Paleontology: Ancient Marine Reptiles is a four-lesson course teaching a comprehensive overview of the evolutionary changes that occur when air-breathing terrestrial animals return to water. This course examines the diversity, adaptations, convergence, and phylogenetic relationships of extinct marine reptiles. Students will explore three major groups of marine reptiles: ichthyosaurs, plesiosaurs, and mosasaurs. Watch a preview of the course here: https://uofa.ualberta.ca/courses/paleontology-marine-reptiles
1.2 The Aquatic Problem - Part 1
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來自 University of Alberta 的課程
Paleontology: Ancient Marine Reptiles
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從本節課中
Introduction to Marine Reptiles
Welcome to the first lesson of Palaeontology: Ancient Marine Reptiles. In this lesson we will explore the main theme of the course: the aquatic problem. In other words, what happens when a terrestrial animal returns to the water permanently? How do air-breathing, land-lubbing creatures once again adapt to life in the sea? Life in water is very different than life on land. Water is much denser than air, which affects all aspects of an animal's life including movement, sight, and hearing. In addition, animals that return to the water cannot breathe water, and so must return to the surface for air. Water also conducts heat much better than air, making staying warm and active a challenge. Despite all these obstacles, many land animals have returned to the water throughout the course of evolutionary history. In fact, many examples of them are living today including whales, seals, crocodiles, sea turtles and penguins. Each of these animals had ancestors that returned to the water. The process of overcoming the challenges associated with this transition is what we refer to as the aquatic problem. This lesson will explore many different types of adaptations that modern and extinct animals have evolved to meet these challenges. You will be introduced to some extinct groups of reptiles you have probably never heard of, and will gain a new appreciation for how well suited modern marine animals are to their environment. 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.
與講師見面
Michael CaldwellProfessor
Department of Biological Sciences
Halle P. StreetSessional Instructor
Faculty of Science
Before we move on, take a moment and try to think of some ways that humans
artificially modify our bodies to be more efficient in the water.
What specific clothing or equipment do we use and how would these modifications
differ if someone is swimming laps in the pool or diving to explore a coral reef?
Human's have invented a number of tools
that enable them to explore the aquatic environment.
The use of scuba gear, goggles and
wet suits, are some of the examples you may have thought of.
Further examples will be introduced in this section,
as we discuss some of the specific factors that contribute to the aquatic problem.
First, let's consider the differences in movement on land
as opposed to movement in the water.
On land, your feet push against the ground to move forwards, but
in water, what do you push against?
Some semiaquatic mammals, like hippos, can walk along the bottom of lakes and rivers.
But what about animals that live out in the open ocean?
In order to move, they have to push against the water itself.
Adapting to push against water instead of land
is the first aspect of the aquatic problem we will address.
If you move your hand through the air in front of you, you feel almost nothing.
But if you were to repeat that motion underwater,
your hand would encounter a lot of resistance.
And this resistance results from the greater density of water, which is what
amniotes push against to generate propulsive force that moves them forwards.
This is known as <b>thrust</b>.
Some semi aquatic amniotes, like ducks and beavers,
have webbing between their toes that increases their surface area.
In this case, more surface area means that they push against more water,
which results in more thrust.
Next time you go swimming, try moving your hand through the water,
first with your fingers spread apart, and then with your fingers pressed together.
You generate much more thrust and movement with a closed hand than an open hand.
And you can generate even more with the aid of a swimming paddle.
Human feet are not particularly efficient at displacing much water to
generate thrust.
So scuba divers can generate more thrust by wearing swim fins.
Another adaptation to increased propulsion is the flipper.
Almost all aquatic and semi-aquatic amniotes have developed <b>flippers</b>,
which are limbs adapted to generate thrust in water.
Now compared to a terrestrial limb, such your own hand,
the bones in the flipper tend to be flatter than the humerus, radius, and
ulna are often proportionately shorter.
On the other hand, the <b>phalanges</b>, or finger bones, are proportionately longer.
Flippers are encased in tissue and take on a smooth, flat appearance like a wing.
Flippers aren't very good at supporting weight on land, although some amniotes
like sea lions, retain some weight-bearing abilities in their front limbs.
Others, like penguins and sea turtles,
use their flippers like underwater wings to generate thrust.
Flippers are part of the <b>appendicular skeleton</b>,
which includes the limb, hip, and shoulder bones.
So we say these animals move using <b>appendicular locomotion</b>.
<b>Axial locomotion</b> is when an animal generates thrust with its <b>axial skeleton</b>,
which includes the head, spine, and tail.
In order to generate more thrust from tail movement, whales and
manatees have <b>caudal flukes</b> that increase the surface area of their tails.
The caudal fluke sits at the end of the tail, is supported by the vertebrae.
They are made out of concentrated bundles of dense,
fibrous connective tissues called <b>collagen</b>.
Sea snakes have also evolved a flattened, paddle-like fluke that
increases the amount of thrust their tails generate as they undulate.
These three animals are representative
of the fossil marine reptiles we will focus in the following lessons.
And based on what you've just learned about the parts of the body that can
generate thrust which of these three animals are axial swimmers?
A, Ichthyosaur.
