VICTOR O. LESHYK
Extinctions
at the end of the Triassic period killed off top predators such as
Redondavenator (left), seen in an artist's impression confronting an
aquatic phytosaur.
It takes a little fiddling, a few missed
turns on the old, labyrinthine lanes and a good deal of folding and
unfolding of an Ordnance Survey map bought that morning, but eventually
Paul Olsen and Dennis Kent manage to locate the unmarked access path
that leads through the woods to a desolate stretch of shoreline on
Lavernock Point, a wild, cliff-lined promontory south of Cardiff in
Wales, UK.
Olsen and Kent park their rental
car in a muddy lay-by. A slow rain begins to fall and low peals of
thunder can be heard in the distance. Grumbling good-naturedly about the
British climate, the two geoscientists shrug into their parkas, sling
their rucksacks over their shoulders and start down the slick path for
another afternoon of getting cold, wet and muddy in the search for clues
to mysterious events that wiped out much of life on Earth 200 million
years ago and allowed the dinosaurs to take over the world.
How
the dinosaurs' mighty reign ended has been fairly well established: a
catastrophic asteroid impact near Chicxulub on the Yucatan Peninsula in
Mexico, just over 65 million years ago, is widely credited with bringing
the age of the dinosaurs to a close and ushering in the age of mammals.
Olsen
and Kent, who both work at Columbia University's Lamont-Doherty Earth
Observatory in Palisades, New York, have long speculated that with
Chicxulub, history might have been repeating itself: that another
asteroid, 135 million years earlier, could have wiped out, or at least
had a hand in wiping out, much of the late-Triassic flora and fauna.
This would have allowed dinosaurs to spread around the globe during the
subsequent Jurassic period (200 million to 145 million years ago),
evolve enormous bodies and dominate the planet until the next great
impact catastrophe.
Sudden death
It
is certain that something drastic did happen at the end of the
Triassic, because in the space of a few thousand years, half of all
genera known to have existed at the time suddenly vanish from the fossil
record. At sea, 20% of all families abruptly disappear, including an
entire class of creatures — the eel-shaped conodonts. On land, the death
toll was even higher. It was one of the greatest mass extinctions in
Earth's history and, at this vast distance in time, one of the least
understood. “The only thing anyone can say with any certainty about the
Triassic–Jurassic mass extinction is that it happened,” says Olsen.
“Whatever it was that caused it, it happened so swiftly that most life
forms never had time to adapt and evolve to meet the changes.”
The prevailing view among scientists these
days is that the extinctions were caused by massive volcanic activity
associated with the break-up of the super-continent Pangaea. The series
of eruptions created a vast geologic formation called the Central
Atlantic magmatic province (CAMP; see
'End of an era').
“We are talking here about volcanic activity on a scale many thousands
of times greater than anything ever witnessed by humans,” says Gregory
McHone, an independent Canadian geologist who has spent much of his
career investigating the CAMP volcanic event and building a convincing
case for its involvement in the Triassic–Jurassic mass extinction.
The
flood-basalt eruption of the Icelandic volcano Laki in 1783 provides
researchers with a scaled-down model of just how bad things might have
been in the late Triassic. An outpouring of sulphurous gases from Laki
created haze that cooled the planet and caused widespread crop failures
and famine. It ultimately contributed to the deaths of an estimated 6
million people, says McHone. But disastrous as Laki's eruption was, it
belched up just 15 cubic kilometres of basalt. The CAMP events produced 2
million cubic kilometres or more, in a series of pulses that alternated
between cooling the climate with sulphurous haze and warming it with
massive emissions of carbon dioxide and methane. The oceans grew acidic
and parts became starved of oxygen, while on land a surge in lightning
sparked extensive fires
1. Many of Earth's life forms simply couldn't recover from that succession of body blows, says McHone.
It's
a plausible theory, as Olsen and Kent both readily concede — even,
perhaps, the most likely one. All the same, the CAMP theory leaves a lot
of unanswered questions, not least of which is the suddenness of the
extinctions. The late-Triassic eruptions span hundreds of thousands of
years, but the die-offs seem swift in the fossil record. And what of the
'fern spike'? Late-Triassic sediments on the US east coast contain huge
quantities of fossilized fern spores.
