144-65 Ma

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§ Cretaceous Period

Extinction of the dinosaurs and many other Mesozoic organisms. Flowering plants appear. Many of today’s mammalian orders originated from 100 million to 85 million years ago.

§Dating

As with other older geologic periods, the rock beds that define the Cretaceous are well identified but the exact dates of the period's start and end are uncertain by a few million years. No great extinction or burst of diversity separated the Cretaceous from the Jurassic. However, the end of the period is most sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 65.5 Ma. This bolide collision is probably responsible for the major, extensively-studied Cretaceous-Tertiary extinction event.

§Divisions

The Cretaceous is usually separated into Early and Late Cretaceous Epochs. The faunal stages from youngest to oldest are listed below; time is referred to as early or late, and the corresponding rocks are referred to as lower or upper:

§Paleogeography

During the Cretaceous, the late Paleozoic - early Mesozoic supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin orogenies that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies.

Though Gondwana was still intact in the beginning of the Cretaceous, Gondwana itself broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, and then started to recede, leaving thick marine deposits sandwiched between coal beds.

The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic. Mid-ocean ridge activity--or rather, the circulation of seawater through the enlarged ridges--enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nannoplankton. These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe and China. In the area that is now India, massive lava beds called the Deccan Traps were laid down in the very late Cretaceous and early Paleocene.

§Climate

The climate was very warm during the Cretaceous; there was no ice at the poles. Sea level was much higher than today, and large areas of the continental crust were covered with shallow seas; sediment cores show that tropical sea surface temperatures may have been 9-12°C warmer than at present, while deep ocean temperatures were as much as 15-20° C higher than today's. The Tethys Sea connected the tropical oceans east to west, which also helped equalize the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.

The planet may not have been much warmer on average than it had been during the Triassic or Jurassic periods, but it had a gentler temperature gradient from the equator to the poles; a side effect of this may have been weaker global winds, contributing to less upwelling and more stagnant oceans than today's, evidenced by widespread black shale deposition.

§Plants

Flowering plants, also known as angiosperms, spread during this period, although they did not become predominant until near the end of the period (Campanian age). Their evolution was aided by the appearance of bees; in fact angiosperms and insects are a good example of coevolution. The first representatives of many modern trees, including figs, planes and magnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoic gymnosperms, like Conifers continued to thrive, although other taxa like Bennettitales died out before the end of the period.

The orchid evolved around this time.

§Living Things

§Terrestrial

On land, mammals were a small and still relatively minor component of the fauna. The fauna was dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they faced growing competition from the adaptive radiation of birds, and by the end of the period only two highly specialised families remained.

The Liaoning lagerstätte (Chaomidianzi formation) in China provides a glimpse of life in the Early Cretaceous, where preserved remains of numerous types of small dinosaurs, birds, and mammals have been found. The coelurosaur dinosaurs found there represent types of the group maniraptora, which is transitional between dinosaurs and birds, and are notable for the presence of hair-like feathers.

During the Cretaceous, insects began to diversify, and the oldest known ants, termites and some lepidopterans appeared. Aphids, grasshoppers, and gall wasps appeared. Numerous exceptionally preserved insects have been found in the Lower Cretaceous Siberian lagerstätte of Baissa.

Dinosaurs that existed in the Cretaceous include:

