Key Features of the Upper Cretaceous

1. Geological Changes

Plate Tectonics: The breakup of the supercontinent Pangaea continued, leading to more isolated landmasses. The Atlantic Ocean expanded, and the Indian Ocean began to open.

Shallow Seas: High global sea levels created vast inland seas, such as the Western Interior Seaway in North America, rich in marine life and significant for the sedimentary record.

2. Climate

The climate was generally warm and humid with minimal polar ice caps. High atmospheric carbon dioxide levels led to a greenhouse climate. Stable temperatures supported lush vegetation even in high-latitude regions.

3. Flora

The Late Cretaceous saw the rapid expansion and diversification of angiosperms (flowering plants), which began to dominate ecosystems. Gymnosperms like conifers, cycads, and ginkgos persisted but were outcompeted in many areas. Dense forests covered much of the land, creating habitats for evolving terrestrial animals.

4. Fauna

Terrestrial Fauna:

Dinosaurs: Renowned for its incredible diversity, including:

Theropods: Carnivores like Tyrannosaurus rex, Velociraptor, and Giganotosaurus.

Ornithopods: Herbivores like Edmontosaurus and other duck-billed hadrosaurs.

Ceratopsians: Horned dinosaurs like Triceratops and Protoceratops.

Ankylosaurs and Pachycephalosaurs: Armored and dome-headed dinosaurs.

Regional differences in dinosaur populations were evident due to continental separation.

Early Birds: Birds diversified with species like Ichthyornis and Hesperornis.

Mammals: Small mammals continued to evolve, setting the stage for post-Cretaceous diversification.

Marine Fauna:

Marine ecosystems thrived with creatures like:

Mosasaurids: Giant marine reptiles that became apex predators.

Plesiosaurs: Long-necked marine reptiles.

Ammonites and Rudists: Important marine invertebrates until the end of the period.

Sharks and Ray-Finned Fish: Significant diversification.

Insects:

Pollinating insects, including early bees and butterflies, co-evolved with flowering plants.

5. Mass Extinction Event

The Upper Cretaceous ended 66 million years ago with the Cretaceous-Paleogene (K-Pg) extinction event.

Causes:

Asteroid Impact: Evidence includes the Chicxulub Crater in the Yucatán Peninsula and a global iridium layer.

Volcanism: Massive eruptions from the Deccan Traps in India contributed to environmental stress.

Impact:

~75% of species, including all non-avian dinosaurs, marine reptiles like mosasaurs, and ammonites, went extinct. Survivors included small mammals, birds, and some reptiles, setting the stage for the Cenozoic Era.