Extinct Species Whether or not the asteroid or comet that carved the Chicxulub crater caused the extinction of more than half the planet's species at the end of the Cretaceous remains a matter of scientific debate. New Dinosaurs Though dinosaurs ruled throughout the Cretaceous, the dominant groups shifted and many new types evolved.
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By the middle of the period, ocean levels were much higher; most of the landmass we are familiar with was underwater. By the end of the period, the continents were much closer to modern configuration.
Africa and South America had assumed their distinctive shapes; but India had not yet collided with Asia and Australia was still part of Antarctica. One of the hallmarks of the Cretaceous Period was the development and radiation of the flowering plants.
The oldest angiosperm fossil that has been found to date is Archaefructus liaoningensis , found by Ge Sun and David Dilcher in China. It seems to have been most similar to the modern black pepper plant and is thought to be at least million years old. It used to be thought that the pollinating insects, such as bees and wasps, evolved at about the same time as the angiosperms.
It was frequently cited as an example of co-evolution. New research, however, indicates that insect pollination was probably well established before the first flowers. While the oldest bee fossil was trapped in its amber prison only about 80 million years ago, evidence has been found that bee- or wasp-like insects built hive-like nests in what is now called the Petrified Forest in Arizona.
These nests, found by Stephen Hasiotis and his team from the University of Colorado, are at least million years old. It is now thought that competition for insect attention probably facilitated the relatively rapid success and diversification of the flowering plants.
As diverse flower forms lured insects to pollinate them, insects adapted to differing ways of gathering nectar and moving pollen thus setting up the intricate co-evolutionary systems we are familiar with today. There is limited evidence that dinosaurs ate angiosperms. Two dinosaur coprolites fossilized excrements discovered in Utah contain fragments of angiosperm wood , according to an unpublished study presented at the Society of Vertebrate Paleontology annual meeting.
This finding, as well as others, including an Early Cretaceous ankylosaur that had fossilized angiosperm fruit in its gut, suggests that some paleo-beasts ate flowering plants.
Moreover, the shape of some teeth from Cretaceous animals suggests that the herbivores grazed on leaves and twigs, said Betsy Kruk, a volunteer researcher at the Field Museum of Natural History in Chicago. During the Cretaceous Period, more ancient birds took flight, joining the pterosaurs in the air.
In the seas, mosasaurs suddenly appeared and underwent a spectacular evolutionary radiation. Modern sharks also appeared and giant-penguin-like polycotylid plesiosaurs 3 meters long and huge long-necked elasmosaurs 13 meters long also diversified. These predators fed on the numerous teleost fishes, which in turn evolved into new advanced and modern forms Neoteleostei. Ichthyosaurs and pliosaurs, on the other hand, went extinct during the Cenomanian-Turonian anoxic event. The Cretaceous—Paleogene extinction event was a large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66 million years ago Ma.
It is widely known as the K—T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the Cretaceous—Paleogene boundary K—T boundary. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period a historical term for the period of time now covered by the Paleogene and Neogene periods.
The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. Non-avian dinosaur fossils are only found below the Cretaceous—Paleogene boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the Cretaceous—Paleogene boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers.
Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and evolutionary radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms. Scientists have hypothesized that the Cretaceous—Paleogene extinctions were caused by one or more catastrophic events such as massive asteroid impacts or increased volcanic activity.
Several impact craters and massive volcanic activity in the Deccan traps have been dated to the approximate time of the extinction event.
Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate. Friday, November 12, Sign in. Forgot your password? Get help. Password recovery. Geology Page.
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