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Comparing Chordates
• A red-billed oxpecker
perches on an impala
• These chordates have a
mutually beneficial
relationship
• Oxpeckers pick ticks and
other external parasites
off the impala, obtaining
food while ridding their
host of parasites
Comparing Chordates
Chordate Evolution
• Ever since the first chordates appeared more
than 500 million years ago, they have been
evolving
• During this continual process, chordates
developed an incredible variety of adaptations
• Some of these traits—scales or hair, for
example—are relatively simple
• Others—such as a four-chambered heart or an
amniotic egg—are far more complex
• All these adaptations were tested and
shaped by natural selection
Chordate Origins
• Much of what scientists know about the
origins of chordates comes from studying
the embryos of living organisms
• Such studies suggest that the most ancient
chordates were closely related to
echinoderms
• Do scientists know what these early chordates
looked like?
• Surprisingly, the answer is yes
Chordate Origins
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The variety of fossilized organisms
preserved in the rich Cambrian
deposits of Canada's Burgess Shale
includes a peculiar organism called
Pikaia, shown in the figure at right
When Pikaia was first discovered, it
was thought to be a worm
On closer inspection, scientists
determined that Pikaia had a
notochord—a flexible, supporting
structure that is found only in
chordates
Pikaia also had paired serial
muscles that were arranged in a
manner similar to those of today's
nonvertebrate chordates, such as
lancelets
On the basis of fossil evidence,
scientists now classify Pikaia as an
early chordate
Chordate Origins
• To better understand the early evolution of
chordates, biologists study a nonvertebrate
chordate that is alive today—the tunicate
• The tadpolelike larvae of tunicates are the
simplest living animals to have a notochord,
a dorsal hollow nerve cord, a tail that
extends posterior to the anus, and
pharyngeal pouches—key features common
to all chordates
• Today, biologists are studying the genes that
control the development of these features
Pikaia
• This is a
reconstruction of
Pikaia, a soft-bodied
animal that lived
during the Cambrian
Period
• Which features did
Pikaia have that were
characteristic of
chordates?
Pikaia
The Chordate Family Tree
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The chordate family tree has its roots in
ancestors that vertebrates share with
tunicates and lancelets
The cladogram in the figure at right
shows chordate phylogeny—how the
different groups of living chordates are
related to one another and to their
invertebrate ancestors
It also shows the evolution of distinctly
vertebrate features, such as jaws and
limbs
Notice that the fishes—from hagfishes to
lungfishes—include six different groups
with long and separate evolutionary
histories
On the other hand, modern amphibians,
reptiles, birds, and mammals share much
more recent common ancestors
Where do extinct groups, such as dinosaurs,
fit into the chordate phylum?
The answer may be found in the fossil
record
Chordate Cladogram
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The phylum Chordata includes both
vertebrates and nonvertebrate
chordates
All of these subphyla share a
common invertebrate ancestor
This cladogram shows the
phylogenetic relationship of modern
chordate groups to that common
ancestor
The different colored lines
represent the traditional groupings
of these animals, as listed in the
key
The red circles indicate some of the
important chordate adaptations
Such adaptations are the results of
evolutionary processes, including
natural selection
Chordate Cladogram
Evolutionary Trends in Vertebrates
• The hard body structures of many
vertebrates have left behind an
excellent fossil record
• As a result, scientists know a great deal
about vertebrates' evolutionary history
• In addition, scientists infer evolutionary
trends by studying the characteristics of
chordates living today
Adaptive Radiations
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The number of species within each chordate
group has changed over geologic time
Look at the figure Chordate Cladogram
again
The red circles in that figure represent
the origin of certain adaptive features
For example, one notable event in
chordate evolution was the development
of jaws
Another event was the development of
paired appendages, including pectoral
and pelvic fin or limb girdles
Paired appendages allowed chordates, such
as the salamander in the figure at right, to
move more efficiently
Over the course of evolution, the
appearance of new adaptations—such as
jaws and paired appendages—has
launched adaptive radiations in chordate
groups
An adaptive radiation is the rapid
diversification of species as they adapt to
new conditions
Chordate Movement
• Amphibians were the
first chordates to have
four limbs
• Limbs allowed animals
like this modern tiger
salamander to crawl on
land
• A rapid increase in the
number and diversity of
land vertebrates followed
the evolution of four limbs
Chordate Movement
Convergent Evolution
• Adaptive radiations sometimes produce
species that are similar in appearance and
behavior, even though they are not closely
