Early Tetrapods & Modern Amphibians by eL3VEnx


									Early Tetrapods & Modern

        Chapter 25
I. Movement Onto Land
• Amphibians are vertebrate transition to land;
  other organisms like plants, gastropods, and
  arthropods made transition earlier
• Since organisms made mostly of water,
  dangerous transition
• Also had to adapt to different oxygen content,
  density, temperature regulation, and habitat
II. Evolution of Terrestrial Vertebrates
• A. Devonian Origin
• Mild temperatures, and periods of flooding or
  droughts creating unstable sources of freshwater
• Fish with lungs were better able to survive
• Early fish fossils that could crawl along mud with
  “walking fins:” Eusthenopteron, Acanthostega,
  and Ichthyostega.
• Land adaptations include: skull, teeth, stronger
  pectoral and pelvic girdles, jointed limbs, stronger
  backbone, muscles to support body in air and
  elevate head, more protective rib cage, ear
  structure, and longer snout
• Many fossils have more than 5 digits
B. Carboniferous Radiation
• Uniformly warm and wet
• Tetrapods radiated in swampy, fern areas eating
  insects, larvae, and invertebrates
• Temnospondyls form lineage from which modern
  amphibians are derived; they have 4 digits on
• Became better adapted to aquatic life; bodies
  flattened, some like salamanders developed
  weaker limbs and stronger tails, and frogs
  developed webbing on hind limbs for better
III. Modern Amphibians (Class Amphibia)
• A. Diversity
• Over 4200 species
• Ears redesigned
• Remain tied to water since eggs are laid in water
  and larvae have gills
• Thin skin loses moisture rapidly restricting them
  to moist habitats
• Ectothermic which also restricts habitat and
B. Caecilians: Order Gymnophiona
• 160 species live in tropical rainforests of South
  America, Africa, and Southeast Asia
• Elongate, limbless, and burrowing
• Some larvae develop in folds of body and in
  others develop in oviduct, eating it for
C. Salamanders: Order Caudata
• 1. Characteristics
• 360 species living in northern temperate areas
• Most are small, under 15 cm, but Japanese giant
  salamander is 1.5 m long
• Limbs are at right angles of body, with fore and
  hind limbs of equal length
• Burrowing and some aquatic species have lost
• Carnivorous, eating high fat and protein foods
  so do not store much fat or glycogen
2. Breeding Behavior
• Fertilize eggs internally
• Terrestrial species deposit
  egg clusters under logs or
  in soft earth; eggs hatch
  out to mini adults
• Most salamanders lay
  strings of eggs in water;
  larvae hatch with gills, and
  then may turn into
  terrestrial or aquatic adults
• Newts have “red eft” stage
  with a terrestrial juvenile,
  that then turns into
  aquatic, breeding adult
• Some newts stay entirely
3. Respiration
• Thin skin easily exchanges gases; also use mouth
• At various stages may also have gills, lungs, both
  gills and lungs, or neither
• If lungs are used, they are present from birth, but
  only become functional following metamorphosis;
  hold nostrils above water to breathe
• Larvae hatch with gills, and lose them following
  metamorphosis, along with fin-like tail; if a
  lineage does not undergo metamorphosis, it
  retains these characteristics
• Those in terrestrial family Plethodontidae have no
  lungs and use only skin
                    • Preservation of larval
4. Paedomorphosis     features into adulthood
                    • Trend found in salamanders
                    • Some never metamorphose,
                      like the mudpuppy
                    • Others typically may not, but
                      can change if conditions
                      change (typically in dry
                    • Another example is
                      Ambystoma tigrinum, which
                      stays in an axolotyl stage;
                      can metamorphose if treated
                      with thyroid hormone
                    • Some have partial
                      paedomorphsis, retaining
                      larval characteristics but
                      switching to lungs
D. Frogs and Toads: Order Anura
•   1. Characteristics
•   3450 species
•   Evolved during Jurassic Period, 150 mya
•   All have tailed larval stage but are tailless as
    adults; none retain larval characteristics as adults
•   21 families
•   Family Ranidae, larger frogs of North America
•   Family Hylidae, tree frogs
•   Family Bufonidae, toads
2. Habitat and Distribution
• 260 species of genus Rana
  found in temperate and
  tropical areas
• Frogs and toads are found
  in damp forested floors,
  although in tropical
  rainforests may live entire
  life in 1 tree
• Swamps, ponds, streams
• Frogs and toads are
  declining worldwide and
  becoming patchy in
  distribution; cause is
3. Life Cycle
• Solitary except during
  breeding season
• Spend breeding season
  swimming around in water
• During winter, burrow into
  mud to hibernate, using
  energy from stored fat and
• Also accumulate glucose
  and glycerol in tissues to
  create an “antifreeze” to
  prevent tissues from
  forming ice crystals
• Easy prey; protects
  themselves by
  concealment, poison
  glands, and aggression
4. Integument and Coloration
• Thin, moist, and attached to body in several points
• Epidermis contains keratin; this is thicker in more terrestrial
• Inner dermal layer has mucous glands, which secrete
  waterproofing agents, and serous glands, which secrete
• Dendrobatid frogs from South America secrete highly toxic
• Chromatophores are pigment cells that produce skin color
• Xanthophores are upper pigments with yellow, orange, and
• Iridophores are middle layer with silvery light-reflecting
  pigment that gives iridescent quality
• Melanophores are deepest layer with brown or black
• Green color is produced by interactions among all of these
• Frogs can adjust color to camouflage themselves
Skin Pigmentation
5. Skeletal and Muscular Systems
                     • Well developed
                       endoskeleton of bone
                       and cartilage with
                       changes to allow for
                       jumping and
                     • Front of skull is light
                       weight and flattened
                     • Limbs have 3 joints:
                       hip, knee, ankle
                     • Foot has 5 rays on
                       hind limb, and 4 on
                       front limb; digits
6. Respiration
• Use skin, mouth, and lungs with skin being
  critical during hibernation
• CO2 is mainly lost across skin whereas O2 is
  mainly absorbed through lungs
• The movement of air into lungs is somewhat
  passive, depending on movement of throat
7. Vocalization
                          • As air enters and
                            leaves lungs, passes
                            over vocal cords, on
                            way to vocal sacs
                          • Both males and
                            females have vocal
                            cords but males have
                            a more developed
                          • Songs are unique and
                            characteristic of the

