Dinosaurs, pterosaurs, mosasaurs, and ammonoids, to name a few, were among the groups lost at this time. The Cretaceous extinction event is marked by the famous K-T boundary and asteroid impact on what is now the Yucatan peninsula. Many believe this impact caused dinosaur and other extinctions. Other possible causes, including extensive volcanic eruptions the Deccan Traps in India occur at this time.
Lobsters Decapoda fossils, for example Hoploparia browni , are relatively rare in Cretaceous deposits. Hoploparia browni. Crabs Decapoda , such as Avitelmessus grapsoideus , which had appeared in the Jurassic, became more abundant. Avitelmessus grapsoideus. Epiaster whitei. Micraster sp. The sharks, one of the two major modern fish families, are represented here by a group of fossil sharks teeth.
Ray-finned fish Actinopterigii , The teleost fish teleosti , for example Rhacholepis buccalis , which dominate modern groups, appeared first in the Jurassic. In the Cretaceous they first outnumber the earlier fish types. Rhacholepis buccalis. A museum-quality model of a Tylosaurus, a huge predatory marine lizard, chasing a large ammonite, sits on top of the case. Tylosaurus was one of the largest of the mosasaurs , reaching lengths of 50 ft or more.
It was a dominant predator of the Western Interior Seaway of the U. Tylosaurus model ; fossil of a complete 29 ft specimen AMNH, The Ornithischia bird-hipped plant eaters are represented by a Hadrosaurus tooth plate and Hadrosaurus teeth.
Hadrosaurus tooth plate life reconstruction. Hadrosaurus teeth. The Theropods predatory bipedal dinosaurs, such as Tyrannosaurus and Birds are represented by a tooth from an Albertosaurus sp. Albertosaurus sp. Bivalves were very common and important in this Period.
One group, the rudist bivalves have an unusual form in which one valve is cone shaped, much like rugose corals of the Paleocene. During the Cretaceous they built huge reefs, cementing themselves together as they grew upward. The rudists became extinct at the end of the Mesozoic. Other bivilves also flourished including the oysters, such as Actinostrea travisana , and extinct oyster relatives. Extinct oysters sometimes had unusual forms with one valve snail-shaped pyritized fossil , or twisted into a spiral.
In each case a second valve covered the opening as seen partial in this second spiral example. Cretaceous: Jurassic: Triassic: Permian: Pennsylvanian: Mississippian: Devonian: Silurian: Ordovician: Cambrian: to Explore This Park. Article Cretaceous Period— Significant Cretaceous events During the Cretaceous Period the first flowering plants appeared and rapidly diversified. Angiosperms flowering plants appeared in the fossil record more than million years ago during the Cretaceous Period.
Once they appeared, they quickly became the dominant type of plant life on land and remain so today. The earliest angiosperms evolved from a specialized group of seed ferns. Since their first appearance, angiosperms have adapted to nearly every terrestrial habitat from mountains to deserts, and some have adapted to shallow coastal waters.
Not all flowering plants produce the kinds of blossoms you find in florist shops, but along with enclosed seeds, flowers are a key reproductive feature of angiosperms. The evolution of flowers, which attract pollinators, especially insects, and the evolution of a modified seed, with a protective coating and a ready supply of nourishment, enabled flowering plants to populate the world. Rise of the Rocky Mountains. Near the end of the Cretaceous Period, western North America was once again rising from the sea.
During a time of mountain-building known as the Laramide Orogeny, a long series of repeated uplifts, periods of volcanism, and episodes of erosion occurred. The Laramide Orogeny continued into the Cenozoic Era and was a time of active tectonics and block-fault mountain building. The deformation and uplift of the Laramide Orogeny affected an area that had already been the site of large block uplift that formed the Ancestral Rocky Mountains during the Pennsylvanian Period.
Preexisting Precambrian faults and shear zones largely controlled Laramide events. In addition to the mountain uplifts, substantial Laramide deformation occurred in the bordering basins. These basins subsided concurrently with the uplift of the adjoining mountains. About 66 million years ago, nearly all large vertebrates and many tropical invertebrates became extinct in what was clearly a geological, climatic and biological event with worldwide consequences.
Geologists call it the K-Pg extinction event because it marks the boundary between the Cretaceous and Paleogene periods. The event was formally known as the Cretaceous-Tertiary K-T event, but the International Commission on Stratigraphy, which sets standards and boundaries for the geologic time scale, now discourages the use of the term Tertiary. The "K" is from the German word for Cretaceous, Kreide. In , a geologist who was studying rock layers between the Cretaceous and Paleogene periods spotted a thin layer of grey clay separating the two eras.
Other scientists found this grey layer all over the world, and tests showed that it contained high concentrations of iridium, an element that is rare on Earth, but common in most meteorites, Kruk said in a class she co-taught on Coursera. The crater site is more than miles kilometers in diameter and chemical analysis shows that the sedimentary rock of the area was melted and mixed together by temperatures consistent with the blast impact of an asteroid about 6 miles 10 km across striking the Earth at this point.
When the asteroid collided with Earth, its impact triggered shockwaves, massive tsunamis and sent a large cloud of hot rock and dust into the atmosphere, Kruk said. As the super-heated debris fell back to Earth, they started forest fires and increased temperatures.
Tiny fragments likely stayed in the atmosphere, possibly blocking part of the sun's ray for months or years. With less sunlight, plants and the animals dependent on them would have died, Kruk said. Furthermore, the reduced sunlight would have lowered global temperatures, impairing large active animals with high-energy needs, she said.
There is also evidence that a series of huge volcanic eruptions at the Deccan traps , located along the tectonic border between India and Asia, began just before the K-Pg event boundary.
It is likely that these regional catastrophes combined to precipitate a mass extinction. The world was a warmer place during the Cretaceous period. The poles were cooler than the lower latitudes, but "overall things were warmer," Kruk told Live Science. Fossils of tropical plants and ferns support this idea, she said. Animals lived all over, even in colder areas.
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