[tt] NS: The Ordovician: Life's second big bang

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I really do wonder how the Creationists offer a competing explanation? The 
time line fits!

The Ordovician: Life's second big bang
http://www.newscientist.com/article.ns?id=mg19826601.700&print=true
11 June 2008
James O'Donoghue

JUST over half a billion years ago, evolution hit a purple patch. In
the space of a few million years, once-empty seas were suddenly
overrun by all manner of newfangled life forms. Animals had arrived
on the scene and life on Earth never looked back.

At least, that's what we originally thought the fossil record was
telling us. It now turns out that this spectacular event - known as
the Cambrian explosion - stuttered to a halt not long after it
began. Around 515 million years ago, evolution ran out of steam and
the increase in biodiversity went into reverse. For the march of
progress to continue, life needed rebooting.

It came in the form of a second explosion of life called the Great
Ordovician Biodiversification Event, a little-heard-of episode which
has been the focus of intense scientific interest in recent years.
Since discovering hints of it around 20 years ago, palaeontologists
have established beyond doubt that the "Ordovician explosion" was
every bit as momentous for animal evolution as the Cambrian one. Now
they are asking a more difficult question: what caused it?

The Cambrian period, from 542 to 489 million years ago, has always
exerted an irresistible lure for palaeontologists. For thousands of
millions of years before it, the most complex life forms on Earth
were marine microbial mats known as stromatolites. These were
briefly superseded by the enigmatic Ediacarans, the first
large-bodied life forms on Earth (New Scientist, 14 April 2007, p
34). Then, in the space of just 20 million years at the start of the
Cambrian, all bar one of the basic types, or phyla, of animals we
see around us today, made their appearance in the fossil record.

Stromatolites became lunch and faded away as trilobites and other
arthropods came to dominate the seas. High levels of dissolved
nutrients supported filter-feeders such as sponges, molluscs,
echinoderms and clam-like brachiopods. Diverse reef systems formed
around stony sponges called archaeocyathids.

But this watery Eden wasn't to last. The fossil record reveals a
decline in biodiversity by the middle of the Cambrian, one so steep
that in the early 1990s Phil Signor of the University of California,
Davis, suggested there had been a mass extinction starting around
515 million years ago (New Scientist, 1 August 1992, p 14).

It later became clear that the decline in biodiversity was not a
mass extinction but resulted from a sharp drop in origination - the
rate at which new species appear. According to a 2004 paper by
Richard Bambach and Andrew Knoll of Harvard University, by the
mid-Cambrian the origination rate in many groups had fallen to
between a fifth and a third of the levels seen earlier, well below
the natural rate at which species were becoming extinct. It's not
clear why this happened, but the result was a decline in
biodiversity (Paleobiology, vol 30, p 522). Many major groups were
hit hard, while the archaeocyathids disappeared altogether and were
replaced by stromatolites. Cambrian life had gone from explosion to
damp squib.

By 510 million years ago, the number of genera had dropped to 450
from a peak of about 600. "The middle to late Cambrian is almost
like an empty quarter. There is a surprising amount of barrenness,"
says Arnie Miller, a palaeobiologist at the University of Cincinnati
in Ohio.

Then, at the start of the Ordovician period, 489 million years ago,
evolution sparked back into life. First came massive algal blooms,
says Marco Vecoli of the University of Lille, France. These provided
a bountiful food supply for filter-feeders, kick-starting an
evolutionary bonanza. The stromatolites were booted out once more,
with sponges re-establishing themselves as major reef-building
organisms. They were joined by coral species as reefs again became
hotspots of biodiversity (see "Life in the Ordovician seas").

The evolutionary tempo picked up and new animal species began to
appear as never before. Though only one new phylum appeared during
the Ordovician - the moss animals or bryozoans - diversity within
existing phyla rocketed, quickly outstripping anything achieved in
the Cambrian. "During the Cambrian there is a fantastic array of new
body plans, but it wasn't until the early to mid-Ordovician that we
got a really striking increase in diversity," says Dave Harper of
the University of Copenhagen in Denmark. By the end of the
Ordovician, biodiversity had reached a level that would not be
surpassed for another 200 million years. Look along the shoreline
today and chances are you will find creatures that first appeared in
the Ordovician, including starfish, sea urchins, oysters and
scallops.

