Paleontology: Theropod Dinosaurs and the Origin of Birds is a five-lesson course teaching a comprehensive overview of the origins of birds. This course examines the anatomy, diversity, and evolution of theropod dinosaurs in relation to the origin of birds. Students explore various hypotheses for the origin of flight. Watch a preview of the course here: https://uofa.ualberta.ca/courses/paleontology-theropod-dinosaurs
4.1 The Dinosaur Renaissance
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From the course by University of Alberta
Paleontology: Theropod Dinosaurs and the Origin of Birds
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From the lesson
Coelurosaurs II
Dinosaurs had long been thought of as overgrown reptiles; cold blooded, swamp bound, with meagre intelligence and little to no social complexity. The ‘Dinosaur Renaissance’ was a revolution in palaeontological thinking that entirely transformed that traditional image of dinosaurs. In Lesson 4, we will see how new research and discoveries over the past fifty years have shaped our modern image of dinosaurs into one of energetic, intelligent animals, that likely displayed many of the complex social behaviours witnessed in modern birds. You’ll also meet the deinonychosaurs, A.K.A. ‘the raptors’, and you will learn the leading theories for how one group of dinosaurs learned to fly.
Meet the Instructors
Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
You will remember from lesson one, that it was Sir Richard Owen, who in 1842,
first recognized that various fossil species of
giant Mesozoic vertebrates belong together in the single natural group.
He called that group the Dinosauria.
That name translates to terrible or 'wondrously great' lizards.
This is my friend, Beans, the bearded dragon and he's here to help explain that.
Originally there were only three species included in the Dinosauria.
And all three were known from only incomplete and scrappy fossil material.
You'll also remember that the first Theropod to be scientifically described
was Megalosaurus.
And it was one of Owen's three original dinosaurs.
Among the few bones known from Megalosaurus was
a section of the jaw with teeth still in place.
These teeth look very similar to those of modern large carnivorous lizards, and
they were set in sockets like the teeth of modern crocodiles.
The two other original dinosaurs were Igaunadon, and
the ankylosaurus Hylaeosaurus.
Iguanodon had teeth that were similar to those of an iguana.
And Hylaeosaurus had small armoured plates, or osteoderms, like a crocodile.
This selection of fossils suggested the group was reptilian.
And with nothing better to go on, Owen and
others envisioned dinosaurs as essentially monstrous reptiles.
In drawings and sculptures that Owen had created to illustrate dinosaurs,
all the missing anatomy was artistically filled in using lizards and
crocodiles as models.
And so that became the starting point for imagining what a dinosaur was.
In retrospect, it's a shame that the original Megalosaurus material
did not include well preserved foot bones, instead of a toothy jaw.
If it had, Owen would surely have recognized the avian pharyngeal formula.
Then, he might well have name the group dinoornithea,
the terrible or 'wondrously great' birds.
As it happened, reptile like became the default assumption for
anything dinosaurian.
Now, I happen to like reptiles and find it regrettable that there were,
and are, a great many reptilian misconceptions.
These negative stereotypes contributed to a caricature of dinosaurs
as overgrown lizards.
Dinosaurs were sluggish, dull witted, anti-social,
swamp-dwelling, Darwinian dead ends.
They were the denizens of prehistory's primordial backwaters.
Preconceived notions are often the hardest alter,
even when contradictory evidence sits directly in front of you.
That's a fact of human nature.
As scientists, we're just as human as anyone else.
Over the 120 years that followed Owen's naming, more dinosaur fossils were found.
The evidence indicating that dinosaurs were something
other than large reptiles grew and grew.
Then, in 1964 an expedition from the Yale Peabody Museum under
the leadership of John Ostrom discovered skeletons from a new genus of theropod.
A theropod that Ostrom realized dramatically shattered
every aspect of the orthodox view of dinosaurs.
Ostrom named the new theropod Deinonychus.
The discovery of which of the following traits would have contradicted
the historical misconception of dinosaurs?
Is it A, short hind legs.
B, a large brain cavity.
C, evidence of a solitary hunting strategy.
And or D, associated fossil feathers.
More than one answer might be correct, so
select all the answers that you think apply.
B and D are both correct answers.
The discovery of a large brain cavity and fossil feathers would have gone
a long way towards countering the classic dinosaur stereotypes.
As it happens, Deinonychus was found to have elongated hind legs.
Although it's legs were not as elongate as in many other theropods,
such as small tyrannosauroids and ornithomimids.
