1. Early Tetrapods & Modern Amphibians Chapter 25

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Salamanders

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Skin Pigmentation

17. 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

18. 6. Respiration Use skin, mouth, and lungs with skin being critical during hibernation CO 2 is mainly lost across skin whereas O 2 is mainly absorbed through lungs The movement of air into lungs is somewhat passive, depending on movement of throat

19. 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

20. 8. Circulation 6 th 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. Frog Life Cycle

27. 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

28. 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

29. Metamorphosis