[meteorite-list] Evidence that Earth's First Mass Extinction Was Caused by Critters, Not Catastrophe

[Ron Baalke]

http://news.vanderbilt.edu/2015/09/evidence-that-earths-first-mass-extinction-was-caused-by-critters-not-catastrophe/

Evidence that Earth's first mass extinction was caused by critters, not catastrophe
by David Salisbury 
Vanberbilt University
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 
itself.

"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," 
said Darroch.

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 
Ediacaran reign.

"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.

Media Inquiries: 
David Salisbury, (615) 322-NEWS 
david.salisbury at vanderbilt.edu