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
3.1 Early Coelurosaurs - Part 1
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Paleontology: Theropod Dinosaurs and the Origin of Birds
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Coelurosaurs I
In the previous lesson, we explored how the various theropod lineages adapted to their role as apex predators. In this lesson, we will explore a new group of theropods, as much characterized by their speed and agility as their predatory prowess. The coelurosaurs were the most successful and diverse of all the theropods, and included herbivores, the smallest of all dinosaurs, and, of course, the mighty tyrannosaurs.
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Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
We're naturally fascinated by the majesty and power of large carnivores,
whether they're lions, sharks, or tyrannosaurs.
We're excited by the thought of them despite how dangerous they are.
But in most ecosystems,
apex predators are vastly outnumbered by a slew of smaller body predators.
Although, the Ceratosaurs, Megalosaurs, and Allosaurs,
ruled at the top of the late Jurassic food chains,
they coexisted with various smaller carnivorous theropods.
Being smaller, such theropods had smaller bones,
and that's made their fossil skeletons much harder to find,
but we know they were around.
Among the middle and late Jurassics' tinier theropods,
where animals like Ornitholestes, Coelurus,
and their old friend Compsognathus.
These little predators belong to a group of theropods known as the Coelurosauria.
The members of the Coelurosauria,
which we call simply Coelurosaurs,
started out in the middle Jurassic at the size disadvantage,
however, they had other things going for them.
In the middle and rear portions, they're slender tails,
they were exceptionally elongated zygapophyses,
and zygapophyses are little forwards and backwards projecting prongs.
And these prongs, overlap with those adjacent in a series,
to strengthen the vertebral column,
and the extra long zygapophyses of Coelurosaur tails,
held their tails extra stiff.
Now, what good was that?
Well, it certainly meant that it was easier for
Coelurosaurs to passively hold their tails up and off the ground.
Additionally, by preventing the back half of the tail from sagging and flopping around,
Coelurosaurs may have been better able to use
their tails as dynamic stabilizers while running.
That is, when trying to make a turn at high speeds,
they could swing their tails to one side to help them
maintain balance and overcome rotational inertia.
Coelurosaurs also had more vertebrae connected directly to their hipbones.
This stiffened their bodies and made the hips slightly stronger anchors for leg muscles.
Their legs and feet were also different.
Their metatarsals were long and narrow,
and the tibia was also long substantially lengthening the leg.
And this meant that every step a Coelurosaur took, covered more ground.
When running at their maximum gate,
this made Coelurosaurs faster.
So far, you've learned about early Coelurosaur adaptations,
the stiff tail and long legs.
This made Coelurosaurs: A, apex predators,
B, fast and agile,
or C, great climbers?
Coelurosaur adaptations made them fast and agile.
So, B is the most correct answer.
Most of the defining features of
the Coelurosauria are adaptations for improved speed and agility.
Coelurosaurs may have been pressured to adapt in these ways because of their size.
Being smaller, Coelurosaurs were forced to feed on smaller, quicker,
and more agile prey,
and they needed to be able to escape from the larger members of other theropod groups.
Coelurosaurs also possess relatively large brains and are
characterized by slender grasping hands with three fingers.
Evidently, the adaptations of Coelurosaurs serve them well.
They survive the drastic and diversified in the Cretaceous.
Some Coelurosaur groups grew larger,
some evolve to bravery, and some took to the skies.
It is within the Coelurosauria,
that the ancestors of birds arose.
And as such, birds are classified as a kind of Coelurosaur.
This makes Coelurosaurs the longest lived of any dinosaur lineage.
But even without birds,
they would easily rank as the most diverse of all major theropod groups.
In this lesson, we'll journey through
the evolutionary history of the Coelurosaur family tree
and meet the many non-avian families that arose prior to the direct bird branch.
Let's begin with the Compsognathidae.
This is the group to which little Compsognathus belongs,
and Compsognathus is a good representative of the group as a whole.
Compsognathus were small and slender with
extremely long tails relative to the rest of their bodies.
Spectacularly well-preserved Compsognathus specimens,
had been discovered in European limestone deposits in Chinese lake-bed shale deposits.
Some of these specimens preserve not only the skeletal anatomy,
but also some soft tissue.
That has made the anatomy of Compsognathus among
the best understood of all non-avian theropods.
Such specimens include preservation of integument, ligaments,
the correctness covering of a claws,
and in particularly rare cases,
even the form of internal organs.
Also included in the digestive tract of
some Compsognathus specimens are the partially digested remains of other animals.
These fossil gut contents have told us a lot about Compsognathid diets.
Which of the following do you think was on a Compsognathid's menu?
More than one answer might be correct,
so select all that you think apply.
Was it: A, small lizards, B,
fish, C, clams, and/or D, birds?
