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
1.2 Bird Anatomy
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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
Bird Anatomy
In Lesson 1, we explore the anatomy and adaptations of birds, and meet the Victorian scientists who first suspected the link between the terrible lizards and modern birds. In order to fly, birds have undergone a series of anatomical specializations that distinguishes them from other vertebrates. However, many of the most striking and anatomically unusual traits of birds originated over 230 million years ago with the very first theropod dinosaurs. Just a quick note before you get started: 'Palaios' is the Greek word for 'ancient', so palaeontology or paleontology is the study of ancient life. Both spellings are correct, with palaeontology used in Britain, and paleontology more common in the US.
Meet the Instructors
Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
To understand the evolutionary path from dinosaur to bird,
we need to first understand just what it means to be a bird.
We're going to spend the rest of this lesson learning about bird anatomy.
Here, you'll discover the most important anatomical features
that dinosaurs needed to evolve in order to become birds.
So, to begin with, the trait we most closely relate to birds is flight.
Birds have numerous adaptations that enable this amazing ability.
First it's easier to fly if you're light-weight.
To lighten their skeletons,
birds have evolved a number of interesting adaptations.
Most bird bones are hollow and have extremely thin walls.
For example, here is the bone from the wing of a bird.
And here, the slightly smaller bone from the arm of a mammal.
Now holding the two I can tell that the bird bone is substantially lighter.
But because this is a science course, don't just take my word for it.
See for yourself.
First, here's the bone from the mammal.
105 grams.
Now here's a slightly larger bone from a bird.
Only 37 grams.
The bird bone is much lighter.
Here's another bird wing bone.
This time we have removed a section of the outer wall
allowing you to see the inside.
You'll notice first how very thin the edges are.
And secondly, you can see that inside there are tiny struts
of bones that criss-cross the internal space.
This is a sophisticated bit of evolutionary engineering
that helps reinforce and strengthen a bird's bones.
Without these struts, the bone would easily break.
Aside from being thin walled, the skeletons of birds are highly pneumatized.
Pneumatization is the development of air pockets within bones.
Air pockets make bones weigh even less.
In birds most of these air pockets are connected directly to
the respiratory system that is responsible for efficient avian breathing.
Think about the great diversity of birds.
Diversity in size, shape, and way of living.
Now think about what pneumatization means for the skeleton of a bird.
It makes them very light.
Which one of these birds will have its skeleton more pneumatized?
Is it A, hummingbird,
B, vulture, C,
sparrow, Or D, Parrot.
The extent of skeletal pneumatization varies between bird species.
Generally, the skeletons of large,
soaring birds like vultures are the most pneumatized.
This helps to lighten their load, as they spend many hours each day
soaring passively, but efficiently, on the wing.
So B is the correct answer.
Another general need of a flyer is a rigid body,
that can stand up to gravity and maintain a constant aerodynamic shape,
without sagging or flopping about with every wing beat.
A good way to hold the skeleton rigid without the need for
constant muscle flexing is to fuse multiple bones together.
Nowhere on the bird is this concept more apparent than at the hips.
The sacrum is a feature common to most terrestrial vertebrates.
It is a typically short series of vertebrae that are all fused together, and
are each fused to the left and right ilia, or the upper hip bones.
But birds take this to the extreme.
They've evolved what is called a synsacrum.
And the synsacrum is essentially an extraordinarily long sacrum.
It incorporates multiple additional vertebrae from the abdomen and
tail, essentially turning the back half of a bird's skeleton
into one solid bone structure.
The front half of a bird is also held together more rigidly
than other vertebrates.
Have a look at the rib cage.
See these backwards projecting processes on each rib?
These are called uncinated processes.
Notice the uncinate process of every rib, overlaps the next rib behind it.
And this helps lock the entire rib cage together.
Behind the synsacrum there's another fused structure.
This is a pygostyle, and
pygostyles are formed by a series of fused vertebrae at the tip of the tail.
This stiffens the tail tip and makes it a good anchor for
rectrices, that is a fan of tail feathers.
Notice that between the synsacrum and the pygostyle,
there's only a very short series of unfused vertebrae,
and this is the only region of significant tail mobility.
Also note, the absolute length of the tail skeleton is extremely short.
