Chordata

The Chordata include animals (including you)
that have (even if only as an embryo) a
notochord and a hollow dorsal nerve cord.

The notochord acts as a flexible stiffening rod in
animals that retain it as adults (e.g hagfish and
lancelets).

In adult humans the notochord forms the
intervertebral disks.
Hagfish
Vertebrata
 During
the Cambrian period approx 500
million years ago chordates evolved a
head with a skull becoming Craniates and
later a more complex nervous system and
a vertebral column.
 Humans,
obviously are both craniates and
vertebrates.
Vertebrata

Next major step in evolution of vertebrates was
the evolution of jaws about 470 million years
ago.

Jaws derived from skeletal rods involved in
supporting the gills.

Jaws an important evolutionary development
that opened huge number of opportunities.
Huge number of aquatic jawed fishes evolved
including ancestors of sharks and the bony fish.
34 .13
Origin of tetrapods

The movement of vertebrates onto the land was
another big step.

First amphibians evolved from a group of bony
fish called the lobe-finned fish. Excellent fossils
(mainly from Greenland) document the
transition.

Limbs appear to have first evolved to enable
organism to move along the bottom and only
later proved useful on land.
34.19
Acanthostega (345 mya fossil from Greenland) early tetrapod
The origin of tetrapods and
movement onto land

There are a suite of challenges associated with
terrestrial living that the first tetrapods has to
overcome.

These included the fact that water supports a
fish’s body but air does not, so stronger skeletal
structures were needed to support the body and
limbs to allow movement.

In addition, gills collapse out of water so an
alternative breathing system (i.e. lungs) was
needed.
The origin of tetrapods and
movement onto land
 In
addition, there was a constant threat of
water loss from the skin or through eggs,
which limited the first tetrapods the
amphibians to a close connection with
water until hard shelled eggs were
evolved.
The origin of tetrapods and
movement onto land
 The
tetrapods (a monophyletic group that
includes the amphibians, reptiles, birds
and mammals) are all descended from an
ancestral lobe-finned fish.
Tetrapod limb
 The
tetrapods are defined by their limbs,
which have a characteristic structure.
 Taking
the forelimb, for example, there is
first a single bone (humerus), then a pair
of bones (radius and ulna), next a series of
wrist bones (carpals) and finally a set of
digits (phalanges).
Tetrapod limb
 The
bones found in modern tetrapod limbs
are homologous to those in lobe-finned
fishes such as Eusthenopteron and
Panderichthys of the late Devonian period
about 380 mya and to the earliest known
tetrapods amphibians such as
Acanthostega and Icthyostega (both
approx. 360-365 mya) .
Figure 25.01a
17.1a Eusthenopteron
Ichthyostega

Ichthyostega was discovered in the 1930’s in
Greenland.

Ichthyostega’s legs probably wouldn’t have
supported it well on land, but would have
enabled it to move around on the bottom in
shallow water.

It has 7 digits rather than the five of modern
tetrapods. It has a strong spine and ribcage, but
the hind legs would not have supported it out of
water.
Acanthostega

A contemporary of Ichthyopstega more recently
discovered is Acanthostega.

It has 8 digits and a very fish like shoulder
structure and no wrist. It would not have been
able to come out of water and it possessed gills.
Like Ichthyostega it probably made its way along
the bottom and crawled over vegetation rather
than crawling on land.
Figure 25.01b
17.1 B and C
The origin of tetrapods and
movement onto land
 The
transition from lobe-finned fishes
fishes to Acanthostega and Ichthyostega
has recently been neatly bridged by the
discovery of a classic intermediate form
Tiktaalik roseae from Greenland.
Tiktaalik roseae
 Discovered
in 2004 in Greenland Tiktaalik
roseae is an extremely important fossil link
in the origin of tetrapods.
 Described
as a “fishapod” Tiktaalik has a
mixture of fish and tetrapod
characteristics.
Tiktaalik roseae

Like a fish it has gills and scales.

Intermediate characteristics include halftetrapod/half fish limb bones. There is a wrist,
but there are fins instead of toes. The inner ear
is also intermediate in structure between fish
and tetrapods.

