Western North America has just about the best record of dinosaurs from the Late Triassic to the end of the Cretaceous. Only large gap is the Middle Jurassic (when the West was Flooded). China, Argentina, and Europe also have very good records. We will follow dinosaur history, with a western North American focus: see table.
In early dinosaur history, all the continents joined together into single landmass called Pangaea ("all Earth"), so that populations of animals and plants were similar all over the world.
Oldest dinosaurs known from earliest Late Triassic. Include herrerasaur predators, and basal sauropodomorph and (very rare) basal ornithischian herbivores. Best example: Ischigualasto Formation, earliest Late Triassic, Argentina.
During Late Triassic, dinosaurs still shared the Earth with many diverse pseudosuchians and therapsids. Were not dominant predators nor most common large herbivores.
Later in Late Triassic into Early Jurassic, dinosaur radiation produces faunas with coelophysoids as top predators and prosauropods and various ornithischian groups (primitive thyreophorans, heterodontosaurids, etc.) as main herbivores. Good Late Triassic example is Chinle Formation, Late Triassic, American Southwest. Famous for Petrified Forest. Early sauropods in the southern continents during latest Triassic.
The Triassic-Jurassic Extinction: almost all non-crocodyliform pseudosuchians and non-mammaliform therapsids die out. Dinosaurs left as dominant group: fully into the 4th Amniote Radiation. Extinction may be result of major geologic changes (see below), but also some preliminary evidence of asteroid impact.
At Triassic-Jurassic boundary, Pangaea begins to pull apart along current Atlantic coastlines of what is now eastern North America and northern Africa. Rifting produces series of rocks called (in eastern North America) the Newark Supergroup: record presence of the coelophysoid-prosauropod fauna, though more from footprints than from bones. The rifting produced huge volcanic eruptions.
In American Southwest at this time: Kayenta Formation: records coelophysoid-prosauropod assemblage after the competition was gone.
By Middle Jurassic: coelophysoids become less important, and are replaced by megalosaurids and carnosaurs as top predators. Prosauropods die out and become replaced by sauropods. Stegosaurs become a major group of herbivores. No good North American examples of this, but changes recorded in Europe, China, and South America.
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