Ferns
are usually the first plant to appear after a natural disaster, says
Kent. “If you look in the fossil record you see a sudden massive spike
in ferns just around the time of the extinctions, but demonstrably
before the great basalt flows — at least in our part of the United
States.” In other spots, the extinctions seem to coincide with the
oldest basalt layers, within the errors of the dating conducted so far.
“The
only way we are ever going to unravel this mystery is to work out a
timeline, as precise as we can make it, of all the various events around
the world that led up to it,” says Kent. Pursuit of that timeline has
taken Olsen and Kent on a global quest, from North Carolina to Nova
Scotia, Canada; China to Germany, Italy, Austria and the High Atlas
Mountains in Morocco; and now to Wales.
Just in time
Olsen
has been thinking about the possibility of an impact connection to the
Triassic–Jurassic extinctions for more than 20 years
2, 3,
but one of the biggest drawbacks to the asteroid theory is that until
recently nobody had found any evidence of such a catastrophe occurring
around the time of the extinctions. Then, last year, French and German
research teams re-dated a badly eroded structure left by a massive
impact near Rochechouart, in western France
4.
Previous work had put the impact at around 214 million years ago, long
before the extinction event. But the revised date of 199 million to 203
million years ago overlaps with the extinctions, which have been dated
to 201.4 million years ago
5.
The
authors of the paper also suggested that the enormous shock waves
generated by the 2-kilometre asteroid, as it slammed to Earth at more
than 25 kilometres per second, could account for unexplained ripples and
disturbances in the late Triassic limestone and shale beds in western
Britain — sedimentary beds that coincide with the mass-extinction event.
“When I read that,” says Olsen, “I decided it was time to come over
here and take a much closer look.”
AP/JIM ROSS
Paul Olsen inspects rocks along a beach in Wales to trace what caused one of the biggest mass extinctions on Earth.
As Olsen scrambles along the base of the
Welsh cliffs, his stories bring to life the Triassic world. There were
the monkey lizards with their beaky faces, long arms and grasping tails;
crocodilian creatures that trotted along like dogs; and shallow
tropical seas teeming with sharks, right where this shingle beach lies
today. “Then suddenly it all came crashing to an end,” says Olsen.
Olsen's
fascination with this lost world goes back to 1968, when he was a
14-year-old in Livingston, New Jersey, and heard that dinosaur
footprints had been found in the rocks of the nearby Roseland quarry. He
and a school friend hopped on their bikes, pedalled over to the quarry
and found that there were fossils everywhere.
By
the time the two friends were in their second year of high school, they
had catalogued thousands of fossils and tracks of reptiles from the
late Triassic and early Jurassic. They became so involved that when the
quarry and its treasures were to be sold off and developed into housing
units, the teens mounted a publicity campaign to save it.
Soon,
Life
magazine was on the phone and their campaign had captured national
attention. Olsen even made a cast of a footprint left by a fearsome
three-toed beast, and sent the fibreglass model to then-US president
Richard Nixon.
The publicity garnered by this
sheer chutzpah helped to protect the fossil-rich site, and the
fibreglass footprint went on to find a place in Richard Nixon's
presidential library.
Olsen's trip through
western Britain with Kent retains some of that sense of schoolboy
enterprise and science on the human scale: the two researchers bicker
amiably over which way to go at the crossroads; stop off in the local
supermarket to pick up more plastic sandwich bags for their rock
specimens; use a self-modified, battery-powered drill to take core
samples; and even get scolded by the waspish landlady at their guest
house when they clomp in with muddy boots at the end of the day.
Explosive force
At
Lavernock Point, the scientists ignore the rain and thunder as they set
about their work. Olsen makes his way to the base of the cliffs and
points to a layer of buff-coloured limestone at about waist height.
