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
5.1 Understanding the Evolution of Birds
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Из курса от партнера University of Alberta
Paleontology: Theropod Dinosaurs and the Origin of Birds
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The Avian World
66 million years ago, an asteroid the larger than Mt. Everest collided with the earth and brought about the extinction of the dinosaurs…except birds! Now that you’re familiar with some of their larger Mesozoic ancestors and their bird-like features, it’s time to meet the avian lineage proper. With the evolution of flight, birds could exploit habitats and resources that were literally unreachable by other animals. The evolution of birds has been one of diversification. Flightlessness has evolved numerous times, as have specializations for insectivory, swimming, and predation. Although theropods may no longer dominate the land, they still rule the skies.
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Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
We now come to the final major branch of the theropod family tree and
the third major group of Eumaniraptora, the Avialae or birds.
In this lesson we're going to examine the fossil record of their evolution and
the morphology of various modern forms.
Trying to pinpoint exactly where in the Eumaniraptora
the Avialae come from is difficult.
If I had to speculate I'd say, based on the similarities in the tails and
the feet of some early birds, that they're most likely descended
from a close common ancestor shared with troodontids.
But others would argue that they're probably more closely related
to dromaeosaurus or to more ancient eumaniraptorans that we don't yet
have a good fossil record of.
Looking at the branching pattern of the theropod family tree at such a fine level
of detail, it is also unclear where exactly Archaeopteryx fits.
Indeed, it's possible that Archaeopteryx is not actually part
of the true Avialae, but just very close to it.
Archaeopteryx could in fact be a troodontid or
it could still very well be the first true bird.
Even at this early stage, the bird branch of the theropod tree is bushy,
making it hard to see specific phylogenetic relationships.
And as you'll learn in this lesson birds only became more and more diverse.
Flight, it seems is a pretty useful power to have.
It gave birds easy refuge from predators and
the ability to exploit hard to reach resources.
Flight removed basically all geographic barriers.
Even with competition from pterosaurs, bird evolution took off.
And birds became the most successful of all dinosaurs.
True Avialae have a scapula fused to the coracoid.
They also have long arms with a humerus that is longer than the scapula and
an ulna that is longer than the femur.
The tails of Avialae are short and are composed of 25 or fewer caudal vertebrae.
The most primitive known Avialae were chicken-sized and
included the Chinese genre Jeholornis.
Jeholornis is important,
not just because of its status as an early member of the Avialae,
but because it seems to show an early stage in the development of one
of the classic bird traits that is unrelated to flight, a beak.
Do you remember which of the following theropod groups evolved beaks?
Is it A, Scansoriopterygidae?
B, Spinosauridae?
C, Therizinosauria?
And or D, Ceratosauria?
More than one answer might be correct.
So select all the answers that you think are correct.
Therizinosauria all had beaks; so too did the Ceratosaurine limosaurus.
So C and D are correct.
You will also recall that beaks evolved in the Oviraptorosauria and
the Ornithomimosauria.
As we have seen many times throughout this course,
beaks often evolved in theropods that switch to herbivorous or omnivorous diets.
Jeholornis still has teeth but they're greatly reduced in number, and
it has a small beak in the front of its mouth.
Fossil seeds preserved inside of one Jeholornis specimen confirm
that it too had made the switch.
Most remaining birds belong to the pygostylia.
These birds possess long rod like pygostyles which
you will remember are fused terminal tail vertebra that support feathers.
Additionally, the pygostylia all have more sacral vertebrae
than their more primitive avian ancestors.
Among the earliest pygostylia was the genus Confuciusornis.
This bird is currently known only from the Early Cretaceous of Eastern Asia.
But thousands of specimens have been excavated from the Laoining fossil beds.
Confuciusornis and it's relatives are about the size of modern pigeons and
are completely toothless.
However, the beaked but toothless jaws of the Confuciusornis and
modern birds or an instance of convergence.
In many other groups of Cretaceous birds
that are more closely related to modern birds, still had teeth.
Confuciusornis offers a good example of sexual dimorphism
in the plumage of early birds.
Some Confuciusornis had a pair of long thin tail feathers with
broadened ovoid tips while others lacked these enhanced tail plums.
In today's birds,
ornamental plumage is often used by males to display and attract females.
Based on this modern analog what conclusion could palaeontologists draw
about Confuciusornis?
Is it A, the long tail feathers were present in juveniles but not in adults?
B, Confuciusornis flock together in large social groups.
C, the Confuciusornis with the long tail feathers were males,
and those without were females.
Or D, the Confuciusornis without the long tail feathers
were likely brightly colored.
The answer is C.
It is usually the males of a species that are most flamboyant.
So it is generally assumed that the long plumed Confuciusornis are males.
The rest of the members of the Pygostylia are grouped in a clade called
Ornithothoraces.
And that word means bird chests and
is a reference to the group's development of a large and keeled sternum.
They evolved a more flexible shoulder joint and a tarsometatarsus condition.
The feet of the Ornithothoraces had a backwards pointing hallux,
which allowed each foot to function better at grasping branches.
Another key adaptation was the arrangement of asymmetrical feathers
that extended from digit one of the hand to form a structure called the alula.
And the alula is easily seen on the wings of all modern birds, like this one.
This is the wing of a pelican and has been prepared so
that you can see the relationship between the wing feathers and the skeleton.
Here is digit one which along with these feathers forms the alula.