related
– This trend is called convergent evolution
• Convergent evolution occurred many times
during chordate evolution when unrelated
species encountered similar ecological
conditions and evolved similar adaptations
– For example, convergent evolution has produced
flying vertebrates as different as birds and bats
Chordate Diversity
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Living chordates are extremely diverse, as
shown in the figure
Yet, the species of chordates that are
alive today are a small fraction of the
total number of chordate species that
have existed over time
Today, vertebrates make up about 96
percent of all living chordate species and
account for more than 50,000 species
throughout the world
The six living groups of chordates are
the:
– Nonvertebrate chordates
– Fishes
– Amphibians
– Reptiles
– Birds
– Mammals
Of these, the largest group by far is
the fishes
Chordate Diversity
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This pie chart shows the diversity
of chorates
The area of each slice
represents the relative number
of living species in each group
of chordates
The inner circle shows the six
major chordate groups and
gives the percentage of species
contained in each
The outer circle breaks down
each major group and shows
the number of known species
Of the total number of fish
species, what percentage is
represented by the ray-finned
fishes?
Chordate Diversity
Controlling Body Temperature
• On a spring morning, after a cold night, a
tortoise lies on a rock basking in the sun
• Nearby, a snake slides out of its burrow beneath
a rotting stump
• In a tree overhead, a young robin puffs up its
downy feathers
• As you walk out of the water after an early swim,
your skin gets goose bumps and you shiver
• All these activities are examples of the
different ways that vertebrates control their
body temperature
Body Temperature and Homeostasis
• Recall from Chapter 2 that many of the chemical
reactions that are important in metabolism are
influenced by temperature
• For this reason, essential life functions can be carried
out most efficiently when an animal's internal body
temperature is within a particular “operating range”
• For muscles to operate quickly and efficiently, for
example, their temperature can neither be too low nor
too high
– If muscles are too cold, they may contract slowly, making it
difficult for the animal to respond quickly to events around it
– If an animal gets too hot, on the other hand, its muscles may
tire easily and other body systems may not function properly
Body Temperature and Homeostasis
• Because most chordates are vertebrates, and mechanisms for
controlling body temperature are well developed among vertebrates,
this section will focus exclusively on that group
• The control of body temperature is important for maintaining
homeostasis in vertebrates, particularly in habitats where
temperature varies widely with time of day and with season
• Vertebrates have a variety of ways to control their body
temperature
• All of these ways incorporate three important features:
– Source of heat for the body
– Way to conserve that heat
– Method of eliminating excess heat when necessary
• In terms of how they generate and control their body heat,
vertebrates can be classified into two basic groups:
– Ectotherms
– Endotherms
Ectothermy
• On cool, sunny mornings, lizards often bask in the
sun
• This doesn't mean that they are lazy!
• A lizard is an ectotherm, which means that its body
temperature is mainly determined by the
temperature of its environment
– Most reptiles, fishes, and amphibians are ectotherms—
animals whose body temperatures are controlled primarily
by picking up heat from, or losing heat to, their environment
• Ectotherms often warm up by basking in the sun,
and may cool down by seeking shelter in
underground burrows
Ectothermy
• Ectotherms have relatively low rates of
metabolism when they are resting
• Thus, their bodies do not generate
much heat
• When active, an ectotherm's muscles
generate heat, just as your muscles do
• However, because its body lacks
effective insulation, the heat is lost to
the environment fairly easily
Endothermy
• An endotherm is an animal whose body temperature
is controlled from within
– Birds and mammals are endotherms, which means they can
generate and retain heat inside their bodies
• Endotherms have relatively high metabolic rates that
generate a significant amount of heat, even when
they are resting
– Birds conserve body heat primarily through insulating feathers,
such as down
– Mammals have body fat and hair for insulation
• Mammals can get rid of excess heat by panting, as
dogs do, or by sweating, as humans do
Comparing Ectotherms and
Endotherms
• In an absolute sense, neither endothermy nor
ectothermy is superior
• Each strategy has advantages and disadvantages in
different environments
– For example, endotherms move around easily during cool nights
or in cold weather because they generate and conserve their
own body heat
– That's how musk ox live in the tundra and killer whales swim
through polar seas
– But the high metabolic rate that generates that heat requires a lot
of fuel
• The amount of food needed to keep a single cow
alive would be enough to feed ten cow-sized lizards!