Compsognathid's a lot of things,
but cracking the shells of clams and other molluscs is
something these little dinosaurs jaws weren't strong enough to do.
However, lizards, fish, and even birds are all known to have been Compsognathid chow.
So, A, B, and D are correct.
The gut contents of the Compsognathid Scipionyx from Italy,
reveal that it ate small lizards and fish.
Scipionyx lives on what was then a series of small islands.
So it's no surprise that it had a taste for seafood.
Gut contents from Compsognathid Sinosauropteryx also include lizard bones,
as well as those of small mammals.
Gut contents are also known for multiple specimens of
the Chinese Compsognathid Sinocalliopteryx.
In one specimen, the bones of early birds litter the inside of the rib cage.
How Sinocalliopteryx caught these birds is unclear.
Like modern cats and foxes,
it may have been a particularly stealthy hunter,
able to sneak up on birds while they were on the ground.
The gut contents of another Sinocalliopteryx specimen,
include the dismembered leg of a small Dromaeosaurid.
Dromaeosaurids are kind of predatory dinosaur better known as raptors.
Although, Dromaeosaurid were predatory dinosaurs themselves,
Sinocalliopteryx is among the largest known Compsognathid.
It measured almost eight feet long,
and its size allowed it to take down
some prey that would have been too dangerous for its smaller cousins.
That's one of the advantages to growing bigger.
Animals that were once a threat to you,
start to become animals you can eat.
That simple principle had a big influence on the evolution of the next coelurosaurs.
When people think of meat-eating dinosaurs,
they generally think first of Tyrannosaurus rex.
And because of the fact that they think this is the typical meat-eating dinosaur,
they forget that in fact,
this is one of the very last and the most-specialized dinosaurs.
We used to think that Tyrannosaurus developed from an earlier dinosaur called Allosaurs,
but now we know that in fact,
it's much more specialized than that.
It's much more closely related to dromaeosaurus and ornithomimids.
And we know that because of
very specialized features throughout the skeleton but especially in the hind legs.
So when we look at the foot,
what we see is a condition called the arctometatarsalian foot.
First of all, the metatarsals,
the bones of the flat of the foot,
are very elongate and relatively narrow.
Secondly, when we look at the middle toe,
the third toe, it in fact,
becomes pinched out between the other two toes.
And here, we have very specialized joints lower in the foot,
which transfer stresses that come from the middle toe when it hits
the ground first into the two toes to either side of it.
What that allows is this bone to become, first of all,
longer for better mechanical advantage when it's running, secondly,
it becomes very spring-like,
because all of the stress is relieved from this toe and pushed into the two side toes.
When we compare a tyrannosaur foot with the foot of a duck-billed dinosaur,
we see the differences instantly.
We see that the duckbill foot is much shorter and broader in all aspects throughout,
and we do not see the arctometatarsalian condition.
What this tells us is that pound for pound,
a tyrannosaur or another one of the coelurosaurs is going to be
much faster than an equivalent-sized plant-eating dinosaur,
and they could almost always catch something about the same size as them.
Which of the following dinosaurs were tyrannosaurs?
Select every correct answer.
A. Velociraptor, B. Albertosaurus,
C. Tyrannosaurus, and/or D. Gorgosaurus.
B, C, and D are all tyrannosaurs.
The velociraptor is not.
By the end of the Cretaceous,
tyrannosaurs dominated all of the ecosystems in the northern hemisphere.
Because there were many environments and many kinds of prey,
tyrannosaurs were quite diverse animals.
This is Daspletosaurus, and Daspletosaurus is a form from
Southern Alberta that was probably the direct ancestor of Tyrannosaurus rex.
But like Tyrannosaurus rex,
it has all of the features that help define the family.
When we look at Daspletosaurus,
we see that the back of the skull is,
in fact, very broad.
The reason for that is that tucked in behind there,
we have the jaw muscles,
and the jaw muscles have become incredibly powerful in tyrannosaurs,
for a reason we'll discuss in a moment.
But another thing that happens because of the broad back of the skull is that the eyes,
in fact, are facing forward.
And when both the left eyes and the right eyes are looking at something in the front,
they have overlapping fields of vision.
So, these animals had stereoscopic vision the same way that we do.
They could be much more precise in terms of judging the distance to
their prey and picking what part of the prey they wanted to attack.
So, these animals are very precise because of their stereoscopic vision.
The powerful jaw muscles are,
in fact, operating the lower jaws.
And we see in the lower jaws that the depth of the jaws is reflecting the powerful bite,
because if it's too narrow,
then they're going to break their jaw.
The same happens in the front of the face.
The height of the skull here is very deep,
and that's because this has to support the bite with these teeth.
There are other things that are happening here that are quite amazing, for example,
these gnarly looking nasal bones have fused
together to give greater basal support for these jaws,
and there is movement in these jaw so that
the teeth don't break when they bite too hard into something.
