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:
- Aglandular skin (lose less water)
- Better water conserving kidneys
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:
- Simple hinge-joint ankle without heel
- Fully upright stance of hindlimbs: inturned head of femur
- Tibia, fibula, and metatarsals all long and slender
- Very long hindlimbs
- Longer, S-shaped neck (cervicals very distinct shape from dorsals)
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:
Dinosauria defined as most recent common ancestor of Iguanodon and
Megalosaurus and all of its descendants
Diagnosis includes:
- Semiopposable manual digit I ("thumb")
- Manual digits IV and V reduced in size
- MAYBE three or more sacrals (but homoplastic, since some primitive forms lack this)
- Semiperforate acetabulum
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