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
3.2 Bird-Like Traits in Coelurosaurs
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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 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.
Meet the Instructors
Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
Oviraptorosaurs are very interesting dinosaurs because of the fact that they
have this combination of characters, things that we expect to see in non-avian
theropod dinosaurs like the long legs that end in claws,
the three fingers in the hand, the long tail, and so on.
But they also have characters that look very, very bird-like.
The skull that has no teeth and has a well-developed beak,
it's very deep, looks almost like a parrot skull.
The very large eyes, the large brain, the long neck is very bird-like as well.
Most theropod dinosaurs only have ten vertebrae in the neck.
Oviraptorosaurs are like birds,
they've added extra vertebrae into the neck as well.
At the expense of the length of the body, which is relatively short.
When you look at the hips,
the hips are in some ways like a normal non-avian theropod.
But in other ways, for example, the number of
vertebrae that are incorporated into the sacrum, they're very bird-like too.
I mentioned the long tail before, and that is a long,
bony tail compared to a modern bird.
But as far as the meat eating dinosaurs are concerned, this is a short tail.
It's been abbreviated, and
there are far fewer vertebrae than you would see in most dinosaurs.
The end of the tail has a specialized
feature in some of the oviraptorosaur species, not all of them.
But they'll take the last couple of vertebrae and fuse them together.
And this happens in birds as well.
It's a feature that we call a pygostyle, or a 'pope's nose'.
It's what supports the fan of feathers at the end of the tail.
And we think in these oviraptorosaurs,
they were doing much the same thing as in modern birds.
So oviraptorosaurs then are animals that, right from their discovery,
have given us some clues about where birds came from.
The interesting thing is though that oviraptorosaurs in the 1920s
already had the answer as to where birds came from.
This bone right here is called a furcula.
It's the equivalent of our collar bone.
It's a bone that's found in most vertebrates including fish.
It's a bone though that had not been identified in dinosaurs in the 1920s.
And because of the fact that everybody assumed that the clavicles had been
lost in dinosaurs, it was assumed that birds could not come from dinosaurs.
Because if you've lost the clavicles, how do you get it back and become a bird?
The irony is that the very first oviraptorosaur specimen that was found
has clavicles in it.
The trouble is that they were misidentified as ribs,
as gastralia, or as other bones.
And so everybody just assumed that dinosaurs did not have clavicles.
But the clavicle on these oviraptorosaurs is in fact almost exactly
like the clavicles that we find in modern birds.
The right one and the left one have fused together into this bow-shaped bone,
which in fact stores kinetic energy when a bird is flapping its wings.
And the furcula gets compressed somewhat and then springs back.
The presence of the furcula then was misidentified, but
even in 1920s would've told us loud and clear that birds came from dinosaurs.
>> Mother oviraptorosaurs were in the habit of sitting on top of their nest and
guarding their eggs.
Now this is a behavior trait they share with a modern animal.
Which one is it?
A, elephants.
B, crocodiles.
C, crows.
Or D, sea turtles.
The correct answer is C.
Crows, like many birds, sit on their nests.
Since the discovery of the Mongolian oviraptorosaur nests,
many fossil eggs from other kinds of dinosaurs have been found.
As far as we know, all dinosaurs laid eggs.
The eggs of modern reptiles and birds vary in shape and size and so
did those of dinosaurs.
Theropods generally had oblong, elliptical eggs.
Along with the more derived and bird-like theropods,
oviraptor eggs were asymmetrical with one blunt and one more pointed end.
Some other variables noted in egg layers are the number of eggs that different
species lay at one time along with the kinds of nests that they construct.
Animals that lay only one or two eggs at a time tend to have relatively large eggs.
While animals that lay multiple eggs in the same clutch,
tend to produce eggs that are much smaller relative to their own body size.
Many birds construct elaborate nests and incubate their eggs by sitting on them.
Most dinosaurs were too large and
heavy to sit directly on their eggs, but some, like oviraptorosaurs,
constructed circular nests on mounds surrounded by a rim of sediment.
They laid their eggs in a ring, and
they sat in the center of the ring to protect and warm the eggs.
Other dinosaurs dug bowl-shaped nests and
then covered their eggs with plucked vegetation.
As the vegetation rotted,
the decomposition released heat which warmed the eggs.
Dinosaur nests are often found near one another
in what seemed to have been large nesting sites.
In some cases, these sites include huge numbers of eggs, sometimes thousands!
And must reflect a large dinosaur colony reproducing all at once.
The nests at such sites are usually evenly spaced in distance from
each other such that the full grown adults could walk safely between the nests.
What's more, these fossil nests are often stacked one on top
of another in a stratigraphic section, which indicates that the dinosaurs
built the nests at the same site year after year.
This practice of nesting at the site for many years is termed 'site fidelity'.
Many bird colonies employ site fidelity today.
