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)

• 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

• 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