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 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 D inosaur Renaissance,
a total re-thinking 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 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 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 short-hand 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.
And in the early days of dinosaur paleontology, people mostly thought dinosaurs were just
big lizards and lived the same basic life as your average iguana.
In fact, the first dinosaur named was Megalosaurus, the “Great Lizard.”
Based on its fossils, including a partial jaw and thighbone, 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 armoured 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 theropods, like Deinonychus, and modern birds.
It had some distinctly bird-like 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 theropod-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
-- 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 it looks kind of woven, like fabric.
It’s also dotted with features known as Haversian canals, which are the pathways the
blood vessels followed 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 was 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 that 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 the 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 theropods
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 pachycephalosaurids from North America and Mongolia.
And in 2018, similar nasal structures 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 those investigations have all 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 this 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 depends on its 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 their 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 non-avian dinosaurs.
But the ancestors of the modern birds survived.
Warm-blooded birds could stay active even 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.
As always a big thanks to this month’s Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng,
and my boy Steve!
Now if go to patreon.com/eons and make a pledge of $15 or more to get access to our new quarterly
Eons livestream!
You’ll get insider updates about the show, as well as the latest paleo news, and special
guests!
And also thank YOU for joining me today, in the Konstantin Haase Studio, if you haven’t
already go to youtube.com/eons and subscribe.