GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
The Origin of Dinosauria

During the Triassic, the last major continental masses finally collided together to form Pangaea, the One World Continent. Drier interior conditions.

Reptiles have several superior derived features (relative to synapsids) with regards to dealing with dry environments:

These may have been factors in the gradual change between the 2nd and 3rd Amniote Radiations, during the Middle Triassic Epoch.

3rd Amniote Radiation Pseudosuchians:

  • Middle to Late Triassic Epochs
  • Pseudosuchian archosaurs dominate, but some therapsids groups still around (small-to-mid sized carnivores, small-to-mid sized omnivores, small insectivores)
  • Most pseudosuchians quadrupeds, but some facultative bipeds

    Pseudosuchian diversity included a number of different clades:

  • Parasuchians: "crocodile-mimics", had similar ecology to modern crocodiles
  • Rauisuchians: largest predators of Triassic; similar appearance to later carnivorous dinosaurs (may be paraphyletic)
  • Aetosaurs: armored plant-eaters; similar in ecology to later armored dinosaurs
  • Crocodylomorphs: true crocodilians and their ancestors (true crocs come much later; early crocodylomorphs were all very small running terrestrial animals)

    Reptile diversity among non-pseudosuchians during Middle and (especially) Late Triassic is VERY high:

  • First turtles
  • First lepidosaurs (lizards and sphenodontians)
  • Tree climbing chameleon-mimics
  • Gliding reptiles
  • Etc.

    Among the new forms of the Middle Triassic: earliest remains of Dinosauriformes:

  • Dinosauriforms comprise dinosaurs, and their primitive relatives
  • Characterized by:
  • Very small to begin with: probably fast running predators avoiding larger predators

    Adaptations that allowed faster, longer sustained running (vertical hindlimb, stronger pelvis) would later allow dinosaurs to achieve giant size.

    By Late Triassic (and maybe during the Middle Triassic), true dinosaurs finally appear:

    Diagnosis includes:

    Most primitive dinosaurs are small (~1 m long, half of which is tail) and bipedal, with grasping hands.

    When dinosaurs first appear, were NOT the dominant forms on land:

  • Rauisuchians still top predators, with therapsids making up a number of the small-to-mid sized forms
  • Aetosaurs and some therapsids common mid-sized herbivores

    By end of Late Triassic, dinosaurs would become larger than any animal which had lived on land before.

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    Last modified: 14 July 2006