ENHS Nature Trails Feb. 98 Article 2

President's Column
by Nathan Tublitz

The World's Greatest Mid-life Crisis: The Mass Extinctions at the Permian-Triassic Boundary*

We all know what happened to the dinosaurs -some 65 million years ago, at the end of the Cretaceous period, they went extinct. The permanent disappearance of the dinosaurs, the dominant land organisms of their period, triggered a new era of life on this planet and enabled mammals -- minor players in the Cretaceous -- to radiate rapidly and fill niches vacated by their reptilian predecessors. This extinction was almost certainly caused by a extraterrestrial asteroid impact in the Yucatan Peninsula which threw up a world-wide dust cloud. The Cretaceous-Tertiary extinction profoundly influenced the course of terrestrial life, yet it is dwarfed by the mother of all extinctions, the one that occurred nearly 200 million years earlier.

Life on earth is thought to have originated about 3 billion years ago, give or take a half billion years. From that primordial ooze emerged the first life in the form of self-replicating, nucleic acid-based single cells. It took another 2.5 billion years for those single cells to figure out how to live together in multicellular array. Multicellularity really took off about 500 million years ago during the Cambrian Explosion with the first appearance of the ancestral forms of many of our current forms of life. But a strange thing happened around halfway along that long path from the Cambrian to our 20th century; over 90% of life on this planet disappeared forever about 250 million years ago, at the end of the geological period known as the Permian.

The cause of this mother of all extinctions is a mystery of Holmesian proportion, a fascinating yarn recounted to me by the local researcher working on this question, Dr. Greg Retallack Professor of Geology here at the University of Oregon. If El Nino gets you depressed, you would have never liked Permian life. Temperatures in the Permian were significantly colder than current Oregon temperatures and its climate was much like that of current day Siberia. During this period there was a single land mass on Earth, the supercontinent Gondwanaland, centered far south of the Equator. The habitat of this land mass was dominated by swampy peat marshes filled with archaic plants like Glossopteris which had large, tongue-shaped leathery leaves. Despite its less than tropical climate, the Permian was biologically very rich since this period was the source of large coal beds found in many locales around the world.

But those happy Permian days didn't last forever. At the end of the Permian there was an abrupt and dramatic change in the environment affecting the entire planet. Temperatures soared, seas warmed, and the atmospheric levels of carbon dioxide increased. On land the coal-forming strata of the Permian were replaced by newer Triassic soils strikingly lacking in coal. Glossopteris and its Permian relatives completely disappeared, replaced by a few coniferous plants with needle-like leaves more suited to temperate climes. The six million year period starting at the end of the Permian and running into the beginning of the Triassic was also marked by a major decline in biological productivity, another measure of the extent of plant life. The fauna of the Permian also suffered heavily during the Permian-Triassic transition. Both terrestrial and marine species were decimated - nearly 90% of all species went extinct at the end of the Permian. Not surprisingly, the Permian-Triassic extinction is the largest ever recorded. The obvious question, the one that intrigues Dr. Retallack, is the cause of this profound devastation.

Dr. Retallack's recent research on this subject has fingered a potential culprit - an extraterrestrial asteroid-like object much larger than the one that blew away the dinosaurs. He estimates that it was at least 5-10 km in diameter and collided with the Earth at such speeds to have produced a 500 km wide impact crater. An impact of this magnitude would have undoubtably dispersed vast amounts of particulate matter into the atmosphere, severely reducing sunlight levels worldwide with obvious negative consequences to the flora and fauna of the period. This intriguing hypothesis is supported by Dr. Retallack's recent research on the Permian-Triassic boundary layers. He has demonstrated that these layers contain unusually high levels of platinum-iridium and shocked quartz, both of which are indicators of an impact event by an extraterrestrial source. Interestingly, much of his data has been collected in the Antarctic, an excellent source of geological information on this period of Earth's history.

Although Dr. Retallack's data strongly suggests that the Permian-Triassic extinctions were the result of an asteroid impact, several key parts of the puzzle are not yet known. For example, the site of the impact crater has not yet been unambiguously determined, although he has some evidence that points to Western Australia as a potential impact site. And the levels of platinum-iridium and shocked quartz at the end of the Permian are not as high as one might predict based on the data from the asteroid impact that killed the dinosaurs. This situation is a wonderful example of science at its best: exciting, new information reveals many more questions. These unanswered questions form the basis of Dr. Retallack's current work. Keep your eyes peeled - this work is clearly front page material for the popular press and will receive much coverage in the not-too-distant future.

More detailed information on this subject can be found in an article by Dr. Retallack in Science magazine in 1995 (volume 267, pages 77-80).

*The title for this month's column was taken from Dr. Retallack's recent lecture on this subject.

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