Excerpt from the Crystal Desert: Memories of Gondwana

I had the pleasure of spending time with David G Campbell during a cruise to the Antarctic. I love his book and will be posting excerpts from it from time to time.

The book is called “The Crystal Desert:  Summers in Antarctica” and is published by the Houghton Mifflin Company.

“A superior personal account of place, a remarkable evocation of a land at the bottom of the world (Boston Globe)

Memories of Gondwana

The south polar region of Earth was just as dark in the winter during the Jurassic (200 million years ago to about 146 million years ago) as it is now, but there was no continent, just ocean. That was soon (at least as measured in geological time) to change. Staring about 150 million years ago, tectonic forces began to tear Gondwana (the more southerly of two original super continents) asunder. The breakup was slow, only a few centimeters per year, but inexorable. Africa separated first, setting off on a path which nudged it against Eurasia, where plants and animals from the north invaded repeatedly. (Madagascar, which separated from Africa about 100 million years ago, was spared these invasions and as a result still supports a unique and relict Gondwana biota.)  A few million years later India broke away from Gondwana, drifted northward across the equator, and crashed into central Asia; the crumpled edge of the collision became the Himalayas. South America, Australia and Antarctica were still connected at the beginning of the Cretaceous, 120 million years ago. What remained of the fractured continent had a hospitable climate; its southern shore was highly seasonal, and its northern edge was subtropical. Gondwana remained stationary in these amiable middle latitudes for nearly 50 million years. The seas that surrounded the continent harbored species that have since disappeared; plesiosaurs (aquatic dinosaurs, fifteen meters long) and ammonites (shelled relatives of squids) as well as sharks and lobsters (extant in other parts of the world today, but absent from Antarctica). But these were also seas of genesis, where the ancestor of the penguins lost its ability to fly in the air and entered the ocean.

By the mid-Cretaceous, about 100 million years ago, flowering plants, including the characteristic southern beeches, began to appear on Gondwana. Dinosaurs were at their peak, and mammals, particularly marsupials, were becoming ever more diverse and ecologically important. When the dinosaurs abruptly went extinct about 66 million years ago, mammals became the dominant vertebrate animals. In 1981, Michael Woodburne and William Daly, paleontologists at the University of California at Riverside, found a 40-million-year-old fossil jaw of an opossum, Polydolops, on Seymour Island, east of the Antarctic Peninsula in the Weddell Sea. This small, furtive animal probably subsisted on berries and insects. It was perhaps the most important paleontological discovery ever made in Antarctica. For more than a century zoologists have been mystified by the disjunct distribution of the marsupials, which are found in Australia (kangaroos, wallabies, koalas, wombats, and the extinct predatory marsupial “wolf”, Thrylacine, to name a few) and the Americas (a diversity of opossums, the extinct doglike borhyaenids, and even a saber-toothed predatory marsupial). The oldest known marsupial fossils, which are from North America, are 100 million years old. How did the marsupials get from North America to Australia? Until the Seymour island discovery theories ranged from crossing the Bering Sea land bridge (unlikely, since there are no marsupials in Asia) to rafting across the Pacific on logs or floating mats of vegetation (extremely improbable for a large mammal). But now it is clear: marsupials walked from North America to Australia via South America and Antarctica.

gondwana

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