125 million years ago in Northwestern China, a  dinosaur laid down to sleep…and never woke up.

And it wasn't alone.

Since the 1990s, paleontologists have been  pulling complete dinosaur skeletons from the   rocks of the Lujiatun in Liaoning Province,  most of them seemingly posed in perfect rest.

These fossils include all kinds of dinosaurs  – like, a carnivorous troodontid that was   found with its neck curled around its body, an  early example of how most birds sleep today.

Nearby, early hadrosaurs were found tucked up   comfortably with their rigid tails  sticking out straight behind them.

All are beautifully preserved whole,   covered gently by a blanket of ashy rock  that has kept them protected all this time.

But not all the residents were  dozing quite so peacefully.

In 2023 a new set of fossils was described that  reveals an early ceratopsian with a porcupine-like   tail, Psittacosaurus, and a weasel-shaped  early mammal, Repenomamus, entombed together.

The animals are entwined.

The fingers of  Repenomamus are locked tightly around the   dinosaur's jaw and its teeth are sunk  deep into the ribs of Psittacosaurus.

This fossilized scene seems to capture a fight  between a voracious mammal and its unlucky   prey, covered and killed together  in the middle of their engagement.

Fossils like this tell us so much  about extinct animals that we never   would otherwise know – things like hunting  technique or even normal body postures.

And Lujiatun has often been compared to a  famous archeological site where behavior   is also well preserved: the site of Pompeii,   a town crystallized by a volcanic  eruption almost 2000 years ago.

But while it seems like something   similar must have happened to freeze these two  places in time, when you look at the details,   it turns out that their residents may have  experienced very different final moments.

In both Pompeii and Lujiatun, residents  were preserved in their daily lives,   with no evidence that they saw  death coming and tried to avoid it.

It’s odd – almost a geological contradiction.

If an eruption or flood is moving fast enough to  catch you off guard, it should also have enough   strength to pick you up and toss your body around  like a ragdoll – and, presumably, wake you up.

If the eruption is so slow that  it wouldn’t disturb your body,   you’d probably see it coming,  even if you couldn’t outrun it.

So, in these two places, something  caused instantaneous death and then   also preservation.

This is exceptionally rare.

And yet, some scientists actually think that  the two events happened in different ways.

Lujiatun's story started when  China was splitting apart.

To the east, a new Pacific plate was forming.

Its birth was not easy for the existing plates,  which got pushed and pulled in uncomfortable ways.

The once stable continental  rock began to thin and crumple,   leading to valleys and lots and lots of volcanoes.

These volcanoes were similar to Mt.

Vesuvius,   the volcano that would later  kill the residents of Pompeii.

In both cases, some of the magma was full of  dissolved water and thick, viscous silica.

And so instead of spewing slow-flowing lava,   these volcanoes could explode  in violent eruptions of ash.

When these eruptions are big enough and the  conditions are right, this ash can flow down   a hillside in a superheated mixture of gas and  rock, a lethal event known as a pyroclastic flow.

These flows can reach speeds of more than  100 kilometers per hour and can travel   many kilometers away from the original volcano.

This is the exact sort of eruption that covered  the residents of Pompeii.

But in Lujiatun,   the rocks point to something a little different.

They seem to have originally been a pyroclastic  flow, but one that was… reshuffled a bit,   with the rocks not quite in the right  order… As if the ash had been laid down,   then picked back up and moved again.

This can happen many ways: if that  pyroclastic flow hits a river,   for example, or erupts through a glacier, or  if a big rainstorm floods down upon loose ash.

In any case, the result is the same:  a volcanic mudslide known as a lahar.

Lahars are also devastating  – they can be hot or cold,   and in steep areas can reach  over 200 kilometers per hour.

But there is one big difference  between lahars and pyroclastic flows,   and that is how they change as they slow down.

By the time the pyroclastic flow from Mt.

Vesuvius  traveled the roughly 8 kilometers to the town   of Pompeii, it had lost some of its original  density and thickness, becoming almost fluffy.

It was still fast enough to cover a person,  but light enough to not move their body.