B, Plesiosaur, and or C, Mosasaur.
The Plesiosaur has limbs that have been modified into large flippers,
and not much of a tail fluke.
Therefore it's most likely an appendicular swimmer. The Ichthyosaur and
Mosasaur have smaller flippers and more developed tails so
it's more likely that they use axial locomotion.
So A and C are the correct answers.
Note, how the tail of the Ichthyosaur and Mosasaur are completely different.
The Ichthyosaur's tail is short and crescent shaped, and
the Mosasaur's tail is long and broad.
Even though both generate thrust using their exoskeleton,
they did so in different ways.
The most common solution to the problem of underwater thrust generation
is to use a high surface area flipper or fluke in a continuous motion.
Think of the foot propelled swimmers such as a duck, or
the flipper propelled swimmers such as sea turtles.
Like a human swimmer doing freestyle,
these animals move their limbs continuously in an efficient,
alternating pattern which prevents them from slowing down.
Animals that use axil locomotion, such as sea snakes,
whales undulate continuously as they swim.
Take a look at this swimmer doing a dolphin kick.
This is a swimming style in which humans use axial locomotion.
You can see that the entire body
undulates with the movement wave starting in the abdomen.
There are three major types of axial locomotion in aquatic animals
differentiated depending on where the wave starts in the body.
<b>Anguilliform</b> locomotion describes a motion where the propulsive wave originates at or
near the head, and then ripples down the entire body.
It's most obvious in sea snakes and eels.
Anguilliform swimmers tend to be very long and skinny.
<b>Thunniform</b> locomotion is seen in several kinds of high speed
predatory fish like tuna and sailfish, and also in some dolphins.
Thunniform swimmers begin their undulatory wave at or
just in front of the cuttle fin or fluke.
So the body doesn't oscillate during swimming.
We tend to see broad torpedo shaped bodies and
crescent shaped fins and flukes in tunniform swimmers.
In between these two extremes are <b>carangiform</b> swimmers.
In this kind of locomotion, the angulatory wave starts
somewhere in the back half of the animal but not quite at the tail fluke.
Seals, crocodiles, and marine iguanas swim using this mode of locomotion.
What kind of swimming mode is being used by a whale?
As a hint, try and look at how much of the body ungulates.
A) Anguilliform.
B) Carangiform.
Or C) Thunniform.
If you look at this humpback whale,
you can see that only the back half of the body undulates.
In anguilliform swimmers, undulation starts at the neck.
So A is not correct.
In thunniform swimmers, it is only the caudal fin or fluke that oscillates.
So C is not correct.
In carangiform swimmers, the undulatory wave starts
in the back half of the animal, but still well before the caudal fluke or fin.
So B, carangiform locomotion is the correct answer.
As we have already discussed, the high density of water allows aquatic
animals to push against the water surrounding them in order to move.
Now a terrestrial animal can push off the ground in four directions,
forwards, backwards, left, and right.
We can jump too, but since we always come back down,
it doesn't really count as sustained directional movement.
Being able to push off of the surrounding water
enables aquatic animals to move up and down at will.
Terrestrial animals only move in two dimensions, but
aquatic animals move in three and this means that
aquatic animals have to be stable in three dimensions, instead of just two.
This is the second aspect of our aquatic problem.
If you ever played with a toy boat on the water,
you know that it can be difficult to stop it from rolling over and
the problem just gets worse underwater, especially at high speeds.
A fully aquatic animal needs to be able to control their up-down,
left-right motions and also needs to be able to control how much they roll.
Whales and dolphins mostly use their flippers for manoeuvring and
stabilizing their body so it doesn't roll sideways while moving forwards.
Sea turtles are flat enough and
slow enough that their bodies don't roll very easily.
Some whales and dolphins have a prominent dorsal fin, which helps stabilize the body
against rolling, especially those that typically move at great speeds.
Think about steering a car or a bike.
At low speeds, you have to use large motions to turn, but at high speeds
a small twitch of the steering wheel or handle bars has a much larger impact.
Therefore, stability is more important for faster moving animals
which is why they tend to have larger dorsal fins.
The high density of water also contributes to the third aspect of
the aquatic problem we will discuss, the problem of drag.
Imagine four swimming pools full of the following substances,
honey, water, air, and cream.
Which would cause the least drag and which would cause the most drag?
<b>Drag</b> resists the movement of our bodies through any medium and
is increased by both higher density and viscosity.
Air is much less dense than water and is also less viscous.
So moving through it causes less drag.
Think about walking in a shallow swimming pool.
The greater density and
viscosity of water means it's much more difficult than walking on the sidewalk.
Cream is denser and more viscous than both air and water and
honey is the densest and most viscous of all four.
So, the easiest substance to walk through is air which causes
the least drag followed by water, cream, and honey, which causes the most drag.
There are two types of drag that an animal experiences in the water,
inertial drag and viscous drag and
combined these two types of drag form the third aspect of the aquatic problem.
<b>Inertial drag</b> is caused by the high density of the water.
As a body moves through water,
it disturbs the water molecules, forcing the water to flow around it.
Once the body has passed through the water,
it leaves an empty space where the body used to be.