Why does a bird need unfused vertebrae between the synsacrum and pygostyle?
Is it to A, facilitate in-flight steering?
B, make walking easier?
Or C, there is no value to this feature, future
evolution of birds will likely see these bones fuse into the rest of the synsacrum.
Unfused vertebrae allow a bird's tail to be moved like a rudder,
improving flight manoueverability so A is the correct answer.
Flight demands strong muscles to keep wings flapping.
Although muscles are themselves soft tissues
they rely on the skeleton for sturdy support and anchorage, thus the need for
strong muscles often necessitates the growth of large,
bony surfaces for those muscles to attach to.
And this breast bone with it's dramatic projection is a keeled sternum.
The keeled sternum provides a bird with an enlarged attachment surface for
wing muscles.
The scapula, or shoulder blade,
the clavicle, or collar bone, and the coracoid.
Form the pectorial girdle.
As you can see here, the coracoids are attached to the breast bone and
their function is to brace the wings apart.
The left and right clavicles of birds,
combine to form a single v-shape bone called the furcula.
And the furcula is more commonly known as the wish bone.
In birds, the furcula serves several important functions.
Can you identify one of them?
Is it to A, stabilize the shoulder?
B, compress avian lungs?
C, House the preen gland, or
D, brace the rib cage?
The correct answer is A.
We'll talk more about the furcula in later lessons, but it does function to stabilize
the shoulder of birds, as well as store elastic energy while the wings flap.
The wings of birds are formed from the same bones
found in the front limbs of other vertebrates.
The upper arm consists of the humerus.
In birds, the humerus articulates with a pectoral girdle
through a special joint that allows a wide range of movement.
The forearm is composed of two bones.
The radius and the ulna.
Look closely at the ulna and you can see a line of small bumps on its surface.
These bumps are quill knobs.
They're sites where the large wing feathers attach.
In the wrist many of the carpals had become
fused together to form one structure.
The fingers of birds are highly modified.
There are only three, and as you can see the first digit is greatly reduced.
Nonetheless, it still plays an important roll in flight.
Which we'll discuss later.
The second digit, is the longest.
It's followed by the third, seen here.
Not every part of a bird's anatomy is strictly adapted for flying.
For instance, one of the most characteristic features of modern birds
has nothing to do with flying.
Birds had beaks with toothless jaws.
Modern birds have a diversity of diets.
Different groups of birds have beak shapes
that are adapted to conform to the food they eat.
Birds of prey have sharp hook beaks for tearing flesh.
Woodpeckers have long chisel shape beaks for boring into tree bark.
Dabbling ducks have broad spoon shaped bills to siphon mud.
Seed eating birds have short thick beaks to break seed shells and so on.
Behind the beak are large orbits that house the eyes.
As a general rule birds have excellent vision.
Behind the orbits is a relatively large brain case.
The elementary school put down,
bird brain, is ironically not a well informed insult.
Birds like parrots and crows, rank among the smartest of all animals.
And even comparatively small brained birds like ostriches,
are far smarter than modern reptiles.
The necks of birds are often long and flexible, and that's because they
typically contain a long series of cervical, or neck, vertebrae.
You and I each have only seven vertebrae in our necks, and
seven is the typical number for mammals.
Although there are a few unusual mammal species that deviate slightly.
Non-avian dinosaurs typically have nine or ten cervical vertebrae.
But bird necks are far more variable.
A relatively short-neck bird, like a parrot, has nine cervicals,
while a long-neck bird, like a swan, can have well over 20.
We've seen how bird wings have
a skeletal layout similar to the forelimbs of other bony land animals.
But what about their legs?
Do you know in what ways the hind limb skeleton of a bird differs from our own?
Unlike your legs, bird legs have A, knee joints that point backwards.
B, proportionately longer femora.
C, long metatarsals that are fused together.
Or D, inflexible toes.
Unlike you and I, birds have very long metatarsals that are fused together.
So, C is the correct answer.
As in the wing, the legs of birds are made up
of the same leg bones that we and other vertebrates posses.
The upper leg is comprised of the femur, and
the lower leg contains two bones, the tibia, and the fibula,
however in birds, the fibula is extremely thin, and
the tibia is fused together with some of the ankle bones,
forming a structure called the tibiotarsus.