Oviraptors laid bird-like eggs and built brooding nests.
They had other bird-like traits as well.
Which of the following is not a modern bird trait?
Is it A, bipedal stance?
B, toothed jaw?
C, A 2-3-4-5-0 phalangeal formula?
Or D, presence of a furcula?
The answer is B, modern birds do not have teeth.
Although oviraptorosaurs have created a lot of confusion and debate, they've also
helped to resolve one of paleontology's biggest issues, the origin of birds.
Take a look at this skeleton.
You can see a number of bird-like traits.
Some of these traits are common to virtually all theropods,
a bipedal stance, an S-curved neck, and three,
forward pointing toes with a phalangeal formula identical to birds.
Some traits are exceptionally birdlike including the maniraptoran wrists,
vertebrae that are highly pneumatic and an extensive series
of rigid vertebrae that are fused on to the very bird-like hips.
Just how closely related oviraptorosaurs are to the true bird line
is still a little hazy and some of these traits may have evolved
convergently in both oviraptorosaurs and in other birdlike theropods.
Nonetheless, oviraptorosaurs have helped reveal the many traits
that birds share with their dinosaur ancestors.
Here is one more very important trait.
This bone is called the furcula, you may know it better as the wishbone.
A furcula is formed by the fusion of the left and right clavicles,
which are our collarbones.
In oviraptorosaurs, the furcula is not very big.
But in birds like this pigeon,
you can see that the furcula has become proportionately enlarged and
that's because a bird's furcula plays a critical role in flight.
It acts like a strut that strengthens a bird's chest and
as a flexible bone that compresses and decompresses as the wings flap.
This stores energy like a spring and
makes a bird's flight more energetically efficient.
Most non-avian theropods could not fly, so what use was their furcula?
Did it A, improve rigidity in the dorsal vertebral column.
B, improve support for the digestive track.
C, improve neck mobility.
Or D, improve ability to grasp and hold prey with the forelimb.
The correct answer is D.
The furcula likely helped theropods to seize and grapple with prey.
But perhaps, the greatest contribution that oviraptorosaurs have made to
our understanding of bird evolution is what they've taught us about feathers.
Oviraptosaurs were not the first non-avian dinosaurs to be discovered with feathers.
That honor belongs to the Chinese compsognathid Sinosauropteryx,
scientifically described in 1996.
This comsagnathid had been fossilized in a lake bottom deposit,
rich in fine volcanic ash.
The preservation was so
good that a feather coating was visible around the skeleton.
This was a spectacular find.
But for many, the discovery that some theropods had feathers was not a surprise.
It had been speculated for
years that dinosaurs evolved feathers before birds and flight.
Sinosauropteryx confirmed that speculation and proved beyond
any reasonable doubt that dinosaurs shared the most iconic of all bird adaptations.
The feather coat of sinosauropteryx was not adapted for flight in aerodynamics.
Its feathers had a simple form similar to mammalian hair and
to the downy feathers of birds.
That made sense.
It was in line with the leading theories that feathers began as simple structures
and didn't develop the leaf-shape form seen in modern birds until feathers
began to function as flight and glide assisting air-foils..a good theory,
until Dr. Curry and a team of colleagues discovered
a new kind of oviraptorosaur: Caudipteryx.
This is Caudipteryx, it was discovered in the same fossil beds as Sinosauropteryx.
And again, the exceptional preservation shows simple,
filamentous feathers covering its body.
However, also preserved are complex leaf-shaped feathers
extending from the forearms and from the tip of the tail.
That was a big surprise, because as birdlike as caudipteryx was
it certainly did not have the wingspan of a glider or a flyer.
So the feathers of some non-avian dinosaurs were far more advanced and
structurally sophisticated than ever suspected.
The evolutionary story of feathers must have been far more complex
than first thought.
When not in flight, what functions can the feathers of modern birds serve?
More than one answer may be correct, so
check all the answers that you think apply.
Is it A, insulation.
B, fat storage.
C, chemical secretion and, or D, social signaling.
The correct answers are A and D.
Feathers do not store fat, nor do they secrete chemicals.
Like the fur of mammals, feathers provide birds with insulation.
They trap a layer of warmed air next to the bird's body,
which reduces how quickly a bird's internally generated heat is lost.
Without insulating feathers, birds would have to burn a lot more
calories in order to replace lost body heat.
Also like mammalian fur,
feathers covering a bird's skin offer a form of lightweight armor.
A feather coat protects against abrasion and provides a natural soft cushion.
Most feathers are not much use as tactile sensors, but
some are specifically adapted for this purpose.
Many birds have whisker-like feathers that project outwards from their
faces called bristles, and bristles have sensory nerves at their bases.
This allows birds to feel when the bristles are disturbed
by contact with solid objects.
Finally, many birds use their feathers in social displays.
