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
1.1 The Link Between Birds and Dinosaurs
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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
Welcome to theropods and the origin of birds.
My name is Scott Persons, and
I'm a paleontologist at the University of Alberta.
Studying dinosaurs is a fascinating pursuit.
And no group of dinosaurs was more diverse and successful than the theropods.
The theropods saga spans over 200 million years of evolution,
and includes the largest carnivores to ever walk the earth, but
also the smallest vertebrates to ever fly.
It's heady stuff and I love it.
Theropods were among the first dinosaurs to arise.
Throughout the Mesozoic Era, the Age of Dinosaurs, theropods became diverse
in size, eating habits, range and socialization.
Theropods occupied ecological niches from tiny prey, to apex predator.
And they never went extinct.
The descendants of Mesozoic theropods are still with us today.
They are all around us.
Look out the nearest window and you just might see one.
They are birds, like this awesome fellow, Leo.
Here's a macaw and believe it or not, he is a dinosaur.
Establishing that the birds we see today are the direct descendants of dinosaurs
ranks as one of paleontology's greatest scientific accomplishments.
So join me on a fascinating journey into our planet's past.
You're going to learn about one of the most magnificent groups of animals
to ever walk the earth.
And you're going to learn about their progeny.
Animals that delight and fascinate us with their diversity,
color, intelligence and song, the birds.
Today, we recognize that birds evolved directly from dinosaurs.
But do you know when the theory that dinosaurs were the evolutionary
ancestors of birds was first proposed?
Was it A, when the first feathered dinosaur
fossils were discovered in China in the late 1990s?
B, when the first bird genome was sequenced in the 21st century?
C, shortly after the publication of On the Origin of Species, in the 1860s?
Or D, during the "Dinosaur Renaissance", in the 1960s?
Although it has taken the discovery of countless fossils and generations of
scientists to compile the mounds of evidence that now conclusively prove
birds evolved from dinosaurs, you might be surprised to learn the idea that
birds evolved from dinosaurs is nearly as old as the theory of evolution itself.
C is the correct answer.
When Charles Darwin published On the Origin Species in 1859,
he created a scientific and social uproar.
It challenged religious dogma.
It threatened pervasive philosophical views.
And it proposed a single unifying theory for all of biology.
Naturally, a fresh idea like evolution did not gain scientific acceptance overnight,
nor should it have.
The theory of evolution was immediately subjected to severe scientific
scrutiny and ruthless critique, but Charles Darwin was a quiet man who
shied away from publicly championing his idea and debating his critics.
He preferred to let his books speak for him.
But that silence didn't cut it with much of Europe's scientific community.
There were serious questions to be answered that weren't
covered in Darwin's book.
He and his theory of evolution were subject to public and
professional ridicule.
But Charles Darwin was not without allies.
Foremost among them was his friend and
fellow natural scientist Thomas Henry Huxley.
Huxley defended evolution theory in writing and in person.
With his thick skin and his rapier tongue,
Huxley earned himself the nickname Darwin's Bulldog.
The theory of evolution has three premises,
which of the the following is not one of them?
Is it A, parents produce more offspring than can be supported by the environment,
B, natural processes randomly determine which individuals survive and reproduce,
C, offspring resemble their parents, or,
D, offspring occasionally exhibit novel traits or variations?
Although it is a central tenet of evolution that the survival and
reproductive success of an individual organism is determined by
natural processes, these processes are not random.
They are selective.
Hence the term natural selection.
So B is the correct answer.
One of the most significant arguments against evolution had to do with
the theory's central tenet that all life shares a single common origin.
As such, all modern animals are part of a single family tree,
and a long train of ancestors connects all organisms on Earth to each other.
And that seemed entirely contradictory to what Victorian naturalists
perceived in the modern world and the fossil record.
All modern animals appear to belong to distinct and well-defined natural groups.
Fish were fish, amphibians were amphibians,
mammals were mammals, and birds were birds.
There were no half mammals, half reptiles, or half fish, half amphibians.
Instead, modern animals each belong to one, and only one, major group.
For Darwin to be right, the transitional links between
modern groups must have once existed, but were simply now extinct.
For that in turn to be true, such transitional forms or
missing links ought to be found in the fossil record.
Up to that point in the 19th century, no such fossil had been recognized.
Instead, Darwin concluded that the fossil record was too incomplete
to document clear transitions from one major kind of animal to another.
And that was not a very satisfying explanation.
When debating other scientists who used the missing link argument,
Huxley was put on the defensive.
Darwin's Bulldog needed a transitional fossil.
Huxley needed a fossil to show that historically,
major animal groups were connected.
In 1861, he got it.
In 1861, a missing link fossil was discovered.
Do you think it showed the link between A,
fish and amphibians, B, reptiles and
birds, or C, reptiles and mammals?
In 1861, a fossil of Archaeopteryx was found in Germany.
