Late 20 th Century Tests of the Continental Drift Hypothesis

Transcription

1 Late 20 th Century Tests of the Continental Drift Hypothesis 7 Biogeography Revisited Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 What to look for: The existence of paleobiogeographic provinces that make no sense with modern barriers to migration was one of Wegner s key lines of evidence for drift. Once paleomagnetism ( polar wandering ) made it possible to reconstruct past geography, a couple more ways to use fossils to test the predictions of drift arose. One was related to the time of separation of two originally contiguous, now rifted, continents. As time since rifting passes, the biotic similarity decreases in a linear fashion. The other was related to biodiversity. Uniformitarianism leads us to expect diversity to be highest at the equator and to decline poleward. Fossil diversity usually does not do this when plotted on a modern map, but does do so on a paleomagnetically generated paleogeographic map.

3 Simpson s Coefficient of Similarity (number of shared species)/(total number in smaller sample) Early Jurassic Middle Jurassic Late Jurassic or Earliest Cretaceous later Early Cretaceous Late Cretaceous Paleogene Neogene Prior to the Middle Jurassic the percentage of shared mollusk species in the faunas of Europe/Africa and North America was around 85%. However, beginning in the Middle Jurassic this proportion gradually and steadily declined to its present value of less than 60%. Let s propose the hypothesis that the Middle Jurassic saw the rifting of North America away from Pangaea. Presumably then different evolutionary histories of the (now) two separate biogeographic provinces should make them progressively more distinct Width of Atlantic (km) (Proxy for Time) We will test this hypothesis (about timing) shortly. (Modified from Wally Fallaw)

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5 MODERN CORAL DIVERSITY Tropic of Cancer Equator 10 species or more Maximum diversity Tropic of Capricorn 5 species or fewer Looking at a map like this might lead you to suspect that corals are tropical animals!

6 The numbers on each continent reflect the number of known coral genera from Devonian rocks. Notice that very few are found in the tropics -- far and away most are extra-tropical. Uniformitarianism should lead us to expect the opposite!

7 This map represents Devonian continental positions as determined from the horizontal and vertical components of their remnant magnetism. Does this map make better sense than a modern one, in light of the principle of uniformitarianism?

8 Take-home message The existence of paleobiogeographic provinces that make no sense with modern barriers to migration was one of Wegner s key lines of evidence for drift. Once paleomagnetism ( polar wandering ) made it possible to reconstruct past geography, a couple more ways to use fossils to test the predictions of drift arose. One was related to the time of separation of two originally contiguous, now rifted, continents. As time since rifting passes, the biotic similarity decreases in a linear fashion. The other was related to biodiversity. Uniformitarianism leads us to expect diversity to be highest at the equator and to decline poleward. Fossil diversity usually does not do this when plotted on a modern map, but does do so on a paleomagnetically generated paleogeographic map.