Herpetology: Class Amphibia and Class Reptilia

CLASS AMPHIBIA

  • Origins:
    • Fossil ancestral amphibians from the Late Devonian/Early Carboniferous Era.
    • They survived in shallow water and on moist land, typical of modern amphibians.
  • Tiktaalik:
    • Best-known example of ancestral amphibians.
    • Referred to as a “tetrapod fish” or early amphibian.
  • Tetrapod Legs:
    • Share skeletal elements with modern amphibians and advanced vertebrates.
    • Homologous structures with comparable arrangement throughout all vertebrates.
    • Anterior appendage: humerus, radius, ulna.
    • Posterior appendage: femur, tibia, fibula.

Modern Amphibians

  • Approximately 8,450 species.
  • Circulation:
    • 3-chambered heart.
    • 2 Atria (anterior): Left atrium (oxygenated blood), Right atrium (de-oxygenated blood).
    • 1 Ventricle (posterior): Contains a combination of oxygenated and de-oxygenated blood (inefficient).
  • 3 Orders of Amphibia:
    • Order Apoda: "leg-less" - Caecilians (~215 species).
    • Order Urodela: "tailed Amphibians" - Salamanders (~760 species).
    • Order Anura: "tail-less" - Frogs & Toads (~4,900 species).

Order Apoda - Caecilians

  • Number of estimated species ~215.
  • Tropical burrowers found in mud and ponds. Superficially resemble earthworms.
    • Legless.
    • Rudimentary vision.
      • These traits are secondary/derived.
      • Developed from more typical amphibian ancestry.
      • Descended from limbed, seeing ancestors.
      • Were out-competed on land.
      • Adapted to move underground.
  • Maternal Dermatophagy:
    • Young feed on the outer layer of the mother’s integument.
    • The mother regenerates the outer layer in 3 days.
    • Tail integument releases toxic substances (similar to Poison Dart Frogs).

Order Urodela - Salamanders

  • Number of estimated species ~760.
  • Motility:
    • Legs emerge from lateral sides.
    • Causes undulating/swishing movement like fish.
  • Varied Lifestyles:
    • Primitive Salamanders: entirely aquatic.
    • Advanced: larvae (aquatic), adult (terrestrial).
  • Axolotl:
    • Does not undergo transition from aquatic larvae to terrestrial adult.
    • Remains aquatic its entire life.
    • Paedomorphosis: retention of juvenile traits into adulthood, i.e., retains gills into maturity.

Order Anura - Frogs / Toads

  • Most evolutionarily successful group of Amphibians: 60% of all amphibian species.
  • Number of estimated species ~ 4,900.
  • More specialized for life on land than other Amphibian groups.
  • Adaptations to avoid predation:
    • Coloration: Camouflage (green/brown), Warning coloration (bright reds, iridescent blue).
    • Toxic skin secretions: Often associated with bright colors.
  • Anura is the best amphibian group for demonstrating alternation between water and land.
  • “Amphibian” = “2 lives”. Aquatic as juveniles, terrestrial as adults.
  • Development includes complete metamorphosis: larvae (tadpoles) to adult frog.

Reproduction (Anura)

  • Amplexus: Act of gamete exchange, initiates reproductive process.
    1. Based on species-specific responses to season, vocalization (pre-zygotic process retaining reproductive isolation).
    2. Male grasps female from behind.
    3. Both release gametes across cloaca into pond-water.
    4. External fertilization in water.
  • Fertilized Eggs:
    • Lack shell, subject to desiccation.
    • Undergo development in water (high mortality, especially in ephemeral ponds).
  • Parental Care:
    • Female and/or male may incubate eggs.
      • Buccal / Sub-lingual Incubation / Gastric Brooding: both males & females incubate eggs within vocal sac, in pharynx region. Maintains eggs under tongue; at maturity – released from mouth.
      • Integumentary Incubation: adult remains under water. Rises with eggs on dorsal surface, integument grows up over eggs protectively. Eggs remain through development, then dorsal integument decomposes releasing tadpoles into the water.
  • Development – following fertilization: