MALE NARRATOR: In 1964, a paleontologist named John Ostrom dug up some fascinating fossils from the mudstone of Montana.

The fossils were of a slim, sleek dinosaur with sharp curved claws on its feet and a long tail supported by tendons that allowed it to work like a rudder.

From the evidence, Ostrom could tell that this new dinosaur was an active agile predator.

But that was totally at odds with the popular interpretations of dinosaurs at the time, which was that they were all slow, dumb, lumbering beasts.

This revolutionary discovery was given the name deinonychus, or terrible claw.

And Ostrom's description of it set the stage for what's known today as the Dinosaur Renaissance, a total rethinking of what we thought we knew about dinosaurs.

And one of the key questions that scientists revisited was whether dinosaurs were warm-blooded or cold-blooded.

For decades, paleontologists have been studying this question, like six ways from Sunday, attacking it from every angle.

And so far the answer seems to be they were both warm-blooded and cold-blooded, but also neither?

And also, also, maybe that's not even the right question to ask, because the fact is almost 50 years after the Dinosaur Renaissance began, this one fundamental function of dinosaurs' bodies, their metabolism, remains mysterious to us.

But what we have learned about dinosaurs' metabolism is that it was probably diverse, and experts are starting to think that maybe it's not a question of either/or, yes or no, hot or cold.

The clues that we have are conflicting and fascinating and surprising.

And they can tell us a lot, not just about how strangely diverse dinosaurs were, but also about the evolution of modern birds, and maybe even why the non-avian dinosaurs are no longer with us.

You've probably heard of metabolism as a kind of shorthand for how many calories your body uses to keep you alive.

But more accurately, metabolism refers to all of the chemical reactions that go on inside an organism, like the ones that convert food into energy and the ones that build compounds that its cells need.

These chemical reactions also produce heat.

And warm-blooded animals use this heat in what's known as endothermy.

Endotherms can make enough heat that they often don't need to rely on their environment to stay warm.

This sets them apart from ectotherms, or so-called cold-blooded animals, like most fish, reptiles, amphibians, and invertebrates.

They mainly rely on their environment to set their body temperature.

This difference in temperature regulation means that endotherms can live in more variable environments and are usually more active than ectotherms.

So figuring out if an animal is warm or cold-blooded can tell us a lot about how it lived, which is why experts have been puzzling for so long over which ones the dinosaurs were.

In the early days of dinosaur paleontology, people mostly thought dinosaurs were just big lizards and lived the same basic life these your average iguana.

In fact, the first dinosaur named was megalosaurus, the "great lizard."

Based on its fossils, including a partial jaw and thigh bone, naturalist William Buckland described it in 1824 as a huge lizard that was probably amphibious.

Within a decade of that find, two more prehistoric reptiles were described-- the duck-billed iguanodon and the armored hylaeosaurus.

And together, these finds prompted paleontologist Sir Richard Owen in 1841 to propose the clade Dinosauria, or the terrible lizards.

But then, in the 1870s, a new discovery challenged the idea of dinosaurs as plodding simple beasts.

It was the discovery of archaeopteryx.

Archaeopteryx belonged to a lineage of flying dinosaurs called avialae that sits between feathered therapods like deinonychus and modern birds.

It had some distinctly birdlike traits, like a wishbone, wings, and flight feathers.

And it could probably fly, at least a little.

But it also looked a lot like other dinosaurs.

It had sharp teeth, three-clawed fingers, and a long, bony tail.

And its ankles and wrists were also distinctly therapod-like.

The discovery of archaeopteryx was the first indication that dinosaurs were more than oversized lizards.

They also seemed to be active animals that might have been related to birds.

But these ideas didn't really take hold until John Ostrom resurrected them in the 1970s, when he described deinonychus as a bigger version of archaeopteryx.

He argued that birds descended from dinosaurs, and that many features associated with birds, like active lifestyles, agility, and most importantly endothermy, got their start in an ancient creature that probably looked like deinonychus.

Ostrom's ideas fundamentally challenged our understanding of dinosaurs.

And because of that, deinonychus may be one of the most important dinosaur fossils ever discovered.

But Ostrom's initial argument was based on the bigger general features of deinonychus.

With the development of new sophisticated tools, paleontologists became able to examine fossils down at the microscopic level.

And this gave experts a tremendous amount of new information.

But still, in many cases, the evidence has been either inconclusive or conflicting.

For example, as early as the 1960s, researchers began studying bone tissue.

Typically, fast-growing active endotherms have a type of bone tissue called fibrolamellar bone.

