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
5.2 Survival of the Neornithes - Part 1
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來自 University of Alberta 的課程
Paleontology: Theropod Dinosaurs and the Origin of Birds
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從本節課中
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.
與講師見面
Philip John Currie, Ph.DProfessor and Canada Research Chair, Dinosaur Paleobiology
Department of Biological Sciences
From this point onwards, we have to say goodbye to all the theropod
groups that we have met previously and to all the rest of Dinosauria.
At roughly 65.6 million years ago,
a giant asteroid collided with Earth and brought on a series of
climactic disasters that doomed the dinosaurs, nearly.
The Neornithine survived although all other Cretaceous birds died out.
Unfortunately there's no good record of early neornithines and
their fossil record throughout the Cretaceous is also extremely spotty.
Neornithines underwent a great radiation in diversity but
it's unclear when this radiation happened.
It may have occurred in the wake of the End-Cretaceous mass extinction
as they diversified in response to the ecological vacuum the extinction created.
However, recent genetic studies of modern birds
have applied a new technique, called the molecular clock, to the problem.
The concept behind a molecular clock is that mutations within the genetic code
of a lineage accumulate at a roughly steady average rate.
By comparing the genetic codes of two lineages, you can add up the number
of genetic discrepancies to calculate how many mutations occurred,
since the two lineages split from a common ancestor and
thereby estimate how long ago that split took place.
When applied to birds, the results indicate that as many as 42
major avian groups had already split before the end of the Cretaceous.
It may be that this result is simply a misleading artifact brought about
by an above average rate of genetic change following the extinction event.
But if it is true the Neonithines had a much greater Cretaceous
diversity than the fossil record currently suggests.
And had a much greater survival record through
the end-cretaceous extinction than many other animal groups.
Based on your understanding of a molecular clock,
which of the following could the technique not be used to estimate?
A, which of three related lineages first diverged from their shared ancestral line.
B, if a radiation of species occurred recently or in the distant past.
C, if two lineages split before or
after an extinction event of known age.
Or D, at what point in time an ancient lineage became extinct.
The answer is D, the molecular clock requires genetic data,
this means it can only be applied to modern surviving lineages for
which a good genetic sample can be taken.
Ancient extinct lineages cannot be analyzed because
good genetic data is not preserved in the fossil record.
This is a rhea, a flightless bird from South America.
As you might suspect, it is a relative of the ostrich and the Australian emu.
Such large flightless birds, along with others like the Cassowary and Kiwi,
belong to the most primitive of all living bird groups, the Palaeognathae.
Most palaeognates are primarily herbivorous
but do occasionally eat small mammals and reptiles.
The Kiwi is a specialized insectivore
that probes into soft soil using its long down turned beak.
Being completely flightless many palaeognates have reduced wings and
a keel-less sternum.
Additionally, because they are freed from the weight restrictions of flight,
palaeognates have grown larger.
The ostrich is the heaviest modern bird, but
the recently extinct palaeognates, Apiornus maximus,
better known as the elephant bird, is thought to have been
the heaviest bird of all time, and to have weighed 1/2 a ton.
A bird that cannot fly, and is as big as a modern ostrich,
is in danger of being easily spotted and hunted by predators.
How do flightless birds, like ostriches, avoid predation?
Is it by, A secreting feather toxins.
B burying its head in the sand.
Or, C running away.
Ostriches and many other flightless birds use speed to escape predators.
C is the correct answer.
For ostriches, which live alongside big predators like lions and
hyenas, flightlessness puts them in real danger.
To avoid being eaten, they've fallen back of the old coelurosaurian trick
of high speed running.
This is the foot of a modern ostrich.
And you can see how long the metatarsals are.
Additionally, you can see that an ostrich foot has two toes.
And that only one of the toes has a claw.
Have a look at the claw's tip.
It has been severely worn by contact with hard ground while running.
To serve as shock absorbers, the two toes have large fleshy pads.
We know from fossil footprints that many non-avian theropods,
including ornithomimids and tyrannosaurs had toe pads similar to an ostrich's.
While ostriches have a real need for
speed, many of their large Palaeognathe relatives did not.
Elephant birds were inhabitants of the island of Madagascar
until as recently as three centuries ago.
Their fellow Paleognathes, the moas once thrived
on the islands of New Zealand until roughly 600 years ago with
some Moa species growing nearly as large as the elephant birds.
The first Moa fossil known to science
was a broken section of femur discovered along a riverbank.
And this curiosity made its way from New Zealand to Australia, and
finally to England, where it landed on the desk of none other than Sir Richard Owen.
Owen pondered the bone for some time.
Then in 1839 he announced rather daringly to the scientific
community that the bone was from a giant flightless bird.
