[meteorite-list] Evidence that Earth's First Mass Extinction Was Caused by Critters, Not Catastrophe
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 15 Sep 2015 17:03:59 -0700 (PDT)
Evidence that Earth's first mass extinction was caused by critters, not catastrophe
by David Salisbury
Sep. 2, 2015
In the popular mind, mass extinctions are associated with catastrophic
events, like giant meteorite impacts and volcanic super-eruptions.
But the world's first known mass extinction, which took place about 540
million years ago, now appears to have had a more subtle cause: evolution
"People have been slow to recognize that biological organisms can also
drive mass extinction," said Simon Darroch, assistant professor of earth
and environmental sciences at Vanderbilt University. "But our comparative
study of several communities of Ediacarans, the world's first multicellular
organisms, strongly supports the hypothesis that it was the appearance
of complex animals capable of altering their environments, which we define
as 'ecosystem engineers,' that resulted in the Ediacaran's disappearance."
The study is described in the paper "Biotic replacement and mass extinction
of the Ediacara biota' published Sept. 2 in the journal Proceedings of
the Royal Society B.
There is a powerful analogy between the Earth's first mass extinction
and what is happening today",
Darroch observed. "The end-Ediacaran extinction shows that the evolution
of new behaviors can fundamentally change the entire planet, and we are
the most powerful 'ecosystem engineers' ever known."
The earliest life on Earth consisted of microbes - various types of single-celled
microorganisms. They ruled the Earth for more than 3 billion years. Then
some of these microorganisms discovered how to capture the energy in sunlight.
The photosynthetic process that they developed had a toxic byproduct:
oxygen. Oxygen was poisonous to most microbes that had evolved in an oxygen-free
environment, making it the world's first pollutant.
But for the microorganisms that developed methods for protecting themselves,
oxygen served as a powerful new energy source. Among a number of other
things, it gave them the added energy they needed to adopt multicellular
forms. Thus, the Ediacarans arose about 600 million years ago during a
warm period following a long interval of extensive glaciation.
"We don't know very much about the Ediacarans because they did not produce
shells or skeletons. As a result, almost all we know about them comes
from imprints of their shapes preserved in sand or ash," said Darroch.
What scientists do know is that, in their heyday, Ediacarans spread throughout
the planet. They were a largely immobile form of marine life shaped like
discs and tubes, fronds and quilted mattresses. The majority were extremely
passive, remaining attached in one spot for their entire lives. Many fed
by absorbing chemicals from the water through their outer membranes, rather
than actively gathering nutrients.
Paleontologists have coined the term 'Garden of Ediacara' to convey the
peace and tranquility that must have prevailed during this period. But
there was a lot of churning going on beneath that apparently serene surface.
After 60 million years, evolution gave birth to another major innovation:
animals. All animals share the characteristics that they can move spontaneously
and independently, at least during some point in their lives, and sustain
themselves by eating other organisms or what they produce. Animals burst
onto the scene in a frenzy of diversification that paleontologists have
labeled the Cambrian explosion, a 25-million-year period when most of
the modern animal families - vertebrates, molluscs, arthropods, annelids,
sponges and jellyfish - came into being.
"These new species were 'ecological engineers' who changed the environment
in ways that made it more and more difficult for the Ediacarans to survive,"
He and his colleagues performed an extensive paleoecological and geochemical
analysis of the youngest known Ediacaran community exposed in hillside
strata in southern Namibia. The site, called Farm Swartpunt, is dated
at 545 million years ago, in the waning one to two million years of the
"We found that the diversity of species at this site was much lower, and
there was evidence of greater ecological stress, than at comparable sites
that are 10 million to 15 million years older," Darroch reported. Rocks
of this age also preserve an increasing diversity of burrows and tracks
made by the earliest complex animals, presenting a plausible link between
their evolution and extinction of the Ediacarans.
The older sites were Mistaken Point in Newfoundland, dating from 579 to
565 million years ago; Nilpena in South Australia, dating from 555 to
550 million years ago; and the White Sea in Russia, dating also from 555
to 550 million years ago million years ago.
Darroch and his colleagues made extensive efforts to ensure that the differences
they recorded were not due to some external factor.
For example, they ruled out the possibility that the Swartpunt site might
have been lacking in some vital nutrients by closely comparing the geochemistry
of the sites.
It is a basic maxim in paleontology that the more effort that is made
in investigating a given site, the greater the diversity of fossils that
will be found there. So the researchers used statistical methods to compensate
for the variation in the differences in the amount of effort that had
been spent studying the different sites.
Having ruled out any extraneous factors, Darroch and his collaborators
concluded that "this study provides the first quantitative palaeoecological
evidence to suggest that evolutionary innovation, ecosystem engineering
and biological interactions may have ultimately caused the first mass
extinction of complex life."
Marc Laflamme, Thomas Boag and Sara Mason from the University of Toronto;
Douglas Erwin and Sarah Tweedt from the Smithsonian Institution, Erik
Sperling from Stanford University, Alex Morgan and Donald Johnston from
Harvard University; Rachel Racicot from Yale University; and Paul Myrow
from Colorado College collaborated in the study.
The project was supported by grants from the Connaught Foundation, National
Science and Engineering Research Council of Canada, NASA Astrobiology
Institute, National Geographic Society and National Science Foundation
grant EAR 1324095.
David Salisbury, (615) 322-NEWS
david.salisbury at vanderbilt.edu
Received on Tue 15 Sep 2015 08:03:59 PM PDT