This type of bone grows really quickly and looks kind of woven, like fabric.

And it's also dotted with these little features known as haversian canals, which are the pathways the blood vessels use to bring nutrients to the fast growing tissue.

By contrast, the bones of ectotherms grow more slowly and have fewer of these canals.

And they're also lined with dark rings that show when the bone growth slowed down, which happens in cold-blooded animals during times of seasonal stress.

So what do dinosaur bones look like?

Do they have the woven pattern or rings?

Well, some dinosaur bones have lots of weaves, and some have lots of rings.

Deinonychus, for example, has no bone rings, and was probably more like an endotherm.

Archaeopteryx, on the other hand, did have rings, indicating that it was slow-growing and probably more of an ectotherm.

So some dinosaurs appear to have endothermic traits while others have signs of being ectothermic.

But getting mixed results from bone tissue is not unheard of.

Fibrolamellar bone has been found in animals we know are cold-blooded, like young alligators.

And rings of slow growth also appear in some modern warm-blooded animals, like deer.

So another way to learn about dinosaurs' metabolism is to look for anatomical features in their fossils that we know are related to endothermy.

For example, mammals and birds both have noses that are lined with features known as respiratory turbinates.

These are little webs of bony tissue that warm up air that's entering the lungs and remove moisture from your breath as it's being exhaled.

It's an adaptation that helps us and other endotherms retain more heat and water during respiration.

So finding respiratory turbinates in a skull is widely considered to be evidence of endothermy.

And in the 1990s, researchers studied the nasal regions of three species of therapods and one ornithischian.

They found that in all cases, the nasal passages were too narrow to have respiratory turbinates, and instead they more closely resembled those of ectotherms.

But because nothing is straightforward on this topic, in 2014, researchers reported what seemed to be respiratory turbinates in fossils of pachycephalasaurids from North America and Mongolia.

And then in 2018, similar nasal features in two different species of ankylosaurs were shown to have functioned like turbinates.

Now, other scientists have used lots of other lines of evidence to answer the same question, like trying to estimate dinosaurs' blood pressure, brain size, predator-to-prey ratios, and other things.

But all of those investigations have also produced mixed results.

So it seems that some dinosaurs were so active that they probably ran a little bit warm, even if they were otherwise cold-blooded.

And we know that this is at least possible, because that's how some fish live, like modern tunas and lamnid sharks.

These animals use a sophisticated network of blood vessels to keep the heat generated from their working muscles inside their bodies, instead of losing it to the cold ocean waters around them.

So even though most animals today are either one or the other, dinosaurs were probably somewhere in between.

Some dinosaurs were more ectothermic, while others were more endothermic.

Animals like this that control their body temperature in a variety of ways are mesothermic.

And today, they're pretty rare.

But of course, I've just been talking about the extinct dinosaurs, the non-avian dinosaurs.

What complicates this picture even more is the fact that avian dinosaurs, AKA birds, are all warm-blooded.

So how did that switch happen?

How did a group of animals completely reorganize its physiology to go from mesothermic to endothermic?

Well, modern birds can tell us some interesting things about the origins of endothermy.

Think of a baby bird.

When it hatches, it's naked and it depends on his parents for warmth.

It's basically a little ectotherm.

But as chicks develop, their tissues start producing more energy through the mitochondria in their cells.

As a result, the mitochondria give off more heat.

It becomes an endotherm.

Now, we don't really understand what causes the mitochondria to kick into high gear.

But experts think it may one day tell us a lot about what mechanisms could be at play in the evolution of endothermy.

We do at least have a sense of when endothermy evolved in birds.

The fossil record shows that in the mid-Cretaceous period, birds had respiratory turbinates, as you'd expect in animals with a very high metabolism needed for flight.

And by the late Cretaceous, birds were fully endothermic, and not a moment too soon.

Stop me if you've heard this before, but 66 million years ago, a combination of volcanic activity and a massive asteroid impact set off a global catastrophe of dust, acid rain, and cold.

At least 75% of all species on Earth vanished, including all the non-avian dinosaurs.

But the ancestors of the modern birds survived.

Warm-blooded birds could stay active as the climate took a turn for the worse.

And even though they were endotherms, they needed much smaller amounts and different kinds of food than the huge dinosaurs did.

Out of the ashes of the extinction event, the avian dinosaurs rose and filled many of the niches left vacant by their more reptilian cousins.

So sometimes, unearthing a revolutionary fossil like deinonychus solves a mystery.

But more often than not, it brings up more questions.

But the questions themselves are incredibly valuable, because they remind us that there's a lot more to these terrible lizards than we once thought.

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