But lahars are much thicker than pyroclastic ash   clouds—so thick that they’re  often described as “concrete.” They can slow down and become more liquid  and gentle, but the problem still remains:   a lahar that was fast enough to catch and  kill a dinosaur without even waking it up   would also have been powerful enough  to fling the dinosaur downstream.

So even if Lujiatun experienced one,  that would only be a piece of the puzzle.

And both the residents of Pompeii and the  dinosaurs of Lujiatun were preserved quickly,   as if the rocks took a snapshot of  their lives… and the explanation   for how that happened might be  hidden in their body posture.

The story is, perhaps, a bit simpler in Pompeii  - or maybe we just know it better.

After all,   people were there to watch  it happen and write about it.

The pyroclastic flow hit the town in the  morning, when most residents were awake.

Approximately 70% of the townspeople  were preserved in relatively   life-like postures - walking, sitting,  crawling, and in some cases, sleeping.

But there are small details in their  postures that are different from daily life.

For example, 64% of the bodies  show flexed arms and fingers.

And although these were initially interpreted as  people trying to defend themselves from the ash,   we now know it happened due to a  phenomenon called pugilistic attitude.

This is a positional change that occurs when  muscles and tendons are rapidly dehydrated.

The pyroclastic flow that  hit Pompeii was so hot that   it caused almost instantaneous  death, frying people in place.

The ash then settled around their bodies,  which were covered in more rocks and ash as   the eruption continued, eventually preserving  the town in around 7 meters of sediment.

But none of the dinosaurs in  Lujiatan show pugilistic postures   or any apparent reaction to their entombment.

This has led some scientists to  theorize that perhaps these dinosaurs   were preserved in their burrows, which may  have collapsed and killed them immediately.

But that doesn't seem to explain all of the cases.

Like, one especially sad fossil shows  a group of 24 juvenile Psittacosaurus.

All of the babies' bodies are rotated slightly,   as if they'd been pushed around by a weak current  - something difficult to do inside of a burrow.

So, this fossil points to a lahar.

And Repenomamus also shows small  signs it was moved by a current.

Its back leg is trapped within the bent knee  of the Psittacosaurus it was trying to eat,   suggesting the two bodies were shifted  slightly before their final preservation.

Yet none of the sleeping dinosaurs  show any signs of waking.

And Repenomamus seems to have  been thoroughly enjoying its   last meal.

So…how did they all miss the lahar?

Well, it's possible that they were already dead.

See, volcanoes can also  have eruptions of pure gas,   including CO2, which is odorless and invisible.

CO2 is normally a safe component of  our planet's atmosphere, but at high   enough concentrations, it's deadly to  breathe.

It can cause unconsciousness   almost instantaneously, then within a  minute: respiratory arrest and death.

And this is what some scientists think  happened to many of the dinosaurs at Lujiatun,   especially those that were preserved sleeping.

They were killed by invisible gas  almost instantly - as quickly as   the residents of Pompeii, if not as dramatically.

Then, before their bodies could collapse or rot,   they were preserved in place  by a slow-moving lahar.

Now, excavations at Lujiatun continue  – with new discoveries each year.

But one problem with Lujiatun dinosaurs is that   they are often found by farmers and locals  and excavated before paleontologists arrive.

So it can be difficult to tell  exactly what layer they came from.

Fortunately, these dinosaurs still have some of  their original sediment stuck to their bones.

And analyses of this sediment  determined that not all of the   sleeping dinosaurs have even come from  the same location or the same eruption.

That means that this deadly combination of  gas and ash may have happened more than once.

The conditions that preserve behavior  in the fossil record are unusual,   but Lujiatun seems to have been a  perfect storm of volcanic influences.

And surprisingly, it was probably a  different perfect storm than Pompeii saw.

Lujiatun’s sleeping dinosaurs  have taught us a lot – like that   Psittacosaurus took care of its young  and was hunted by much smaller mammals,   and even that the tails of some early  hadrosaurs were too rigid to bend!

They may have been caused by different geological  events and took place millions of years apart,   but sites like Pompeii and Lujiatun  give us a unique window into daily   life that we would otherwise never see  – even if that life was taking a nap.