Herpetology: Class Amphibia and Class Reptilia

Origins:
- Existed during the Late Devonian / Early Carboniferous Era.
- Adapted to survive in both shallow waters and moist land, traits seen in modern amphibians.
Tiktaalik:
- A well-known example of an ancestral amphibian, recognized as a "tetrapod fish" or early amphibian.
Tetrapod Legs:
- Contain the same skeletal elements as modern amphibians and advanced vertebrates, indicating homologous structures.
- Anterior Appendage Structure: Humerus, Radius, Ulna
- Posterior Appendage Structure: Femur, Tibia, Fibula
Modern Amphibians:
- Approximately 8,450 species (this figure may be artificially high).
- Circulatory System:
  - All possess a three-chambered heart: 2 atria (Left Atrium: Oxygented; Right Atrium: De-oxygenated) and 1 ventricle containing a mixture of oxygenated and deoxygenated blood (inefficient).
- Orders of Amphibia:
  1. Order Apoda: Legless - Caecilians
  2. Order Urodela: Tailed - Salamanders
  3. Order Anura: Tail-less - Frogs and Toads

Species Count: Approximately 215 species
Habitats: Found primarily in tropical regions, typically burrowing in mud or ponds.
Characteristics:
- Superficially resemble earthworms (legless, rudimentary vision).
- Traits such as being legless and possessing poor vision are considered derived traits from typical amphibian ancestors.
Maternal Dermatophagy:
- Young feed on the outer skin layer of the mother, with regeneration taking about three days.
- Tail integument releases toxic substances similar to those of poison dart frogs.

Species Count: Approximately 760 species
Motility:
- Legs emerge from the sides, resulting in undulating movements reminiscent of fish.
Lifestyle Diversity:
- Primitive Salamanders: Entirely aquatic.
- Advanced Salamanders: Larvae are aquatic while adults are terrestrial.
Axolotl:
- A unique individual that remains aquatic without transitioning to a terrestrial adult, exhibiting paedomorphosis by retaining juvenile characteristics (e.g. gills).

Species Count: Approximately 4,900 species (accounting for about 60% of all amphibians).
Adaptations for Life on Land:
- Includes coloration for camouflage or warning (e.g., bright reds and iridescent blues) often tied to skin toxicity.
Development:
- Complete metamorphosis from larvae (tadpoles) to adult frogs.
- Reproduction involves external fertilization during the process called Amplexus.
- Males clasp females, both release gametes into water for fertilization.

Lifespan Stages:
- 1st Life: Tadpole Stage
- Characteristics: Herbivorous, gills, long finned tail, no legs, aquatic lifestyle.
- 2nd Life: Adult Frog Stage
- Characteristics: Carnivorous, lungs develop, legs emerge, lateral line sensory systems disappear.

Origins: Derived from amphibians in the early Carboniferous period; dominated during the Carboniferous era.
Adaptations to Terrestrial Life:
- Amniotic Egg:
- Shelled egg that retains moisture, crucial for reproduction on land.
- Variability: Reptiles (calcareous), Birds (calcareous), Mammals (retained in female).
Reptile Characteristics:
1. Dry, Scaly Skin:
- Provides waterproofing, composed of keratin.
1. Thoracic Breathing: Replaces buccal pumping.
2. Ectothermy: Regulating body temperature behaviorally.
Orders of Reptiles:
1. Order Testudines: Turtles and Tortoises
2. Order Squamata: Lizards and Snakes
3. Order Crocodilia: Includes crocodiles, alligators
4. Order Sphenodonta: Tuatara, found only in New Zealand

Species Count: Approximately 11,500 species.
Unique Adaptations:
- Lizards regenerate tails as a defense strategy.
- Snakes exhibit unique skeletal traits, including the disarticulating mandible for consuming larger prey.
Senses and Reproduction:
- Utilize Jacobson’s organ for olfactory sensation and pre-fertile internal fertilization.