Water enters this space, swirling and spinning and creating turbulence.
The empty space which pulls the water in to fill it,
also pulls on the object that created it.
This causes inertia drag, sucking the object backwards.
To minimize it, a body should be shaped so
that it disturbs the water as little as possible as it passes through.
Less disturbance means less swirling water, less suction,
and therefore, less wasted energy and decreased speed.
An object that decreases inertial drag by minimizing
disturbance to the water is described as <b>streamlined</b>.
Which of these bodied positions would be most streamlined in the water?
Would it be a swimmer with his arms.
A) Held out sideways away from the body.
B) Held against the side of the body.
Or C) Extended out front above the head.
Streamlining is easily visualized in the pool.
When pushing off the wall, swimmers want to be as streamlined as possible so
they can maintain maximum speed.
Swimmers pushing off of the wall with their arms wide and legs apart can
barely move a meter before the significant inertial drag pulls them to a stop.
With their arms held against their sides, they can move farther.
However, if they stretch their arms straight out over their heads,
keep their legs together, and their bodies straight,
they can move several meters before their inertial drag slows them to a stop.
So the most streamlined body position and the correct answer is C.
A well streamlined body has a small amount of surface
area in the direction they are moving resulting in a smooth, elongate shape.
In the most derived cases, streamlining results in a smooth,
torpedo-shaped body, referred to as <b>fusiform</b>.
It can be seen in animals like sharks, tuna, and dolphins.
Different aquatic animals may be less elongate and smooth, but
we tend to see some degree of streamlining in the body shape
of amniotes that spend a lot of time in the water.
The second type of drag is called <b>viscous drag</b>.
Viscous drag is due to the higher viscosity of water.
A highly viscous material resists, flow which causes a lot more
friction as it moves over a surface and slows an animal down.
To decrease viscous drag,
the surface needs to be as smooth and frictionless as possible.
Once again, let's picture this in a pool.
Swimmers wear tight fitting swimsuits and
swim caps to cover their hair in order to minimize friction.
Many olympic swimmers even shave their legs and arms before racing.
Whales and manatees have lost all or most of their hair,
resulting in smooth skin that produces much less drag.
And this allows them to swim using far less energy.
Many marine reptiles have smaller, smoother scales than their terrestrial
relatives, which again, causes less friction, and therefore less viscous drag.
It's important to note here,
that being surrounded by a high density medium, such as water,
doesn't just cause new locomotion problems for terrestrial amniotes.
It also confers a major benefit.
On land, all of an animal's weight is carried by its feet,
but in the ocean, water supports the body on all sides.
A terrestrial skeleton needs to be very strong and
heavy to support an animal's weight.
But in the water, a much smaller,
more loosely attached skeleton can support a much larger body.
And this is why the earth's largest animals are found In the oceans.
Any amniote that swims, including us, still needs to breath air.
No aquatic amniote has ever re-evolved gills or
similar fish like respiratory structures.
This means that although locomotion and feeding occur in the water,
they still must return to the surface to breathe.
But how often?
Which of these organisms can hold their breath underwater the longest?
A, human.
B, Sperm Whale.
C, Emperor Penguin.
D, Salt Water Crocodile.
The average human can probably hold their breath for
up to a minute to dive underwater.
Animals like crocodiles, marine iguanas, penguins, and
most whales can go about 10 to 30 minutes between breaths.
Sperm whales can hold their breath for about 90 minutes.
That makes B the correct answer.
Currently the record holder for longest dives is a whale you
might not have heard of called Cuvier's beaked whale.
They can stay underwater without breathing for more than two hours.
So how do these aquatic amniotes manage to stay under water for
such long periods of time?
Like us, all amniotes have lungs,
which means they have to return to the surface periodically to breathe.
This is the fourth aspect of the aquatic problem.
Aquatic and semi-aquatic animals have evolved
several different methods for dealing with breathing in aquatic environments.
Crocodiles can tolerate much higher levels of <b>lactic acid</b> in their blood,
which is formed when amniotes do strenuous activity without getting enough oxygen.
This is what happens when we run for a long time and
get sore legs, or when we hold our breath while exercising.
Whales take series of rapid breaths at the surface
to super charge their blood and muscles with oxygen.
Then, they use the oxygen stored in their blood, rather than
air in their lungs, to maintain their muscle activity during deep dives.
Penguins slow their heart rate when they dive so
that blood doesn't move around the body as fast, and the oxygen is used slower.
They also have an amazing ability to divert blood away from some organs and
towards more critical areas, when their oxygen levels are depleted.
The need to surface for air also results in skull shape adaptations.
In most terrestrial amniotes, the nostrils, or
<b>nares</b>, are located towards the tip of the snout, like you can see on this dog.
In many aquatic amniotes,
the nares migrate towards the eyes, or the top of the head.
So that the animal only has to lift a little bit of its body
above the water in order to breath.
This is easiest to see in whales and dolphins, like this beluga whale skull.
In marine mammals,
these dorsally positioned nostrils are called the <b>blowhole</b>.
However, this is not the case for every aquatic animal.
For instance, crocodiles’ nares are on the tips of the snout, as you can see here.