Like a tetrapod it has lungs, tetrapod rib bones
and a mobile neck.
Tiktaalik roseae 375 mya
Amphibians and reptiles
 Lobe-finned
fishes gave rise to
amphibians which in turn gave rise to
amniotes which evolved the hard-shelled
(amniotic egg) liberating them from need
to lay eggs in water as amphibians do.
 Also
evolved less permeable skin.
Modern amphibians: Class
Amphibia
 Amphibians
are characterized by a moist
permeable skin and this limits the
environments in which they typically can
live as they are constrained by the threat
of water loss.
 Typically
they inhabit wet or damp habitats
where the humidity is high (e.g., ponds.
forest floors in leaf litter).
Class Amphibia
 The
moist skin is used extensively for gas
exchange and some species have lost
their lungs over evolutionary time and
depend exclusively on gas exchange
across the skin and oral cavity.
 All
amphibians are ectothermic.
Class Amphibia
 The
word amphibian means “two lives”
and is a reference to the fact that frogs go
through metamorphosis from a tadpole
stage.
 In
most amphibians, fertilization is external
and the male fertilizes the soft eggs as the
female sheds them from her body.
Class Amphibia
 Amphibian
eggs do not have a hard shell
and dry out quickly if not kept in a moist
environment.
 Many
species lay their eggs directly in
water, but some frogs show more parental
care and brood eggs in (depending on the
species) the mouth, stomach or on their
back.
Figure 25.04
Class Amphibia
 Eggs
in frogs develop into tadpoles that
have a fishlike tail and external gills.
 As
the tadpole develops, often very quickly
in a race against time to escape a pool
before it dries up, limbs develop, the tail
shortens by reabsorption and the gills are
lost as the tadpole metamorphoses into a
miniature frog.
Figure 25.26
Class Amphibia
 The
amphibians are represented by about
4800 species and include the familiar
frogs, toads and salamanders and the less
well-known caecilians, which are legless,
burrowing amphibians.
Amniote origins and classification
 The
possession of a shelled egg unites the
mammals, birds and reptiles into a
monophyletic group the amniotes.
 The
shelled egg freed the amniotes from
the need to reproduce in water that
hampered the amphibians ability to spread
into harsh environments.
The Amniotic egg

The amniotic egg is hard shelled and is called
an amniotic egg because the embryo develops
within a sac called the amnion.

The embryo feeds on yolk from a yolk sac and
deposits its waste into another sac called the
allantois. The allantois and another membrane
the chorion together lie against the shell and
being richly supplied with blood exchange gases
with the outside through the pores in the shell.
Figure 26.04
Amniote origins and classification
 There
is considerable disagreement
between cladistic and traditional
classification of the amniotes.
 Traditional
classification recognizes three
classes:



Reptilia: reptiles
Aves: birds
Mammalia: mammals
Amniote origins and classification

Because the class Reptilia does not include all
the descendents of their most recent common
ancestor (i.e. the birds are excluded) the reptiles
are a paraphyletic group.

