In-depth Notes on Reptiles and Amniotes

Learning Outcomes
  • At the end of this mini-lecture, students will:
    • Describe the main characteristics of amniotes.
    • Explain the differences between anapsids, synapsids, and diapsids, and provide examples for each.
    • Identify the characteristics unique to reptiles.
    • Discuss the evolution of reptiles.
Characteristics of Amniotes
  • Terrestrially adapted egg (Amniotic Egg)
    • Provides a stable environment for embryo development.
  • Extra-embryonic membranes:
    • Yolk sac: Nutrient transfer to the embryo.
    • Chorion: Facilitates O2 & CO2 exchange between the embryo and the external environment.
    • Allantois: Stores nitrogenous wastes; aids in respiration.
    • Amnion: Protects the embryo from mechanical shocks and maintains hydration.
Evolution of Amniotes
  • Early tetrapod ancestor (approx. 340 million years ago) diverged into two main lineages:
    • Synapsids:
    • Therapsids, which are the ancestors of mammals.
    • Sauropsids:
    • Comprised of Anapsids and Diapsids.
  • Temporal fenestrae:
    • Openings in the skull:
    • Anapsids: No fenestrae.
    • Synapsids: One fenestra.
    • Diapsids: Two fenestrae.
Diapsids
  • Divergence into major lineages:
    • Archosauromorpha: Includes groups like
    • Crocodilians,
    • Ichthyosaurs,
    • Pterosaurs,
    • Dinosaurs,
    • Birds.
    • Lepidosauromorpha: Includes groups such as
    • Lizards,
    • Snakes,
    • Tuataras.
Characteristics of Reptiles
  • Classification: Tetrapods, many possessing amniotic eggs (some are ovoviviparous, some viviparous).
  • Physical Attributes:
    • Scaly skin composed of keratin and waxy lipids.
    • Lungs ventilated using muscles.
    • Ectothermic organisms (cold-blooded), with some exceptions:
    • Crocodilians: Regional endotherms surviving on approximately 10% of the energy required by similar endothermic species.
  • Behavior:
    • Burmation: A condition where metabolism slows down during cold temperatures.
Evolution of Reptiles
  • Archosaurs:
    • Dinosaurs
    • Split into:
      • Saurischia: Lizard-hipped dinosaurs.
      • Ornithischia: Bird-hipped dinosaurs.
    • Pterosaurs: Aerial reptiles with over 200 described species.
    • Over 100 species existed on land.
    • The age of dinosaurs ended approximately 65 million years ago.
Modern Reptiles
  • Crocodilia:
    • Evolution noted in the Middle Triassic; includes alligators, crocodiles, gharials, and caimans.
    • Found in freshwater and saltwater across various continents.
  • Sphenodontia:
    • Originating in the early Mesozoic period with two extant species today (tuatara).
  • Squamata:
    • Evolved in the Late Permian; includes:
    • Lizards (approximately 6,000 species),
    • Snakes (approximately 3,600 species):
      • Possess specialized jaws with 8 rotational joints.
      • Have a single long lung for their slender body design.
      • Equipped with venom glands.
      • Lack eyelids but have a transparent scale covering their eyes.
  • Testudines:
    • Existed for about 200 million years.
    • Characterized by a carapace (dorsal shell) that includes the backbone and ribs.
    • A plastron (ventral shell) and scutes (scales covering both shells).