The Cretaceous interval was the final and longest phase of the Mesozoic period. It lasted roughly 79 million years, from the minor extinction occasion that closed the Jurassic interval about 145 million years in the past to the Cretaceous-Paleogene (Okay-Pg) extinction occasion 66 million years in the past. The identify comes from “creta,” the Latin phrase for chalk, due to widespread chalk deposits courting from the interval, in response to the Nationwide Park Service (opens in new tab).
Within the early Cretaceous, the continents had been in very totally different positions than they’re in the present day, in response to the Australian Museum (opens in new tab). Sections of the supercontinent Pangaea had been drifting aside. The Tethys Ocean nonetheless separated the northern continent Laurasia from the southern continent Gondwana. The North and South Atlantic had been nonetheless closed, though the Central Atlantic had begun to open up within the Late Jurassic interval. By the center of the Cretaceous interval, ocean ranges had been a lot larger (opens in new tab); many of the landmasses we’re acquainted with had been underwater (opens in new tab). By the tip of the interval, the continents had been a lot nearer to their fashionable configuration. Africa and South America had assumed their distinctive shapes. However India had not but collided with Asia, and Australia was nonetheless a part of Antarctica.
Cretaceous interval crops
One hallmark of the Cretaceous interval was the event and radiation of flowering crops, or angiosperms, which “quickly diversified,” in response to the Nationwide Park Service. This radiation “gave rise instantly and mysteriously to beautiful angiosperm variety within the mid-Cretaceous,” an evolutionary improvement that troubled Charles Darwin, who noticed evolution occurring way more slowly, in response to a evaluation within the journal Proceedings of the Royal Society B (opens in new tab). Darwin proposed that flowering crops will need to have began growing lengthy earlier than the Cretaceous, probably on “a misplaced island or continent,” William E. Friedman, an evolutionary biologist at Harvard College, wrote within the American Journal of Botany (opens in new tab) in 2009. Nonetheless, the Cretaceous-era burst of floral improvement might as an alternative reveal how evolution can occur in a short time, Friedman wrote.
Although Darwin’s misplaced continent by no means confirmed up, some flowering crops might have appeared within the Jurassic, latest analysis has proven.
Nonetheless, Jurassic-era flowering crops would have been unusual and may additionally have been evolutionary hyperlinks between older crops that resembled angiosperms and the true factor, discovered within the Cretaceous, researchers mentioned. Scientists typically place “the oldest uncontested” angiosperm fossils at about 125 million to 130 million years in the past, within the early Cretaceous, in response to the Brooklyn Botanic Backyard (opens in new tab). These embrace crops of the genera Archaefructus and Montsechia, which present the primary proof of ovaries in crops however might have lacked petals.
Since Darwin, scientists have thought that pollinating bugs, equivalent to bees and wasps, performed a key position within the Cretaceous explosion of flowering crops, in response to latest (opens in new tab) and foundational (opens in new tab) analysis. That is incessantly cited for example of co-evolution, in response to the Washington Native Plant Society (opens in new tab).
The mid-Cretaceous noticed ample populations of each bugs and flowering crops, and up to date finds lastly caught Cretaceous-era insect pollinators frozen within the act. In 2019, scientists reported within the journal Proceedings of the Nationwide Academy of Sciences (opens in new tab) the primary direct fossil proof of insect pollination within the Cretaceous: a tumbling flower beetle, Angimordella burmitina, preserved in amber for the reason that mid-Cretaceous, 99 million years in the past, and coated with pollen grains. The beetle sports activities a number of physique components specialised for feeding on flowers, together with pollen-feeding mouthparts, and the pollen grains have traits, like clumping traits, related to insect pollination, the researchers reported.
And in a 2020 paper revealed within the journal BioOne (opens in new tab), scientists reported on the oldest bee discovered bearing pollen, the 100 million-year-old Discoscapa apicula. Additionally discovered encased in amber, this insect shared some traits with fashionable bees, equivalent to hind legs laden with pollen, and a few traits with wasps, equivalent to its wing vein options.
Because of pollinating bugs, flowering crops had large benefits over crops that unfold pollen solely by wind, spurring the explosion of angiosperms, in response to Illinois Extension on the College of Illinois Urbana-Champaign (opens in new tab). Competitors for insect consideration in all probability facilitated the comparatively speedy success and diversification of the flowering crops, “lead[ing] to the event of many alternative measurement, shapes, colours and fragrances of flowers we see in the present day,” together with the manufacturing of nectar to draw hungry bugs. As numerous flower kinds lured bugs to pollinate them, bugs tailored to alternative ways of gathering nectar and shifting pollen, thus organising the intricate co-evolutionary programs discovered to at the present time.
