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 look at the history of dinosaurs with a focus on western North America, but some additional information from the rest of the world.

Dinosaurs are present by the late Middle Triassic or earliest Late Triassic.

Oldest known dinosaur community: the Ischigualasto Formation of Argentina:

• Dinosaurs only a small part of the fauna: assemblage is dominated by pseudosuchians and therapsids
• Ischigualasto dinosaurs include:
  • Eoraptor, a possible basal theropod or basal saurischian
  • Herrerasaurus, a herrerasaurid
  • Pisanosaurus, the basalmost ornithischian
  • and Saturnalia, the basalmost sauropodomorph, is known from nearly the same age in Brazil

The Ischigualasto is similar to many basal dinosaur communities: herrerasaurid predators (as secondary to pseudosuchians), and small basal ornithischians and small prosauropods.

This communities are replaced by the typical coelophysoid-prosauropod communities:

• First appear in Late Triassic, communities persist throughout Early Jurassic
• Coelophysoid ceratosaurs are the dominant dinosaurian predators (and in Early Jurassic, the top predators of all)
• Prosauropods, including giant ones, are the most common herbivores
• Also present: basal ornithischians, heterodontosaurid ornithopods, and basal thyreophorans (like Scutellosaurus and Scelidosaurus
• Dinosaurs of the coelophysoid-prosauropod communities are very similar all over the world
  • During the Late Permian and Early Triassic, the continents had assembled into a single supercontinent called Pangaea: so dinosaurs, other animals, plants, etc. could travel from one end of the world to the other

The Chinle Formation and Kayenta Formation show us two different parts of the history of the coelophysoid-prosauropod community.

The Chinle Formation:

• Late Triassic of the American Southwest
• Formed in a forested enviroment; the permineralized trees from this unit form The Petrified Forest, and the soils preserved as The Painted Desert of Arizona
• Included the coelophysoid Coelophysis as the top dinosaur predator, but pseudosuchians still ruled
• Some prosauropods and basal ornithischians, but therapsids and pseudosuchian herbivores are also present

At the Triassic-Jurassic Boundary (c. 200 Ma), Pangaea began to break apart. Rift formed between what is now eastern North America plus Europe and western Africa and South Atlantic. The central Atlantic Ocean was being formed. The rift that opened up contains lots of volcanic rocks, lake sediments, and poorly sorted sandstones that run up and down the eastern part of North America: the Newark Supergroup.

There are some dinosaur bones from the Newark Supergroup, but mostly lots of footprints.

Gigantic widespread lava flows called flood basalts were produced by the break-up of Pangaea and the birth of the Atlantic. Climatic effects of these volcanoes may be the cause of the Triassic-Jurassic extinction:

• Victims include:
  • All pseudosuchians except for crocodylomorphs
  • All therapsids except for mammals and some very close relatives
  • Many other amniote groups
  • Herrerasaurids
• Survivors included pterosaurs, mammals, and (most impotantly) most members of the coelophysoid-prosauropod community.

The Kayenta Formation represents a post-extinction expression of the coelophysoid- prosauropod community:

• Early Jurassic of the American Southwest (stratigraphically above the Chinle)
• Somewhat drier environment: eventually replaced with desert environment
• Largest predator is Dilophosaurus, a coelophysoid
• Also present is the smaller coelophysoid Syntarsus
• Scutellosaurus, the basal thyreophoran, is present
• Prosauropods and possible heterodontosaurids are also present
• Dinosaurs make up almost all the large animals in the Kayenta

At the end of the Early Jurassic:

• Coelophysoids and heterodontosaurids become rare
• Basal thyreophorans and prosauropods die out and are replaced by more advanced thyreophorans and sauropods, respectively

Dinosaurs of the Middle Jurassic are not known in North America, other than footprints. In Europe, South America, and China we see the rise of the carnosaur-sauropod-stegosaur community:

• Main predators are "megalosaurs" and true carnosaurs
• Main herbivores include diverse sauropods and stegosaurus
• Coelurosaurs, ankylosaurs, ceratopsians (in Asia only), and hypsilophodontian ornithopodsalso appear at this time, but rare

• Found over most of the American West, from Montana to Arizona, and from Idaho to Oklahoma
• Included a mixture of forest, parklands, fernlands (no grasslands before Cenozoic!), rivers, lakes, and swamps
• Main predator was the carnosaur Allosaurus, but also coelophysoids (Elaphrosaurus), neoceratosaurs (Ceratosaurus), "megalosaurs" (Torvosaurus), and coelurosaurs (Ornitholestes)
• Diverse sauropods, including diplodocids (Diplodocus, Apatosaurus, Barosaurus), the basal macronarian Camarasaurus, and the brachiosaurid Brachiosaurus
• Best known thyreophoran is Stegosaurus, but early ankylosaurs are also present
• Most common ornithopod is the primitive iguanodontian Camptosaurus, but hypsilophodontians are also present
• The Morrison Formation has produced thousands of fossils, many of them articulated
• Our best glimpse into Late Jurassic life

• Many of the same dinosaurs as the Morrison: Allosaurus (maybe), Ceratosaurus, Elaphrosaurus, Barosaurus, Brachiosaurus, the iguanodontian Dryosaurus, etc.
• Additionally, the stegosaur Kentrosaurus, the diplodocoid Dicraeosaurus, and possible primitive titanosaurs

So Late Jurassic dinosaurs were still relatively global. This would change in the Cretaceous.

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