Early tetrapods and modern amphibians

Presentation

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