Evolution of the Animal Body Plan

1
Evolution of the Animal Body Plan

• Part 2 Chordates and Vertebrates
• E. Q. How has the chordate body plan evolved
• over time to better suit these animals to their
• ecological niches?

2
Chordate Characteristics

• The members of Phylum Chordata exhibit many
  different kinds of body plans, but they all have
  certain characteristics in common.
• All chordates, at some time in their lives,
  have
• A dorsal hollow nerve cord
• Pharyngeal gill slits
• A stiff rod ventral to the nerve cord called the
  notochord
• A muscular post-anal tail.
• Some chordates show these characteristics for
  their entire lives, while in other chordates,
  they develop into other structures.

• In vertebrates, the dorsal hollow nerve cord
  develops into the brain and spinal cord.
• In most vertebrates, the notochord degenerates
  during the embryonic stage of development. The
  only remnants appear in adults as the
  gelatinous disks between the vertebrae.
• The pharyngeal gill slits develop into gills in
  aquatic vertebrates, jaw support, hearing
  elements, or other functions in higher
  vertebrates.
• Most vertebrates retain the muscular post anal
  tail as adults.

3
The Simplest Chordates

• Tunicates are some of the simplest chordates.
  They belong to the subphylum Urochordata.
• Most tunicates are sessile marine filter feeders.
  Because they squirt water when disturbed, they
  are often called sea squirts.
• Some others are zooplankton. Some species are
  colonial.
• Adult sea squirts no not resemble other
  chordates, but as embryos, all chordate
  characteristics are present.

• Lancelets belong to the subphylum
  Cephalochordata. They are shaped like the blade
  of a spear, and all of the chordate
  characteristics appear in adults.
• Lancelets burrow into the sand with their mouths
  exposed. Water enters the mouth and exits the
  gill slits, while food is filtered out.
• Although they dont swim well, they show the
  side-to-side swimming motion of fishes.

4
Simple Chordates
5
Subphylum Vertebrata

• The axial skeleton makes large body size possible
  as well as fast movements.
• Vertebrates have two pairs of appendages (arms
  and legs).
• The vertebrate skeleton is made of bone or
  cartilage, and is capable of growing with the
  animal.
• Vertebrates have a closed circulatory system with
  a ventral, chambered heart.
• Vertebrates have muscles in their intestinal
  walls to propel food through the digestive
  system.
• Vertebrates reproduce sexually.

• Vertebrates keep the primitive chordate
  characteristics, but they also have additional
  characteristics that separate them from the
  non-vertebrate chordates.
• Many of these features are associated with large
  body size and active lifestyle.
• One vertebrate trait is an enlarged brain
  enclosed within a cranium and a concentration of
  the sense organs in the head.
• Another characteristic of vertebrates is the
  vertebrate axial skeleton, the main support of
  the central trunk of the body.

6
Vertebrate Diversity
Osteichthyes
Chondrichthyes

• Agnatha

Reptilia
Mammalia
Aves
Amphibia
7
Agnatha (Jawless Fishes)

• Lampreys feed on live fish by attaching with
  their round mouth and using their rasping tongue
  to penetrate the skin of their host. They then
  ingest the hosts blood. Lampreys are considered
  both predators and parasites. They can be
  freshwater or marine.
• Hagfish look like lampreys, but they are mainly
  scavengers. Some hagfish eat dead or dying fish,
  but most eat marine worms. Hagfish are entirely
  marine. Fertilization is external.

• The oldest vertebrate fossils are jawless
  creatures resembling fish and are about 400-500
  m.y.a.
• Superclass Agnatha includes some extinct fishlike
  animals called Ostracoderms. They had circular
  or slit-like mouths without jaws and were
  probably mudsuckers or suspension feeders.
• Lampreys and hagfish are the only agnathans
  living today.
• Like extinct agnathans, they lack paired
  appendages.
• Agnathans also lack scales.

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Jawless Fishes
9
Chondrichthyes

• The origin of jaws was a major event in
  vertebrate history. Vertebrate jaws evolved from
  changes in the skeletal supports for the most
  anterior gill slits.
• During the Devonian Period (417-354 m.y.a.),
  Chondrichthyes and Osteichthyes both evolved. At
  the end of the Devonian, the first amphibians
  also appeared.
• Sharks, skates, and rays make up Class
  Chondrichthyes.
• They are called cartilaginous fish because their
  skeletons are made of cartilage, rather than
  bone. They have tooth-like scales embedded in
  their skins.