The teeth are amazing.
The teeth are very deep, number one.
Secondly, although they have serrations on the front and the back,
and the temptation is to call them steak-knife equivalents,
the reality is that these teeth are more like bananas.
They have serrations down the front and the back so that they can,
in fact, bite through the flesh of their prey,
but they're really thick banana-like teeth so
that they can break the bones of the animal that they're biting into, as well.
All the teeth are large, thick,
and very, very powerful,
except for the teeth at the front of the jaws.
The premaxillary teeth, the teeth in the premaxilla,
are a little bit specialized.
First of all, when you look at the premaxillary teeth of a tyrannosaur,
you can see that they're very small,
compared to the teeth in the cheeks of the animal.
Secondly, when we look at the serrations,
they don't run down the front and the back of the tooth,
like they do in most theropod dinosaurs,
both rows of serrations are on the back of the tooth.
And when we look at the cross-section of the base of the tooth,
we see it's D-shaped,
and the D-shaped cross-section is somewhat similar,
at least functionally, to our incisors.
So these animals were using their incisors in much the same way that we use them.
They were using it to nip the flesh off of bone.
The back teeth, again,
were incredibly well-adapted for biting into flesh and breaking bones,
and processing animals' front teeth are very different.
Tyrannosaurids didn't just suddenly get big.
They bulked up gradually over evolutionary time,
and are descended from smaller Tyrannosauroids.
Early Tyrannosauroids, like Guanlong and Dilong from China,
and Eotyrannus from Europe,
were about the size of the modern leopard.
Living alongside many gigantic carnivorous dinosaurs,
these Tyrannosauroids were both hunters and hunted.
At such a size,
and with long arms and lightweight skulls,
they bore only a faint resemblance to later and more advanced tyrannosaurs.
To survive, early Tyrannosauroids relied on their coelurosaurian,
long hind legs, which helped them to outrun both prey and predators.
As later tyrannosaurs evolved,
and gradually fought their way to the top of the food chain,
speed and agility remained key to their success.
An extreme increase in absolute body size across
the evolutionary history of a lineage is referred to as gigantism.
We've already met many other theropod groups that display a trend towards gigantism.
You recall which other theropod groups show gigantism?
Is it, A. Spinosaurids?
B. Carcharodontosaurids?
C. Compsognathids?
And/or D. Coelophysoids?
More than one answer may be correct,
so select all that apply.
A and B are the correct answers.
Some spinosaurs and carcharodontosaurs grew to be very large predators.
On the other hand, compsognathids and coelophysoids did not.
Among Tyrannosauroids, gigantism eventually led to the advanced Tyrannosauridae.
This is the group that includes such colossal predators as Albertosaurus,
Tarbosaurus, and, of course, Tyrannosaurus rex.
The Tyrannosauridae first appeared in the late Cretaceous,
and ruled as the dominant large carnivores across North America, Asia,
and Europe, displacing, it seems,
previous top predators like the carcharodontosaurids.
The size wasn't the only thing that changed over the course of tyrannosaur evolution.
Tyrannosaurs are famous for having tiny arms relative to their great body size.
This arm belongs to the 11-meter-long carcharodontosaurid, Acrocanthosaurus.
While this is the arm of the 12-meter-long Tyrannosaurus.
You can see that the Acrocanthosaurus arm has three fingers,
while the Tyrannosaurus has just two.
And that, despite coming from a somewhat smaller dinosaur,
the Acrocanthosaurus arm is substantially longer.
Not only is the Tyrannosaurus arm shorter,
but the individual arm bones are much skinnier,
and the ridges for muscle attachment are much shorter.
This indicates that tyrannosaur arms were proportionately less-muscled.
However, the ancestors of tyrannosaurs had
normal-sized and muscled forelimbs, with three fingers.
Why would tyrannosaurs have evolved short arms with reduced muscles,
and one less finger?
How is that a beneficial adaptation?
The answer has to do with teeth and balance.
Here is another comparison.
And this is the tooth from Giganotosaurus,
which you will recall is an extremely large carcharodontosaurid.
And this is a tooth from Tyrannosaurus.
Typical of most carnivorous dinosaur teeth,
the Giganotosaurus fang has a sharp point and is laterally compressed,
so that it has a flattened, knife-like shape.
The tyrannosaur tooth is different.
It is a relatively blunt tip and is thick.
As you can see, it's not laterally compressed.
These differences give the tyrannosaur tooth the potential to do
more than simply slice through soft flesh.
Which of the following could the thick,
blunt teeth of Tyrannosaurus do better than the sharp,
compressed teeth of Giganotosaurus?
Was it, A.
Capture fish? B. Crush bone?
C. Chew?
Or D. Sever tendons?
B is the correct answer.
Tyrannosaur teeth were adapted for bone crushing.
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