                 • 6th aortic arch present in gills
8. Circulation     was converted into
                   pulmonary artery to lungs
                 • Frog heart has 2 atria and 1
                   single ventricle
                 • Blood from body enters right
                   atrium and blood from lungs
                   enters left atrium; both
                   deoxygenated and
                   oxygenated blood mix in
                 • Valves do control blood
                   though so mainly
                   deoxygenated blood goes to
                   lungs and oxygenated goes
                   to body
9. Feeding and Digestion
• Carnivorous
• Catch prey with tongue
  that is hinged at front of
• Free end is glandular with
  sticky secretions that
  cause prey to adhere
• Teeth are used to hold
  prey, not to chew or bite
• Digestive tract produces
  enzymes to break down
  carbohydrates, proteins,
  and fats
• Tadpoles are herbivorous
  and have longer tracts to
  digest plant material
10. Nervous System and Senses
• a. Brain
• Consists of 3 parts: forebrain (sense of smell),
  midbrain (vision), and hindbrain (hearing and
• Forebrain has cerebrum, but not really used.
  Olfactory lobe most important part
• Midbrain has optic lobe
• Hindbrain has anterior cerebellum (not used) and
  posterior medulla which controls auditory
  reflexes, respiration, swallowing, and circulatory
            • Dominant sense in many
b. Vision     amphibians (except
            • Eyelids keep eyes moist,
              protected, and free of
              dust; upper is fixed and
              lower has clear
              nictitating membrane
            • Cornea and lens bend
              light to focus image on
              retina, with both rods
              and cones for color
            • Iris changes to adjust to
              different light levels
            • At rest, frog focuses on
              distant objects
c. Other Senses
• Pressure sensitive lateral line is only found in
  larvae and aquatic adults
• Ear is sensitive to airborne sounds; tympanic
  membrane passes vibrations to structure similar
  to cochlea
• Chemical receptors in skin, on tongue, and in
  nasal cavity
11. Reproduction and Development
• a. Egg
• In spring, males migrate to same pond or stream
  and call females
• This is dependent on temperature, humidity, and
  hormonal changes
• Male clasps female in water and as she releases
  eggs, male discharges sperm over them
• Eggs usually laid in masses
• Eggs absorb water and swell; development begins
• Tadpoles hatch in 6-9 days
Frog Life Cycle
b. Tadpoles
• Tadpole has horny jaws for grazing and a
  ventral adhesive disc for clinging to objects
• 3 pairs of external gills develop into internal gills
  covered by flap on right side fused to body wall
  and a spiracle on left side
• Water flows through mouth, over gills, and out
c. Metamorphosis
• Hind legs appear first; forelimbs are temporarily
  hidden by operculum
• Tail is resorbed
• Intestine becomes shorter
• Mouth transforms into adult version
• Lungs develop and gills resorbed

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