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Early tetrapods and modern amphibians
- 1. Early Tetrapods and Modern Amphibians 1
- 2. Moving onto land Transition from an aquatic life to terrestrial life took millions of years Amphibians repeat this transition by ontogenetically and phylogenetically - I.e., they transition in their own lives but also in life history Modern amphibians number 7,500 species of frog, toad, salamander, and caecilian - Even the most terrestrial of amphibians is still closely tied to water 2
- 3. Moving onto land Land greatly influences the remaining vertebrate classes - Together, the amphibians and amniotes form the clade Tetrapoda - “Four limbs”; though many tetrapods have lost limbs Movement onto land is perhaps the most dramatic change life has made - Life originated in water - Animals are mostly water - All cellular activities occur in water 3
- 4. Moving onto land Life on land would require adapting to: - Increased oxygen content - Air hold more oxygen than water - Decreased fluid density - Air is less dense than water - Less stable temperatures - Temperature regulation required - Greater habitat diversity - Aquatic system much more homogenous 4
- 5. Devonian Origins of Tetrapods Freshwater habitats are unstable - Oxygen content varies - Water may dry during summer or drought Fishes living in these waters may have possessed traits that could be exapted for terrestrial life - Air filled cavities (i.e., swim bladders) - Paired nostrils leading to pharynx - Boney elements of fins 5
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- 7. Devonian Origins of Tetrapods Eusthenopteron - lived 385 mya - Bones of forelimbs are recognizable - Humerus, radius, ulna - Unlikely that it could walk, but undoubtedly could use limbs to push through shallow water 7
- 8. Devonian Origins of Tetrapods Tiktaalik - lived 375 mya - Morphologically intermediate between lobe-finned fishes and tetrapods 8
- 9. Devonian Origins of Tetrapods Acanthostega - lived 365 mya - Well-formed tetrapod limbs with clearly formed digits - Probably still unable to walk on land 9
- 10. Devonian Origins of Tetrapods Ichthyostega - lived 300 mya - Limb morphology suggest terrestrial walking was possible - Retained some aquatic traits - Fins on tail - Gill operculum 10
- 11. Modern Amphibians Three living amphibian orders - Show general adaptations for life on land - Skeletal strengthening Ancestrally: - Eggs are aquatic - Larvae are aquatic and use gills for respiration - Adults are terrestrial and use cutaneous and lung respiration - Ectothermic 11
- 12. Modern Amphibians Many deviations from this ancestral plan - Some salamanders do not complete metamorphosis - Retain gills (neoteny or paedomorphosis) - Some are entirely aquatic - Some are entirely terrestrial 12
- 13. Modern Amphibians Among the most terrestrial amphibians, water still looms large - Skin requires moisture to prevent desiccation - functions as respiratory surface - Eggs are highly susceptible to desiccation - Cool/moist climates have greatest diversity 13
- 14. Order Gymnophiona Caecilians - 200 species - Elongate amphibians - Limbless burrowers - South America, India, Africa, Southeast Asia 14
- 15. Order Gymnophiona Caecilians - Most species are blind or nearly so - Sensory tentacles occur on the snout - Eat small worms and invertebrates 15
- 16. Order Gymnophiona Caecilians - Internal fertilization - Eggs deposited in moist soil - Larvae may develop within eggs - Others are viviparous - Nourished by eating walls of oviduct 16
- 17. Order Urodela Salamanders - 700 species - Tailed amphibians (as adults) - Occur in northern temperate regions - Do occur elsewhere, but a biogeographic oddity - Generally small, but Japanese giant salamander may be 1.5m 17
- 18. Order Urodela Salamanders - Limbs set at right angles - Hind and forelimbs of the same size - May be absent in burrowing and aquatic forms - Carnivorous as adults and larvae - Worms, arthropods, and molluscs comprise diet 18
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- 20. Order Anura Frogs and toads - 6,600 species - Most familiar of the amphibians - Fossils from 190 mya - Reproduction and moist skin keep them close to free water - Body morphology specialized for jumping - Unlike some urodelans, never perennibranchiate 20
- 21. Order Anura Frogs and toads - Integument and coloration - Frog skin always moist - Contains epidermis and dermis - Epidermis endowed with keratin to toughen it - Toad have more keratin than frogs - All anurans have mucus glands that waterproof the skin - Granular glands produce a whitish poison - Present in all amphibians, but potency is variable 21
- 22. Order Anura Frogs and toads - Integument and coloration - Chromatophores are located in the dermis - Modification of their shape produces different colors - Familiar green frogs are blue light filtered through yellow chromatophores https://howlingpixel.com/wiki/Chromatophore 22
- 23. Order Anura Frogs and toads - Respiration - Adults use three surfaces - Skin, mouth, and lungs - Larvae use gills - Depend on lungs more than salamanders - Cutaneous respiration important during hibernation and expelling carbon dioxide 23 https://www.researchgate.net/publication/285385704_History_ and_recent_progress_on_chytridiomycosis_in_amphibians
- 24. Order Anura Frogs and toads - Respiration - Positive-pressure respiration - Lungs filled by forcing air into them - Amniotes have negative-pressure breathing 24
- 25. Order Anura Frogs and toads - Circulation - Closed circulation - Double circuit - Pulmonary and systemic circuits - Three chambered heart - Separate atria, single ventricle 25
- 26. Order Anura Frogs and toads - Feeding - Carnivorous as adults - Short digestive tract - Herbivorous as larvae - Long digestive tract 26
- 27. Order Anura Frogs and toads - Reproduction - Amplexus - males clasps onto females back - Fertilizes eggs as they are laid - Gelatinous layer surrounding eggs swells with water contact - Eggs anchored to vegetation 27