It was along this land bridge which the ancestors of the Notungulata traveled, and when in South America, due to their isolation, developed all the peculiarities of the group. This must have been not later than the latter part of the Cretaceous.
Either this bridge remained until into the early Tertiary; so the Pyrotheria and Hystricomorpha made their migration later, or these two groups did not reach the isolated Patagonian section until later than the first invasion. I am inclined to believe in the migration being at a later period. This bridge does not explain the presence of the edentates, for which there is every reason to believe that they developed in situ. The Marsupial invasion must have been from some other direction, or their presence in Africa has not yet been discovered.
CHAPTER IV
Ungulata
The systematic arrangement of the South American ungulates is of such a nature that scarcely two students of these forms have agreed. I feel that the Pyrotheridae are proboscideans as did Ameghino, but there my agreement ends. The other varied groups I believe have a common ancestry, their great divergencies being due to adaptations to the greatly varied characters of the country they occupied. In spite of the great variation they have certain features in common so that I agree with those who have developed the term Notungulata to include them all.
From what source they originally came is not clear, but it seems to me that these notungulates have more in common with what we know of the African fauna of the Fayum than with any other fauna; so that my feeling would be that these two faunas had a common ancestry at least, and possibly the South American ungulates are derived from the African. The lophiodont upper dentition, the bicrescentric lower molars with a “pillar” in the posterior crescent, the development of the tympanic bulla with the extension of the inflated cavity up into the squamosal bone, the development of the post-tympanic portion of the squamosum, and the general arrangement of the basicranial foramena indicate in my mind that these notungulates have all risen from the same stock, and that that stock had much in common with the hyracoids.
I should therefore arrange the various groups as follows.[11]
NOTUNGULATA
Order I. Upper molars composed of an external longitudinal crest and two transverse crests, the posterior the less developed; lower molars composed of two joining crescents with a “pillar” in the posterior crescent; structure of the feet and limbs varying.