Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
  High-Bandwidth Version
Search Evolution  
 
Click to return to the Evolution Home Page
darwin change extinction survival sex humans religion
Darwin    
   
Deep Time

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Mesozoic Era: (248-65 mya)

Triassic | Jurassic | Cretaceous

border

border

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.

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.

End Cretaceous extinction

Date:

65 mya

Intensity:

1

Affected:

About 60-80 percent of all species, including dinosaurs, marine reptiles, and flying reptiles go extinct

Hypotheses:

Meteor impact, volcanic activity, sea-level change

Summary:

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.



border

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.

border

-> Learn more about the Cenozoic Era

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

  related web activities  
   
Origins of Humankind
See the humanlike species that came before us.
 
 
A Modern Mass Extinction?
Are we in the midst of one? And if so, did we trigger it?
 
 
Life's Grand Design
Are nature's complex forms evidence of "intelligent design?"
 
   
  related topics  
   
  Deep Time/History of Life  
   
  Evolution of Diversity  
   
  Evidence for Evolution  
 
 
       
         
 
Videos Web Activities Site Guide About the Project FAQ Glossary Site Map Feedback Help Shop