Why did the ice ages start at that time?
This is a point of much discussion, but presently many think the closing of the Panamanian Isthmus may have been the trigger. The Isthmus of Panama closed at about 4-3 Ma ago. • Changing ocean circulation: water no longer from Atlantic into Pacific at equator • More humidity (more snow) to northern high latitudes (Gulfstream) • Contact N and S America: faunas meet! example of influence of plate tectonics on life South America • Separate from other continents (early Cenozoic) • Fauna evolved independently, but migrations from Africa (monkeys, rodents); many marsupial animals (convergence in evolution) • Glyptodonts, armadillos, anteaters, marsupial horse-like litopterns, marsupial carnivores, giant carnivorous flightless birds (Phorusracids), and terrestrial crocodiles (Sebecids). (see also Gould, S. J., 1986. Play it again, life. Natural History, February 1986, p. 18-26) Great American Interchange (2-3 Ma) • From North to South America went: Bears, camels, cats, deer, dogs, elephants,