Birds and crocodilians share a most recent
common ancestor and thus form a monophyletic
group (the Archosauria), which includes the
extinct dinosaurs, but neither is more closely
related than the other to the members of the
Reptilia
Figure 26.02
18.2
Amniote origins and classification
 Traditional
classification considers birds
because of their endothermy and feathers
to be members of a different grade to the
crocodilians and reptiles and so places
them in their own class the Aves.
 Cladistic
classification in contrast groups
the amniotes on the basis of common
ancestry.
Amniote origins and classification
 One
of the major characteristics used to
classify the amniotes is the structure of the
skull and the number of holes it contains.
 The
stem group of amniotes diverged into
three lineages in the Carboniferous period
(approximately 350 mya). These were the
synapsids, anapsids and the diapsids.
Figure 28.01
20.1
Figure 26.02
18.2
Figure 26.01
18.1
Modern reptiles
 The
modern reptiles being a paraphyletic
group include anapsids and diaspids.
 The
anapsid representatives are the
turtles (order Testudines).
 Most
of the remaining reptiles are diapsids
and members of the order Squamata
which includes the lizards and snakes.
Differences between reptiles and
amphibians
 Reptilian
skin is dry and scaly, which limits
water loss.
 The reptiles’ amniotic egg frees reptiles
from the need to lay eggs in water. Thus
they can occupy much drier habitats.
 Reptilian jaws are more powerful and can
apply a crushing grip.
Order Squamata: Suborder
Sauria the lizards
 Lizards
are a very diverse group that
includes terrestrial, burrowing, aquatic,
arboreal and even aerial members.
 Lizards
have good vision and an external
ear, which snakes lack. They also have
eyelids, also a trait that snakes lack.
Order Squamata: Suborder
Sauria the lizards
 Most
lizards have four limbs, although
some species are completely legless.
 Well
known species include: chameleons,
geckos, iguanas and monitor lizards,
which include the largest species, the
Komodo dragon.
Figure 26.12
Gecko (note the flattened pads on the toes. Ridges on these pads enable the gecko
to cling to smooth surfaces).
Figure 26.14
Chameleon catching an insect with its sticky extensible tongue.
Order Squamata: Suborder
Sauria the lizards
 Lizards
have invaded many of the world’s
hottest areas by evolving a suite of
adaptations that make survival in deserts
possible.
 These
include a thick skin that contains
lipids, which reduce water loss, and the
excretion of uric acid which minimizes
water loss.
Reptiles

Reptiles are ectothermic and adjust their body
temperature by moving from one microclimate to
another to bask or cool down.

Cold climates do not suit reptiles as there are
too few opportunities to warm up.

Because they spend relatively little energy
keeping warm, ectotherms in general do well in
low productivity ecosystems such as tropical
deserts and grasslands.
Snakes
 Snakes
are limbless and usually lack both
the pectoral and pelvic girdles.
 They
have numerous vertebrae, which are
shorter and wider than those in other
vertebrates and allow them to make
undulatory movements.
Snakes

Snakes are an extremely successful group of
predators. Although most have poor vision (with
the exception of arboreal species) and limited
hearing ability they use other sense organs to
track prey.

Snakes have pit-like Jacobson’s organs in the
roof of the mouth, which are olfactory organs.
The forked tongue when extended samples the
air and picks up molecules that are delivered to
the Jacobson’s organ when the tongue is
withdrawn.
Snakes

Crotaline vipers (pit vipers such as rattlesnakes)
have heat-sensitive pit organs on their heads
between the nostrils and eyes.

These are very sensitive to radiant heat and can
detect temperature differences as slight as
0.003ºC. The vipers use the organ to track prey
and to aim their strike when biting.
Figure 26.24
18.22
Predation

Snakes use one of three methods to catch and
kill prey.

Most catch prey by grabbing it and swallowing it
alive. Most such species are quick and
concentrate on small easy to handle prey.

The other two group kill their prey either by
constriction or with venom.
Constrictors

A variety of snakes including pythons and boas
kill by constriction.

They coil around their prey and every time the
prey breathes out they tighten their coils a little
more until the prey can no longer breathe and
suffocates.

Most constrictors are large, slow-moving
ambush predators and the largest snakes, the
Anacondas and pythons are all constrictors.
Venomous snakes

About 20% of all snakes are venomous (although in
Australia 80% of snakes are venomous). About 50,00060,000 people die annually worldwide from snake bite,
most of them in the Indian subcontinent.

Snakes with venom lethal to humans include the
 vipers (including the American pit vipers) which have
large movable tubular fangs at the front of the mouth;
 elapids (cobras, mambas, coral snakes kraits which
have shorter, but permanently erect fangs in the front
of the mouth;
 sea snakes (family Hydrophiidae).
Figure 26.25
18.20
Venomous snakes

Snake venoms are highly modified salivas and
complex in constitution including a variety of
proteins and enzymes.

Elapid venom is neurotoxic and works by
shutting down the respiratory system whereas
viper venom is more painful and attacks the
vascular system bringing about coagulation of
blood and clotting of arteries.
Crocodiles and Alligators: Order
Crocodilia
 Modern
crocodiles and birds are the only
survivors of the Archosaurian lineage that
included the dinosaurs.
 Crocodiles
have changed little in almost
200 million years a testament to the
success of their design.
Crocodiles

Crocodiles have their teeth set in sockets a trait
found otherwise only in mammals and fossil
birds and also like mammals have a complete
palate which enables them to breathe even if the
mouth is filled with water or food.

