Scientists solve mystery of how world’s best-preserved dinosaurs died

By Stephen Beech

The world's best-preserved dinosaurs found at a site dubbed the “Chinese Pompeii” didn't actually die due to volcanic activity, suggests new research.

Fossilized remains at The Yixian Formation in China were instead preserved by far more mundane events, say scientists.

These include the collapse of burrows and long rainy periods that built up sediments that buried the dead in oxygen-free pockets.

Researchers say that between around 120 million and 130 million years ago, during the age of dinosaurs, temperate forests and lakes hosted a "lively" ecosystem in what is now northeast China.

Diverse fossils from that time remained mostly undisturbed until the 1980s when villagers started finding "exceptionally preserved" creatures, which fetched high prices from collectors and museums.

The finds led to a fossil "gold rush" with so much digging - at what is believed to be the most extensive palaeontological excavations anywhere on the planet - that the site can now be seen from space.

By the 1990s, it had become clear that the Yixian Formation contained uniquely well-preserved remains of dinosaurs, birds, mammals, insects, frogs, turtles and other species.

Unlike fossils unearthed in most other locations, many animals came complete with internal organs, feathers, scales, fur and stomach contents.

It suggested some kind of sudden preservation process at work.

The finds even included a cat-size mammal and a small dinosaur locked in mortal combat when they died.

The world’s first known non-avian feathered dinosaurs were also discovered - some so intact that scientists worked out the feathers’ colors.

The leading theory for how the fossils came to be so well preserved was a sudden burial by volcanism, perhaps like the waves of hot ash from Vesuvius that entombed citizens of Pompeii in 79 AD.

But new research suggests that the "Pompeii" hypothesis is wrong.

Earlier studies have suggested that multiple Pompeii-type events took place in pulses over around a million years.

The new study, published in the journal Proceedings of the National Academy of Sciences (PNAS), uses state-of-the-art technology to date the fossils to a compact period of less than 93,000 years when nothing particular happened.

Study co-author Dr. Paul Olsen, of Columbia University in New York, said: “These are probably the most important dinosaur discoveries of the last 120 years.

“But what was said about their method of preservation highlights an important human bias.

"That is, to ascribe extraordinary causes, i.e. miracles, to ordinary events when we don’t understand their origins.

"These [fossils] are just a snapshot of everyday deaths in normal conditions over a relatively brief time.”

Study lead author Dr. Scott MacLennan, of the University of the Witwatersrand, South Africa, analyzed tiny grains of the mineral zircon, taken from both surrounding rocks and the fossils themselves.

Within these, he measured ratios of radioactive uranium against lead, using a new, extremely precise method called chemical abrasion isotope dilution thermal ionization mass spectroscopy, or CA-ID-TIMS.

The fossils and surrounding material consistently dated to 125.8 million years ago, centered around a period of less than 93,000 years, though the exact number is not clear.

Further calculations showed that the timeframe contained three periods controlled by variations in the Earth’s orbit when the weather was relatively wet.

The research team says that caused sediments to build up in lakes and on land far more rapidly than previously had been thought.

Dr. MacLennan said: "Many deceased creatures were quickly buried, and oxygen that normally would fuel decomposition was sealed out.

"The sealing effect was fastest in lakes, resulting in the preservation of soft tissues."

The researchers ruled out volcanic activity.

Some previous studies have suggested that animals were encased by lahars, fast-moving concrete-like slurries of mud that flow off volcanoes following eruptions.

But Dr. Olsen said lahars are "extremely violent" and likely to rip apart any living or dead thing they encounter.

Others have suggested pyroclastic flows - fast-moving waves of searing ash and poisonous gases like at Pompeii - were responsible.

But while there are layers of volcanic ash, lava and intrusions of magma in the Yixian Formation, the remains there don’t match those at Pompeii.

The researchers say feathers, fur and everything else would almost certainly have been burned in a pyroclastic flow.

And many of the dinosaurs were found with arms and tails tucked cozily around their bodies, as if they were sleeping, rather than trying to avoid a horrific death.

The researchers say the evidence points instead to sudden burrow collapses.

Dr. Olsen said that even today, burrow collapses are a common cause of death for birds such as penguins.

He says the frozen-in-time mammal-dinosaur battle may well have happened as the mammal invaded the dinosaur’s burrow, while exceptionally rainy times could have helped destabilize the ground.

Dr. Olsen believes the Yixian Formation is not unique.

He said: “It’s just that there is no place else where such intense collecting has been done in this kind of environment."

China has tried to limit for-profit fossil sales, but researchers say the market is still thriving, and huge government resources are going into the development of tourism around the fossil sites.

Dr. Olsen's personal Holy Grail is feathered dinosaurs, but he says they are "exceedingly rare" even in the richest deposits.

He said: “You have to dig out, say, 100,000 fish to find one feathered dinosaur, and no one is digging on the Yixian scale."

He added: “It takes enormous effort, which is expensive. And land is valuable.

“So no one is doing it. At least not yet.”