History of Vertebrates Ch. 20

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History of VertebratesCh. 20
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• Scientists divide the earths past into different
  time periods
• large blocks of time are called eras
• eras are divided into blocks of time are called
  periods
• some periods are divided into epochs, which in
  turn can be divided into ages

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Figure 26.1 An evolutionary timeline
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The Paleozoic Era

• Virtually all of the animals that survive at the
  present time originated in the sea at the
  beginning of the Paleozoic era
• the diversification of animal life began soon
  after the Cambrian period (545-490 M.Y.A.)
• the first vertebrates evolved about 500 M.Y.A.

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Life in the Cambrian
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The Paleozoic Era

• While most of the animal phyla that evolved in
  the Cambrian remained marine, a few phyla have
  successfully invaded land
• fungi and plants were the first terrestrial
  organisms, appearing over 500 M.Y.A.
• arthropods were the first terrestrial animals,
  invading land about 410 M.Y.A.
• vertebrates invaded the land during the
  Carboniferous period (360-280 M.Y.A.)
• amphibians were the first terrestrial
  vertebrates, followed by the reptiles, which were
  successful and dominant

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An early reptile the pelycosaur
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The Paleozoic Era

• Mass extinctions are particularly sharp declines
  in species diversity
• five mass extinctions have occurred during the
  history of life
• the most drastic occurred during the last ten
  million years of the Permian period, which marked
  the end of the Paleozoic era
• an estimated 96 of all species of marine animals
  became extinct
• the most well-studied extinction occurred at the
  end of the Cretaceous period (65 M.Y.A.)
• it was probably triggered by a large asteroid
  hitting the earth
• dinosaurs went extinct at this time

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The Mesozoic Era

• The Mesozoic era (248 to 65 M.Y.A.) has
  traditionally been divided into three periods
• Triassic
• Jurassic
• Cretaceous
• During the Jurassic period, the super-continent
  of Pangaea began to break up, sea levels were
  rising, and the worlds climate became warmer and
  wetter.

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Some dinosaurs were truly enormous
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Dinosaurs
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The Mesozoic Era

• About 65 M.Y.A., at the end of the Cretaceous
  period, dinosaurs disappeared
• this loss included flying reptiles (pterosaurs)
  and the great marine reptiles
• mammals occupied the niches left open by the loss
  of the dinosaurs

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An extinct flying reptile
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Extinction of the dinosaurs
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The Mesozoic Era

• Many explanations have been advanced to explain
  the demise of the dinosaurs
• the most widely accepted, proposed by Luis W.
  Alvarez, blames an asteroid impact
• iridium is an element rare on earth but abundant
  in meteorites
• a layer of iridium is abundant in many parts of
  the world in a layer of sediment that dates to
  the end of the Cretaceous period

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The Cenozoic Era

• Early in the Cenozoic era (65 M.Y.A. to present),
  the climate was relatively warm compared to
  todays colder and drier climate
• A gradual cooling caused ice caps to form at the
  poles
• many very large mammals evolved during the ice
  ages, including
• mastodons, mammoths, saber-toothed tigers, and
  cave bears

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Fishes Dominate the Sea

• A series of key evolutionary advances allowed
  vertebrates first to conquer the sea and then the
  land
• About half of all vertebrates are fishes
• fishes provide the evolutionary base for the
  invasion by land by amphibians

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Fishes Dominate the Sea

• All fishes have four important characteristics
  in common
• gills
• gills are used to extract dissolved oxygen from
  water
• vertebral column
• all fishes have an internal skeleton with a
  spine
• single-loop blood circulation
• blood is pumped in a single loop, from the heart
  to the gills, then to the body, then back to the
  heart
• nutritional deficiencies
• fishes are unable to synthesize the aromatic
  amino acids and must consume them in their diet
• this trait has been inherited by all of their
  vertebrate descendants

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Fishes Dominate the Sea

• The first fishes were jawless ostracoderms and
  appeared in the sea about 500 M.Y.A.
• agnathans are the only jawless fishes found today
• they include hagfish and lampreys

Figure 26.10 Specialized mouth of a lamprey
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26.4 Fishes Dominate the Sea

• Jawed fishes appeared around 410 M.Y.A.
• jaws evolved from the frontmost of a series of
  cartilaginous arch supports that reinforced the
  tissue between gill slits

Figure 26.11 A key adaptation among fishes
evolution of the jaw.
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Fishes Dominate the Sea

• The earliest jawed fishes had spines
  (acanthodians) or heavy armor (placoderms), and
  some reached enormous sizes
• Sharks and bony fishes appeared about 400 million
  years ago and shared the seas with spiny fish and
  placoderms for 150 million years
• Eventually, the less maneuverable spiny fish and
  placoderms went extinct
• Sharks and bony fish have dominated the seas for
  the last 250 million years

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Fishes Dominate the Sea

• Chondrichthyes
• Sharks, skates, and rays
• Sharks have flexible skeleton made of cartilage
  and are fast and maneuverable swimmers
• while some are filter feeders, most sharks are
  predators and have a mouth armed with rows of
  sharp teeth

