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Title: Chordata

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  • 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.

  • 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

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  • Next major step in evolution of vertebrates was
    the evolution of jaws about 470 million years
  • 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
  • Limbs appear to have first evolved to enable
    organism to move along the bottom and only later
    proved useful on land.

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
    fishs 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

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

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) .

17.1a Eusthenopteron
  • Ichthyostega was discovered in the 1930s in
  • Ichthyostegas legs probably wouldnt 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.

  • 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.

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
    half-tetrapod/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.

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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.

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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.

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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

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.

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Amniote origins and classification
  • There is considerable disagreement between
    cladistic and traditional classification of the
  • Traditional classification recognizes three
  • 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

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.

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.

Gecko (note the flattened pads on the toes.
Ridges on these pads enable the gecko to cling to
smooth surfaces).
Chameleon catching an insect with its sticky
extensible tongue.
Order Squamata Suborder Sauria the lizards
  • Lizards have invaded many of the worlds 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 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 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 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 Jacobsons 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
    Jacobsons organ when the tongue is withdrawn.

  • 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.

  • Snakes use one of three methods to catch and kill
  • 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.

  • 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
    50,000-60,000 people die annually worldwide from
    snake bite, most of them in the Indian
  • 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).

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

  • 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.

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

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
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  • 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.

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

How birds fly
  • A birds wing is an airfoil and is cambered with
    a slightly convex upper surface and concave under
  • 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.

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 birds
    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 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,
  • 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)

Modern mammals
  • After the Cretaceous extinction mammals radiated
    to occupy niches previously occupied by the
  • 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

  • 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.

  • 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.

Echidna or Spiny anteater
  • There are seven orders of marsupials with just
    under 300 species that include such animals as
    possums, kangaroos, wombats, koala and Tasmanian
  • 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.

  • Most marsupials are found in Australia (isolated
    from eutherians for about 65 million years) and
    the remaining species (opossums) occur in the

Rock Wallaby
Convergent evolution of eutherian and marsupial
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

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,

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.

Humpback Whale
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.

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.

Red howler monkey
20.22 20.23
Olive baboon
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.

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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.