Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Cretaceous
Terrestrial Life through the Cretaceous
Life in the Cretaceous Seas
Extinction of the Dinosaurs
Continents and Greenhouse Climates
Leaf related to the gooseberry family
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Tyrannosaurus rex
Triceratops prorsus
Edmontosaurus annectens
Bennettitalean trunks
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Foundational Concepts
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Earth Processes
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Terrestrial Life through the Cretaceous
Many new groups of organisms (plants and animals) evolved during the Cretaceous. Near the beginning of the Cretaceous, around 120 million years ago, new groups of dinosaurs originated, including ceratopsians (plant-eating dinosaurs with horns on the head, such as Triceratops), pachycephalosaurs (smaller dome-headed, plant-eating dinosaurs), and hadrosaurs (‘duck-billed’ dinosaurs such as Edmontosaurus). The earliest marsupial, monotreme, and placental mammals appeared during the middle part of the Cretaceous. In addition, a now-extinct group of rodent-like forms, the multituberculates, known from the Jurassic, had become common in Asia and North America. Although many types of mammals were present, they were a relatively inconspicuous part of the fauna until the succeeding Paleocene Epoch. The first birds had appeared in the Late Jurassic, but they began to diversify during the Cretaceous. They still shared the skies with flying reptiles, the pterosaurs. Terrestrial reptiles such as snakes, lizards, and sphenodontians became more common as well.

The terrestrial environments in which Early Cretaceous animals lived were still dominated by ferns, seed ferns, Bennettitaleans, conifers, and cycads, but angiosperms (flowering plants) also became part of the flora at this time, about 135 million years ago. Angiosperms diversified rapidly, and by the end of the Cretaceous were by far the most diverse group of terrestrial plants. Many explanations have been offered for their rapid diversification, but the true explanation is probably quite complex. Some Cretaceous flowering plants may have had their seeds dispersed by animals, but that was also true of other kinds of plants. Today, some species of flowering plants grow rapidly compared with living conifers and cycads, so perhaps the success of their Cretaceous ancestors was related to their rapid growth. Another explanation is that many angiosperms are pollinated by insects; insect pollination is thought to increase the rate at which new species evolve. However, several other groups of Cretaceous plants were also insect-pollinated. Whatever the reasons for the success of the angiosperms, many new groups of insects evolved during the Cretaceous, including the oldest known ants and bees as well as newly evolved groups of pollinating species such as flies, beetles, wasps, and moths. Some paleontologists think that the coincident evolution of these insect groups and the diversification of flowering plants is an example of the process of coevolution, in which two different types of organisms (such as an insect and plant) become specifically adapted to one another. Insects also evolved more types of feeding behavior both in quiet-water habitats such as lakes and in flowing-water habitats.

Overview | Terrestrial Life through the Cretaceous | Life in the Cretaceous Seas
Extinction of the Dinosaurs | Shifting Continents and Greenhouse Climates

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