• Abelisaurus - a genus of abelisaurid theropod dinosaur from the Late Cretaceous Period (Maastrichtian) of what is now South America. It was a bipedal carnivore that probably reached 7 to 9 meters (25 to 30 feet) in length, although it is known from only one partial skull.
• Acanthopholis - a genus of ankylosaurid dinosaur in the family Nodosauridae that lived during the Early Cretaceous Period (Albian to Cenomanian stages) around 100 million years ago. The dinosaur's name refers to its armour (Greek akantha meaning 'spine' or 'thorn' and pholis meaning 'scale'). Acanthopholis's armour consisted of oval plates set almost horizontally into the skin, with spikes protruding from the neck and shoulder area, along the spine. Acanthopholis was quadrupedal and herbivorous. Its size has been estimated to be in the range of 3 to 5.5 meters (10 to 18 ft) long and approximately 380 kilograms (840 lb) in weight.
• Achelousaurus - a genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous Period of what is now North America. It was a quadrupedal herbivore with a parrot-like beak, a rough boss (raised bony area) on the snout and two more behind the eyes, and two horns on the end of its long bony neck frill. With a total body length of 6 meters (20 feet), Achelousaurus was a medium-sized ceratopsian.
• Achillesaurus -- a genus of alvarezsaurid theropod dinosaur from the Santonian-age Upper Cretaceous Bajo de la Carpa Formation of Rio Negro, Argentina. It was a relatively large, basal alvarezsaurid, and a contemporary of Alvarezsaurus. Achillesaurus is based on MACN-PV-RN 1116, a partial skeleton including a sacral vertebra, four tail vertebrae, part of the left thighbone, shin and foot, and the left ilium.
• Achillobator (pronounced /əˌkɪlɵˈbeɪtɔr/; "Achilles' warrior/hero") is a genus of dromaeosaurid theropod dinosaur from the late Cretaceous Period of what is now Mongolia, about 90 million years ago. It was probably an active bipedal predator, hunting with the large sickle-shaped claw on the second toe of each hind foot. It was a large dromaeosaurid, between 15 and 20 feet (4.6 to 6 meters) from snout to tail.
• Acrocanthosaurus - a genus of theropod dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. Its fossil remains are found mainly in the U.S. states of Oklahoma and Texas, although teeth attributed to Acrocanthosaurus have been found as far east as Maryland.
• Adamantisaurus - a genus of titanosaurian sauropod dinosaur from the Late Cretaceous Period of what is now South America. It is only known from six tail vertebrae but, as a sauropod, it can be assumed that this dinosaur was a very large animal with a long neck and tail. It may have been armored but, until more material is discovered, its exact appearance will remain a mystery.
• Adasaurus - a genus of dromaeosaurid theropod dinosaur from the Late Cretaceous Period of what is now Central Asia. It was a small bipedal carnivore with a sickle-shaped claw on the second toe of each hind foot. An adult was probably about 8 ft (2 m) from the snout to the end of the tail.
• Aegyptosaurus - a genus of dinosaur believed to have lived in what is now Africa, around 95 million years ago, during the mid- and late-Cretaceous Period (Albian to Cenomanian stages). This quadrupedal sauropod was a herbivore and its fossils have been found in Egypt, Niger and in several different locations in the Sahara Desert. All known examples were discovered before 1939. The fossils were stored together in Munich, but were obliterated when an Allied bombing raid destroyed the museum where they were kept in 1944, during World War II.

§Marine

In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs in the early and middle of the Cretaceous, plesiosaurs throughout the entire period, and mosasaurs in the Late Cretaceous.

Baculites, a genus of straight-shelled form of ammonite, flourished in the seas. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera and echinoderms such as sea urchins and starfish (sea stars) thrived. The first radiation of the diatoms (generally siliceous, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene.

§Possible Extinction Event in the Cenomanian

Undersea volcanic activity triggered a mass extinction of marine life and buried a thick mat of organic matter on the sea floor about 93 million years ago, which became a major source of oil, according to a new study.

"It certainly caused an extinction of several species in the marine environment," said University of Alberta Earth and Atmospheric Science researcher Steven Turgeon. "It wasn't as big as what killed off the dinosaurs, but it was what we call an extreme event in the Earth's history, something that doesn't happen very often."

Turgeon and Creaser found specific isotope levels of the element osmium, an indicator of volcanism in seawater, in black shale-rocks containing high amounts of organic matter-drilled off the coast of South America and in the mountains of central Italy.

