Continental Drift Biology 4974/5974 Evolution Figs.: Hall and Hallgrimsson 20014, Strickberger’s Evolution

Continental drift explains a number of challenging anomalies in plant and animal distribution. Coupled with evolution, the explanatory power is tremendous. Reading: Chapt. 24 and Chapt. 5

Learning goals Know and understand The basis for identifying biogeographical regions. How acceptance of continental drift illustrates the scientific method. The supporting evidence for continental drift. The plate movements and boundary types. The formation and timing of Rodinia, Laurasia, Gondwana, Pangaea. The processes of vicariance and dispersal and how they modify endemism.

Biogeographical regions and endemism Biogeographers divide the world into biogeographical regions, based on unique associations of plants and animals or “endemism.” Nearctic Palearctic Neotropical Ethiopian Oriental Australasian Antarctic E.C. Pielou.1979. Biogeography. John Wiley & Sons, Fig. 1.1.

Biogeographical regions and endemism These regions are identified based on the concept of endemism: a species or taxon found in one region and no other is considered endemic to that region. The explanation for endemism, and biogeographic regions, is continental drift and isolation. Examples: Order Primata is found in the Neotropical, Oriental, and Ethiopian regions with endemic families in each. Family Antilocapridae of the Order Artiodactyla is only found in the Nearctic. Order Monotremata is only found in the Australasian region.

Brief history In 1912, Alfred Wegener (1880-1930), German meteorologist, proposed that continents have changed position over time. Provided evidence for supercontinent Pangaea in the Paleozoic. Scientists were skeptical. Finally accepted in the 1960s. Modern theory of plate tectonics developed. Cumulative evidence: Fits for shorelines of continents. Similar geological formations and fossils in different parts of the world. Magnetic orientation of rock formations match. Young age of sea floor indicates recent origin. Now sophisticated GPS data show movement.

Fig. 5.5

Plate movements (p.92) Movements are driven by spreading sea floor ridge (where magma upwells. Plates move apart from the ridge. Plates slide past one another along a common boundary or fault, resulting in earthquakes. A plate may rotate. One plate moves into another (convergent boundary): the forward edge sinks into the mantle below the other plate. The descending plate produces mountain uplift. These motions produce three kinds of plate boundaries: Spreading ridge Transform fault Subduction zone C.J. Avers 1989. Process and Pattern in Evolution, Oxford University Press, Fig. 10.13

Pre-Cambrian: The first super-continent: Rodinia Continental drift began ~4.5 billion years ago. Many, small plates. Plates consolidated beginning 2.5 billion years ago. The supercontinent of Rodinia formed about 1 billion years ago. Rodinia began breaking up by 750 million years ago. T.T. Torsvik. 2003. Science 300:1379-1381.

Paleozoic era: Formation of Pangaea The dispersed plates came together again during the Paleozoic. Formed two large continents, Laurasia (No. America and Eurasia) and Gondwana (India, So. America, Africa, Antarctica, Australia), 375 mya. Formed supercontinent of Pangaea by the end of the Paleozoic, about 250 mya. T.T. Torsvik. 2003. Science 300:1379-1381.

Mesozoic era Pangaea gradually broke up during the Mesozoic. Continents gradually assumed their current position in the Cenozoic. Fig. 5.7

Fossil reconstructions as evidence: e.g. Gondwana Fig. 5.7 Seed fern Glossopteris: Gondwana distribution, late Paleozoic. Freshwater reptile Mesosaurus & mammal-like reptile Cynognathus So. America/Africa origin, late Paleozoic/early Mesozoic. Mammal-like reptile Lystrosaurus: India, Antarctica, So. Africa, and China, early Mesozoic. How is China explained? (India plate carried a piece which become China beyond the Himalayas)

Vicariance and dispersal Patterns of endemism are altered by dispersal and vicariance because of continental drift. New contacts between continents, and species disperse both ways. Example: No. and So. America were connected by Isthmus of Panama in the Pliocene: 15 families of mammals migrated from N to S and 7 families migrated from S to N. Continents separate, vicariance splits a species, resulting in separate evolutionary histories. Example: Australian marsupials vs. So. American marsupials. After Gondwana broke up, South America had early Eutheria, which outcompeted marsupials. Australia had separated already with only marsupials.

Fig. 5.9

Study questions How are biogeographical regions identified? How do different continents come to share related species? Alfred Wegener’s idea that continents drift did not gain acceptance until the 1960s. How did the process of acceptance illustrate the scientific method? What makes them move? What are the different movements they may experience? Define: Rodinia, Pangaea, Laurasia, Gondwana In which era did the continents start to assume their current position? What processes alter patterns of species distributions with respect to continental drift? (Hint: Explain what happens when new continents form and old ones break up.)