Way back in the early Cretaceous Period, 130 million years ago, a small dinosaur breathed its last breath near a large lake in the hot, forests of China.

Or, wait.

Maybe none of that is true.

Maybe it died at the very end of the Cretaceous, 70 million years ago.

And maybe it was wandering around a river system when it met its end, instead of a lake.

And maybe it didn't even live in China!

Maybe it was in Mongolia!

Welcome to the paleontological firestorm that is LH PV18.

That's the name of a dinosaur specimen that paleontologists have been studying -- and drawing totally different conclusions about - - for almost a decade.

Depending on how you interpret the evidence, you may say that this dinosaur is just one of many specimens of a theropod called Tarbosaurus bataar, probably a juvenile.

Or, you might say it's the original and currently only specimen of an entirely different theropod, named Raptorex kriegsteini All that scientists can agree on about this creature is that it was a dinosaur; it was small and carnivorous; it lived somewhere in central Asia; and it had itty-bitty arms.

That's it.

So, how can there be such hugely different interpretations about what this thing was?

Well, a lot of it comes down to the great disparity over the actual age of the specimen.

Figuring out the age of fossils is an incredibly important field in paleontology, one that we haven't talked about very much.

Because it's complicated.

But in order to answer the question about whether this dinosaur was Tarbosaurus, or whether Raptorex really existed as its own genus, you have to understand the many ways in which we can -- and can't -- determine the age of a fossil.

The first thing to know about LH PV18 is that it was excavated by private collectors, probably illegally, and not by professional paleontologists.

So the exact spot where the collectors dug it up is unclear.

And without that critical piece of information, everything else about this fossil is controversial.

The specimen was first described as a new genus, Raptorex, in 2009, by Dr. Paul Sereno and his coauthors, who figured that the fossil was about 130 million years old.

This finding got a lot of press, because the fossil had the tiny arms of T. rex, but it was many millions of years older than when T. rex lived.

That meant that the tiny arms we now associate with T. rex must have appeared far earlier in the history of tyrannosaurs than we thought.

But another group of scientists, led by Dr. Denver Fowler, took a second look at the fossil in 2011.

And they determined that the fossil was more likely to be 70 to 75 million years old, about 60 million years younger than Sereno's estimate.

Why the disparity?

Because dating specimens involves a complex combination of chemistry, geology, and frankly a fair amount of inference.

In general, there are two main ways to date a fossil.

There's absolute dating, where you can get an exact number for how old a specimen is.

And there's relative dating, where you compare a fossil or rock to others that are similar, and get an approximate age.

The gold standard, the thing every paleontologist longs for, is an absolute date, because they're the most precise.

But they're only available under certain circumstances.

Take, for example, a really popular form of absolute dating called carbon dating.

There are a few different types of carbon atoms, called isotopes.

And most of the carbon in the atmosphere is Carbon 12, so-called because it has 6 protons and 6 neutrons.

The other is found in trace amounts, a heavier isotope called Carbon 14.

Carbon 14 has 6 protons and 8 neutrons, which makes it unstable - aka, radioactive.

And because it's unstable, over time, it decays, releasing an electron and an electron antineutrino to become the stable Nitrogen 14.

Now, both Carbon 12 and Carbon 14 are always in the atmosphere, and they're attached to Oxygen atoms, making CO2, which you breathe in all the time, as do all the animals and plants that you eat.

So, at any given moment, a tiny percentage of the carbon in your bones, teeth, and skin is Carbon 14.

And inside your body, its atoms are decaying - just slowly enough, and releasing few enough particles, that it's not especially dangerous.

But it's also always getting replaced, by new Carbon 14 from the atmosphere and from the food you eat, making it a steady component of your body.

That is, until you die.

When you stop breathing in and eating Carbon 14, it's like starting a chemical stopwatch.

That's because Carbon 14 decays at a steady rate.

Every 5,700 years or so, about half of the Carbon 14 in a specimen decays into Nitrogen 14.

It's what we call a halflife - the length of time it takes for half of the original isotope to have decayed away.

And by studying the ratio of Carbon 14 to Carbon 12 in your bones, scientists can get a pretty good sense of how long ago you died.

So why didn't paleontologists just Carbon-date Raptorex?

Because Carbon 14 decays so fast that after about 40,000 or 50,000 years, there's pretty much no Carbon 14 left.

Even in a big ol' dinosaur.

So carbon dating can't be used for anything you think might be older than 50,000 years.

Thankfully, there are other elements that are even more useful for paleontologists.

Uranium, Lead, and Argon all produce radioactive isotopes.

And even though there's not much of them, if any, in your bones, they are common in certain kinds of rocks -- specifically, volcanic or igneous rock.

And, much like the levels of Carbon 14 in your body, molten lava has a pretty consistent amount of those isotopes moving in and out of it.

But: Once the lava cools, those isotopes can't be replenished.

So, just like when that Carbon-14 clock starts ticking down the instant you die, the moment that volcanic rock cools starts the radioactive stopwatch.

And the good news is, radioactive Isotopes of Uranium, Argon, and Lead have much longer half-lives than Carbon 14, so their isotopes are measurable after millions or even billions of years.

So, they can be used to date much older fossils.

But the bad news is, fossils rarely form in volcanic rocks, like lava.

So, paleontologists have to look for volcanic rock layers that have covered the softer, sedimentary rocks that fossils are often found in.

