New Work on th Cambrian Explosion

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New Work on th Cambrian Explosion

Postby Roger Stanyard » Sun Jan 06, 2008 9:35 pm

Two Explosive Evolutionary Events Shaped Early History Of
Multicellular Life

The Ediacara fossil Fractofusus andersoni from the approx 565
million year old Mistaken Point Formation in Newfoundland, Canada,
represents the earliest Ediacara assemblage, known as the Avalon
assemblage. Credit: Bing Shen
by Staff Writers
Blacksburg VA (SPX) Jan 04, 2008
Scientists have known for some time that most major groups of
complex animals appeared in the fossils record during the Cambrian
Explosion, a seemingly rapid evolutionary event that occurred 542
million years ago. Now Virginia Tech paleontologists, using rigorous
analytical methods, have identified another explosive evolutionary
event that occurred about 33 million years earlier among macroscopic
life forms unrelated to the Cambrian animals. They dubbed this
earlier event the "Avalon Explosion."
The discovery, reported in the January 4 issue of Science, suggests
that more than one explosive evolutionary event may have taken place
during the early evolution of animals.

The Cambrian explosion event refers to the sudden appearance of most
animal groups in a geologically short time period between 542 and
520 million years ago, in the early Cambrian Period. Although there
were not as many animal species as in modern oceans, most (if not
all) living animal groups were represented in the Cambrian oceans.

"The explosive evolutionary pattern was a concern to Charles Darwin,
because he expected that evolution happens at a slow and constant
pace," said Shuhai Xiao, associate professor of geobiology at
Virginia Tech. "Darwin's perception could be represented by an
inverted cone with ever expanding morphological range, but the
fossil record of the Cambrian Explosion and since is better
represented by a cylinder with a morphological radiation at the base
and morphological constraint afterwards."

Darwin reckoned that there should be long and hidden periods of
animal evolution before the Cambrian Explosion, Xiao said.

But paleontologists have not found such evidence, and recently
scientists have learned that biological evolution has not been
moving on a smooth road. "Accelerated rates may characterize the
early evolution of many groups of organisms," said Michal
Kowalewski, professor of geobiology at Virginia Tech.

To test whether other major branches of life also evolved in an
abrupt and explosive manner, Virginia Tech graduate students Bing
Shen and Lin Dong, along with Xiao and Kowalewski, analyzed the
Ediacara fossils: the oldest complex, multicellular organisms that
had lived in oceans from 575 to 542 million years ago; that is,
before the Cambrian Explosion of animals. "These Ediacara organisms
do not have an ancestor-descendant relationship with the Cambrian
animals, and most of them went extinct before the Cambrian
Explosion," said Shen. "And this group of organisms - most species -
seems to be distinct from the Cambrian animals."

But how did those Ediacara organisms first evolve, Shen asked. Did
they also appear in an explosive evolutionary event, or is the
Cambrian Explosion a truly unparalleled event"

"We identified 50 characters and mapped the distribution of these
characters in more than 200 Ediacara species. These species cover
three evolutionary stages of the entire Ediacara history across 33
million years," said Shen.

The three successive evolutionary stages are represented by the
Avalon, White Sea, and Nama assemblages (all named after localities
where representative fossils of each stage can be found). The
earliest Avalon stage was represented by relatively few species.

Surprisingly, however, as shown by Shen and colleagues, these
earliest Ediacara life forms already occupied a full morphological
range of body plans that would ever be realized through the entire
history of Ediacara organisms.

"In other words, major types of Ediacara organisms appeared at the
dawn of their history, during the Avalon Explosion," Dong
said. "Subsequently, Ediacara organisms diversified in White Sea
time and then declined in Nama time. But, despite this notable
waxing and waning in the number of species, the morphological range
of the Avalon organisms were never exceeded through the subsequent
history of Ediacara."

Kowalewski said their research team had not anticipated the
discovery. "Using the scientific literature, we were trying to
create a more rigorous reconstruction of the morphological history
of Ediacara organisms," he said.

The process involved adapting quantitative methods that had been
used previously for studying morphological evolution of animals, but
never applied to the enigmatic Ediacara organisms.

"We think of diversity in terms of individual species. But species
may be very similar in their overall body plan. For example, 50
species of fly may not differ much from one another in terms of
their overall shape - they all represent the same body plan. On the
other hand, a set of just three species that include a fly, a frog
and an earthworm represent much more morphological variation. We can
thus think of biodiversity not only in terms of how many different
species there are but also how many fundamentally distinct body
plans are being represented. Our approach combined both those
approaches," said Kowalewski.

"In addition, the method relies on converting different morphologies
into numerical (binary) data. This strategy allows us to describe,
more objectively and more consistently, enigmatic fossil life forms,
which are preserved mostly as two-dimensional impressions and are
not understood well in terms of function, ecology, or physiology,"
Kowalewski said.

Scientists are still unsure what were the driving forces behind the
rapid morphological expansion during the Avalon explosion, and why
the morphological range did not expand, shrink, or shift during the
subsequent White Sea and Nama stages.

"But, one thing seems certain -- the evolution of earliest
macroscopic and complex life also went through an explosive event
before to the Cambrian Explosion," Xiao said. "It now appears that
at the dawn of the macroscopic life, between 575 and 520 million
years ago, there was not one, but at least two major episodes of
abrupt morphological expansion." ... y_Events_S
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