Such a bizarre conjecture from a small piece of evidence
led to serious harrumphing from the scientific community at large.
Three years later, a new shipment of bones was sent from New Zealand to Owen.
This was a full skeleton of a moa,
which Owen triumphantly mounted and placed on public display.
This was a grand victory for Owen and
greatly advanced his scientific reputation.
Moas and elephant birds descended independently from flight-capable,
palaeognath ancestors.
And that ability to fly is how their ancestors got to each of the islands,
which large, earthbound mammals had been unable to reach.
So, the colonizing paleognathes found themselves
on lands where the niche of large herbivore was unfilled.
Both groups of paleognathes evolved to fill that niche,
eventually growing into a diversity of large flightless species.
Why did Moa and Elephant Birds not follow the same evolutionary
path as ostriches, and develop the capacity for high-speed running?
Is it because they A, were herbivorous and did not need to run to catch their prey?
B, lived on islands where there were no large predators to chase them.
C, had perching feet and could not evolve large foot pads.
Or D, weighed too much.
The correct answer is B.
New Zealand and Madagascar were also lands uninhabited by large mammalian carnivores.
So Moas and Elephant Birds evolved thick,
stout limbs that helped to support their increasing body weight.
But they had little evolutionary incentive to evolve long,
ostrich-like legs for running.
This would ultimately be the downfall of both lines of great palaeognaths.
When humans eventually colonized Madagascar and New Zealand,
they found the large and relatively slow birds to be easy prey.
The Moas were quickly hunted to extinction.
The sister group of the Palaeognathae is the Neognathae.
And these birds are characterized by the fusion of their metacarpals,
elongate third fingers.
and 13 or fewer caudal vertebrae.
The Neognathae are divided into two subgroups:
the Galloanserae and the Neoaves.
We'll start with the Galloanserae, which in turn is
composed of two major groups, the Anseriformes and the Galliformes.
The Anseriformes includes modern ducks, geese, swans and many other water fowl.
It also includes some giant flightless forms,
which evolved not on isolated islands like Moa and Elephant birds, but
on major continental bodies.
Sixty-five million years ago,
life was in the process of recovering from the End Cretaceous extinction.
Perhaps largely because they were both warm blooded, mammals and
birds, rather than reptiles, began to fill the large herbivore and
carnivore vacancies left by non-avian dinosaurs.
One group of Anseriformes was among the first to grow large.
The gastornithidae were large flightless birds known from throughout Asia,
Europe, and North America.
Particularly large species stood two meters tall.
Like elephant birds and moas, they had relatively short legs and
probably few if any natural predators.
Gastornithids had thick large beaks, with powerful jaw muscles,
but the precise ecological role of Gastornithids is debated.
Their beaks only have a slightly down curved tip,
making it unclear if they were predators or herbivores.
Never the less, these birds with their horse-size heads have been known for
a long time by palaeontologists.
And museum displays of forms like Diatryma,
capture the imagination of the public. Regardless Gastornithids
were never highly diverse, and
they're thought to of lasted about 11 million years.
Like many birds, ducks are extremely vocal.
They are also highly social and use sound to convey
all sorts of information to other members of their flock.
Listen closely to these bird calls, and
see if you can identify which belong to a duck.
More than one answer might be correct, so select all that apply.
A, [SOUND],
B [SOUND],
C, [SOUND],
D, [SOUND].
In fact all those calls belong to ducks.
Every answer is correct.
The standard quack,
quack is just one of the many vocalizations most ducks can make.
On land ducks are a comical, awkward sight.
Their hind limbs have been modified for swimming.
The feet and legs of swimming Anseriforms are strongly muscled, and
have lengthened second, third, and fourth toes.
These toes support a web of skin for paddling.
This pattern of webbing is termed the palmate foot type.
In many cases, the first toe has no webbing, and makes little or
no contribution to the paddle.
However, in some diving ducks, the first digit,
sports a fleshy lobe that provides an extra swimming boost.
The feet of the second group of Galloanserae are also modified,
but for the opposite purpose.
The Galliformes include modern chickens, turkeys, peafowl,
quail, and various other game birds, like this pheasant.
Primitive Galliformes have feet adapted for perching.
Each foot has a long, backwards oriented and
low positioned, hallux, which helps the foot to firmly grasp branches.
More derived forms have feet modified for walking and running,
with a hallux reduced in size and positioned high on the feet.
And this prevents the hallux from contacting the ground.
Most Galliformes spend far more time foraging on the ground
than they do in trees or on the wing.
The wings of Galliformes are also adapted for their mostly terrestrial habits.
This is the wing of a pheasant, and for
comparison, here's the wing of a goose of roughly similar body size.