A volcanic layer above or below a fossil-bearing layer isn't going to give you the exact age of the fossil, but it's often very close.

So, great - we just need to find a layer of lava or ash above or below Raptorex, and the mystery is solved, right?

Ahh, if only.

Some, but not all fossils are found with dateable volcanic rocks on top of them.

And with Raptorex, we don't even know where it was found exactly, so we can't associate it with a certain layer.

So, when there's no way of getting an absolute date, paleontologists can use relative dating instead.

In relative dating, you can get an approximate age range for a fossil from a stratum of rock, by finding other rocks with similar composition that you can date reliably.

So, say you find a dinosaur fossil in a layer of sedimentary rock.

Paleontologists can study the mineral and chemical content of that rock, the shapes and sizes of its grains, and the way that it has settled into layers.

These are all geologic fingerprints that help link different types of sedimentary rock together.

Then, you can look for strata that have those same fingerprints, in other areas where there are volcanic layers nearby.

And that can give you an approximate date for your fossil.

This is called lithostratigraphy8 the use of matching rocks to get an idea of how old something is.

And guess what!

We can't do this with Raptorex either!

Remember, we don't know exactly where the fossil was found, and most of the sediment around its bones had been removed, so the formation was hard to determine.

And without knowing what formation the dinosaur came from, lithostratigraphy can't help you.

So another form of relative dating is biostratigraphy, or determining the age of a fossil by using other fossils found around it.

You've probably noticed by now that animals and plants tend to go extinct, or radiate into different species.

That means there can be short, distinct periods of time when they existed.

So if fossils with known ages are found in other deposits, they can be clues to a rock's approximate age.

Because Raptorex isn't known from any other specimens with known ages, Raptorex itself couldn't be used for biostratigraphy.

But fortunately, that dinosaur didn't fossilize alone!

Buried in the sediment between its bones was the single vertebra of a fish, and this bone is what paleontologists have used to try to determine Raptorex's age.

Sereno's team concluded that the vertebra was from a freshwater fish called Lycoptera, which is known from the late Jurassic to the Early Cretaceous, or about 150 to 130 million years ago.

Since they'd been told that the fossil was found in an area of China that had Early Cretaceous rocks, and which had a lot of fossils of Lycoptera, this made sense.

So Sereno's team concluded that this dinosaur was probably from the Yixian Formation, a formation that, thanks to a layer of volcanic ash, has been absolutely dated using Argon to between 122 and 139 million years in age.

Problem solved, right?

That time frame is millions of years before we know Tarbosaurus existed, so the dinosaur must be a different genus: Raptorex.

But Fowler and his team looked at the same vertebra, and concluded that it was from a totally different kind of fish, a double-armored herring.

And these fish are found all the way from the Early Cretaceous, 130 million years ago, well into the Eocene, 50 million years ago - - which is 15 million years after non-avian dinosaurs went extinct!

So by that logic, the dinosaur could be basically anywhere between 130 and 65 million years old - which is a really big range.

Fowler's team also spoke to a private fossil collector who suggested that the dinosaur was not from China, but possibly Mongolia, which has rocks from the Late Cretaceous, about 75 to 70 million years old.

But the story of Raptorex wouldn't be complete without talking about the final wrinkle in this saga: In addition to the dispute about the age of the fossil, there's also a disagreement about whether the dinosaur was an adult or a juvenile.

And this is important, because there have been times in the past when specimens have been mistakenly assigned to a new genus, before we figured out that they were just a baby version of known species.

And one way to tell an adult from juvenile dinosaur is to look at the fusion of the pieces of its vertebrae.

Young animals often have vertebrae and other bones that aren't fully fused, because being in separate chunks allows them to grow.

Sereno's team concluded that the specimen's bones were fused enough that it was maybe 5 or 6 years old, a subadult.

Fowler's team disagreed -- again!

-- saying that the bone fusion was inconclusive, and it could have been anywhere between 3 and 6 years old.

And given the similarities between this specimen and Tarbosaurus, Fowler's team concluded that Raptorex was just a baby Tarbosaurus - - a genus that we know lived 70 million years ago -- rather than the adult of a new genus that lived longer ago.

So is Raptorex, 130 million years old or 70 million years old?

An adult of its own unique genus, or is it a juvenile Tarbosaurus?

Can we not science our way out of this?!

Well, with enough time and money, there may be other, newer ways to fix a date to this animal.

For example, we could try Detrital Zircon testing, which studies the isotopes in zircons found in the surrounding rock to come up with an age range.

Or, we could have another run at biostratigraphy, but this time, looking for pollen.

Pollen, like other fossils, has shapes that are specific to different species of plants, which means it can be used to help establish a date.

And of course, there's always the possibility that someone will discover another dinosaur that has all of the same basic traits, and found in a known location.

Hey, we can always dream.

For now, though, the age of Raptorex - or Tarbosaurus -- is hung up on a fish fossil - so you'll have to decide for yourself what age it is.

In the end, we don't know for sure whether that dinosaur died in China or Mongolia, or by a river or a lake, or 70 million years ago or 130 million years ago.

So the story of LH PV18 is a story that doesn't have a setting.

We can't talk about it in a certain place or time.

Instead, it's more of a story about what we don't know, and why science can often be inconclusive, and how much we still have to learn.