2 November 2019 | New Scientist | 15THE discovery of thousands of
exquisitely preserved plant and
animal fossils in Colorado gives
an unprecedented look at life
after the asteroid smash that
wiped out the dinosaurs.
We know that an asteroid hit
what is now Mexico’s Yucatán
peninsula about 66 million years
ago, killing three-quarters of living
organisms, including all dinosaurs
except for birds. But the first
million years after this mass
extinction has a notoriously poor
fossil record, says Tyler Lyson at
the Denver Museum of Nature
and Science. “I’ve spent 15 years
looking for fossils in this interval
and have only found a handful of
scrappy mammal jaws,” he says.
That changed when Lyson and
his colleagues stopped hunting for
naked bones and started searching
for concretions: types of rocks
that can form around organic
materials, including bone.
Inside the concretions, they
discovered thousands of fossils of
plants and vertebrate animals. “We
were finding whole skulls, and in
some cases skeletons, of mammals,
crocodiles and turtles,” says Lyson.
Dating the rocks allowed
them to come up with a detailed
chronology of life’s recovery
following the asteroid collision
(Science, doi.org/dc9j).
In the aftermath of the
extinction, the most common
plants were ferns, and the
surviving mammals weighed
no more than half a kilogram. But
within 100,000 years, recovery
was well under way: mammalshad rebounded to almost the
same size as they were before the
asteroid strike, which was up to
8 kilograms, and the forests had
become dominated by palm trees.
Within 300,000 years, the
diversity of plants in the forests
was rising and mammals grew
to up to 25 kilograms. At the
700,000-year mark, mammals
reached up to 50 kilograms in size.
This is also the time that legumes
appeared. Lyson calls this the
“protein bar moment” since it
probably provided the calories
needed to drive this rapid growth.When the researchers looked
back through the climate record,
they found that these three bursts
of mammal evolution seem to
coincide with temperature rises
of about 5°C. This suggests that the
rise of mammals was helped along
by a more tropical climate that
enhanced plant growth and hence
increased the food supply for
animals, says Lyson.
The fossil findings are now
“unquestionably the best record
of what happened after the
[dinosaur] extinction”, says
P. David Polly at Indiana
University. “They show that
the diversity of mammals and
plants recovered within about
100,000 years of the event,
which is the blink of an eye
in geological terms.”
Having such a detailed picture
of the way ecosystems rebounded
after the last extinction event
may help us predict what will
happen following the mass
extinction that many people
think is happening now as a result
of rapid climate change, says
Gregory Webb at the University
of Queensland, Australia. ❚Palaeontology
Ruby Prosser Scully
HHMI TANGLE BANK STUDIOSSolar system
Strange domes
on Venus are shiny
‘peanut butter’ hills
VENUS’S unusual domes may
sparkle. Our neighbouring world
is sprinkled with bulges that are
several hundred metres high and
appear different from the rest of
its surface. They may be made
of shiny “crystal mush” that is
squeezed up from underground.
Most of Venus’s surface is flat,
formed by lava that has flooded
the land over time. But amid these
relatively unremarkable lava flows
sit domes with unexpectedly
steep sides. They have confused
researchers for decades because it
seems unlikely they were produced
by the same volcanic processes that
shaped the rest of the surface.
Geoffrey Bromiley and Sally
Law at the University of Edinburgh,
UK, have come up with a potential
solution. They think the steep-sided
domes are probably formed by lava
that is thicker than the runny type
that flooded the rest of the surface.
“If you poured olive oil onto the
table, it would just flow away and
run all over the table, and you’d still
have a flat surface,” says Bromiley.“If you poured something a lot
thicker, like peanut butter, that
would stay there and form a dome.”
That thicker substance could
come in the form of a crystal mush
similar to what makes up some
landforms that the researchers
have studied in Cyprus, he says.
The mush that formed those hills
is full of green olivine crystals up
to 1 centimetre in size.
Those large crystals are createdwhile magma is still deep
underground. They are then
left behind when the rest of the
still-liquid magma rises up through
the crust because it is hotter and
more buoyant than the surrounding
rock, says Bromiley. But when
faults rip Earth’s crust apart, the
crystallised magma mush can be
squeezed out like toothpaste from
its tube (Icarus, doi.org/dc73).
“In Cyprus, we see these faults
that allow the mush to get to the
surface, and we’re suggesting
the same thing could happen
on Venus,” says Bromiley. ❚
Leah CraneFossils show how mammals rose
from the dinosaurs’ ashes
Some of the delicate fossils
collected from the Corral
Bluffs in Colorado“ When faults rip the crust
apart, crystallised mush
can be squeezed out like
toothpaste from its tube”