Geologists tie up 'loose end' in fossil record


When studying the predecessors of the Cambrian explosion, the period 530 million years ago when evolution suddenly sprang forward with incredible diversity, scientists have had a strange loose end--the fossil record of Ediacarans, flat jellyfish-like organisms not classifiable as plant or animal that could range up to a meter in length.

Recent work by geologists John Grotzinger and Samuel Bowring, both associate professors in the Department of Earth, Atmospheric and Planetary Sciences, MIT graduate student Beverly Saylor and Alan Kaufman of Harvard, has shown that Ediacarans could actually have participated in the development of organisms in the Cambrian period-that its evolutionary life span continued up to that time. Their work is reported in the October 27 issue of Science.

Professor Grotzinger and Ms. Saylor went to the deserts of Namibia where Ediacaran fossils are plentiful and through uranium-lead dating of zircon and analysis of the layering of the beds of volcanic sediment, determined that Ediacaran fossils are found very near the top of the latest Precambrian rocks and that they co-existed with fossils of small shell-bodied creatures that were previously not thought to have lived coincident with the Ediacarans. Ediacaran fauna were considered a "broken link" of the evolutionary chain, most likely an evolutionary dead end. It was thought they became extinct before the end of the Precambrian period.

This new evidence leaves open the possibility that Ediacarans played a role in the development of the multitude of flora and fauna that characterize the Cambrian period and which are the predecessors of life on Earth today.

"What this shows is that evolution likely proceeded smoothly as opposed to having a period of evolution followed by an extinction, which would open ecological niches allowing other life forms to develop," Professor Bowring said.

The dating was done on grains of the mineral zircon found in the Namibian volcanic ash. Zircon occurs in trace quantities in some volcanic rocks. Zircons allow uranium into their crystal structure and over time, the uranium decays to lead. This technique allows dating of rocks that are 500 million years old to an accuracy of better than plus or minus one million years.

"Five to 10 years ago, being able to date something to within five million years was a major achievement," Professor Bowring said. "The more precisely we can resolve time, the more sophisticated the evolutionary questions we can address. Rock and fossil records provide relative ages. Only absolute dating, like what we have done with uranium/lead dating, will provide us with the precision necessary to determine rates of evolution."

"Another exciting outcome of this work is that by dating carbon isotopic variations in Namibia, we can by inference date rocks in other parts of the world that lack ash beds," Ms. Saylor added. The same pattern of changes in carbon isotopes is found in rock layers throughout the world, permitting the dating in this research to have widespread applicability.

A version of this article appeared in MIT Tech Talk on November 1, 1995.


Topics: Earth and atmospheric sciences

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