The origin and early evolution of tetrapods
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The Origin and Early Evolution of Tetrapods

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The origin and early evolution of tetrapods

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)

Fig 25 2

Fig. 25.2

Fig 25 3

Fig. 25.3

Fig 25 1

Fig. 25.1

Extant amphibians diversity and life cycles

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)

Fig 25 4


Fig 25 8 and fig 25 9

Fig. 25.8 and Fig. 25.9

Fig 25 5

Fig. 25.5

Fig 25 6

Fig. 25.6

Fig 25 7

Fig. 25.7

Fig 25 11 and fig 25 12

Fig. 25.11 and Fig. 25.12

Fig 25 25

Fig. 25.25

Fig 25 13 and fig 25 14

Fig. 25.13 and Fig. 25.14

Anurans form and function

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

Fig 25 15

Fig. 25.15

Fig 25 18

Fig. 25.18

Fig 25 17 and fig 25 19

Fig. 25.17 and Fig. 25.19

Fig 31 22

Fig. 31.22

Fig 25 20

Fig. 25.20

Fig 25 21 and fig 25 22

Fig. 25.21 and Fig. 25.22

Fig 25 24

Fig. 25.24

Fig 25 26

Fig. 25.26

Fig 34 9

Fig. 34.9

Amphibians and environmental issues

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)

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