They are ambush predators that kill by grabbing
and drowning their prey. The largest Nile and
Estuarine crocodiles (called “salties” in Australia)
can exceed 1000 kgs in weight and can attack
and kill almost anything.
Figure 26.27a
18.24
Modern Birds
 The
other remaining Archosaurs are birds
given their own class Aves by traditional
taxonomists.
 There
are approximately 8,600 species of
birds ranging is size from the tiny bee
hummingbird to the ostrich.
Characteristics of the birds





Feathers are a unique character of birds
among living animals, but also found in
dinosaurs.
Endothermic
Skeleton modified for flight. Bones hollow,
forelimbs support the wing, ribs with uncinate
processes, beak but no teeth, reduced tail.
Breathing by lungs and associated air sacs
Internal fertilization and hard-shelled amniotic
egg
Evolution of birds

Birds evolved from a group of theropod
dinosaurs in the Jurassic period. The oldest
known bird fossil is Archaeopteryx lithographica
which has a mix of “reptilian” and avian features.

Reptilian: long tail, teeth, long clawed fingers

Avian: feathers, ribs with uncinate processes,
avian shoulder girdle.
Archaeopteryx
(oldest known
fossil bird)
Jurassic
150mya
Feathers
 Feathers
are what enable birds to fly, but
originally are believed to have evolved as
a thermoregulatory device.
 Feathers
are lightweight, but strong. The
surface of the feather is made up of tightly
spaced, overlapping filaments that hook
together. Overlapping feathers form the
wings with which birds fly.
Figure 27.05e
19.4
Structures and adaptations for flight

To generate the power to fly birds have evolved
very large flight muscles (the supracoracoidueus
and the pectoralis), which attach to an enlarged
keeled sternum

In addition birds have strengthened the skeleton
by fusing bones together and reduced their
weight in several ways by:




Evolving light, hollow bones
Reducing the tail
Evolving a toothless bill
Dispensing with a bladder and excreting uric acid.
Figure 27.08
19.6
How birds fly

A bird’s wing is an airfoil and is cambered with a
slightly convex upper surface and concave
under surface.

Because air must travel further over the upper
surface of the wing than below it must travel
faster and thus exerts less pressure above the
wing than it does below.

The increased pressure below generates lift, the
force which keeps the bird up.
Figure 27.16
19.13
Lift and thrust

In order to fly both horizontal thrust and vertical
lift are required.

Thrust is mainly generated by the primary
feathers (the long ones at the end of the bird’s
hand), which on the downstroke twist and acting
like a propeller push the air backwards.

Lift is mainly generated by the secondary
feathers (the inner portion of the wing), which
form an airfoil.
Early mammals

About 300 million years ago amniotes split into
two lineages one, the synapsids, ultimately
produced the mammals and the other lineage
produced the “reptiles,” dinosaurs, and birds.

First mammals arose about 200 million years
ago. Small and inconspicuous.

Dinosaurs dominated Earth until mass extinction
65 million years ago caused by an asteroid
impact wiped them out and mammals diversified.
Mammals

Mammals are characterized by a suite of shared
derived characters including









Milk
Hair
Differentiated teeth
Single jaw bone (dentary)
Middle ear with three bones: incus, malleus, stapes
Muscular diaphragm
Two occipital condyles (bones at base of skull that
articulate with vertebrae).
Endothermic (maintain constant high body
temperature; evolved independently by birds)
Four chambered heart (also evolved independently by
birds)
Figure 28.02
20.2
Modern mammals

After the Cretaceous extinction mammals
radiated to occupy niches previously occupied
by the dinosaurs.

Today there are 26 orders of mammals that can
be divided into three groups on the basis of
differences in reproduction:



The Monotremes: one order
Marsupials: seven orders
Placental mammals: eighteen orders
Monotremes

The monotremes are a small order of four species the
duck-billed platypus and three species of spiny anteater
or echidna found in Australia and New Guinea. They
diverged from the line leading to the other mammals in
the Jurassic period about 180 mya.

