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Digital Studios. 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 lithostratigraphy¸ 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. Thanks for joining me! And I want to thank CuriosityStream for supporting
PBS Digital Studios. With CuriosityStream you can stream documentary
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promo code EONS. Now, extra-big thanks to our four eontologists,
David Reed Rasmussen, Jon Ivy, Eric Lawrence, and … Steve. Thank you so much for your support! If you’d like to join them, head over to
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Pbs eons is truly such a gift
They have a bunch of great videos!
A fascinating look at how paleontologists try and discern details from fossils of naturewasmetal to determine what they are, how they lived and where they fit into the tree of evolution
Uploads PBSeons
Here come the upvotes :)
The thing is, carbon dating is incredibly fucking innacurate anyways. There were different levels of carbon in the atmosphere back then and it changed a lot so you'd need to be able to know when they died to be able to compare it to how much carbon they have now, it can get contaminated, and there's so much more like scientists have carbon tested living pengiuns to be 8,000 years ago. There's so many examples of this it's incredible.