Feather crests, fans and extravagant plumes are all bobbed, waved and
flashed about in courtship rituals amd territorial displays.
The simple hair-like body feathers of both Sinosauropteryx and Caudipteryx
would have served well as insulation and could have provided a little protection.
The bristles of modern birds have a simple hair-like form, so it is possible
that some of the feathers on the faces of dinosaurs served that function as well.
However, we should note that no extra elongate and
particularly 'whiskery' feathers have been identified in dinosaurs to date.
What about social display?
Here, Caudipteryx would seem better endowed.
The complex feathers on its tail and arms form broad fans,
which would have been excellent attention getting display structures.
Similar adaptations are found in many ground dwelling birds,
such as turkeys and peacocks and are used in courtship displays.
So it is generally thought that the early simple feather coverings of
dinosaurs served primarily as insulation with perhaps, an additional
minor protective benefit and they might have developed a few tactile bristles.
However, the first complex leaf shaped feathers evolved not for
flight, but for social display.
Thanks to additional feathered specimens from Liaoning,
China and other exceptional fossil localities, we now know
that all major groups of Coelurosaurs had simple feathers.
A few flightless and non-avian forms had complex feathers as well.
Up to this point in the course,
we've seen a lot of different hand and finger adaptations in theropods.
Which of the following is not an arm and
hand configuration that has been described so far.
Is it A, strong arms with three clawed, grasping fingers?
B, short arms with a single digging claw?
C, long arms with three fingers, one much longer than the rest?
Or D, tiny arms with two fingers.
A describes the Allosaurs.
B describes the Alvarezsaurids.
D describes Tyrannosaurs.
That leaves C as the correct answer.
However, we're about to introduce you to another group of
Maniraptoran Coelurosaurs that have this very odd arm and hand configuration.
While the Alvarezsaurids evolved reduced forelimb length for
the purposes of digging,
another group of Maniraptoran Coelurosaurs developed in the opposite direction.
The Scansoriopterygidae is a newly discovered group
with the longest arm to body ratio of any dinosaur.
Among the Scansoriopterygids are Epidexipteryx, Scansoriopteryx,
the newly discovered Yi and, well, for the moment that's it.
Just three genera.
All of them are known from the feather preserving fossil beds of Liaoning,
China, and they are all among the smallest known dinosaurs.
Epidexipteryx was only about the size of a robin.
Scansoriopteryx was probably not much bigger,
although its full-size is currently unknown.
Because only two tiny specimens have been found and
both show the characteristically unfused skeletal structures of juveniles.
Not only were the arms of Scansoriopteryx elongated, but so were their fingers.
In fact the third finger of each hand was longer than the rest of the arm.
What were they doing with these long arms and even longer fingers?
Well, a hint comes from the hind limbs.
The hind feet of Scansoriopteryx resemble those of early birds.
With the rear hallux modified for grasping.
The Scansoriopteryx appeared to have been adapted for life in the trees,
with gripping hind feet and long reaching forearms,
these small dinosaurs would've been excellent climbers.
Some paleontologists have even compared Epidexipteryx to the modern Aye Aye.
An Aye Aye is it kind of lemur that also has an exceptionally long finger
which it uses to probe into narrow insect borings in tree trunks.
They skewer grubs with a claw at the end of the finger and
pull out the insect for a juicy meal.
Epidexipteryx and other Scansoriopterygids may have done the same.
However the newest Scansoriopterygid Yi suggests that the long fingers and
arms may have served an even more surprising purpose.
A long bony rod called a 'styliform element' extended from each of Yi's wrists.
And these two rods each supported webbings of skin
which were found in a fossilized specimen.
The skin stretched between the body, the rod and the long finger.
And this formed a kind of primitive wing.
Although styliform elements have not been discovered in other Scansoriopterygid's
it's entirely possible that they did have them.
With these fleshy wings the tiny tree-dwelling dinosaurs
could have glided from branch to branch like modern flying squirrels.
But understand that the wings of Scansoriopterygids are very different
from those of birds and it seems that the one did not give rise to the other.
The feathers of Scansoriopterygid's include the simple hair-like forms
seen in other Coelurosaurs but also, some complex branching feathers.
In Epidexipteryx four long skinny and
complex feathers protruded from the tip of the tail.
Like the tail feathers of Oviraptorosaurs,
these are thought to have served as sexual display structures.
We now only have one major group of non-avian Coelurosaurs to go,
the Deinonychosauria.
This group is so birdlike and their anatomy and inferred ecology so
different from what was once considered typical of a dinosaur,
the Deinonychosauria sparked a revolution in paleontological thinking.
That revolution is a story of our next lesson.
We're now at the end of the third lesson.
This is a good opportunity to,
once again, investigate our interactive phylogenetic tree.
Have a look at the Coelurosaurs and
see if you can recall all the different types you've just learned about
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