Two perfectly articulated skeletons of the sheep-size dinosaur Psittacosaurus, found in China’s Yixian Formation. New research suggests they died in burrow collapses, not via volcanism, as previously thought. Credit: Jun Liu, Institute of Paleontology and Paleoanthropology, Chinese Academy of Sciences

The world’s ‘best-preserved dinosaurs’ are thought to have died in Pompeii-type events, but new research may suggest a more ordinary explanation.

Between about 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 pretty much undisturbed until the 1980s, when villagers started finding exceptionally preserved creatures, which fetched high prices from collectors and museums.

This started a fossil gold rush. Both locals and scientists have now dug so much, their work can be seen from space – perhaps the most extensive palaeontological excavations anywhere.

By the 1990s, it was clear that the so-called Yixian Formation contained uniquely well-preserved remains of dinosaurs, birds, mammals, insects, frogs, turtles and other creatures.

Unlike the skeletal and often fragmentary fossils unearthed in most other places, many animals came complete with internal organs, feathers, scales, fur and stomach contents. It suggested some kind of sudden, unusual preservation process at work.

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

The world’s first known non-avian feathered dinosaurs showed up – some so intact that scientists worked out the feathers’ colours.

The discoveries revolutionised palaeontology, clarifying the evolution of feathered dinosaurs, and proving without a doubt that modern birds are descended from them.

Leading hypothesis

How did these fossils come to be so perfect? The leading hypothesis up to now has been sudden burial by volcanism, perhaps like the waves of hot ash from Mt Vesuvius that entombed many citizens of Pompeii in AD 79.

The Yixian deposits have been popularly dubbed the “Chinese Pompeii.”

A new study says the Pompeii idea is highly appealing – and totally wrong. Instead, the creatures were preserved by more mundane events including collapses of burrows and rainy periods that built up sediments that buried the dead in oxygen-free pockets.

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

The current study uses newly sophisticated technology to date the fossils to a compact period of less than 93,000 years when nothing particular happened.

Study co-author Paul Olsen, a palaeontologist at the Columbia Climate School’s Lamont-Doherty Earth Observatory, 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, ie 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 coauthor Paul Olsen at the site where the first feathered non-avian dinosaur was found, in 1996. It had excellently preserved filamentous feathers, showing that non-bird dinosaurs developed them first for insulation, not flight. Credit: Paul Olsen.

Perfectly articulated

The Yixian Formation fossils come in two basic varieties: intact, perfectly articulated 3D skeletons from deposits formed mainly on land, and flattened but highly detailed carcasses found in lake sediments, some containing soft tissues.

To come up with fossil ages, the study’s lead author, Scott MacLennan of South Africa’s University of the Witwatersrand, analysed 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 ionisation 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 this timeframe contained three periods controlled by variations in the Earth’s orbit when the weather was relatively wet.

This caused sediments to build up in lakes and on land far more rapidly than previously had been thought.

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 rule out volcanism on multiple counts. Some previous studies have suggested that creatures were encased by lahars, fast-moving concrete-like slurries of mud that flow off volcanoes following eruptions.

But lahars are extremely violent, said Olsen, and apt to rip apart any living or dead thing they encounter, so this explanation does not work.

Others have said pyroclastic flows – fast-moving waves of searing ash and poisonous gases á la Mt Vesuvius – were responsible.

These struck down residents of Pompeii, then wrapped the bodies in protective layers of material that preserved them as they were at the moment of death.

Even when remains decayed, voids in the ashes remained, from which investigators have made lifelike plaster casts.

The remains characteristically are curled in so-called pugilistic positions, torturously doubled over and with limbs severely drawn up, as blood boiled and bodies crumpled in the explosive heat. Victims of modern fires exhibit similar poses.

While there are in fact layers of volcanic ash, lava and intrusions of magma in the Yixian Formation, the remains there don’t match those of the unfortunate Pompeiians.

For one thing, feathers, fur and everything else would almost certainly have been burned in a pyroclastic flow.

For another, the dinosaurs and other animals are not in pugilistic positions; rather, many are found with arms and tails tucked cosily around their bodies, as if they were sleeping, perhaps dreaming dinosaur dreams, when death found them.

Imminent attack: An artist’s rendition of a Psittacosaurus dinosaur with babies being hunted by Repenomamus, a mammal. One fossil assemblage from the Yixian Formation preserved the remains of these species in mortal combat, frozen in mid-action. The dinosaur here is shown with bristly proto-feathers on its tail. Credit: Illustration by Alex Boersma.

Burrow collapse

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

Cores of rock surrounding the skeletonised fossils generally consist of coarse grains, but grains immediately around and within the skeletons tend to be much finer.

The researchers interpret this to mean that there was enough oxygen around for a while for bacteria or insects to degrade at least the animals’ skin and organs, and as this happened, whatever fine grains were in the surrounding material preferentially seeped in and filled the voids; the more decay-resistant bones remained intact.

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

The frozen-in-time mammal-dinosaur battle may well have happened as the mammal invaded the dinosaur’s burrow to try and eat it or its babies, he said.

As to what caused the burrow collapses, this is speculation. One thought: bigger dinosaurs (whose remains don’t appear here but who were almost certainly around) could have squished burrows simply by tromping around.

Exceptionally rainy times could have helped destabilise the ground.

Olsen believes the Yixian Formation is not unique: “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 the market is still thriving, and huge government resources are going into development of tourism around the fossil sites.

Olsen’s personal Holy Grail is feathered dinosaurs, but these 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.”

The study was just published in the journal Proceedings of the National Academy of Sciences.