It is a common misconception that the knees of birds point
backwards rather than forwards.
As you can see, this is not the case, but
this misconception stems from the characteristically short femora of birds.
This arrangement commonly results in the true knee
being obscured by the feathers of the body.
Meanwhile bird ankles are visible and are raised
well above the ground, making us think that they are actually the bird's knees.
Metatarsals in humans are the long bones that run through the soles of our feet.
In birds, however, they're elevated off the ground.
Therefore, only a bird's toes make significant contact with the ground.
The metatarsals of each foot
are also fused together along with the remaining ankle bones.
And this structure in birds is known as the tarsometatarsus.
The relatively short femora in long lower leg bones of birds,
reflect how they typically walk and run.
Unlike us and most non-avian dinosaurs,
birds swing their upper legs relatively little when taking steps.
Instead they bend their knees and swing their lower legs.
And this biomechanical difference in locomotion,
stems from both a major reduction in upper leg musculature.
and the typical bird constraint of having a centre of body mass
that is positioned well in front of the hips,
both of these traits relate to the evolution of shorter bony tails.
That brings us to feet.
As Dr.Currie explained, the feet of birds and some dinosaurs are extremely similar.
Do you know what a phalangeal formula is?
Let me quickly explain.
Here's a skeleton of a human foot.
Starting with the inside or big toe, we state the number of bones in each digit.
Moving our way to the outermost toe.
So in a human foot we would count, two, three, three, three and three.
Two, three, three, three, three is the phalangeal formula of the human foot.
Now I want you to figure out the phalangeal formula of a bird foot.
Here is the skeletal image of a bird's right foot.
What is the phalangeal formula?
Is it A, 2-3-4-4.
B, 2-3-4-5-0.
Or C, 0, 5, 4, 3, 2.
This is the right foot of a bird.
So we count from the left, or inside of the foot, clockwise to the right,
or outside of the foot, in order to determine the phalangeal formula.
Counting the bones starting at the back side and
moving clockwise, we discover that B is the correct answer.
The feet of birds and some dinosaurs are very similar.
A typical bird foot has a phalangeal of 2-3-4-5-0.
The first toe of birds which we call the hallux,
and which typically points backwards in birds, contains two bones.
The second toe contains three, the third has four, and the fourth has five.
The fifth toe, which is an ancestral trait of all tetrapods, has been lost.
And here we denote that with a zero.
Note that although there are more bones in the fourth toe,
the third toe is typically longest.
The four bones that comprise the third toe are each
proportionately longer than the others in the foot.
Finally, here's one more important trait.
Like all vertebrates, bird hips are composed of left and
right sets of three bones.
The upper hip bone is called the ilium, and
this is the bone that sacral vertebrae fuse to providing rigidity.
Below the ilium is the ischium, which projects towards the rear.
Finally, there's the pubis, which in birds also projects towards the rear.
If you've previously taken a course on dinosaurs,
you no doubt learned about important differences in hip structure.
Let's see what you've remembered.
One of the following dinosaurs has a pubis that projects
towards the rear of the animal.
Which one is it?
Is it the A, Tyrannosaurus,
B, Stegosaurus, C,
Diplodocus, or D, Allosaurus.
If you haven't learned about dinosaur hip structure in the previous course,
then just make a guess and move on.
Tyrannosaurus, Diplodocus and
Allosaurus are all Saurischian or Lizard-Hipped dinosaurs.
So they have forward-pointing pubis bones.
The Stegosaurus, however, is an ornithischian, or
bird-hipped dinosaur, and has rearward pointing pubis bones.
Therefore, B Stegosaurus is the correct answer.
However, this similarity in hip structure is strictly convergent.
And birds are not actually closer related to any of the ornithischian dinosaurs,
as you'll soon discover.
You're now familiar with many signature anatomical traits of modern birds.
To make the connection with their theropod ancestors,
we're going to survey all the major groups of theropod dinosaurs.
As we move through the lessons of this course, we will bring you nearer and
nearer to the theropods most closely related to birds.
So, to start, we'll go way back
into the Triassic Period when theropod dinosaurs first arose.
What were these first bipedal dinosaurs like?
And what set them up to become so
successful through millions and millions of years of evolution?
Let's take a look.
I'll see you in the next lesson.
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