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Deep Time |
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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Mesozoic Era: (248-65 mya)
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Triassic | Jurassic | Cretaceous
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Cretaceous Period (144-65 mya)
As Pangaea continues to break apart, the present-day continents
begin to take shape. Over time, flora and fauna acquire different characteristics
across geographic areas now separated from one another. The first flowering plants and
pollinating insects appear, and the evolution of these two groups is intimately connected.
Modern insect groups that debut in the Cretaceous include bees, ants, and butterflies.
Dinosaurs dominate the landscape, adding, among others, club-tailed
ankylosaurs and horned ceratopsians such as Triceratops to the herbivores, and
tyrannosaurs to the carnivores. As birds diversify, several pterosaur lineages end. It
is not known whether competition between the two groups drives their demise.
Frogs and salamanders continue to diversify, and turtles and
tortoises flourish. New mammal groups include the placentals,
marsupials, and monotremes.
In the oceans, as ammonites, belemnites, and other
mollusks thrive, brachiopods continue to decline. Marine diatoms
evolve, and crustaceans (such as lobsters and crabs), corals, and
snails take on modern appearances.
One of the most cataclysmic mass extinction events closes out the
Cretaceous. Most plankton, many tropical invertebrates and land plants, and perhaps
one-half of all animal families go extinct. The disappearance of the incredibly
well-adapted dinosaurs (except birds) means that other creatures, like mammals,
have new opportunities to evolve and flourish.
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130 mya: Flowering plants
Angiosperms, which include hardwood trees and grasses, are
distinguished from other plants by the flowers many produce. While some are wind
pollinated, most use color, scent, or both to attract insects (and later, birds
and bats) that transfer pollen to female pollen collectors. Nectar may have
evolved as a reward for performing this function. Animals deliver pollen more
efficiently than wind, so plants that attract them improve their chances of
reproducing. Angiosperms are not the only plants to evolve animal pollination,
however, as some cycads are pollinated by insects such as beetles. Angiosperms
become the dominant forms of plant life on land.
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End Cretaceous extinction
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Date:
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65 mya
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Intensity:
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1
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Affected:
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About 60-80 percent of all
species, including dinosaurs, marine reptiles, and flying reptiles go extinct
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Hypotheses:
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Meteor impact, volcanic
activity, sea-level change
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Summary:
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Following the devastating
Permian extinction, biodiversity rebounds and reaches higher levels in the
late Jurassic and Cretaceous than ever before. At 65 mya, however, as many
as 80 percent of all species are wiped out in the second-worst extinction
event in the history of life. Extensive evidence from sites around the world
suggests a meteor strike near present-day Mexico triggers the killing; its
impact drives a series of catastrophic changes in climate and sea level.
Major volcanic activity might also have disrupted atmospheric conditions and
contributed to the extinction. Fish and plankton sustain a major hit but
survive; up to 35 percent of land plants don't. While the dinosaurs are the
poster children of this extinction, pterosaurs, marine reptiles, and several
mollusk groups also perish. Groups that suffer heavy losses but survive include
mammals, amphibians, crocodiles, turtles, and insects on or near land,
and diatoms, bottom-dwelling foraminifera, and unicellular
dinoflagellates in marine waters.
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Read more
Opportunity knocks (65 mya)
Time and again through the history of life, extinction offers
surviving groups a chance to populate vacated ecological niches. These beneficiaries
of extinction are opportunists -- simply in the right place at the right time and
in no significant way responsible for their good fortune. Dinosaurs and mammals are
among evolution's great opportunists.
Dinosaurs succeeded the mammal-like reptiles as the dominant
land vertebrates in the Triassic, a relatively short time after the Permian
extinction. Though the mammal-like reptiles maintained sizable populations for
several million years after the extinction, their diversity had been significantly
diminished by the event. The minor extinction in the middle of the Triassic that
sealed their fate provided a window of opportunity for the dinosaurs.
As the dinosaurs radiated across the planet, the much-smaller
mammals led a solitary, stealthy existence, probably adopting a nocturnal lifestyle.
No one knows for sure just why, but mammals did not suffer the extinction dinosaurs
did at the end of the Cretaceous. Once the dinosaurs were gone, however, mammals,
like the dinosaurs before them, adapted to diverse habitats and filled nearly every
corner of the globe.
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-> Learn more about the Cenozoic Era
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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