Deinonychus appeared to be adapted for agility and speed.
The skull of Deinonychus did indicate a surprisingly large brain,
suggesting a level of intelligence that exceeded any modern reptile.
Ostrom's team found several specimens of Deinonychus all in the same quarry.
He interpreted this as evidence that the dinosaurs had died together.
And therefore had probably been living together in a social group.
As it happened, the Deinonychus material discovered by the Peabody crew
did not include any evidence of feathers.
Although we now know that Deinonychus did have them.
While actual feathers were absent, Deinonychus had a lightly built skeleton,
long arms, each with three long fingers.
In these regards, Ostrom realized that Deinonychus
was extremely similar to Archaeopteryx and he revitalized
Thomas Henry Huxley's old theory that birds are the descendants of dinosaurs.
Deinonychus was the catalyst that brought on chain reaction of
palaeontological revaluations.
Dinosaurs could now be reimagined as quick and athletic not slow and sluggish.
Some were relatively intelligent and
social, perhaps even cooperative pack hunters.
Far from extinct failures, they were the ancestors of the more
than 10,000 species of birds that are still alive today.
Other scientists looked with fresh eyes at different kinds of dinosaurs.
And suddenly,
all the evidence that had been sitting directly in view came into focus.
We call this sudden turning point in the general conceptualization of dinosaurs,
the Dinosaur Renaissance.
Ostrom began it, but
it took the work of many paleontologists to bring it fully about.
One of the other leading lights of the renaissance was Dr. Robert T Bakker.
Bakker was one of Ostrom's graduate students and
part of the original team that found and prepared the first Deinonychus material.
He is famous for his argument that all dinosaurs had a high metabolic
physiology that was more similar to that of mammals and birds then to reptiles.
This is one of the most important scientific arguments
in our struggle to understand non-avian dinosaurs.
It's often referred to as the warm-blooded, cold-blooded debate.
The terms: warm and
cold-blooded are not generally used in the formal scientific literature.
Their meaning can be ambiguous.
So we need to cover so basic physiology terms.
Endotherms are animals that regulate their body temperature internally.
When the external environment is too cool, they use energy to create heat.
When it's too hot, they sweat or pant to cool down.
Mammals, like you and I, are examples of endotherms.
Ectotherms are animals that cannot regulate their body temperature internally
and so must regulate it using temperature variations of external microclimates.
To warm up, they can bask in the Sun and lay on top of warm rocks.
To cool down, they can wade in the cool water or seek out shady spots or burrows.
Reptiles, like lizards are examples of ectotherms.
Homeotherms are animals that maintain a constant internal body temperature.
Heterotherms are animals that have fluctuating internal body temperatures.
Tachymetabolic animals are those that have high metabolic rates, even when at rest.
Finally, bradymetabolic animals are those that
have low metabolic rates when at rest.
Metabolic types can be combined within a single creature,
which of these animals is both an ectotherm and a heterotherm?.
Is it A, tortoise.
B, dolphin or C, hawk.
A, the Tortoise is the correct answer.
As a reptile, it is an ectotherm since it regulates its body temperature using micro
climates like a Sun warmed rock.
It also a heterotherm, because its internal body temperature fluctuates.
In simplified terms, a tortoise is warm during the day and cool at night.
Naturally, these physiology terms are not mutually exclusive.
In fact, they're often linked to each other.
For instance, an animal that we classically consider to be warm-blooded
such as a monkey or a hawk have high internal body temperatures.
Because these internal temperatures are usually higher than the external
environment, internal metabolic processes must maintain them.
So the animal is an endotherm.
Internal regulation prevents the animal's body temperature from strongly
fluctuating and thus, it is also a homeotherm.
Maintaining a constant high body temperature via metabolism requires
a constant high metabolic rate, even when resting, and so
the animal is also tachymetabolic.
By comparison, a classically cold-blooded animal, like a rattlesnake or
a tortoise, relies on the external environment for heat and
must seek shelter when needing to cool down.
So, it is ectothermic.
Waiting for its temperature to rise while basking in the Sun or
to cool while in the shade results in fluctuating body temperatures.
So it is a heterotherm.
But there are many exceptions to these general associations.
Many aquatic animals are ectotherms, but because large bodies of
water are resistant to temperature fluctuations, these animals
live in environments where the external temperature does not fluctuate.
And so neither does their body temperature and
they could be classified as homeotherms.
Here's a tricky question.
Hummingbirds regulate their temperature internally,
they've incredibly high metabolic rates when awake.
