by George Gaylord Simpson (1946)
DOCTOR GEORGE GAYLORD SIMPSON, Chairman, Department of Geology and Paleontology, The American Museum of Natural History; and Professor of Vertebrate Paleontology, Columbia University, New York, N. Y.: Tertiary Land Bridges. (This lecture was illustrated by lantern slides.)
An early discovery of paleontology was that animals now confined to one part of the globe may occur as fossils in different and distant regions. In some cases, such as that of the elephants and their allies, it would now be impossible for the animals to migrate between the regions where they currently occur, and regions where they were formerly abundant. It was necessary to conclude that regions now separated by the oceans have been connected by land, at various times in the geological past. It has also been found that continents now connected by land must have been separated during some geological epochs.
The problems involved in these varying land connections and separations are among the borderland studies that overlap two, or more, scientific disciplines. Solutions of these problems are essential to both geologists and zoologists, and such solutions can only be reached by combined study, from both the geological and the zoological point of view. Some zoologists have concluded that the recognition of related, or supposedly related, animals in two widely separated regions justifies the postulation of a direct land route between those regions, even though this may be in reckless violation of geological principles and [[p. 256]] probabilities. Some geologists, particularly a few of the advocates of continental drift, have postulated continental connections of a sort and at times and places indicated as practically impossible by the zoological data.
There have also been numerous sober and careful studies of intercontinental connections, or their absence, but even these have necessarily been incomplete and inconclusive. My efforts to evaluate these studies, and to contribute to making them more precise, have been based primarily on the re-examination of faunal relationships that are known, beyond reasonable doubt, to have involved the rise and fall of intercontinental land bridges. The simplest and clearest case available is the relationship between North and South American land faunas during the Cenozoic. This has been analyzed elsewhere, in some detail.1 Among other things, there emerge, from this and similar studies, certain faunal criteria for the existence, position, and nature of land bridges, especially that postulation of a given bridge requires evidence of:
1. Exchange of varied types of land life, not only of one kind of animal.
2. Faunal interchange in both directions.
3. Interchange involving or maintaining ecologically balanced faunas.
When these conditions are not met by a postulated land bridge, the alternatives to be considered are:
1. Indirect migration, e.g., via the well-established North American-South American and North American-Eurasian bridges, rather than by a direct trans-Pacific Asian- (or Australian-) South American bridge.
2. Migration without a continuous bridge, by what I have called a sweepstakes route, e.g., between Eurasia and Australia, or the American continents and the West Indies.
More recently, I have been working on the application and extension of these and other principles, with regard to Eurasian-North American Cenozoic faunal and continental relationships, a much more [[p. 257]] complex problem than that of South American-North American relationships. There is a very large literature on this subject, but the best general reviews of it are now far out of date,2 and much of the literature is fragmentary, excessively subjective, or otherwise inadequate. There are two main lines of evidence, which are frequently confused: first, earliest dates of common appearance of given groups on both continents, which are limiting dates for the migrations of these groups; and, second, fluctuations in faunal resemblance of the continents. In each case, not only the overall statistical picture, involving numbers of migrant groups and degrees of resemblance, but also the nature of the animals concerned, especially their ecological and climatic preferences, are pertinent.
An attempt has been made to place this study on a more objective and quantitative basis than heretofore. There are various difficulties and shortcomings in the quantitative approach. These have to be studied carefully, and applied with due caution, but preliminary results are encouraging. The work is still under way, and it would be premature, now, to give any detailed conclusions. Some of the preliminary results, however, seem likely to stand up.
As regards frequency of migration between Eurasia and North America, preliminary study indicates an early Eocene climax, a mid-Eocene low, a dual late Eocene-early Oligocene climax, a middle-to-late Oligocene low, slow resumption in the early Miocene, building steadily to a dual late Miocene-early Pliocene climax, another dual late Pliocene-Pleistocene climax, and then a drop to the Recent low. Migration is only one of at least five separable factors involved in faunal resemblance. Faunal resemblance does not vary directly with the frequency of intermigration, and its measurement and analysis are particularly complex and difficult, although reasonably good results have been obtained and better are in sight.
The presence, or absence, of a land bridge and, when present, its position, do not follow as directly from the evidence as to frequency of migration and degree of faunal resemblance as has commonly been supposed. As far as this new analysis of the faunal data has yet been followed, it suggests that there was a Eurasian-North American land bridge almost continuously throughout the Cenozoic, with probable [[p. 258]] interruptions of appreciable duration only in the middle Eocene, middle to upper Oligocene, and Recent. As regards the position of the bridge, or bridges, it is a surprising result of this study that no good evidence is found for any Atlantic, direct European-North American, bridge during the Cenozoic, contrary to my own former opinion and that of most paleogeographers. The evidence throughout is consistent with a single Siberian-Alaskan bridge, as adequate to explain all the known facts.
1. See, especially, Simpson, G. G., Mammals and land bridges. J. Washington Acad. Sci. 30: 137-163. 1940; and Simpson, G. G., Turtles and Origin of the fauna of Latin America. Am. J. Sci. 241: 413-429. 1943. [[on p. 256]]