[meteorite-list] Giant Dinosaurs Arrived With A Bang

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:49 2004
Message-ID: <200205161956.MAA00092_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/news/news.jsp?id=ns99992290

Giant dinosaurs arrived with a bang
New Scientist
May 16, 2002

Dinosaurs may have arrived with a bang, as well as gone out with one.
Scientists have found the hallmarks of a meteorite impact and mass
extinction in rocks just below strata containing the earliest footprints of
large meat-eating dinosaurs.

The finding of high levels of iridium metal and fossilised fern spores
suggests that a sudden extinction cleared the ecological stage, leaving room
for meat-eating dinosaurs to grow suddenly larger. A subsequent, massive
meteorite impact about 65 million years ago resulted in the extinction of
the creatures.

Dinosaurs evolved about 230 million years ago and competed with many other
reptiles until the Triassic period ended about 202 million years ago. Then
most of the competitors vanished and dinosaurs grew to their
characteristically monstrous proportions in the Jurassic period that
followed.

The key to how the dinosaurs came to dominate the land in this way may now
have been discovered in the sedimentary rocks laid down over the
Triassic-Jurassic boundary in what is now northeastern North America.

Devastated landscape

The rocks contain few fossil bones, but they preserve both fossil pollen and
the footprints of animals that walked beside ancient lakes. Lake levels rose
and fell with periodic climate cycles, so the rocks can be finely dated,
says Paul Olsen of the Lamont Doherty Earth Observatory of Columbia
University.

Surveying 80 sites, Olsen's group found that fossil footprints changed from
typical Triassic to typical Jurassic groupings in a period of just 50,000
years. In between lay the boundary between the periods, defined by a change
in pollen type, and including the layer rich in iridium and fern spores.

The fern spores are indicative because ferns spread rapidly over devastated
landscapes - sharp peaks of spores also occurred just after the final,
cataclysmic impact 65 million years ago.

Eat anything

The faunal change was also sharp. "In the late Triassic, there were lots of
different footprints representing many different reptile groups," Olsen told
New Scientist. Yet at the start of the Jurassic "all you see are dinosaurs,
lizards and very small, fully terrestrial crocodiles".

And the size of the dinosaurs jumps sharply. Just 50,000 years after the
start of the Jurassic, there are tracks of Eubrontes giganteus, a six-metre
long predator that Olsen says was nearly twice as massive as the biggest
Triassic dinosaur.

The meat eaters survived the disaster probably because of their adaptable
diets, says Olsen. It is typical for a "decimated ecosystem" to become
dominated by animals that can survive on whatever they can find, he says.

Instead of hunting plant-eaters, "they're primarily hunting other carnivores
and things in the water," such as fish. Not until about 100,000 years after
the extinction did a few small plant-eaters start leaving their footprints
by the lakes.

Michael Benton of the University of Bristol aggress that the rapid change in
seen in the dinosaurs suggests "it was more of a catastrophic event than
people had thought". However, he warns that Olsen's group has studied only
one area, while iridium-rich deposits from the impact 65 million years ago
have been found at 200 different sites.

Journal reference: Science (vol 296, p 1305)
Received on Thu 16 May 2002 03:56:39 PM PDT