But while sleeping at night, their metabolism slows to a crawl and
their body temperature plummets.
So which of the following terms accurately describes the metabolism of a hummingbird?
More than one answer might be correct, select all the answers that apply.
A, Endothermic.
B, Ectothermic.
C, Tachymetabolic and, or D, Bradymetabolic.
A and D are the correct answers.
A hummingbird is an example of a bradymetabolic endotherm.
Unfortunately, we're 66 million years too late
to take a dinosaur's temperature or monitor its daily metabolic rate.
So what evidence did Robert Bakker have to argue for high mammal and
bird-like metabolic temperature regulation in dinosaurs?
For starters, he considered the complex teeth of hadrosaur and
ceratopsian dinosaurs.
These teeth are clearly adapted for intense and efficient chewing.
This, Bakker argued indicated a high food consumption need.
Modern mammals and birds have high food consumption needs, because
of the energetic demands of maintaining high constant internal temperatures and
this high food requirement also means that it takes more prey to satisfy
a mammalian or avian predator than a reptilian predator of equivalent size.
Therefore, for any number of potential prey species,
an ecosystem can have fewer carnivorous mammalian and
avian predators than it can reptilian predators.
When Robert Bakker started adding up the number of dinosaur predators and
the number of dinosaur prey in well sampled fossil localities,
he found a low ratio of predators to prey and this indicates that
predatory dinosaurs also had high energy requirements.
Bakker also theorized that although cold-blooded animals
are far more numerous than warm-blooded animals,
being warm-blooded imparted a distinct competitive advantage when
it came to the specific niches of terrestrial large bodied animals.
Think for a moment about the modern world.
How many terrestrial mammals can you name that weigh over 20 kilograms or 50 pounds?
Well, there are a lot.
There's you, me, deer, lions, elephants, llamas, wombats and so on.
And birds?
Birds mostly fly, but there are still various large,
flightless birds like ostriches in Africa and Asia,
rhea in South America and emu and cassowaries in Australia.
Now how about any modern 50-pound, fully terrestrial reptiles?
Not so many.
Crocodiles and various large snakes are semi-aquatic.
The only real terrestrial reptilian heavyweights
are animals like Galapagos tortoises and Komodo dragons and
these giant reptiles live only on small island chains where they're
isolated from competition with large-bodied warm bloods.
The comparative scarcity of modern large terrestrial
cold-bloods has been interpreted as evidence that they have been
excluded from such niches by competition with mammals and birds.
Now the first primitive proto-mammals evolved at roughly
the same point in time as the first proto-dinosaurs.
And true non-avian dinosaurs and
true mammals coexisted for nearly 150 million years.
Yet in all that time, it was the mammals that never broke the 20-kilogram mark.
Bacher argued that this lack of large Mesozoic mammals was
the result of competitive exclusion on the part of dinosaurs.
And that exclusion could not have happened if dinosaurs were cold blooded.
Dinosaurs had an erect posture with legs positioned underneath their bodies,
just like mammals and birds.
They did not have splayed out legs like lizards.
An erect posture is a trait characteristic of warm blooded animals.
Do you think this is because A, straight legs carry blood more efficiently?
B, an erect posture allows endotherms to reach higher food resources?
Or C, an erect posture facilitates an active lifestyle?
As you're about to see, active animals benefit from an erect posture.
That makes C the most correct answer.
Another important line of evidence that suggests a high metabolic rate in
dinosaurs comes simply from the erect posture of their limbs.
And this is trait that Owen realized dinosaurs shared with mammals and birds.
Although, he failed to recognize the full implications.
Erect limbs are better at passively supporting the body's weight
while standing and walking.
They are therefore associated with animals that spend more time being up and active.
With no need to sit still and absorb heat, or wait to cool down,
combined with high food requirements, warm blooded animals are generally more active.
Obviously all these lines of evidence are indirect, and
testing an extinct animal's physiology is extremely difficult.
However, since Doctor Bakker first made his case for mammal and
bird-like metabolic temperature regulation, additional lines of evidence
have come to light that lend more support to his theory.
As we continue to examine evidence that supports mammal and bird-like metabolisms
in dinosaurs, we've begun to discover dinosaur fossils within the polar circles.
So think about this.
There are very few examples of arctic reptiles.
Why is that?
Is it because, A, ectotherms require warm microclimates
to raise their body temperature, and these are scarce in the arctic?
B, because arctic climates make food resources scarce.
Or C, continental drift prevented reptiles from populating arctic regions.