The Ordovician also saw an increase in ecological complexity. During
the Cambrian, animals had largely been confined to the sea floor.
There may have been a few worms poking around in the mud and the odd
animal in the water immediately above, but the real action was on
the sediment itself. That largely two-dimensional world was
fundamentally altered by the Ordovician newcomers. Burrowing animals
went deeper than before, churning up the sea floor and creating new
habitats. "You are pushing more and more species into the same
space, and they are having to start to do things in a different
way," says Harper. "One of the ways is to dig deeper and deeper, but
they are also building higher and higher tiers [of reefs] above the
sediment."

As competition increased, more ingenuity was required to survive.
Whereas filter-feeders once sat back and waited for food to arrive
on the currents, they were now having to actively filter the water
for nutrients. "Wow, there's a whole new food source, and this group
takes advantage," says Mary Droser, a palaeobiologist at the
University of California, Riverside.

Another way to survive was to escape the sea floor altogether. A
recent study by Kevin Peterson of Dartmouth College, New Hampshire,
found that by the mid-Ordovician the larvae of several different
animal groups had developed the ability to swim away from the sea
bottom. This may have been driven by the need to dodge the swaying
tentacles of the filter-feeders (Geology, vol 33, p 929).

Life also staked its first claim on land. Simple plants colonised
damp areas while scorpion-like eurypterids took their first
tentative steps along the shoreline.

This boom was like nothing the world has seen since. The Ordovician
is the only time in the history of animal life that huge numbers of
new species appeared without a mass extinction to clear the decks
beforehand. "It's a real enigma in the history of life, and from my
perspective it's unique," says Droser.

So what triggered it? "There isn't a simple, sexy explanation," she
adds. One possibility is that there was no evolutionary need to
clear the decks: the stalled Cambrian explosion had left plenty of
ecological space to be filled.

It's certainly clear that environmental and geological conditions
during the Ordovician were especially suited to biodiversification,
says Alan Owen of the University of Glasgow in the UK, one of the
leaders of a team trying to pin down the causes of the Ordovician
explosion. "It was a time when the Earth's continents were widely
dispersed and of extreme provincialism," he says (see Maps). "The
seas around each continent had their own fauna. It was a time of
mountain-building, which generated uplift, erosion and the
introduction of nutrients into the sea. It was also a time of
intense volcanic activity which generated yet more nutrients and
created local environments where things could evolve."

The surge in biodiversity also coincided with a warm, stable
climate. On top of that, sea levels were some of the highest the
world has ever seen, covering continental shelves with great
expanses of warm, shallow water that provided light, oxygen and
nutrients for life to thrive in. "There are no modern analogues for
these very big, broad, shallow water areas," says Miller. "That sort
of sea does not really exist now."

These general environmental triggers are reasonably well accepted.
But Birger Schmitz, a geologist at the University of Lund in Sweden,
goes further, suggesting that there could indeed be a simple, sexy
explanation for the diversity boom: the Ordovician explosion may
have been triggered from space.

Asteroid impacts are usually seen as the harbingers of doom,
bringing death and destruction. Not so for Schmitz. He argues that
colossal impacts during the Ordovician created the conditions for
biodiversification, and believes he has evidence to prove it (Nature
Geoscience, vol 1, p 49).

The story begins 470 million years ago, when a large asteroid broke
up in the solar system, creating a swarm of smaller asteroids and
meteoroids (Meteoritics & Planetary Science, vol 42, p 113). Most of
the fragments are still out there and, even now, they make up the
majority of meteorites which fall to Earth.

"If you are going to be hit by a meteorite, the probability is it
will have an age of 470 million years," says Schmitz. The only
confirmed case of a human being hit by a meteorite was when a young
boy was struck by a fragment of this asteroid in Uganda in 1992.
Luckily it didn't do him any harm.

The same probably could not be said of Earth in the period
immediately after the asteroid break-up. In 1988, Jan Nyström of the
Swedish Museum Of Natural History in Stockholm announced a rare find
- a fossil meteorite embedded in a slab of mid-Ordovician limestone
from the Österplana quarry in Kinnekulle nature reserve, Sweden
(Nature, vol 336, p 572). Since then, around 50 beautifully
preserved fossil meteorites have been found in the area (Earth and
Planetary Science Letters, vol 194, p 1). Recent dating studies show
that these crash-landed into the Ordovician sea 467 million years
ago, where they came to rest on the sediment and were fossilised
alongside Ordovician creatures.

