Evolution of Terrestrial Vertebrates and Modern Amphibians

1. AMPHIBIANS – A Tetrapod Delivered By SHITTU Olalere Dept of Zoology University of Ilorin, Nigeria

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. All sorts of “amphibians” in the fossil record

4. 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

5. Limbs with digits strengthened limb girdles ? “Fish” Tetrapod

6. 1st Terrestrial tetrapods – Ichthyostegans, Labyrinthodonts †Acanthostega Early tetrapod trackway

7. Labyrintodonts Ichthyostega

8. 1st terrestrial tetrapods

9. 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

10. Internal choanae (nares)

11. 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

12. 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

13. C. Salamanders: Order Caudata 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

14. C. Salamanders: Order Caudata • Aquatic: • “Taste” chemicals in water • Vibrations – Retain lateral line system • Terrestrial: • Smell – Olfactory epithelium (volatile) • Smell – Vomeronasal organ (Non-volatile)

15. C. Salamanders: Order Caudata • Sexually dimorphic (larger in males) • Courtship / Identification (species / sex / individual) • Highly advanced in Plethodontidae • Nasolabial Grooves = Non-ciliated grooves; upper lip to nares • Aids in collection / delivery of chemical cues (capillary action) • Sexually dimorphic (esp. during reproductive season)

16. Amphiumidae(Amphiums): Aquatic: SE United States Proteidae(Waterdogs): Aquatic: Eastern United States / SE Europe Amphibians • terrestrial egg-laying • aestivate Morphology: • Eel-like; maintain four reduced limbs • Paedomorphic; lack gills (lungs present) Habitat: • Sluggish streams / rivers; swamps Reproduction: • Internal fertilization; female nest guarding • dissection specimen • nocturnal Morphology: • Paedomorphic; feathery gills / caudal fins Habitat: • Lakes / streams • Limestone caves (drastic reduction in numbers) Reproduction: • Internal fertilization; ♂ / ♀ nest guarding

17. Pheromone: Chemical cue that affects the behavior and / or physiology of a conspecific Amphibians Plethodontidae(example = Ensatina escholtzii): Anatomy: • Constriction at tail base = tail autotomy • Very costly (tail = large fat reserve) • ~ 10 – 12% tail regeneration (~ 2 years) Terrestrial: North / Central / South America & Europe Habitat: • Prefers cool forests with litter / dead trees • Tolerates logging better than most plethodons Reproduction: • Mating begins (Nov.) and ends (March) with rains • ♂ use hedonic glands / pheromones to attract ♀ • Methods of pheromone administration: • Slap ♀ nares with hedonic gland (video) • Innoculate ♀ with sharp teeth • Elaborately patterned courtship “dance” • “Tail straddle-walking” (unique to plethodons) • ♂ repeats dance (~ 1 – 5 hours) • Pathway / movement unique to each species

18. Sperm Cap Gelatinous Base Amphibians Plethodontidae(example = Ensatina escholtzii): Reproduction: • ♂ lays down spermatophore; ♀ picks it up (video) • ♀ lay 8 – 12 eggs (~ 5 mm) • Terrestrial – in logs; under logs; in burrows Terrestrial: North / Central / South America & Europe • ♀ guards eggs ( predation / fungal infection) • Direct development of eggs (~60 – 120 days) Growth / Maturation: • Sexual Maturation ~ 4 years • Life Span ~ 10 years (best guess)

19. Habitat: Anti-predator Display Amphibians Rhyacotritonidae: (example = Rhyacotriton olympicus) Anatomy: • Full metamorphosis • No operculum / opercular muscle ( hearing) • Highly reduced lungs (cutaneous respiration) Terrestrial: NW United States • Cold, clear streams / seepages / waterfalls • Very desiccation intolerant • Low heat tolerance (susceptible to logging) Reproduction: • Internal fertilization • Spermatophore deposition • Tail-wagging display • Aquatic egg-laying • No nest guarding Growth / Maturation: • Sexual Maturation ~ 4 years • Life Span ~ 10 years (best guess)

20. Dicamptodontidae Rhyacotritonidae Plethodontidae Ambystomatidae Salamandridae Amphibians Larval Anatomical Specializations: Stream Larvae Pond Larvae • High body profile • broad tail fin extending up to head • long, plume-like gills • Only front legs at hatching • Depressed body profile • Narrow tail fin (not onto trunk) • Short, thread-like / curly gills • All four legs at hatching

21. Habitat: Red-spotted Garter Snake “The Arms Race” Amphibians Salamandridae(example = Tarichagranulosa): • Prefer older growth forests during “newt” phase Warning Coloration: • Flashes tail & brightly colored stomach Terrestrial: North America / Europe / NW Africa / Asia Chemical Defense: • Tarichatoxin (non-protein – VERY poisonous) • Neurotoxin – blocks NA+ channels (paralysis) • Small dose lethal to birds / mammals • Large dose lethal to humans

22. C. Salamanders: Order Caudata • Hearing – limited high frequency (no ear drum); primarily ground vibration • Vision – acute; especially in Plethodontidae

23. Sirenidae (sirens): Aquatic: SE United States / NE Mexico Cryptobranchidae(Hellbenders): Aquatic: Central China / Eastern United States Amphibians • Nocturnal • Vocalization Morphology: • Eel-like; lack hind limbs (lack pelvic girdle) • Paedomorphic; gills present Habitat: • Swamps / lakes / marshes (slow-moving water) Reproduction: • External fertilization; female nest guarding • largest individuals • live 50+ years Morphology: • Dorsal-ventral compressed body (paedomorphic) • Multiple folds in skin (cutaneous respiration) Habitat: • Clear, cold mountain streams / lakes • Severely reduced ranges (loss of habitat) Reproduction: • External fertilization; Males = “den masters”

24. Salamanders

25. 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

26. 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

27. 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 paedomorphosis, retaining larval characteristics but switching to lungs

28. D. Frogs and Toads: Order Anura 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

29. 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

30. 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

31. 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

32. Integument and Coloration • 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

33. Skin Pigmentation

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. Frog Life Cycle

45. 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

46. c. Metamorphosis • Hind legs appear first; forelimbs are temporarily hidden by operculum • Tail is reabsorbed • Intestine becomes shorter • Mouth transforms into adult version • Lungs develop and gills reabsorbed

47. Metamorphosis

48. Problems with Terrestrial Living Solutions

49. Solutions Senses Olfaction and hearing

50. Solutions Senses Hearing