Download Early Tetrapods & Modern Amphibians

Early Tetrapods & Modern
Amphibians
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
diversity
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
forelimbs
• 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
swimming
III. Modern Amphibians (Class Amphibia)
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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
range
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
nourishment
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
limbs
• Carnivorous, eating high fat and protein foods so
do not store much fat or glycogen
Salamanders
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
aquatic
3. Respiration
• Thin skin easily exchanges gases; also use mouth
cavity
• 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
4. Paedomorphosis
• Preservation of larval
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
conditions)
• 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
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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
unknown
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
glycogen
• 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
ones
• Inner dermal layer has mucous glands, which secrete
waterproofing agents, and serous glands, which secrete
poisons
• Dendrobatid frogs from South America secrete highly toxic
poisons
• Chromatophores are pigment cells that produce skin color
• Xanthophores are upper pigments with yellow, orange, and
red
• Iridophores are middle layer with silvery light-reflecting
pigment that gives iridescent quality
• Melanophores are deepest layer with brown or black
melanin
• Green color is produced by interactions among all of these
pigments
• 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
swimming
• 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
jointed
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
larynx
• Songs are unique and
characteristic of the
species
http://www.naturesound.com/frogs/frogs.html
8. Circulation
• 6th aortic arch present in gills
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
ventricle
• 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
mouth
• 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
balance)
• 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
system
b. Vision
• Dominant sense in many
amphibians (except
caecilians)
• 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
vision
• 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
immediately
• 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
spiracle
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
Metamorphosis