VE Lecture 5

Early Amniote Evolution

  • Evolution of Amniotes during the Permian and Triassic periods.
  • Overview of major groups, their skull types, and adaptations.

Amniotic Egg

  • Key characteristics of the amniotic (or cleidoic) egg:
    • Protective shell (either calcareous or leathery).
    • Semi-permeable chorionic membrane for gas exchange.
    • Amnion: Facilitates gas transfer to the embryo.
    • Yolk sac: Provides nutrition to the developing embryo.
    • Allantois: Stores waste products generated by the embryo.

Major Amniote Groups (Late Carboniferous)

  • Anapsids (polyphyletic): No temporal fenestrae.
  • Synapsids: One temporal fenestra; includes mammal-like reptiles.
  • Diapsids: Two temporal fenestrae; dominant group of reptiles.
  • Skull Abbreviations:
    • p = parietal
    • sq = squamosal
    • po = postorbital
    • j = jugal

Basal Amniote Groups

  • Neodiapsida: Includes turtles, dinosaurs (birds), pterosaurs, marine reptiles, crocodilians, snakes, and lizards.
  • Synapsida: Represents ‘mammal-like reptiles’, characterized by synapsid skulls.
  • Parareptilia: Anapsid skulls found in certain primitive reptiles.
  • Eureptilia: true diapsids.
  • Cynodontia: Includes cynodonts and mammals.

Late Permian Environment

  • Timeline: Late Permian period (Approx. 255 Ma).
  • Geological Features:
    • Pangaea: Ancient supercontinent.
    • Panthalassic Ocean and Paleotethys Ocean.
  • Climate:
    • Ice cap at North Pole.
    • Transition from tropical zones to cooler regions.

Dominant Late Permian Synapsids

  • Pelycosaurs:
    • Example: Dimetrodon (carnivore).
    • Group includes various herbivores and some with distinctive 'sail backs'.
  • Gorgonopsids:
    • Dominant carnivorous synapsids marked by wide gapes and large canine teeth.

Permo-Triassic Mass Extinction

  • One of the largest mass extinctions (~95% species loss, ~75% tetrapod families).
  • Potential causes:
    • Volcanic activity and oceanic anoxia.
  • Surviving groups include Cynodontia and Neodiapsida.

Post-Permian Radiation of Neodiapsids

  • Groups include:
    • Lepidosauromorpha: Includes Mesozoic marine reptiles.
    • Archosauromorpha: Includes crocodiles, dinosaurs, birds, and pterosaurs.

Early Triassic Environment

  • Period: Early Triassic (~237 Ma).
  • Pangaea undergoing geological changes:
    • Sea floor spreading and subduction zones defined.
  • Climate:
    • Hottest period with arid interiors in Pangaea.

Triassic Diapsids (Rhynchosaurs and Archosaurs)

  • Rhynchosaurs:
    • Herbivorous with beak-like jaws and claws for digging.
  • Archosaurs:
    • Early Triassic: Primarily carnivores; mid Triassic saw the emergence of herbivorous species.
    • Key evolution includes a shift from sprawling to erect postures.

Unique Triassic Forms

  • Examples of unique Triassic diapsids include:
    • Sharovipteryx
    • Megalancosaurus
    • Longisquama
    • Kuehneosaurus

Late Triassic Extinctions

  • Major extinctions impacting:
    • Parareptilia, most Synapsids (except Cynodontia), and many Diapsids excluding pterosaurs, crocodiles, and dinosaurs.
    • Survivors include Testudines and Lepidosauromorphs (modern lizards and snakes).