Comparing Ectotherms and
Endotherms
• Ectothermic animals, such as the gila monster, need
much less food than similarly sized endotherms
• In environments where temperatures stay warm and
fairly constant most of the time, ectothermy is a
more energy-efficient strategy
– But large ectotherms run into trouble in habitats where
temperatures get cold at night or stay cold for long periods,
such as boreal forest biomes
– It takes a long time for a large animal to warm up in the sun
after a cold night
• Most large lizards and amphibians live in warm areas
such as tropical rain forest biomes
Evolution of Temperature Control
• There is little doubt that the first land vertebrates were
ectotherms
• But there is some doubt as to when endothermy evolved
– Although modern reptiles are ectotherms, some biologists
hypothesize that at least some of the dinosaurs were endotherms
– Others hypothesize that endothermy evolved a long time after the
appearance of the dinosaurs, so that all the dinosaurs were
ectotherms
• Evidence suggests that endothermy has evolved more than
one time
– It developed once along the evolutionary line of reptiles that led to
birds and once along the evolutionary line of reptiles that led to
mammals
Form and Function in Chordates
• The nonvertebrate chordates that are
alive today represent a simple and
ancient stage in the development of
chordate body systems
– However, the fact that the organ systems
are simple does not mean they are inferior
– After all, lancelets and tunicates have
survived to the present day, so their body
systems are well equipped to perform the
essential functions of life
Form and Function in Chordates
• Among vertebrates, organ systems exhibit a
wider range of complexity than those of
nonvertebrate chordates
– Many adaptive radiations of vertebrates have
produced a variety of specialized organ systems
that perform essential functions and maintain
homeostasis
• The complexity of vertebrate organ systems can
be seen in the different ways that vertebrates
feed, breathe, respond, move, and reproduce
Feeding
• Feeding and digestion help maintain
homeostasis by providing the body with a
continuing supply of needed nutrients
• Most tunicates, and all lancelets, are filter
feeders
• These chordates remove small organisms called
plankton from the water that passes through
their pharynx
• A few adult tunicates feed on deposited material
from the surface of the sediments on which they
dwell
Feeding
• The skulls and teeth of vertebrates are adapted for feeding on a
much wider assortment of foods, ranging from insects to large
mammals, and from leaves to fruits and seeds
• Some vertebrates—such as baleen whales, flamingoes, and
manta rays—are filter feeders with sievelike mouth structures that
enable them to strain small crustaceans and fish from the water
• The long bill of the hummingbird and the narrow snout of the
honey possum are both adaptations that enable them to feed
on nectar
• Other vertebrates, such as the crocodile are adapted to eating
meat
• Many mammals have sharp canine teeth and incisors that they use
to tear and slice their food
Crocodile Feeding
• The blunt, broad jaws and
numerous peglike teeth of
this crocodile help it catch
large prey—such as
zebra—even in thick
vegetation
• How do the mouth
structures of a filterfeeding vertebrate differ
from those of a carnivore
like this reptile?
