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!
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