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.

The central Atlantic Ocean continues to widen during the Cretaceous, dividing the world up into the northern supercontinent of Laurasia (comprised of North America, Europe, and Asia except for India) and the southern supercontinent of Gondwana (comprised of South America, Africa, Antarctica, India, Madagascar, and Australia). Because of the physical separation, the dinosaur communities of the Cretaceous diverge from each other. Laurasia itself was divided up by shallow seaways.

Another big change in the early part of the Early Cretaceous: the rise of angiosperms:

• The clade of flowering and fruiting plants
• Flowers developed to attract insects for pollenation
• Fruit developed to attract herbivorous vertebrates (including dinosaurs!) to eat them and deposit the seeds elsewhere
• Not all flowering plants have big showy flowers (grass is an angiosperm, for example)
• Angiosperms began as as weedy plants: diverse but not very common
• Some of the best early angiosperm fossils are from Prince George's County, MD!!

During the early Early Cretaceous, in Europe and North America (and to a degree, some most of the rest of the world), the carnosaur-sauropod-stegosaur dinosaur community is replaced by a carnosaur-iguanodontian-ankylosaur community:

• Carnosaurs, and to a lesser degree "megalosaurs", are the dominant predators, but coelurosaurs present as smaller predators
• Iguanodontian ornithopods and ankylosaurs are the most common herbivores, but sauropods and hypsilophodontians are still present in reasonable numbers, and stegosaurs rare (and die out before the end of the Early Cretaceous)
• Best example of this community is the Wealden Group of England and continental Europe:
  • Predators include a carnosaur (Neovenator), a spinosaurid (Baryonyx), and various coelurosaurs
  • Herbivores include the iguanodotian Iguanodon, the hypsilophodontian Hypsilophodon, the nodosaurid ankylosaur Hylaeosaurus, the ankylosaurid ankylosaur Polacanthus and various sauropods and stegosaurus.
  • Environment was forests and coastal swamps
• Western North American equivalent was the Lakota Formation and related units:
  • Very similar fauna to the Wealden
  • Predators include some carnosaur material and the giant dromaeosaurid Utahraptor
  • Hebivores include Iguanodon, Hypsilophodon, Gastonia (the sister taxon of Polacanthus), and various sauropods.

In eastern Asia during the Early Cretaceous, early tyrannosaurids, therizinosauroids, dromaeosaurids, oviraptorosaurs, ceratopsians (especially Psittacosaurus), and ankylosaurine ankylosaurids are present. This represents the early precursor of the coelurosaur-hadrosaurid- marginocephalian-ankylosaurian community (more about it below).

Not too much is known for Gondwana during the early Early Cretaceous: some carnosaurs, possible neoceratosaurs, diplodocoids, and titanosaurs.

By the later part of the Early Cretaceous, North American and European dinosaurs began to diverge. In western North America, the Cloverly Formation and its equivalent:

• Top predator is the carnosaur Acrocanthosaurus, but most famous is the dromaeosaurid Deinonychus
• Various sauropods, including the brachiosaurid Sauroposeidon (one of the largest of all North American dinosaurs)
• Ornithischians include hypsilophodontians, the iguanodontian Tenontosaurus, and the nodosaurid Sauropelta

Although known only from fragments, the Arundel Formation of Prince Georges County, MD, is the eastern equivalent to the Cloverly. Some possible Deinonychus teeth found.

In Gondwana (especially Argentina and northern Africa during the late Early Cretaceous and early Late Cretaceous, the spinosaur-carnosaur-titanosaur community:

• Top predators include spinosaurids and giant carnosaurs like Giganotosaurus
• Most common herbivores are titanosaurian sauropods, but (at least in some cases) large iguanodontians are also present
• Includes both the largest predators and the largest herbivores ever to live on land
• Represented in both coastal and inland environments

The Late Cretaceous saw some big changes in dinosaur communities. Asian dinosaurs migrate into North America, replacing many of the local forms. Because of this, dinosaurs (and other animals and plants) of Asia and western North America (together called Asiamerica) have a typical coelurosaur-hadrosaurid-marginocephalian- ankylosaur community:

• Top predators are tyrannosaurids; other theropods are also all coelurosaurs (dromaeosaurids, ornithomimosaurs, therizinosauroids, oviraptorosaurs, troodontids)
• Iguanodontians represented by hadrosaurids; hypsilophodontians present in North America but not Asia
• Marginocephalia represented by pachycephalosaurs and neoceratopsians; in America only, the Ceratopsidae are present
• Stegosaurs are extinct; ankylosaurids common throughout Asiamerica, and nodosaurids in North America
• An extremely diverse community
• In general, dinosaurs are more sophisticated (more intelligent, more highly specialized) than previous communities
• Many environments represented: deserts, uplands, forests, lakes, rivers, swamps, etc.

Represented in western North America by the Moreno Hills Formation in the early Late Cretaceous and the Montana Group in the late Late Cretaceous.

In eastern North America, more primitive dinosaurs survive. Eastern and western North America separated by the Western Interior Seaway.

In South America, India, and Madagascar (and possibly Africa, but no good fossils from there yet), an abelisaur-titanosaur community replaces the spinosaur-carnosaur-titanosaur community:

• Top predators are abelisaurid neoceratosaurs; carnosaurs and spinosaurids are extinct
• Main herbivores are titanosaurs

In Europe (an archipelago at the time), a mixture of primitive forms and the abelisaur- titanosaur community

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