Cretaceous: Terrestrial Life through the Cretaceous

Leaf related to the gooseberry family
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Tyrannosaurus rex
Triceratops prorsus
Edmontosaurus annectens
Mosasaurs
Plesiosaurs
Pterosaurs
Ferns
Cycads
Bennettitalean trunks
Coccolithophores
Ammonites
Belemnites
Inoceramids
Rudistids
Diatoms
Foraminifera

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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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Ceratopsia

A suborder of dinosaurs that includes the "horned" dinosaurs. Ceratopsians are a Cretaceous group.

Cretaceous Period

The third and final period of the Mesozoic Era; it lasted from 146 to 65.5 million years ago.

Pachycephalosauria

A group of "dome-headed" dinosaurs. They are represented by the smallest number of specimens among all the major groups of dinosaurs.

hadrosaur

Any of a lineage of herbivorous, "duck-billed" ornithischian dinosaurs common in the Late Cretaceous. Hadrosaurs were one of the few groups of dinosaurs to evolve dentitions that could shred food in a manner like that of herbivorous mammals.

monotreme

a member of the mammalian subclass Monotremata. Monotremes differ from other mammals in laying eggs and having a single opening for urine, feces, and reproduction.

placental mammal

Any member of the mammalian group Eutheria. Placental mammals bear live young that are nourished in the mother's uterus before birth by a placenta. The placenta was derived during evolution from the same membranes that surround the embryos in the eggs of other amniotes. The only living non-eutherian mammals are monotremes and marsupials. The earliest fossil placental mammals are from the Cretaceous.

multituberculate

Any member of the extinct mammalian group Multituberculata. Multituberculates were the most diverse and common mammals of the Mesozoic. Most species were omnivorous, and they ranged from mouse to woodchuck in size.

Paleocene

The first of the seven epochs of the Cenozoic Era; it lasted from 65.5 to 55.8 million years ago.

sphenodontian

Any of a group of archosaurian reptiles called Sphenodontia. Sphenodontians peaked in diversity during the Triassic, and only the tuatara (Sphenodon), survives to the present. Tuataras are restricted to islands off the coast of New Zealand.

bennettitalean

Any member of a group of Mesozoic seed plants, the Bennettitales, with somewhat flower-like reproductive structures, and typically with stout trunks and cycad-like foliage. Bennettitaleans went extinct in the Cretaceous and may have been closely related to angiosperms (flowering plants).

conifer

Any member of the conifer lineage of seed plants that have cone-like reproductive structures, and typically have needle or scale-like leaves. Pines and redwoods are examples of living conifers. Confiers were common trees in the late Paleozoic and Mesozoic and remain abundant in some environments today, although they are much less diverse than the flowering plants.

cycad

Any member of a group of seed plants with somewhat cone- or leaf-like reproductive organs and leaves that superficially resemble those of some palms. Cycads were moderately common in the late Paleozoic and Mesozoic, and a small number of types survive to the present.

angiosperm

Any member of the highly diverse lineage of plants (~200,000 living species) that have their seeds enclosed in a carpel. The "pod" of a bean is a carpel, although carpels show a wide array of forms in different species.

diversification

evolutionary increase in the number of species within a lineage of organisms. Diversification can be geologically rapid or slow.

paleontologist

A scientist who studies ancient life.

pollination

The transfer of pollen from male to female reproductive organs in seed plants.

evolution

Descent with modification. A lineage of organisms evolves when its genetic makeup changes, as the result of either natural selection for particular genes or random accumulation of mutations. Evolution is also sometimes used in a more general sense to indicate change over time - either within lineages or in ecological communities or physical environments.

marsupials

Any member of the mammalian group Metatheria. Common name for the members of the mammalian subclass Marsupialia, which includes kangaroos and koalas, among others. Most marsupials have a pouch in which the young complete early development. Most marsupials lack a true placenta, instead giving birth to an underdeveloped fetus that grows outside the mother's womb in a protective pouch. Marsupial fossils are known from the Cretaceous and Cenozoic. They were common on many continents during the Cretaceous, but today dominate only in Australia.