Just a few finds over the a long time have estimated that some pollinating bugs arrived earlier than flowering crops. In 2009, researchers discovered that 11 species of scorpionflies current beginning within the center Jurassic boasted the elongated mouthparts and pollen-centric diets attribute of pollinators, as reported within the journal Science (opens in new tab). These seemingly pollinating bugs, nevertheless, ate up nonflowering crops, or angiosperms, “lengthy earlier than the same and unbiased coevolution of nectar-feeding flies, moths and beetles on angiosperms,” the research mentioned. These critters went extinct throughout the Cretaceous, across the time of the “world gymnosperm-to-angiosperm turnover,” the researchers mentioned. Within the Nineteen Nineties, researchers reported that bee- or wasp-like bugs constructed hive-like nests in what’s now known as the Petrified Forest in Arizona, courting again to greater than 200 million years in the past. Nonetheless, later re-evaluations discovered that the buildings lacked defining traits of bee nests and most probably got here from beetle larva chambers or different creatures, as reported within the journal Palaeogeography, Palaeoclimatology, Palaeoecology (opens in new tab). That analysis of the buildings “eliminates them as proof that decouples bee origins from the Cretaceous origin of angiosperms,” the scientists wrote.
Some proof exhibits that dinosaurs ate flowering crops. Two dinosaur coprolites (fossilized excrements) found in Utah include fragments of angiosperm wooden, in response to an unpublished research offered on the 2015 Society of Vertebrate Paleontology annual assembly. An Early Cretaceous ankylosaur was discovered with fossilized angiosperm fruit (opens in new tab) in its intestine.
Nonetheless, for probably the most half, proof means that dinosaurs ignored angiosperms within the Cretaceous, sustaining a weight-reduction plan centered on ferns and conifers, College of Bristol researchers mentioned in 2021, summarizing their work on angiosperm evolution within the journal New Phytologist (opens in new tab). The form of some tooth from Cretaceous animals means that the herbivores grazed on leaves and twigs, mentioned Betsy Kruk, previously a volunteer researcher on the Subject Museum in Chicago and now a principal investigator and venture supervisor at Materials Tradition Consulting, a California-based firm that consults on compliance companies together with archaeology and paleontology.
Cretaceous interval animals
The Cretaceous was an age of reptiles. Dinosaurs dominated the land, whereas marine reptiles just like the mosasaurs — which may span 56 ft (17 meters) — swam the oceans. Pterosaurs plied the skies, together with the most important flying animal ever, Quetzalcoatlus, whose wingspan may stretch to 36 ft (11 m).
The largest-ever land predator (opens in new tab), the well-known Tyrannosaurus rex, additionally reigned throughout the Cretaceous. By the tip of the Jurassic, some massive sauropods, equivalent to Apatosaurus and Diplodocus, had gone extinct. However different large sauropods, together with the titanosaurs, flourished, particularly towards the tip of the Cretaceous, Kruk mentioned. Titanosaurs had been probably the most profitable sauropods of the interval, and the previous twenty years have seen a “growth” in titanosaur discoveries, in response to the journal Nature Ecology & Evolution (opens in new tab).
Massive herds of herbivorous ornithischians additionally thrived throughout the Cretaceous. These included Iguanodon (which belongs to the identical group as duck-billed dinosaurs, also referred to as hadrosaurs), Ankylosaurus, and the ceratopsians, like Triceratops. Duck-billed dinosaurs had been the most typical kind of ornithischians, a bunch of largely herbivorous dinosaurs with bird-like hips, in response to the Cal Poly Humboldt Pure Historical past Museum (opens in new tab). Theropods, together with T. rex, continued as apex predators till the tip of this era.
In the course of the Cretaceous, extra historic birds took flight, becoming a member of the pterosaurs within the air. Consultants have lengthy debated (opens in new tab) the origin of flight. In accordance with the so-called timber down concept, small reptiles might have advanced flight from gliding behaviors. The floor up speculation (opens in new tab) posits that flight advanced from the flexibility of small theropods to leap excessive to know prey or evade predators. Early analysis urged that feathers advanced from elongated scales (opens in new tab) whose major operate, no less than at first, was thermoregulation. They may very well be moved to soak up extra photo voltaic warmth in cool circumstances and supply safety from the solar when it was sizzling, in response to a 1975 research in The Quarterly Assessment of Biology. Newer research counsel that signaling and tactile sensing may additionally have performed a task within the evolution of those feather precursors, in response to a research within the Worldwide Journal of Natural Evolution (opens in new tab).
The earliest fossilized chicken, Archaeopteryx, swooped by way of Cretaceous skies 150 million years in the past, although it resembled small dinosaurs greater than the birds we see in the present day, in response to the Australian Museum (opens in new tab). A wide range of birds arrived on the scene quickly afterward sporting a spread of options that may very well be extra like these of present birds. A few of these creatures advanced into birds of the trendy kind by the late Cretaceous, which implies that “bird-like dinosaurs, primitive birds and early fashionable birds all co-existed” for a stretch of the Cretaceous, the Australian Museum added.