Monotremes have several “reptilian” traits

Monotreme means single hole and refers to the fact that
the urinary tract and reproductive tract empty into a
single opening the cloaca just as is the case in lizards
and birds.
Monotremes

In addition, the monotremes lay eggs with a
tough leathery shell and possess an interclavicle
bone, one found in reptiles, but not in other
mammals.

However, they clearly are mammals possessing
a single dentary, three ear bones, hair, and milk
although they lack nipples and the milk seeps
from pores over a relatively wide area. Are
endothermic, but have a lower metabolic rate
than marsupials and eutherians.
34.33
Echidna or Spiny anteater
Marsupials
 There
are seven orders of marsupials with
just under 300 species that include such
animals as possums, kangaroos,
wombats, koala and Tasmanian devil.
 All
marsupials have an abdominal pouch
in which the young are raised having been
born very underdeveloped, moving to the
pouch and latching onto a nipple.
Figure 28.24
20.18
34.34
Marsupials
 Most
marsupials are found in Australia
(isolated from eutherians for about 65
million years) and the remaining species
(opossums) occur in the Americas.
Rock Wallaby
Convergent evolution
of eutherian
and marsupial mammals
34.35
Eutherian mammals
 The
Eutherian or placental mammals
support their developing young using a
chorioallantoic placenta which brings the
blood supplies of mother and offspring into
close contact so that food and gases can
be effectively exchanged.
Major orders of eutherian
mammals
 There
are about 5000 species of placental
mammal that belong to 18 orders.
 Major
orders of mammals include:
Insectivora: shrews, hedgehogs, moles.
Small, primitive insect eating mammals
with many pointed teeth. 440 species
Figure 28.16
20.28
Major orders of eutherian
mammals

Chiroptera: Bats. Flying mammals with forelimbs
modified into wings. Membrane stretched
between elongated fingers. Many species use
echolocation. 977 species second only in size to
Rodentia.

Rodentia: Rodents. The largest group with 2052
species. Possess chisel like upper and lower
incisors. Includes rats, mice, squirrels,
woodchucks.
Major orders of eutherian
mammals

Carnivora: large flesh eating mammals. Cats,
dogs, weasels, seals, sealions. Teeth
specialized for hunting. Canines used for killing.
280 species.

Cetacea: whales, dolphins. Aquatic diving
mammals. Anterior limbs modified into flippers,
posterior limbs absent, possess large rear fluke
for propulsion, nostrils modified into blowhole on
top of head. 78 species.
Figure 28.42
Humpback Whale
Figure 28.40
Figure 28.39
Sealion
Grizzly Bear
Major orders of eutherian
mammals

Artiodactyla: even-toed ungulates. Pigs, deer,
cattle, antelopes, hippopotamuses. Two or four
toes sheathed in hoofs. Many are ruminants
and possess multi-chambered stomachs. 221
species.

Perissodactyla: odd-toed ungulates. Horses,
zebras, tapirs, rhinoceroses. One or three toes.
All herbivorous with teeth adapted to chewing
vegetation. 17 species.
Figure 28.41
20.32
Major orders of eutherian
mammals

Primates: the order to which humans belong
includes prosimians, monkeys, apes and us.
Largely arboreal with binocular vision and
grasping hands. 279 species.
Figure 28.31
Tarsier
Red howler monkey
Figure 28.32b
Olive
baboon
20.22
20.23
Figure 28.32a
Humans are primates

The order Primates split from other mammals
about 55 mya.

Humans are apes and we are most closely
related to chimpanzees. Human and chimp
lineages separated about 7 million years ago.

Primates have grasping hands with a thumb
(monkeys apes and humans have an opposable
thumb) and binocular vision. These are
adaptations to a tree-dwelling existence.
Human derived characteristics
 Humans
are bipedal and have an
extremely well developed brain.
 Humans
have reduced jawbones and a
short digestive tract
 Genomes
of humans and chimps about
99% identical.