I think that
what happened here is that the animals were caught in the wetlands by the
pyroclastic flows which killed them outright.
This led to immediately submersion in the mud and lake bottoms followed
by substantial ash deposition to completely seal in the bodies in a low oxygen
environment.
The water
table remained intact and may in fact have even risen securing an environment
suitable for fossilization.
This is a
remarkable hoard of dinosaurs that has contributed hugely to our understanding
of these creatures.
Feathered
dinosaur death site is an “animal Pompeii”
Exquisitely preserved
fossils come from a catastrophic event.
A series of fossil discoveries in the 1990s changed our understanding
of the lives of early birds and mammals, as well as the dinosaurs they shared
an ecosystem with. All those discoveries had one thing in common: they came
from a small region in northern China that preserved what is now called the
Jehol Biota.
Until now, however, no one knew why so many well-preserved
fossils were found in that region. In a new study published in Nature Communications, researchers
discovered that this remarkable preservation might have been the result of a
Pompeii-like event, where hot ash from a volcanic eruption entombed these
animals.
Dino colors
According to Leicester University's Sarah Gabbott (who
wasn’t involved in the study), “Unravelling the environments in which
fossilisation took place, as the authors do in this paper, is very important.
It places the fossils within the context of their habitat and it allows us to
determine what filters and biases may have played a part.” These biases may
affect which organisms get preserved.
The fossils of the Jehol Biota are from the Early Cretaceous
period, about 130 million years ago, and they comprise a wide variety of
animals and plants. So far, about 60 species of plants, 1,000 species of
invertebrates, and 140 species of vertebrates have been found in the Jehol
Biota.
One of the most remarkable discoveries to arise from these
fossils came in 2010, when Michael Benton of the University of Bristol found
color-banding preserved in dinosaur fossils. These stripes of light and dark
are similar to stripes in modern birds, and they provided further evidence that
dinosaurs evolved into birds. Benton also found that these fossils had
intact melanosomes—organelles
that make pigments. This discovery allowed paleontologists to tell the colors of
dinosaurs' feathers for the first time.
Rising from the ashes
Baoyu Jiang of Nanjing University, the lead researcher of the
new study, has been studying fossils from Jehol Biota for more than a decade.
“About two years ago, we realized that the sediments and their enclosing
skeletons may provide key clues about what happened to these animals when they
were killed and buried,” he said.
The fact that so many fossils were found exquisitely preserved
from the same time period suggested some form of mass death. Even before Baoyu
started this work, there were suggestions that volcanoes may have been
responsible.
Using 14 different fossils from five locations within the
deposits, Baoyu found marks of fast-moving ash and hot gas, known as
pyroclastic flows, that can only result from a nearby volcanic eruption. The
bones showed black streaks, which suggests charring had occurred.
Almost Pompeii
Now, questions remain. The area that supported the Jehol
Biota is suspected to have been a wetland with many lakes. Most fossils are
found in lakebeds, suggesting that either the fossils were washed into these
lakes by floods or that the animals were in the lakes before fossilization took
place.
Baoyu believes that if fossils don't separate bone joints, it
means the animals must have been in the lake before dying. But that's not a
convincing argument, Gabbott said. "A freshly dead carcass, buoyed by
decay gases which collect in the stomach, can be transported for tens if not
hundreds of kilometers without such disarticulation (separation of bones at
joints)."
No other fossil location,
let alone that which produced so many well-preserved samples,
has ever been suggested to have undergone a similar event. However, a
comparison can be made to what happened in Pompeii in 79 AD when Mount Vesuvius
erupted. The ensuing destruction led to the preservation of the city’s
architecture and objects but not of people or animals. The human and animal remains
we see from Pompeii are plaster casts of the empty spaces their decomposed
bodies left in the ash.
Still, Baoyu’s site and Pompeii both show how mass tragedies at
the feet of volcanoes can preserve the past for future generations to discover.
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