One Cretaceous-era chicken, Confuciusornis sanctus, lived about 125 million years in the past. It was a crow-size (opens in new tab) chicken with a contemporary, toothless beak, in contrast to the fanged Archaeopteryx; claws (opens in new tab) much like these of recent, tree-dwelling birds; and flight-worthy feathers. A research of pigment-storing cell organelles in C. sanctus within the journal Science (opens in new tab) discovered that these historic birds seemingly sported darkish feathers on their torsos, with lighter-colored wings, in response to the California Academy of Sciences (opens in new tab). Iberomesornis, a up to date of Archaeopteryx solely the scale of a sparrow, was able to flight and will have been an insectivore.
Associated: Are birds dinosaurs?
Sea creatures additionally thrived throughout the Cretaceous, with many marine teams reaching their peak ranges of variety, in response to the Cal Poly Humboldt museum. Past the mosasaurs, ocean sea life included mollusks that constructed reefs similar to in the present day’s coral reefs, together with sharks, lobsters and crabs, sand dollar-like creatures often known as echinoids, and a sort of bony fish often known as ray-finned fish (named for his or her fins shaped from spines draped with webs of pores and skin).
Although reptiles dominated the Cretaceous world, early mammals did exist on the time. Historically, scientists have considered mammal evolution as constrained by the dominant dinosaurs (opens in new tab); mammals could not evolve many species sorts, as a result of dinosaurs occupied most niches, this view suggests. Solely after the mass extinction that killed off all nonavian dinosaurs may mammals “radiate,” or evolve into many numerous kinds. However mammals might have gone by way of radiations even throughout the dinosaur age (opens in new tab), together with the Jurassic and Cretaceous durations, a 2019 research within the journal Tendencies in Ecology and Evolution (opens in new tab) discovered. And a 2021 research within the journal Present Biology (opens in new tab) discovered that evolutionary suppression of therians, the ancestors of in the present day’s mammals, might have come from not solely dinosaurs, but in addition historic relations of mammals often known as mammaliaforms.
How did the Cretaceous interval finish?
About 66 million years in the past, almost all massive vertebrates and lots of tropical invertebrates grew to become extinct in one in all Earth’s 5 nice mass extinction occasions, in response to former College of California, Davis, Earth and planetary sciences professor Richard Cowen (opens in new tab). Scientists have linked that mass extinction with an monumental asteroid that collided with Earth in what’s now Mexico. The occasion killed off all nonavian dinosaurs, all pterosaurs (which weren’t dinosaurs) and lots of marine reptiles, together with mosasaurs and plesiosaurs, in addition to many early mammals and “a number of amphibians, birds, reptiles and bugs,” in response to the American Museum of Pure Historical past (opens in new tab) in New York. An estimated three-quarters of species alive on the time met their finish.
Associated: What occurred when the dinosaur-killing asteroid slammed into Earth?
Geologists name this mass die-off the Okay-Pg extinction occasion as a result of it marks the boundary between the Cretaceous and Paleogene durations; the “Okay” is from “Kreide,” the German phrase for Cretaceous. The occasion was previously often known as the Cretaceous-Tertiary (Okay-T) (opens in new tab) occasion, however the group that units requirements for geologic nomenclature now considers Tertiary old-fashioned with present science, in response to the Nationwide Fee for Stratigraphy Belgium (opens in new tab).
The Chicxulub (CHEEK-sheh-loob) crater within the Yucatán Peninsula, which spans greater than 110 miles (180 kilometers) in diameter, is the seemingly touchdown spot of the dinosaur-killing asteroid. This crater dates to inside 33,000 years of the Okay-Pg occasion, Reside Science beforehand reported. “We have proven the affect and the mass extinction coincided as a lot as one can presumably exhibit with current courting strategies,” Paul Renne, lead scientist in that research and a geochronologist and director of the Berkeley Geochronology Middle in California, beforehand informed Reside Science.
Scientists had first related the Okay-Pg extinction with an extraterrestrial affect a long time in the past, nevertheless. In 1979, a geologist found (opens in new tab) that the skinny layer of clay separating the Cretaceous and Paleogene durations contained excessive concentrations of iridium. This aspect is uncommon on Earth however way more widespread in meteorites and asteroids, in response to the Lunar and Planetary Science Institute (opens in new tab). Different researchers discovered (opens in new tab) “shocked quartz,” a type of the mineral created beneath intense strain, and tiny, glass-like globes known as tektites that kind from droplets of melted rock. Each of those geological options kind when an extraterrestrial object strikes Earth with nice power.
