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
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
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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
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
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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
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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
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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
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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
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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
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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
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