by William Diller Matthew (1908)
This means practically continuous interchange in the late Tertiary, more interrupted in the early part. As for the route, as I recall Depéret's remarks, he spoke of that across Bering straits as the most probable, and regretted that our lack of knowledge of the Tertiary land formations of Asia prevented us from proving or disproving this route.
I admit quite freely that Depéret's conclusions follow from his data, on the face of the evidence. But when the returns are as fragmentary as in the European Eocene, I think we have a right to go behind them. His assumption is that because a [[p. 69]] group of admittedly American origin does not appear in Europe until the Middle Eocene, that there must have been a connection in the Middle Eocene. But, unless we find also groups of European (Old World) origin which first appear in America in the Middle Eocene, we should look into the matter and test the possibility of the first group having migrated from America to Europe in the Lower Eocene, but not being as yet reported from the Lower Eocene formations of Europe. The test would of course be identity of the Middle Eocene genus in Europe and America. If divergent, or merely paralleling in certain respects the evolution of the North American series as observed in Lower and Middle Eocene stages, I think its occurrence may be explained as a consequence of Lower Eocene rather than of Middle Eocene migration.
In illustration of these statements, the following example will explain my meaning: Sinopa and Tritemnodon occur in Lower and Middle Eocene in North America,--fairly distinct genera in the Bridger; almost indistinguishable in Lower Eocene. American species have peculiarly elongate premolars, throwing them a little out of the direct ancestry of later Hyænodonts. Sinopa occurs in the Middle Eocene of Switzerland. "Sinopa" ethiopica occurs in the Upper Eocene of Africa. The European and African species apparently do not have the elongate premolars; they are known only from jaws but apparently correspond with Sinopa and Tritemnodon, respectively, in stage of evolution towards Hyænodon. Cynohyænodon of European Oligocene is very little advanced over "S." ethiopica, and might be directly descended from the European or African species.
Now if we simply take the recorded occurrence of genera as a basis, we must conclude that Sinopa originated in North America in Lower Eocene, crossed over to Europe in the Middle Eocene and thence to Africa in the Upper Eocene. A more detailed study brings out the following points:
(1) The American species of Sinopa and Tritemnodon paralleled the Old World series, but developed independently from their sudden appearance in the Lower Eocene (Wasatch) to their disappearance in the Middle Eocene (Washakie). They cannot have been directly ancestral to the later Hyænodonts, but represent side branches which left no descendants.
(2) The European and African species are more nearly in the line of ancestry of the later Hyænodonts, but are too conservative to be ancestral to the large and highly specialized Hyænodon and Pterodon.
(3) The latter genera appear in the Upper Eocene and Oligocene of Europe and Africa, and Oligocene of North America, accompanying and soon displacing smaller and more [[p. 70]] conservative genera (Cynohyænodon, Quercyrtherium) in Europe, which may be of autochthonic origin.
(4) No Hyænodonts are found in the Middle Eocene of Africa.
These facts seem to indicate an outside region, probably Asia, for the center of development and dispersion of the Hyænodonts. Thence they reached North America in Lower Eocene, developed till Middle Eocene and became extinct. They reached Europe in Middle or2 Lower Eocene and Africa in Upper Eocene. In Upper Eocene higher stages in the evolution of the race (Pterodon, Hyænodon) invade Europe and Africa and spread to America in Lower Oligocene. The distribution of this family is more or less completely paralleled by several other faunal groups.
Inference from the above as to early Tertiary continental connections: [ || = separation; <==> = union permitting intermigration; ? = doubtful.]
Middle Oligocene, North America || Asia <==> Europe ? Africa
This is of course only a working hypothesis which, in my opinion, accords best with the data as far as I know them. In brief, it is that Asia is and has been the great center of evolution and dispersion of the dominant mammalian types; in the other continents, the course of evolution has been--aside from a few well-known exceptions--alternately an autochthonic faunal development and a series of waves of migration from the highly progressive faunas of the great Asiatic land mass, according as the continents were separated from or connected with it. The principal exceptions are the Proboscidea, of African origin, the true Edentates of South American development and doubtful origin, the Camels, of North American origin,--probably other groups, if we knew something about the fauna of the early Tertiary of Asia.
1. This important statement is in response
to a letter asking for further information in regard to the view expressed
by Depéret, at the recent Seventh International Zoological Congress,
that there was constant intermigration of mammals between Europe and America.
The same authority further believes that the great similarities between
the faunæ of Europe and America can not be explained on the basis
of considerable parallel development. --C. Schuchert.