Crocodile Feeding
Feeding
• The digestive systems of vertebrates have
organs that are well adapted for different
feeding habits
• Carnivores such as sharks typically have
short digestive tracts that produce fastacting, meat-digesting enzymes
• Herbivores such as cows, on the other hand,
often have long intestines that harbor
colonies of bacteria
– These bacteria are helpful in digesting the tough
cellulose fibers in plant tissues
Vertebrate Digestive Systems
• The digestive systems of vertebrates are
adapted for a variety of feeding modes
• As you can see, these systems differ in their
degree of complexity
Vertebrate Digestive Systems
Respiration
• Chordates typically have one of two basic
structures for respiration, or gas exchange
• As a general rule, aquatic chordates—such
as tunicates, fishes, and amphibian larvae—
use gills for respiration
• Land vertebrates, including adult
amphibians, reptiles, birds, and mammals,
use lungs
• However, some animals “break the rules”:
– For example, several fishes, such as lungfishes,
have both gills and lungs
Respiration
• Some chordates have respiratory structures
in addition to gills and lungs
– Many bony fishes, for example, have accessory
organs for respiration, such as simple air sacs, that
are derived from the gut
– All lancelets and some sea snakes respire by the
diffusion of oxygen across their body surfaces
• Recall that diffusion is the process by which molecules move
from an area of higher concentration to an area of lower
concentration
– Many adult amphibians use their moist skins and
the linings of their mouths and pharynxes to
respire by diffusion
Gills
• The figure below shows
how gills function in
chordates
• As water passes over
the gill filaments,
oxygen molecules
diffuse into blood in
tiny blood vessels
called capillaries
• At the same time,
carbon dioxide diffuses
from blood into the
water
Gill Function
• Fishes and many
other aquatic
chordates use gills
for respiration
Gill Function
Lungs
• Although the structure of the lungs varies,
the basic process of breathing is the same
among land vertebrates
• Inhaling brings oxygen-rich air from outside
the body through the trachea and into the
lungs
• The oxygen diffuses into the blood inside the
lung capillaries
• At the same time, carbon dioxide diffuses out
of the capillaries into the air within the lungs
• Oxygen-poor air is then exhaled
Lungs
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As you move from amphibians to
mammals, the surface area of the lungs
increases
Observe this trend in the figure at right
The typical amphibian lung is little more
than a sac with ridges
Reptilian lungs are often divided into a
series of large and small chambers that
increase the surface area available for
gas exchange
In mammals, the lungs branch
extensively, and their entire volume is
filled with thousands of bubblelike
structures called alveoli (singular:
alveolus)
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Alveoli provide an enormous surface area
for gas exchange
This lung structure enables mammals to
take in the large amounts of oxygen
required by their endothermic
metabolism
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However, because air must move in and out
through the same passageways, there is
always stale, oxygen-poor air trapped in the
lungs of mammals and most other vertebrates
Lungs
• In contrast, in the lungs of birds, air
flows in only one direction
– A system of tubes in a bird's lungs, plus
air sacs, enables this one-way air flow
• Thus, gas exchange surfaces are
constantly in contact with fresh air that
contains a lot of oxygen
– This supply of oxygen enables birds to fly at
high altitudes, where there is less oxygen in
the atmosphere than at lower altitudes
Vertebrate Respiration
• Unlike most aquatic
chordates, land
vertebrates—like
salamanders, lizards,
birds, and primates—
use lungs to breathe
• A few aquatic
chordates, such as
sea turtles and
marine mammals,
use lungs as well
Vertebrate Respiration
Circulation
• Circulatory systems maintain homeostasis
by transporting materials throughout
animals' bodies
• The first chordates, like tunicates and lancelets
of today, probably had simple circulatory
systems
• Tunicates have short, tubelike hearts with a
simple pump but no true chambers
• Lancelets have a fairly well-developed
circulatory system but no specialized heart
Single- and Double-Loop Circulation
• As chordates evolved, more
complex organ systems and
more efficient channels for
internal transport developed
• The figure at right shows the
main transport systems in
vertebrates
• Those that use gills for
respiration have a singleloop circulatory system
• In this system, blood travels