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Fishes Dominate the Sea

• Bony fishes have a heavier internal skeleton made
  of bone
• but they achieve maneuverability through the aid
  of a swim bladder, a gas-filled sac that allow
  fish to regulate their buoyancy
• the swim bladder allows a bony fish to remain
  suspended at any depth in the water without
  expending effort
• sharks gain buoyancy with oil in their livers,
  but they must keep swimming to counteract their
  denser-than-water bodies

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Figure 26.14 Diagram of a swim bladder
Figure 26.13 Bony fishes
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Fishes Dominate the Sea

• Bony fishes comprise the class Osteichthyes
• some bony fishes are lobe-finned (subclass
  Sarcopterygii)
• this group includes the ancestors of the first
  tetrapods (four-legged animals)
• other bony fishes are ray-finned (subclass
  Actinopterygii)
• this group includes the vast majority of todays
  fishes
• bony fishes are the most successful of all
  fishes, indeed of all vertebrates
• there are nearly 30,000 species of bony fishes

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Major Classes of Fishes
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Fishes Dominate the Sea

• Bony fishes have several adaptations that have
  helped make them such evolutionary successes
• lateral line system
• a special sensory system that enables fish to
  detect changes in water pressure
• also present in sharks
• operculum
• a bony covering on top of the opening of the
  gills
• this allows the fish to ventilate the gills while
  remaining stationary

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Amphibians Invade the Land

• The amphibians include frogs, salamanders,and
  caecilians
• they were the first terrestrial vertebrates and
  evolved from the lobe-finned fishes

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Figure 26.15 A key adaptation of amphibians the
evolution of legs.
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Amphibians Invade the Land

• Amphibians have five key characteristics that
  allowed them to invade land successfully
• legs
• lungs
• cutaneous respiration
• pulmonary veins
• partially divided heart

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Reptiles Conquer the Land

• Reptiles are more fully terrestrial than
  amphibians
• All living reptiles share the following
  fundamental characteristics
• amniotic egg
• this innovation is a watertight environment that
  offers the embryo protection against drying out
• dry skin
• reptiles are covered by scales or armor in order
  to prevent drying out
• thoracic breathing
• reptiles increase their lung capacity by
  expanding their chest cavity when breathing in air

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The watertight egg
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Reptiles Conquer the Land

• Early archosaurs resembled crocodiles, but later
  forms called thecodonts were the first reptiles
  to be bipedal
• Early archosaurs gave rise to four groups
• dinosaurs, many of which grew to immense sizes
• crocodiles, which have changed little
• pterosaurs, the flying reptiles
• birds
• Dinosaurs were the most successful of all land
  vertebrates but became extinct around 65 million
  years ago, along with the marine reptiles and
  pterosaurs

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Orders of Reptiles
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Birds Master the Air

• Birds evolved from bipedal dinosaurs about 150
  M.Y.A.
• they only became common after the pterosaurs
  became extinct
• many scientists consider birds to be feathered
  dinosaurs, given their similarity in so many
  respects to dinosaurs

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Birds Master the Air

• Modern birds lack teeth and have only vestigial
  tails
• They retain many reptilian characteristics
• birds lay amniotic eggs (but with hard shells)
• birds have reptilian scales on their feet and
  lower legs
• Birds are different than reptiles in that they
  have
• feathers
• these are derived from reptilian scales but
  adapted for flight
• flight skeleton
• the bones of birds are thin and hollow, reducing
  weight while providing enhanced points for flight
  muscle attachment

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26.7 Birds Master the Air

• Birds are endothermic
• their high body temperatures enhance metabolism,
  satisfying the large energy requirements of
  flight
• The oldest bird of which there is a clear fossil
  is Archaeopteryx
• There are about 8,600 species of birds in the
  class Aves today

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Table 26.5 Major Orders of Birds
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Mammals Adapt to Colder Times

• Mammals evolved about 220 M.Y.A. and belong to
  the class Mammalia
• members of this class share three key
  characteristics
• mammary glands
• hair
• middle ear

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26.8 Mammals Adapt to Colder Times

• The first mammals evolved from therapsids
• Early mammals were small shrew-like creatures
• they lived inconspicuously in an age dominated by
  dinosaurs

Figure 26.22 A therapsid
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Mammals Adapt to Colder Times

• Mammals were a minor group during the time of the
  dinosaurs, but rapidly diversified when dinosaurs
  and many other land and marine animals became
  extinct 65 million years ago
• Over 4,500 species of mammals exist now, half of
  them rodents, and one-quarter of them bats

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Table 26.6 Major Orders of Mammals
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Mammals Adapt to Colder Times

• Modern mammals have a number of characteristics
  that make them successful
• endothermy allows for mammals to be active at any
  time of day or night and to colonize harsh
  environments
• teeth type varies in mammals, which have
  heterodont dentition, allowing specialization to
  eating habits
• placenta is an adaptation for nourishing
  developing young that will be born live
• hooves help with locomotion in running mammals
  hooves, claws, and fingernails are made of
  keratin horns are composed of a bony core
  surrounded by a sheath of compacted keratin

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The placenta
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Mammals Adapt to Colder Times

• Todays mammals include
• Monotremes
• egg-laying mammals
• Marsupials
• pouched mammals
• placental mammals

Figure 26.24 Todays mammals