Fossil meteorites are very rare - in fact more than 95 per cent of
them originate from Kinnekulle - but ancient impacts leave other
traces which can be detected in sedimentary rocks, in the form of
grains of the mineral chromite found in extraterrestrial material.
Schmitz uncovered this in abundance in mid-Ordovician strata
throughout Sweden (Science, vol 300, p 961) and also in China, then
- as now - several thousand kilometres away.

Based on the abundance of fossil meteorites and chromite in
mid-Ordovician rocks, Schmitz concluded that the rate of impacts was
around 100 times what it is today. And if meteorites were raining in
thick and fast, enormous, destructive asteroids could not have been
far behind, he says. This theory is backed up by the relative
abundance of Ordovician impact craters among the 170 or so known
craters on Earth, which suggests that large impacts were more common
by a factor of 5 to 10 during the Middle Ordovician than at any
other time in the past 540 million years.

Could this be linked to the biological events that were unfolding at
the time? Both Schmitz and Harper think so. Between 2000 and 2002,
Harper and his team compiled a detailed record of fossil brachiopods
- the most abundant and diverse animals of the Ordovician - in
western Russia, Sweden, Norway and Estonia, all part of the region
geologists call Balto-Scandinavia. This revealed a sudden rise in
biodiversity around 467 million years ago, just as the area was
being bombarded. "It matches the asteroid data more or less
precisely," says Harper.

Schmitz now believes that the destruction wrought by asteroids might
have created the right conditions for diversification. He speculates
that in the early Ordovician, the occupants of the sea were both
widely dispersed and generalists, unspecialised in their ecological
preferences. The asteroid impacts would have caused localised
extinctions and thus created lots of vacant ecological niches for
life to evolve into.

Palaeontologists have given Schmitz's idea a cautious welcome. "It's
an intriguing idea and a lot more could be done to test it," says
Miller. Lots more is being done - the trick now is to replicate the
findings elsewhere. So far, Schmitz only has evidence of a link
between impacts and diversification from one region,
Balto-Scandinavia, and only in one group, the brachiopods. The
Chinese chromite shows increased meteorite flux, but so far there
isn't any fossil evidence to go with it.

The great biodiversification event was not to last indefinitely. The
climate, for so long warm and conducive to life, became more erratic
in the late Ordovician. A short-lived ice age marked the beginning
of the end. There was a one-two extinction punch: the cold brought
major changes to ocean circulation which killed off many animals,
and when all the ice melted the oceans were flooded with poorly
oxygenated water. The mass extinction at the end of the Ordovician
period, 443 million years ago, was one of the most devastating ever
to hit the planet.

Fortunately, all was not lost. The arrival of so many new species
and specialised ecosystems was not to be undone lightly. Life was
now in much better shape to weather an extinction storm and bounced
back fairly quickly. Diversity and complexity in the living world
have stayed with us, the legacy of that long-ignored primeval boom.

Evolution - Learn more about the struggle to survive in our
comprehensive special report.

Life in the Ordovician seas

Marine fauna thrived in the long, hot summer that was the Ordovician
period. Most abundant were brachiopods - shelled creatures vaguely
resembling clams - together with bushy colonies of bryozoans
covering the sea floor. There were also flowery crinoids, sea
urchins, corals, sponges, clams and snails. Planktonic graptolites,
shaped like letters from a defunct alphabet, drifted in the water.

The stars of the show, though, were the trilobites and the shelled
cephalopods. Most trilobites were just a few centimetres in size but
one predatory species, Isotelus rex, grew up to 70 centimetres long,
well over half as big again as the next-largest trilobite (Journal
of Paleontology, vol 77, p 99). There was also a tremendous
diversity of cephalopods in all shapes and sizes: tightly or loosely
coiled and straight or curly-shelled. Many of these creatures were
highly mobile and formidable predators at the top of the food chain.
Cameroceras, measuring up to 5 metres long, is among the largest
invertebrates ever to have lived.

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Ediacarans: the 'long fuse' of the Cambrian explosion?
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Weblinks

The Great Ordovician Biodiversification Event (book)
http://cup.columbia.edu/book/978-0-231-12678-6/the-great-ordovician-biodiversification-event