• There are about 750 species in this group. They
  all have jaws and paired fins.
• Sharks have streamlined bodies. Continual
  swimming makes water pass though the mouth and
  out the gills.
• Sharks have keen eyesight, an excellent sense of
  smell, and a lateral line system to sense prey.
• In sharks and rays, fertilization is internal.
  Males use claspers to transfer sperm to the
  female.
• Sharks have the largest brain/body size ratio of
  any fish.

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Chondrichthyes
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Osteichthyes

• A bony skeleton, an operculum, and a swim bladder
  are all characteristic of Class Osteichthyes.
• There are more members of Class Osteichthyes than
  of any other class of vertebrates.
• They are abundant in the seas and in almost all
  freshwater habitats.
• Members of this class have a skin covered with
  flattened bony scales different from the
  tooth-like scales of sharks or rays.
• Glands in the skin secrete mucus that makes fish
  feel slimy but protects from parasites.

• Bony fish breathe by drawing water in through the
  mouth, over the gills, and out through the
  operculum.
• The muscles attached to the operculum allow the
  fish to breathe while staying in one place.
• Another adaptation is the swim bladder, an air
  sac that controls the buoyancy of the fish.
• Bony fish are maneuverable swimmers because their
  fins are better for propulsion and steering than
  those of sharks.

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Osteichthyes
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Amphibia

• Modern amphibians have several key
  characteristics
• Most change from an aquatic larval state to a
  terrestrial adult.
• Most have a moist thin skin without scales used
  for gas exchange.
• Feet, if present, are often webbed and lack
  claws.
• Fertilization is external. Eggs lack shells.
  They are laid in water or moist places.
• Amphibian hearts have 3 chambers2 atria and 1
  ventricle.
• Modern amphibians include Order Anura (frogs and
  toads), Order Urodela (newts and salamanders),
  and Order Apoda (caecilians, legless amphibians
  that resemble snakes).

• According to current theories, aquatic
  vertebrates moved onto the land as shallow pools
  they live in dried up.
• These animals faced several critical problems
  (1) keep from drying out (2) support the body on
  land (3) reproduce on land.
• The oldest known amphibian fossils date from
  about 370 m.y.a.
• Amphibians are thought to have evolved from
  lobe-finned fish. The earliest amphibians had
  four strong limbs, which evolved from the fishs
  fins.
• The earliest amphibians breathed air through
  lungs and had sense organs adapted to work on
  land.

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Amphibia
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Reptilia

• Earliest reptile fossils date to the
  Carboniferous Period, about 350 m.y.a.
• They diversified rapidly and by the Permian
  Period had become the dominant land vertebrates.
• The Mesozoic Era (286-245 m.y.a) is called the
  Age of Reptiles because nearly all of the large
  vertebrates on Earth were reptiles.
• During the Mesozoic, dinosaurs appeared and
  evolved to fill many niches in a variety of
  environments.
• Representatives of the 4 modern orders of
  reptiles survived the mass extinction at the end
  of the Cretaceous Period (1)turtles and
  tortoises, (2)lizards and snakes, (3) tuataras,
  and (4) crocodilians.

• The success of reptiles on land resulted from
  their ability to solve the major problems facing
  land animals preventing desiccation, locomotion,
  respiration and excretion of N wastes, and
  reproduction on land.
• A thick skin covered by keratin-containing scales
  prevents water loss.
• Sturdy limbs with clawed feet support and propel
  the reptile body.
• Efficient lungs perform gas exchange, while water
  is conserved because most reptiles excrete uric
  acid.
• Internal fertilization and the amniotic egg keep
  the embryo from drying out as it develops in a
  self-contained watery environment.

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Reptile Characteristics

• Like all terrestrial vertebrates (amphibians,
  reptiles, birds, and mammals) reptiles have a
  double loop circulatory system.
• The pulmonary loop takes blood from the heart to
  the lungs to be oxygenated, and the systemic loop
  takes blood from the heart to the tissues of the
  body where O2 is absorbed and CO2 is released.
• In all reptiles except crocodilians, the heart
  has 2 atria and 1 partly divided ventricle.
  Crocodiles have 2 atria and 2 ventricles.
• Reptile lungs are large with many small sacs
  called alveoli that make them much more efficient
  than amphibian lungs.