from the heart to the gills,
then to the rest of the body,
and back to the heart in one
circuit
Vertebrate Circulatory Systems
• Most vertebrates that use gills
for respiration have a singleloop circulatory system that
forces blood around the body
in one direction
• Vertebrates that use lungs
have a double-loop system
• The hearts of fishes have
two chambers
• Amphibians and most
reptiles have threechambered hearts
• Crocodilians, birds, and
mammals have hearts with
four separate chambers
Vertebrate Circulatory Systems
Single- and Double-Loop Circulation
• Vertebrates that use lungs for respiration
have a double-loop circulatory system
• The first loop carries blood between the heart
and lungs
– Oxygen-poor blood from the heart is pumped to the
lungs, while oxygen-rich blood from the lungs returns
to the heart
• The second loop carries blood between the
heart and the body
– Oxygen-rich blood from the heart is pumped to the
body, while oxygen-poor blood from the body returns
to the heart
Heart Chambers
• Chordate hearts are adapted to the complexity of internal
transport for each of the different groups
• During the course of chordate evolution, the heart
developed chambers and partitions that help
separate oxygen-rich and oxygen-poor blood
traveling in the circulatory system
– In vertebrates that use gills for respiration, such
as fishes and larval amphibians, the heart
consists of two chambers: an atrium that receives
blood from the body, and a ventricle that pumps
blood to the gills and then on to the rest of the
body
Heart Chambers
• The hearts of most amphibians have three chambers: two atria
and one ventricle
• The left atrium receives oxygen-rich blood from the lungs
• The right atrium receives oxygen-poor blood from the body
• Both atria empty into the ventricle
– There is some mixing of oxygen-rich and oxygen-poor
blood in the ventricle
– However, the internal structure of the ventricle directs the
flow of blood so that most oxygen-poor blood goes to the
lungs, and most oxygen-rich blood goes to the rest of the
body
Heart Chambers
• Most reptiles have a three-chambered
heart
– However, unlike amphibians, most reptiles
have a partial partition in their ventricle
• Because of this partition, there is even less
mixing of oxygen-rich and oxygen-poor blood
than there is in amphibian hearts
Heart Chambers
• Birds, mammals, and crocodilians have
hearts that are completely partitioned
into four chambers
– This type of heart is sometimes described
as a double pump
• One pump moves blood through the lung loop
and the other moves blood through the body
loop
– The two loops of the circulatory system are completely
separated
– There is no mixing of oxygen-rich and oxygen-poor blood
Excretion
• Excretory systems eliminate
nitrogenous wastes from the body
– In nonvertebrate chordates and fishes,
gills and gill slits play an important role in
excretion
– However, most vertebrates rely on
kidneys—excretory organs composed of
small filtering tubes that remove wastes
from the blood
Excretion
• Nitrogenous wastes—formed from the
breakdown of proteins—are first produced in
the form of ammonia
• Ammonia is a highly toxic compound that
must quickly be eliminated from the body or
changed into a less poisonous form
– In tunicates, ammonia leaves the body through the
outflow siphons
• Other waste byproducts, such as uric acid, are stored
within the tunicate's body and released only when the
animal dies
Excretion
• In vertebrates, excretion is carried out mostly by the
kidneys
• Aquatic amphibians and most fishes also excrete
ammonia directly from the gills into the surrounding
water through simple diffusion
• In mammals, land amphibians, and cartilaginous
fishes, ammonia is changed into urea, a less-toxic
compound, before it is excreted
• In most reptiles and birds, ammonia is changed into
uric acid
• Besides filtering wastes, vertebrate kidneys help
maintain homeostasis by regulating the amounts of
water, salt, and other substances dissolved in body
fluids
Response
• Compared with invertebrates, most
chordates have elaborate systems that
allow them to respond to stimuli in their
environment
– Nonvertebrate chordates have a relatively
simple nervous system with a mass of
nerve cells that form a brain
– Vertebrates have a more complex brain
with distinct regions, each with a different
function
Response
• Nonvertebrate chordates do not have
specialized sensory organs
• In tunicates, however, sensory cells in and on
the siphons and other internal surfaces may
help control the amount of water passing
through the pharynx
• Lancelets—which have a more defined head
region—have a small, hollow brain with a
pair of eyespots that detect light
