As I see this fauna it is composed of several distinct elements, representing different invasions and an element which arose in situ. The reasons for the affinities expressed in the different groups will be found in the introductory paragraphs of the systematic discussion of each group.
The Notungulata, including the Typotheria, the Toxodontia, the Litopterna, the Homalodontotheria, and the Astrapotheria are a group with apparently a common ancestry. In Patagonia they have specialized into the various subdivisions as we find them in the Deseado. This group was in Patagonia as early or earlier than the Casamayor. Their relationships appear to me to be with the Hyracoidea which are generally credited with originating in Africa.
The Pyrotheria are related to the early elephants which also arose in Africa, but it seems to me that this form came to Patagonia at least at a later period, making its first appearance in the upper part of the Astraponotus period. Ultimately the elephants and Hyracoidea had a common origin in Africa.
The Rodentia are all hystricomorphs and appear in South America for the first time in the Deseado. They also occur in the Oligocene of Europe and the Fayum of north Africa. They never reached North America so must have come to South America by some southern route.
The Edentata are an element of the Casamayor fauna and as there is no evidence of their originating anywhere else it would seem that they were indigenous to South America, where they later flourished and developed the greatest variety and profusion of numbers.
The group of marsupials is an element the origin of which presents a most difficult problem. Some belong to the opossum series which could well have been developed from some remnant of the Mesozoic marsupial fauna that had a world wide distribution; but the presence of diprotodonts, which are characteristic of Australia, and of the Borhyaenidae which are closely related to the Thylacinidae of Australia, suggests a migration from that continent as late as Tertiary times; but to my mind this involves a connection which is most too difficult to postulate. There is no evidence that they came to South America in company with other faunas, for they have not been found associated with any other fauna outside of Southern Patagonia. The explanation of the affinities of the Patagonian marsupials with the Australian marsupials is a problem which is not yet in position to be settled.
The birds probably came from Africa with the invasion of the ancestors of the Notungulates.
The idea of an invasion from Africa in Upper Cretaceous times, and possibly another at a later time is correlated with the other evidence of a land bridge between these two continents, as deduced by students of other groups.
- Eigenmann, working on the freshwater fishes,[6]
- Lydekker, studying the hystricomorphs,[7]
- Von Ihering, studying the freshwater mussels,[8]
- Ortmann, studying the freshwater crabs,[9]
not to mention several others studying mullocks, insects, plants, etc., have all postulated a land connection from Brazil to northern Africa during Cretaceous time to explain the distribution of their various groups. The divergence is in the time when this land bridge sank, some believing it to have lasted into Tertiary times, most feeling that it sank in Upper Cretaceous times. Another body of evidence is presented to show that a land bridge connected the West Indies with the Mediterranean regions.[10] There was presumably but one such transatlantic connection. Its position further to the south would seem to me to explain the distributional facts found in the West Indies, but the striking resemblances between the faunas of Africa and South America require a connection from the South Upper American Continent and Africa.