This fossil clearly showed a small carnivorous reptile, but
with wings and feathers.
It was a link between reptiles and birds.
So B is the correct answer.
As we shall soon see, Archaeopteryx showed a more specific link
between dinosaurs and birds.
This is Archaeopteryx.
Excavated from a Jurassic limestone quarry in southern Germany,
this little creature took center stage in the evolution debate.
Having been buried in extremely fine and anoxic marine sediments,
any Archaeopteryx specimens preserve the fine details of feathers.
On this specimen, you can see long flight feathers on the arms and on the tail.
Sir Richard Owen originally described the first skeleton of Archaeopteryx.
Owen was a brilliant naturalist whose
contributions to Victorian natural history had earned him knighthood.
Among his many accomplishments was his recognition that various
fossil remains attributed to extinct giant reptiles
all belonged to a single and unique natural group.
And this was a group that he named the Dinosauria.
Because Archaeopteryx had feathers,
it seemed clear to Owen that it was the oldest known bird.
After all, what could be more bird-like than feathers?
But Huxley realized that there was more to the fossil.
And you can see these important features as well.
Look closely at the forelimbs.
On each hand, Archaeopteryx has three long fingers, tipped with hooked claws.
Now look at the tail.
Archaeopteryx has a long series of tail vertebrate that accounts for
nearly half of its total body length.
Finally, look at the mouth.
Instead of a beak,
the jaws of Archaeopteryx are lined with small carnivorous teeth.
Archaeopteryx, Huxley argued, had the defining features of a bird,
but also strong characteristics of a reptile.
It was a hodgepodge of reptile and bird anatomy, and thus,
it bridged the gap between the two groups.
It was just the sort of transitional fossil that Darwin's theory predicted,
and exactly the sort of missing link that Darwin's skeptics had demanded.
Huxley didn't stop there.
He went one major step further, and confidently offered an explanation for
exactly which branch of the reptilian family tree Archaeopteryx had arisen from.
To tell you more about why Huxley picked the branch that he did,
let me introduce you to University of Alberta Professor of Paleontology and
holder of the Canadian Research Chair in Dinosaur Paleobiology,
Dr. Philip J Currie.
>> Hi, I'm Phil Currie and
I like to work on theropod dinosaurs as my main research area.
When I was a kid, I was fascinated by dinosaurs, like most kids are.
And one of the dinosaurs though, that caught my attention was Compsognathus.
Compsognathus had been discovered in the same limestone deposits in
Germany as Archaeopteryx.
But for many, many years this was the smallest known dinosaur.
And when we think that dinosaurs are fascinating because they're large,
we have to also look at the small ones because they may tell us more about
the evolution of theropod dinosaurs.
Huxley realized this, and when Huxley looked at Archaeopteryx, he realized
that if you plucked the feathers off an Archaeopteryx, you're essentially left
with a little dinosaur that looks almost exactly like Compsognathus.
Both Archaeopteryx and Compsognathus have these small carnivorous-style teeth
that are flattened from side to side with serrations on them.
When we look at the hands, they both have three fingers,
three functional fingers with claws on them.
When we look at any part of the skeleton,
we can see that they're very closely related to each other.
In fact, it's quite evident that because they're so
closely related to each other, Huxley realized that almost certainly,
birds originated from theropod dinosaurs like Compsognathus.
I'm not sure he would've made that leap if he was comparing
Archaeopteryx with Tyrannosaurus rex.
Although those characters are in fact there to relate the two as well.
Compsognathus, like Archaeopteryx, is a little bipedal animal.
And when we look at the feet, we can see that they're very similar to each other,
including the phalangeal formulas.
These are almost identical,
and no other animals have feet like birds and theropod dinosaurs.
>> Ignoring wings and
fancy feathers, Compsognathus and Archaeopteryx have a great deal in common.
But which of the following skeletal traits did Archaeopteryx have but
Compsognathus lack?
Was it A, cervical ribs fused to the neck vertebrae,
B, gastralia, C, a keeled sternum,
or D, a crescent moon-shaped wrist bone?
More than one answer might be correct, so
check all the answers that you think apply.
Cervical ribs that are fused to the neck vertebrae are found in modern birds and
in Archaeopteryx.
But they are also found in Compsognathus and many other predatory dinosaurs.
Gastralia, or
belly rib,s are an anatomical feature that modern birds have done away with.
Dinosaurs, including Compsognathus, have them, but so does Archaeopteryx.
A keeled sternum is critical for
modern birds because it provides increased attachment for flapping flight muscles.
But this feature had not yet evolved in either Compsognathus or Archaeopteryx.
The correct answer is D, a crescent moon-shaped wrist bone.
Like all modern birds, the wrist of Archaeopteryx is specially adapted so
that the wing can fold backwards.
Compsognathus lacks such a wrist.
However, as we'll see later,
this adaptation first evolved in non-avian dinosaurs.
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