“Right there is where it happened: there's the extinction line,” he
says. To a layman, it is innocuous: just another of the alternating
bands of limestone and shale that are brightened here and there by
clumps of flowering purple valerian. But close examination reveals
sand-filled cracks and deposits of grainy, irregularly sized material —
disturbances that might have been the work of a tsunami or an
extraordinary earthquake.
The disturbed layers
here and at other late-Triassic sites in the United Kingdom have a
distinctive orientation, all angled as if the source of the unrest was
in the vicinity of Rochechouart, says Olsen.
It
certainly would have been an awful day in this part of the world when
the Rochechouart crater formed. The researchers who re-dated it estimate
4
that the impact would have generated an earthquake up to magnitude 11.5
— 100 times more powerful than any quake in recorded history.
“It happened so swiftly that most life forms never had time to adapt and evolve.”
Gareth Collins, a geoscientist at Imperial
College London, says that the quake would probably have been much
smaller, although still massive. Collins and researchers from Purdue
University in West Lafayette, Indiana, have developed an online
calculator to model the effects of impacts. Their algorithm paints a
vivid picture of what would have been going on at this spot in Wales
after the asteroid hit with the explosive force of more than one million
megatonnes of trinitrotoluene (TNT). Even at a distance of more than
600 kilometres, the beach would have endured hurricane-force winds and a
hail of debris.
That debris would have
carried a chemical signature of the impact that might still reside in
the layers of sedimentary rocks. Stratigrapher Stephen Hesselbo and
geochemist Ken Amor, both at the University of Oxford, UK, have joined
Olsen and Kent on their outing, and are taking samples to analyse with a
mass spectrometer. They will measure the ratios of chromium isotopes,
and look for one that is characteristic of meteorites. Olsen will also
send samples from this location to another lab, to be searched for other
tell-tale impact markers.
If those tests
detect an extraterrestrial signal in the late-Triassic sedimentary
layers in Wales and elsewhere, it will be the first substantive link
between an impact and the Triassic–Jurassic mass-extinction event. But a
coincidence in time won't prove that an impact was the cause. “The two
things might occur at approximately the same moment but have nothing
whatever to do with each other,” says Olsen.
More
troublingly, the Rochechouart impact doesn't seem to have been anywhere
near big enough to have accounted for the mass extinctions around the
globe — at least, not on its own. The 25-kilometre-wide buried crater
may have been 40 to 50 kilometres across originally, but it is just a
pockmark in comparison with the 180-kilometre-wide scar at Chicxulub.
“Based on our estimates, Rochechouart is quite small in terms of global
environmental consequences,” says Collins.
Picking up the pieces
For
now, says Olsen, it is too early to make any definitive statements
about what Rochechouart did or didn't do. That will take much more data
from late-Triassic sites around the world. This year alone, he has
crossed the Atlantic three times to collect samples from the United
Kingdom and Morocco. He is back in the Atlas Mountains this week, to
examine yet more sites. He and his colleagues are not only looking for
signs of an extraterrestrial impact in the sediments, but are also
searching for other clues, such as the extinction layer and a chemical
signature that could be linked to the CAMP eruptions. All those data,
says Olsen, will help the team to sort out the relative timing of events
around the world, and to create a fuller picture of what happened and
how life responded.
He suspects that
Rochechouart may have been “a piece of a much bigger puzzle”. Perhaps it
was one of a series of asteroids that hit around the same time.
Alternatively, a lone French crash might have been the final straw for a
world already reeling from volcanic eruptions. Or the impact may have
come first, weakening ecosystems enough that when the eruptions started,
life took a nosedive.
Teasing out the answer
will take some time, says Olsen, as he trudges, wet and muddy, back up
the trail at the end of another wearying afternoon in the cold Welsh
rain. “There is no easy way to do this,” he says. “But I believe that
eventually we will be able to put the pieces together and know what
happened and why.”
Este novo tipo de titanossauro pertence ao grupo dos saurópodes 
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