Notice first that the Galliform wing is far shorter in absolute length.
And ends in a rounded edge, rather than an extended, pointed tip.
This makes the wing much less efficient in long, sustained flights.
But it makes it easier for the Galliform to manouever in tight spaces like forests.
Notice too, that in side view, the Galliform wing has a strong arch.
This makes it extremely good at generating lift,
even when moving at relatively slow speeds.
It is a wing form that is beneficial for frequent takeoffs and landings.
This is important,
because wild Galliformes still rely on flight to escape from predators.
Think back to our previous lesson, and
you will recall us discussing some of the Galliformes.
We talked about some of them in the context of what unusual adaptation?
Was it A, raised raptorial claws.
B, insulating feathers,
C, insect probing, or D, wing assisted incline running.
The correct answer is D,
many Galliformes display wing assisted incline running or WAIR.
The second group of Neognates is the Neoaves.
And recent genetic studies have divided the Neoaves into two groups.
The first of these is Columbea, which includes pigeons,
doves, sand grouses, flamingoes, and odd birds like this fellow.
This is a greeb.
Greebs are water birds with long necks and lance-shaped beaks for spearing fish.
A greeb does not have a webbed foot like a duck.
Instead each toe is separate and has large
flattened lobes that give the foot increased surface area for paddling.
This foot for is termed lobate.
Grebes are excellent swimmers and typically poor flyers.
A few grebes are completely flightless.
Columbea also contains one of the most famous extinct birds, Raphus cucullatus.
Better known as the Dodo.
The Dodo is famous for its comic look
and its extermination at the hands of humans in 1662.
This extinction came less than a century after its discovery.
Dodos inhabited the island of Mauritius, east of Madagascar.
They were generally thought to have been primarily fruit eaters.
Dodos were probably descended from pigeon-like ancestors that made
their way to the isolated island and, as in previous cases,
evolved flightlessness in the absence of terrestrial predators.
Dodos were far less adapted for
flightlessness than many of the other forms we've met.
The sternum still had a keel for big chest muscles.
And the hallux was still long and made contact with the ground.
Having been in a predatorless environment for so long,
Dodo's had no intrinsic fear of humans and were easy game for
sailors who visited the island and later, for colonists.
Perhaps more importantly, human colonization introduced pigs and
dogs onto Mauritius, and these domestic animals are thought to have
pillaged dodo nests, which were built on the open ground.
Dinosaurs are famous for being big.
But do you know what dinosaur holds the record for being the smallest?
Is it, A, Compsognathus B,
Argentinosaurus C, Microraptor,
or D, The bee hummingbird?
The bee hummingbird from the Cuban archipelago is the smallest of
all dinosaurs, so D is the correct answer.
The other group of Neoaves is the Passerea.
Passerea contains a huge menagerie of bird groups
that genetic studies have shown all descended from a shared ancestor.
It's far beyond the scope of this course to try and cover all the modern and
extinct groups within Passerea.
Instead, let me introduce you to a few select lineages.
You can find more information in the course notes.
The Opisthocomidae is a group that we could not possibly overlook in this
course.
It's not a very big clade, it only has one living species.
The chicken sized Hoatzin.
The Hoatzin is a herbivorous inhabitant of South American swamps and jungles.
It has an enlarged crop and an odorous digestive system
that has earned it the unflattering nickname of 'stink bird'.
The reason the Hoatzin is so important to include in this course is that it has
an unusual adaptation found only in its juveniles.
Young Hoatzin hatch with clawed hands.
And these claws are used by the juveniles to climb trees
until their wing feathers and flight muscles are fully developed.
Clawed Hoatzin chicks are important when discussing bird and flight origins.
They offer an analog for how a small arboreal might
have behaved, supporting the trees down theory.
Cranes, rails, seagulls, puffins and sandpipers all belong to yet
another group of Passerea, the Cursorimorphae.
The early evolution of this group is uncertain.
Many members of the Cursorimorphae have long legs adapted for wading.
And fragmentary avian fossils from the Cretaceous
indicate the presence of birds with similar wading legs.
However, these Cretaceous waders may belong to different lineages
with their long legs simply being convergent.
The oldest definitive Cursorimorphae fossils
date back to just after the End Cretaceous mass extinction.
Aside from long legs, many of he Cursorimorphae have feet
that are adapted to life in their particular habitats.
Which of the following traits would you expect them to share?
A, Padded feet for running over solid ground.
B, Wide-spreading toes to support their weight on soft, wet ground.
C, Gripping feet for clinging to rocky slopes.
D, Insulated feet for slogging through snow.
The correct answer is B.
As inhabitants of marshes and
shorelines, many of the Cursorimorphae have wide-spreading toes.