• The reptile brain is about the same size as an
  amphibian brain, but the cerebrum, which controls
  behavior, is much larger.
• Most reptiles rely on their senses of sight and
  hearing to detect prey and predators.
• Reptiles are ectotherms, and control their
  internal body temperature by changing behavior to
  control how much heat they absorb.
• Reptiles have three reproductive patterns They
  may be oviparous (lay eggs), viviparous (embryo
  is incubated within the mother), or ovoviviparous
  (produce shelled eggs that incubate within the
  mothers body).

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Reptilia
18
Aves

• All birds have feathers, and no other living
  animals have them.
• The forelimbs of birds are modified into wings.
• All birds have a lightweight, rigid skeleton with
  a keel on the sternum. Many bones are
  thin-walled, hollow, and riddled with air sacs
  from the respiratory system.
• All birds are endotherms with a high metabolic
  rate and a 4-chambered heart..
• All birds have the most efficient respiratory
  system yet evolved, that eliminates dead space
  from the lungs.
• No bird has teeth, and all birds have a beak.
• All birds lay amniotic eggs with a hard
  calcium-rich shell.

• Because birds lack teeth, they have a crop for
  food storage, a proventriculus where digestion
  begins, and a gizzard for grinding of food.
• Birds are thought to have evolved from raptor
  dinosaurs because of three characteristics which
  both groups share an S-shaped flexible neck, a
  unique ankle joint, and hollow bones.
• The first birds are thought to have descended
  from small fast-running carnivorous dinosaurs
  during the Jurassic Period (208-144 m.y.a.)
• Current theories are that feathers evolved before
  wings (for insulation), and wings before flight
  (for stabilization while running).

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Aves
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Mammalia

• There are about 4,400 species of mammals, and
  they live on every continent and in every ocean.
  Some can fly.
• Like birds, all mammals are endotherms, with
  metabolic control of body temperature.
• All mammals (even whales and dolphins) have hair,
  which can serve as an insulator or for
  camouflage.
• Like birds and crocodilians, all mammals have a
  4-chambered heart.
• All female mammals produce milk to nourish their
  young.
• All mammals have a single jawbone with teeth
  specialized to perform different functions.

• The ancestors of mammals appeared more than 300
  m.y.a.
• A major split occurred in the existing reptiles
  of the time. One group produced the dinosaurs,
  birds, and all living reptiles. The other group,
  called synapsids, led to mammals and their
  closest fossil relatives.
• Synapsids like Dimetrodon had skulls that were
  different from pther reptiles and showed some
  specialized teeth.

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Mammals

• In the Permian Period, a sub-group of the
  synapsids, the therapsids, appeared and later
  gave rise to the earliest mammals.
• Many features of mammals first appeared in the
  therapsidsspecialized teeth, endothermy, limbs
  located beneath the body, and hair are examples.
  
• The first mammals and the first dinosaurs
  appeared at about the same time, during the
  Triassic Period. Early mammals were small and
  resembled shrews. With the extinction of the
  dinosaurs 65 m.y.a. the mammals diversified to
  fill newly available niches.

• Mammals are classified into more than 20 orders.
  Important orders of mammals are the monotremes
  and the marsupials. All other mammals are
  placentals.
• Monotremes are the only mammals that lay eggs.
  They are represented by the duck-billed platypus
  and the echidna, or spiny anteater.
• Marsupials like kangaroos and the opossum exist
  today only in Australia, New Guinea, and the
  Americas. Marsupials give live birth to very
  underdeveloped young that migrate to the mothers
  pouch where they complete development.
• Placentals include rodents, insectivores,
  primates, carnivores, proboscideans, and
  cetaceans, among other groups.

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Mammalia
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• E. Q. How has the chordate body plan evolved
  over time to better suit these animals to their
  ecological niches?
• How have the following systems changed from
  Urochordates and Cephalochordates through the
  fishes, amphibians, reptiles, mammals, and birds?
  
• Nervous system
• Digestive system
• Reproductive system
• Circulatory system
• Respiratory system
• Skeletal system