Analysis in 2020 discovered that the item that carved out Chicxulub hit Earth on the most damaging potential angle, Reside Science beforehand reported. The 7.5-mile-wide (12 km) asteroid, touring at about 27,000 mph (43,000 km/h), would have vaporized rocks, sending 325 gigatons of sulfur and 435 gigatons of carbon dioxide into the ambiance within the type of pulverized rock and sulfuric acid droplets, researchers estimated.
When the asteroid collided with Earth, its affect would have triggered a ten.1-magnitude earthquake, despatched a shock wave with “hurricane-force winds” rippling throughout the Americas, and spawned a 330- to 820-foot-high (100 to 250 m) tsunami, in response to a 2021 College of Maryland course (opens in new tab). As particles ejected by the affect fell again to Earth, the fabric would have cooked the ambiance to 2,700 levels Fahrenheit (1,482 levels Celsius), portray the sky purple for a number of hours and igniting forest fires throughout the planet, Reside Science reported in 2013. The warmth pulse was like a worldwide broiler oven, not solely burning vegetation, but in addition cooking residing issues unable to burrow or dive, the researchers mentioned.
“This rain of sizzling mud raised world temperatures for hours after the affect and cooked alive animals that had been too massive to hunt shelter,” Kruk mentioned. “Small animals that might shelter underground, underwater, or maybe in caves or massive tree trunks, might have been capable of survive this preliminary warmth blast.”
Rock vaporized by the asteroid seemingly stayed within the ambiance, blocking a part of the solar’s rays for months or years, in response to the College of Maryland. This will likely even have lasted so long as 16 years, with a 30-year restoration interval. With much less daylight, crops would have died, with penalties touring up the meals chain to herbivores depending on crops and carnivores depending on these herbivores, in response to the Pure Historical past Museum (opens in new tab) in London.
Moreover, the lowered daylight would have drastically lowered world temperatures, which plunged within the tropics from 81 F (27 C) to 41 F (5 C), Reside Science beforehand reported. The newly frigid local weather would have impaired massive energetic animals with high-energy wants, Kruk mentioned.
“Smaller, omnivorous terrestrial animals — like mammals, lizards, turtles or birds — might have been capable of survive as scavengers feeding on the carcasses of useless dinosaurs, fungi, roots and decaying plant matter, whereas smaller animals with decrease metabolisms had been finest capable of wait the catastrophe out,” she mentioned.
The final part of the asteroid fallout, greenhouse warming, might have lasted round 100,000 years, in response to the College of Maryland. Carbonite rocks oxidized by the affect would have launched massive quantities of the greenhouse fuel carbon dioxide (CO2) into the ambiance. Simply earlier than the affect, a sequence of what might have been the second-largest volcanic eruptions ever on land went off on the Deccan traps in western India, in response to the American Museum of Pure Historical past (opens in new tab). These regional catastrophes had already spewed large ranges of CO2 and so seemingly mixed with the asteroid fallout to warmth up the planet as soon as the sun-obscuring mud settled, in response to the College of Maryland.
Cretaceous interval local weather
Even earlier than world cataclysms spurred world warming, the world was a hotter place throughout the Cretaceous interval than it’s in the present day, in response to Local weather Coverage Watcher (opens in new tab). The poles had been cooler than the decrease latitudes, however “total, issues had been hotter,” Kruk informed Reside Science. Fossils of tropical crops and ferns help this concept, she mentioned. Heat ocean currents, unfrozen poles and ranges of CO2 that had been comparatively excessive even earlier than the extinction occasion all mixed to provide a sizzling planet, in response to Local weather Coverage Watcher.
Animals within the Cretaceous lived throughout, even in colder areas. As an example, Hadrosaur fossils courting to the Late Cretaceous had been uncovered in Alaska. And in a 2020 paper within the journal Nature (opens in new tab), scientists reported on a temperate rainforest in Antarctica courting to the mid-Cretaceous.
Study and go to a solid of a titanosaur, the large sauropods of the Cretaceous period, on the American Museum of Pure Historical past (opens in new tab). Discover the Cretaceous-Paleogene extinction and Earth’s 4 different mass extinction occasions, together with the chance that we have entered a brand new one, on the Pure Historical past Museum (opens in new tab) in London. Uncover how pollinators and flowers have co-evolved on the New England Primate Conservancy (opens in new tab). Learn Richard Cowen’s essay on the Okay-Pg mass extinction occasion and different matters in his ebook “Historical past of Life (opens in new tab)” (Blackwell Scientific Publications, 2000).
This text was initially written by Reside Science contributor Mary Bagley with contributions from Reside Science editor Laura Geggel.
Initially revealed on Reside Science on January 8, 2016 and up to date on July 26, 2022.