Response
• Vertebrates display a high degree of cephalization,
or concentration of sense organs and nerve cells at
the front of the body
– The head contains a well-developed brain, which is situated
on the anterior end of the spinal cord
• The vertebrate brain is divided into several parts,
including the cerebrum, cerebellum, medulla
oblongata, optic lobes, and olfactory bulbs
• The medulla oblongata controls the functioning of many
internal organs
• The optic lobes are involved in vision and the olfactory
bulbs are involved in the sense of smell
Response
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The figure shows how the size and complexity of the cerebrum and cerebellum
increase from fishes to mammals
The cerebrum is the “thinking” region of the brain
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It receives, interprets, and determines the response to sensory information
The cerebrum is also involved in learning, memory, and conscious thought
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In fishes, amphibians, and reptiles, the cerebrum is relatively small
In birds and mammals, especially primates, the cerebrum is greatly enlarged and may contain
folds that increase its surface area
The cerebellum, which coordinates movement and controls balance, is also
most developed in birds and mammals
Vertebrate Brains
• The size and complexity of the cerebrum and
cerebellum increase as you move from fishes to
mammals
• Each region of the vertebrate brain serves a
different function
Vertebrate Brains
Movement
• Unlike most other chordates,
nonvertebrate chordates lack bones
– They do, however, have muscles
• Lancelets and larval tunicates swim with a fishlike
movement of their muscular tails
• Some adult tunicates use their siphons to swim by
jet propulsion
• However, most adult tunicates lose their
tails and attach to a hard surface on the
ocean floor for life
Movement
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The skeletal and muscular systems
support a vertebrate's body and
make it possible to control
movement
Vertebrates are much more mobile
than nonvertebrate chordates. With
the exception of hagfishes, all
vertebrates have an internal skeleton
of bone—as shown in the figure—or, in
the case of certain fishes, cartilage
The skeleton includes a backbone
made up of individual bones called
vertebrae
In most vertebrates, tough yet flexible
tissues called ligaments connect the
vertebrae and allow the backbone
to bend without falling apart
Most vertebrates have fin girdles or
limb girdles that support the fins or
limbs
Vertebrate Skeleton
• Like the skeletons of
most vertebrates, this
lizard's skeleton has
two pairs of
appendages
• Muscles and
ligaments attach the
appendages to the
backbone and help
control movement
Vertebrate Skeleton
Movement
• In many fishes and snakes, the main body muscles
are arranged in blocks on either side of the
backbone
– These muscle blocks contract in waves that make the
body bend back and forth, generating forward thrust
• In many amphibians and reptiles, the limbs stick out
sideways from the body in a position resembling a
push-up
• Most mammals stand with their legs straight under
them, whether they walk on two legs or on four
– In this position, the legs can support the body weight
efficiently
Reproduction
• Chordates are diverse in the ways they
reproduce and develop
• Almost all chordates reproduce sexually
– Vertebrate evolution shows a general trend from
external to internal fertilization
• The eggs of most nonvertebrate chordates—
and many fishes and amphibians—are
fertilized externally
• The eggs of reptiles, birds, and mammals are
fertilized internally
Reproduction
• After fertilization, the development of chordates can be
oviparous, ovoviviparous, or viviparous
• In oviparous species, which include most fishes and amphibians
and all birds, the eggs develop outside the mother's body
• In ovoviviparous animals, such as sharks, the eggs develop
within the mother's body and the embryos receive nutrients
from the yolk in the egg
– The young of ovoviviparous species are born alive
• The developing embryos of viviparous species—including most
mammals—obtain nutrients directly from the mother's body
– As with ovoviviparous species, the young of viviparous animals are born
alive
Reproduction
• Some vertebrates, such as most amphibians,
produce many offspring but give them little
or no care
– This reproductive strategy is successful in
circumstances favoring populations that disperse and
grow rapidly
• Mammals and birds, in contrast, usually care
for their young but produce few of them
– This helps young survive in crowded, competitive
environments