders is impracticable.
At the present time, marsupials are very largely confined to Australia and adjoining islands, where they constitute nearly the whole mammalian fauna, and it is in the Australian region that the remarkable diversity already mentioned is to be observed. There are found the phalangers, kangaroos, bandicoots, Tasmanian “devil” and “wolf,” and banded anteaters, not to mention many other curious creatures. In the western hemisphere only the opossums (Didelphis, Chironectes) and one very interesting relic of a long vanished assemblage, Cænolestes of Ecuador and Colombia, are in existence to-day. The opossums, of which some twenty-three species are recognized, have their headquarters in South America, to which nearly all of the species are confined, North America having but two or three.
The more important American marsupials are given in the table below:
Suborder POLYPROTODONTA
I. Didelphiidæ. Opossums.