The Origin and Early Evolution of Tetrapods

1. The Origin and Early Evolution of Tetrapods • SuperclassTetrapoda: four-limbed vertebrates; a monophyletic group comprising amphibians, reptiles, birds, and mammals; terrestrial, some with secondary modifications of limbs and transitions to aquatic/marine habitats (ex. sea snakes, sea turtles, cetaceans) • Characteristics of the Terrestrial Environment: oxygen ~20x more abundant in air than in water; little support against gravity; great fluctuations in temperature; great variety of habitats • The Transition from Lobe-finned Fishes to Terrestrial Vertebrates • Romer’s “Gap”: historical paucity of vertebrate fossils between early Devonian freshwater lobe-finned fishes and first fully terrestrial tetrapods (late Devonian); gap now filled by several transitional specimens • Tiktaalik (2006): transitional form between ancestral lobe-finned fishes (ex. Eusthenopteron) and first tetrapods (early forms aquatic, with gill arches and paddle-shaped tails, ex. Ichthyostega, Acanthostega) • Transitional Characteristics (Tiktaalik): fish-like scales, fin webbing, and fish-like tail, but limbs with joints (shoulder, elbow, wrist), flat head with raised eye sockets, muscular chest and neck lift neck out of water, primitive ribs • Tetrapod lung likely evolved from air-filled cavity in response to low or unstable levels of dissolved oxygen (typical in swamps and mudflats)

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5. Extant Amphibians – Diversity and Life Cycles • Amphibian Diversity and Characteristics: ~6,000 species; ectothermic; strengthened skeletons; most with aquatic larvae (with gills and lateral lines) and metamorphosis to terrestrial adults (with primitive lungs and cutaneous respiration)  dependent on standing water; toxic skin secretions (some extremely venomous) • Order Apoda: Caecilians (~173 species) • Elongate, limbless, burrowing; found in tropical rainforests (esp. South America); terminal anus; sensory tentacles on head; eat worms; internal fertilization via spermatophores; limbless salamanders • Order Urodela (Caudata): Salamanders and Newts (~553 species) • Tails, lateral limbs, carnivorous; diversity greatest in North America; terrestrial species lack aquatic larvae; most with both gills and lungs during life cycle, but cutaneous respiration important; some exhibit paedomorphosis, retain gills as adults (ex. mud puppies, axolotl); many capable of regenerating limbs • Order Anura: Frogs and Toads (~ 5,283 species) • Hind legs modified for jumping (anti-predation) and swimming (webbed), tails absent in adults; metamorphosis and major changes in lifestyles from larvae (tadpoles) to adults; hibernate during winter in temperate climates; most with external fertilization • Families include Ranidae (bullfrogs, leopard frogs), Hylidae (tree frogs), and Bufonidae (toads, with short legs and thick skin)

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14. Anurans: Form and Function • Integument and Coloration: skin thin, loose, and glandular (mucous and serous glands), with chromatophores (Tyndall scattering from deep layers  blue color filtered by over- lying yellow layer  green hues) • Poison-dart frogs (Dendrobatidae) among most venomous animals (tetradotoxin); secretions used on blowgun darts; warning coloration • Skeletal and Muscular Systems: force transferred to hind limbs by urostyle (fused trunk vertebrae); five-rayed hind limb, four-rayed front limb; skull reduced in rear (gill area) • Epaxial (dorsal) muscles: support the head and brace vertebral column • Hypaxial (ventral) muscles: support viscera (more than in fishes) • Respiration and Vocalizations: gas exchange via skin, mouth, and lungs; frog lung a positive- pressure breather (air forced into lung via throat muscles); vocal cords and sacs well developed in males (species-specific mating calls) • Circulation: shift to lung breathing required blood circuit to lungs (pulmonary arteries and veins); partition down center of atria separates pulmonary from systemic flow; two atria in frogs contract asynchronously  single ventricle has some separation between oxygenated and deoxygenated blood (aided by spiral valve in conusarteriosis) • Feeding and Digestion: adults with varied diet (mainly insects) and protrusible tongue (attached at anterior end of mouth); tadpoles mainly herbivorous • Nervous System and Senses: forebrain (olfaction and cerebrum), midbrain (vision), and hind- brain (hearing and balance; cerebellum and medulla); hearing with tympanic membrane, stapes (single ear bone), and inner ear with semicircular canals; eyes with rods and cones (color vision) and nictitating membrane • Reproduction: eggs usually fertilized externally during amplexus and attached to vegetation; some species tend eggs and/or brood larvae; metamorphosis from tadpoles ranges from three months to three years

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23. Fig. 34.9

24. Amphibians and Environmental Issues • Pollution and Sentinel Species • Sentinel Species: first to show effects from pollution (“canary in the coal mine”) • Amphibians exposed to both air and water pollutants during life cycle; vulnerable due to porous skin; developmental abnormalities noted in recent decades (especially extra and missing limbs) • Introduced Species • Introduced amphibians include African clawed frog and cane toad (plague level in Australia) • Endangered Species and Extinction • Threats include epidemic fungal pathogen, climate change (ex. golden frog), and introduced predators/competitors (ex. rainbow trout)