Amphibians

Learning Outcomes
  • Describe the important difference between the lifecycle of amphibians and life cycles of other vertebrates.
  • Distinguish between characteristics of Urodela, Anura, and Apoda.
  • Describe the evolutionary history of amphibians.
Characteristics of Amphibians
  • Four Limbs: All amphibians have four limbs, with an evolutionary exception for Caecilians.
  • Skin: Amphibians have moist and permeable skin, which is essential for gas exchange.
  • Respiration: They can respire through multiple methods: skin, buccal cavity, and lungs.
  • Diet: All amphibians are carnivores and have teeth; they possess vomerine teeth located in the roof of the mouth.
  • Vision: They have image-forming eyes and exhibit color vision.
  • Ears: Amphibians have ears that include an additional bone known as the operculum.
Evolutionary History
  • First Tetrapods: Amphibians are recognized as the first tetrapods, appearing around 400 million years ago (MYA).
  • Ancestry: Evolved from lobe-finned fishes, marking a significant transition to air-breathing and terrestrial life approximately 50 MYA.
  • Early Examples: Early aquatic tetrapods emerged during the Devonian period, primarily adapted to living in water without weight support.
  • Carboniferous Period: This era saw amphibians transition to land with few predators and competitors, promoting terrestrial adaptation.
Modern Amphibians
  • Total of 6,770 species classified into three clades of the subclass Lissamphibia:
    1. Urodela (Salamanders) - 620 species
    • Movement: Utilize lateral undulation.
    • Respiratory Structures: Lack lungs or possess primitive lungs; some have gills.
    • Reproductive Behavior: Engages in courtship rituals, uses spermatophores, is oviparous, and lays eggs in water.
    • Metamorphosis: Typically undergo metamorphosis prior to hatching.
    1. Anura (Frogs and Toads) - 5,965 species
    • Limbs: Adapted for jumping.
    • Skin Glands: Some glands are modified for toxin production.
    • Reproductive Strategy: Practice external fertilization.
    • Parental Care: Varies widely from no care to carrying tadpoles/eggs on their backs.
    • Metamorphosis: Takes place after hatching.
    1. Apoda (Caecilians) - 185 species
    • Physical Features: Limbless; evolved from ancestral species with limbs, resembling earthworms with distinct teeth and jaws.
    • Reproduction: Internal fertilization; may be oviparous or viviparous.