Notice that these feathers are on the leading edge of the wing because they
attach to digit one, movement of that finger allows the the alula to shift and
adjust independently of the rest of the wing.
That is a critical advantage because it lets birds
fine tune the curvature of their wings.
The alula comes into play most frequently during the landing and take off.
When birds need to generate lift in spite moving at relatively slow speeds.
There are two main branches or ornithothoracines,
Enantiornithes and Euornithes.
Enantiornithes appeared in the Cretaceous and
became highly successful and globally distributed.
Some Enantiornithines were as small as modern sparrows, and
others were as large as eagles.
All Enantiornithes still had teeth, and most had reduced hand claws.
The feathers on a bird's wings are not the only ones that help it to fly.
A bird's tailfeathers are also important.
In what way does a bird make use of its feathery tail?
Is it as A, a stabilizer and rudder.
B, a counterweight.
Or C, a secondary paddle?
The correct answer is A.
In flight, bird tails are used as stabilizers and
can be adjusted like a rudder to help steer it through the air.
The Euornithines also became widespread in the Cretaceous and
they marked an important change in the form of the avian tail.
In less derived theropods like Archaeopteryx,
the long tail feathers were ranged like the frond of a fern,
with feathers projecting laterally all along the rear quarter of the tail.
Among primitive pygostylians, the length of the tail is reduced and
the long tail feathers projected like a tuft from the tail tip.
In the Euornithines the tail feathers all project from the pygostyle
in a more organized fan shape, which allow the tail to function better
as a single unit to create lift and improve aerial manoeuverability.
Among the Late Cretaceous Euornithines were the first birds to take
a surprising evolutionary twist, the loss of flight.
Why might a lineage of birds lose the power of flight?
is it because flightlessness A, allows them to increase absolute body size.
B, requires less energy.
C, improves their ability to migrate.
And or D, improves their ability to avoid predators.
More than one answer might be correct so
check all the answers you think are correct.
Flightlessness can evolve for many reasons.
Increase body size is one.
Flight requires a lightweight body, so if the evolutionary benefits
of large body size outweigh the benefits of flight, flightlessness can result.
A big drawback to flight is its high energy requirements.
So A and B are both correct.
Migratory ability and
predatory avoidance are usually benefits associated with flight.
Patagopteryx was a chicken sized bird from South America with wings so
small they could not possibly have been used in flight.
Gargantuavis is a European bird known from only a small collection of bones however.
Those few bones indicate a bird that was robustly built, and
near the size of a modern ostrich.
It would have been too large and heavy to have flown.
The return of flightlessness of such birds in the Cretaceous so
soon after the development of avian flight,
should be no more surprising than the various flightless birds of today.
These grounded Cretaceous birds serve as a reminder that although
we have progressed through the theropod family tree,
with flight and the archetype of modern birds in mind.
Evolution is a process without foresight or long term goals.
Evolution works to optimize animals for their current environment.
It cannot anticipate future environmental changes or challenges.
As long as an adaptation is beneficial in the present,
it can evolve even if that adaptation will doom the species to extinction later.
Advanced Euorithines are classified within the Ornithurae,
which continue the trend in tail reduction.
Generally the tails of Ornithurae contain six or fewer
free vertebrae and the total tail length is less than the length of the femur.
Do you recall from the previous lessons what effect tail reduction had on birds?
Did the reduction result in A, increased total body mass?
B, a centre of mass positioned in front of the hips.
C, increased flight stability.
And or D, weaker femoral retraction musculature.
Multiple answers might be correct so check all the answers you think are correct.
Naturally a shorter tail weighed less than a longer one, so
total body mass was reduced not increased.
Because this weight reduction took place behind the hips,
it resulted in a forward shift in the centre of mass.
A more forward centre of mass and
the lack of a streaming tail reduced the stability of flying birds,
however that was not a bad thing because it granted birds improved manoeuverability.
Lastly, the reduction in the tail included reduced femoral retraction musculature.
So, B and D are the correct answers.
The Hesperornitheans were a Cretaceous group of Ornithurae that
also had greatly reduced wings and most were also flightless.
However, they were not ground birds like Patagopteryx and Gargantuavis.
Instead, they were flightless water birds like modern penguins and
the Galapagos cormorant.
Hesperornitheans still had teeth and to suit their piscivorous diets,
these teeth evolved into long conical form.
It should be remembered that the fossil record of Mesozoic birds is poor.
Although some spectacular fossil quarries have yielded beautifully
preserved specimens, they are the exceptions and not the rule.
Because birds are generally so small bodied and
have such thinly walled bones, their skeletons are not durable.
And this gives them very poor odds of ever becoming fossils.
For instance, at some point in the Cretaceous,
we don't know if it was early or late, a new group of birds evolved.
The Neornithes, and these include all modern birds.
Neornithes are characterized by the complete absence of teeth.
The mass extinction at the end of the Cretaceous and the beginning of
the Cenozoic wiped out many different kinds of animals and plants.
It ended lineages of all major dinosaur groups, except birds.
Which of the following traits of birds might have made them more resilient
than other dinosaurs, during the stressful period of this extinction event?
Was it because birds were A, well armoured.
B, small and therefore had lower food requirements.
C, limited in their habitat distribution.
Or D, grew and reproduced slowly.
One clear pattern of the End Cretaceous mass extinction is
that it hit big animals the hardest.
During periods of extreme environmental stress, the low food requirements and
often faster reproductive rates of small organisms can be a big advantage.
So the correct answer is B.
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