CAMBRIDGE, Mass.--When studying the predecessors of the Cambrian
explosion, scientists have had a strange loose end--the fossil record of
Ediacarans, flat jellyfish-like organisms, not able to be classified as
plant or animal, that could range up to a meter in length. The recent
work of Geologists John Grotzinger, Samuel Bowring, and Beverly Saylor
of MIT and Allan 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 ran right up to the
Cambrian period as opposed to becoming extinct before this important
evolutionary period. Their work is reported in the October 27 issue of
Science.
Grotzinger and 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, they
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 the
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," said
Bowring.
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 1 million years.
"Five to 10 years ago, being able to date something to within five
million years was a major achievement," said Bowring. "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."