The correct answer is A.
The arctic provides few opportunities for warming microclimates to exist.
So ectotherms, like reptiles, have difficulty adapting to that environment.
Histological studies of dinosaur bones have revealed that they grew very quickly.
And this is another trait associated with birds and mammals, but not reptiles.
Chemical analysis of oxygen isotopes throughout dinosaur skeletons indicate
that their temperatures throughout the body were essentially the same.
This is suggestive of homeothermy. Large blood vessel pores throughout the interior
dinosaur bones had been interpreted as attesting to high oxygen and
nutrient demands, which are associated only with modern tachymetabolic animals.
Regulating your body temperature ectothermically only works if you can
reliably find microenvironments with both warm and cool climates.
But that's not always possible, particularly in cold regions.
And that's why there are no reptiles or amphibians in the Arctic or
Antarctic, and why there are very few here in Northern Canada.
During the Mesozoic, global temperatures were much higher than they are today.
Nevertheless, the discovery of abundant large dinosaurs in polar regions,
including Alaska and southern Australia, show that at least some
dinosaurs lived in environments that experienced seasonably cold climates.
By far, the most powerful piece of evidence supporting endothermy and high
metabolic rates in dinosaurs has been the discovery of non-avian dinosaur feathers.
You will remember from lesson three, that the covering of hairlike feathers
preserved in many coelurosaur specimens was adapted to serve as insulation.
This supports the argument for endothermic metabolisms because,
A, ectotherms seldom display during courtship and
early feathers were used in social display.
B, feathers reflect heat while basking in the sun.
Or C, endotherms generate their own body heat and feathers would keep that heat in.
C is the correct answer.
Insulation only works if you're generating internal heat.
Otherwise there's no heat to hold in.
So there's no reason for an animal to have insulation if it isn't endothermic.
A covering of insulation indicates high internal
metabolic heat generation and the need to maintain a constant internal temperature.
Coelurosaurs were not the only dinosaurs to have insulation.
It's now known that megalosaurids also had hair-like coverings and so
did several varieties of small ornithopods.
It remains to be proven if these coverings were true early feathers or
similar but evolutionary unrelated adaptations.
There is one last concept that is critical to the current status of the debate.
That's the idea that many dinosaurs, particularly the enormous Sauropods, might
have maintained constant high internal body temperatures through gigantothermy.
Because ectotherms lose and gain heat from the outside environment, the rate at which
that heat is lost or gained is influenced by their body's exposed surface area.
Now large animals have less surface area relative to their internal
volume than smaller animals with the same overall shape.
This means that once warmed, extremely large animals lose heat slowly.
If an animal were large enough, the rate of heat loss would theoretically be so
low that it could be compensated for by only low level metabolic processes.
In other words, it's theorized that big dinosaurs could have been
ectothermic, but still homeothermic by virtue of their great mass.
This is an interesting theory that many paleontologists think
explains a lot about dinosaur physiology.
However, it should be pointed out that the theory is, of course,
not applicable to the many varieties of small dinosaurs.
And today, we can observe that the largest land mammals,
like elephants and rhinos, are still very much endothermic.
Where does all of this leave the warm-blooded/cold-blooded debate?
The general consensus among paleontologist is that most Coelurosaurs
were probably endothermic, tachymetabolic, homeotherms.
And many paleontologists would argue that most other theropods
probably were as well, though, perhaps, to a lesser degree.
And some paleontologists, myself included, think the evidence is sufficiently
compelling to confidently label virtually all dinosaurs as warm blooded.
Simple feathers for
the presumed function of insulation are common among Coelurosaurs.
Structurally complex feathers that are leaf shaped for
the presumed function of sexual display are known from
specimens of Oviraptorosaurs and scansoriopterygians.
Which of the following feather morphologies have we
already encountered in non-avian theropods in this course so far?
A, gaps between the primary flight feathers.
B, a stiff central shaft.
C, an elongated form, unique to the arms and
tail, and/or, d, an asymmetrical form.
Multiple answers might be correct, so select all that apply.
Gaps between the primary flight feathers are an adaptation seen
in may modern birds that helps to prevent stalling while in flight.
This is an advanced trait.
We have yet to see it in our trek through the theropod family tree.
In addition, it's not a feature required for simple early flight.
We also haven't encountered asymmetrical feather shapes yet.
However, the display feathers of Theropods we've studied have had
stiff central rachis and feathers on the arms and tail already had elongate forms.
So B and C are the correct answers.
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