"Because the climate was so warm back than, the oceanic current was very sluggish and it initially buffered this magmatic pulse, but eventually it all went haywire," said Turgeon. "The oxygen was driven from the ocean and all the organic matter accumulated on the bottom of the sea bed, and now we have these nice, big, black shale deposits worldwide, source rocks for the petroleum we have today."

According to their research, the eruptions preceded the mass extinction by a geological blink of the eye. The event occurred within 23 thousand years of the extinction and the underwater volcanic eruption had two consequences: first, nutrients were released, which allowed mass feeding and growth of plants and animals. When these organisms died, their decomposition and fall towards the sea floor caused further oxygen depletion, thereby compounding the effects of the volcanic eruption and release of clouds of carbon dioxide in to the oceans and atmosphere. The result was a global oceanic anoxic event, where the ocean is completely depleted of oxygen.

§Extinction

In the extinction event that defines the end of the Cretaceous, a significant number of species (~50%) and known families (~25%) disappeared. Plants were nearly unscathed, while marine organisms were hit the hardest. These include a large number (~95%) of types of planktic foraminifers (excepting the Globigerinida), an even larger number of Coccolithophores, all the ammonite and belemnite cephalopods, and all reef-forming rudist molluscs and inoceramid clams), as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most famous victims of the Cretaceous extinction. Dinosaurs that were unique to the very end of the period (such as Tyrannosaurus rex, Triceratops, and Ankylosaurus) were wiped out. The last of the pterosaurs became extinct and the vast majority of birds did as well, including the Enantiornithes and Hesperornithiformes.

The intensive mid-Cretaceous insect extinction began during the Albian.

The extinction event was once thought to have been the trigger for a diversification in animal species. This, according to an article in the Journal Nature did not begin for another 10 to 15 million years after the extinction event at 65 Ma and lasted until about 35 Ma. The article stated that analysis of more than 40 lineages of existing mammals showed that diversification rates “barely changed” in the aftermath of the extinctions at the boundary of the Cretaceous and Tertiary periods.

Research published in the Journal Science in 2010 suggests that all evidence points to a large extinction event from the impact of the asteroid at Chicxulub. A group of 41 researchers have pored over the evidence and decided that—in accordance with the original postulate put forth 30 years ago by a team led by father and son researchers Luis and Walter Alvarez—it was, indeed, a massive asteroid that slammed into Earth, creating Chicxulub Crater on Mexico's Gulf Coast, that killed off many of the species on the planet, including the non-avian dinosaurs.

Based on the size of material from rocky shrapnel and the crater diameter, researchers have estimated the dino-demolishing object to be some 10 kilometers across. And when it struck—at about 20 kilometers per second—it created an instant crater about 100 kilometers wide and 25 to 30 kilometers deep almost piercing the crust of the Earth. The final crater that formed after the initial impact was about 180 kilometers across and two kilometers deep, which is still close to the depth of the Grand Canyon.

The impact spewed rock so high, some of it likely was shot into orbit, whereas other pieces entered the upper atmosphere, reheating as they fell back to the ground. The jolt would have spurred massive earthquakes—some surpassing magnitude 11—tsunamis and landslides. While examining ammonoid fossils in southeastern Missouri Landman found a shallow water site that was immediately covered over by a jumble of stuff. He believes it was a tsunami which might have reached as far from the Yucatan impact site as southern Illinois.

Perhaps most devastating, however, the crash would have caused acid rain and darkness, as particulate matter blocked sunlight, prohibiting photosynthesis in both land and water ecosystems, effectively shutting down large swaths of the food chain. Directly after the extinction event, ferns (which reproduce from spores) proliferated and species that depended on detritus seemed to survive.

§ Sources

• The Columbia History of the World, Harper & Row, 1972
• Wikipedia:Cretaceous
• Nature March 29, 2007 Olaf R. P. Bininda-Emonds of the Technical University of Munich in Germany and Andy Purvis of Imperial College in London
• Ross D. E. MacPhee, a curator of vertebrate zoology at the American Museum of Natural History
• http://www.sciencedaily.com/releases/2008/07/080717095027.htm

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