Cretaceous

• 1. Introduction

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  The Cretaceous period stretched across roughly 77.1 million years, making it the longest geological period of the entire Phanerozoic. It ran from about 143.1 million years ago to 66 million years ago, closing with one of the most dramatic mass extinctions in Earth's history. Its name traces back to the Latin word creta, meaning chalk, a rock type that piled up in vast beds across Western Europe during the period's second half. Scientists use the abbreviation K, drawn from the German word Kreide.

  At its height, a world without permanent ice caps stretched from pole to pole. Forests grew at high latitudes. Shallow inland seas flooded one-third of what we now call dry land. Dinosaurs ruled the continents while giant reptiles ruled the seas. And yet, buried inside this warm and teeming world lay the seeds of everything familiar: the first flowering plants, the first ants and termites, the earliest relatives of modern mammals and birds.

  How did all of this arise, thrive, and then collapse in the span of a geological eyeblink? And what do the chalk cliffs of Dover, the oil fields beneath the Persian Gulf, and the survival of crocodilians all have to do with a single asteroid?

• 2. Naming and Discovery

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  Belgian geologist Jean d'Omalius d'Halloy first defined the Cretaceous as a distinct period in 1822, calling it the Terrain Cretace. He drew on strata in the Paris Basin and named the period for the thick chalk beds found across the upper portion of the sequence in Western Europe. Chalk forms from the microscopic calcite skeletons of coccolithophores, a type of algae, whose shells accumulated on ancient seafloors.

  Also in 1822, Conybeare and Phillips introduced the twofold division of the Cretaceous into an early and late section, a framework still in use today. Then, in 1840, Alcide d'Orbigny refined things further by dividing the French Cretaceous into five stages: the Neocomian, Aptian, Albian, Turonian, and Senonian. He later added two more, the Urgonian and the Cenomanian, slotting them between his original groupings.

  Today the Cretaceous is subdivided into twelve stages, all drawn from European stratigraphy, and used worldwide. The oldest stage, the Berriasian, is named for Berrias in France, while the youngest, the Maastrichtian, takes its name from the Maastricht Formation in the Netherlands. The lower boundary of the period, where the Cretaceous begins, remains officially undefined, making the Jurassic-Cretaceous boundary the only system boundary in the geological timescale without a formally agreed Global Boundary Stratotype Section and Point.

• 3. A World of Shallow Seas

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  At the peak of the Cretaceous marine transgression, one-third of Earth's present land area sat beneath water. The supercontinent Pangaea, already breaking apart since the Triassic, completed its fragmentation during this period. The South Atlantic and Indian Oceans were newly formed as South America, Antarctica, and Australia rifted away from Africa. India and Madagascar only separated from each other around 80 million years ago.

  Active rifting along the ocean floors raised massive underwater mountain chains, pushing sea levels higher across the globe. Across North America, a vast interior sea called the Western Interior Seaway divided the continent from north to south, separating the western landmass of Laramidia from the eastern landmass of Appalachia. At the same time, bivalve fossil patterns suggest that Africa itself was split in half during the Coniacian and Santonian ages by a shallow sea running through the central Sahara, connecting the Tethys to the South Atlantic.

  The chalk that named this period formed on the floors of these shallow, warm seas. Mid-ocean ridge activity circulated seawater through the enlarged ridges, enriching the oceans in calcium and creating ideal conditions for calcareous nanoplankton. The result was a rock record of exceptional richness, preserved today in formations like Kansas's Smoky Hill Chalk Member and the marine limestones of the Chalk Group, whose deposits form the white cliffs of Dover on the south coast of England and matching cliffs along the Normandy coast of France.

• 4. Temperature Swings of a Hothouse World

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  Palynological evidence, meaning the study of fossil pollen, reveals that the Cretaceous climate passed through three broad phases: a warm-dry phase in its early ages, a warm-wet phase through its middle stretch, and a cool-dry phase in its final ages. The dominant orbital rhythm governing carbon flux throughout was the 400,000-year eccentricity cycle.

  The middle portion of the period saw temperatures reach extraordinary heights during what researchers call the Mid-Cretaceous Hothouse, which lasted from the early Albian until the early Campanian. Mean annual polar temperatures during this interval exceeded 14 degrees Celsius, warm enough for ectothermic reptiles to inhabit the poles. Tropical sea surface temperatures during the Cenomanian-Turonian Thermal Maximum, the most extreme hothouse interval of the entire Cretaceous, reached at least 30 degrees Celsius, with some estimates as high as 33 to 42 degrees. Deep ocean temperatures were between 12 and 20 degrees Celsius, far warmer than the cold abyssal waters of today.

  This heat flattened the temperature gradient between the equator and the poles. During the Cenomanian-Turonian Thermal Maximum, that gradient was just 0.54 degrees Celsius per degree of latitude in the Southern Hemisphere, compared to a present-day value of 1.07. Weaker gradients meant weaker winds, weaker ocean currents, and widespread ocean stagnation. The stagnant middle Cretaceous seas became oxygen-depleted, and organic matter settled to the seafloor undecomposed. That anoxic environment is why roughly half of the world's known petroleum reserves were laid down during this interval, in what would later become the Persian Gulf and the Gulf of Mexico.

• 5. The Rise of Flowering Plants

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  Flowering plants, called angiosperms, make up around 90 percent of living plant species today. At the start of the Cretaceous, they barely existed. The earliest widely accepted fossil evidence consists of monosulcate pollen grains from the late Valanginian, around 134 million years ago, found in Israel and Italy, present only in small quantities.

  Among the oldest known macrofossils of flowering plants are Montsechia, recovered from Barremian-aged beds in Spain, and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen, the distinctive marker of the eudicot lineage that includes most modern flowering trees and shrubs, first appeared in the Late Barremian. Monocots, the group that includes grasses and orchids, are first known from the Aptian.

  From those modest beginnings, angiosperms underwent a rapid radiation through the middle Cretaceous, eventually displacing the gymnosperms, conifers, cycads, and their relatives that had previously dominated the land. By the end of the Cretaceous, flowering plants were the dominant group on Earth. The oldest known angiosperm grasses date to the Albian. The oldest large angiosperm trees yet discovered come from the Turonian, around 90 million years ago, in what is now New Jersey; one specimen had a trunk diameter of 1.8 meters and an estimated height of 50 meters. Meanwhile, storms, hurricanes, and wildfires driven by the intense Cretaceous heat damaged trees enough to trigger unusually heavy resin production, creating the Cretaceous Resinous Interval, which lasted from 125 to 75 million years ago.

• 6. Life on Land and Sea

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  Dinosaurs were at their most diverse during the Cretaceous, dominating every continent, including the high polar latitudes. Dinosaur fossils have been found within 15 degrees of the Cretaceous south pole. Yet mammals were also a significant presence. Cimolodont multituberculates outnumbered dinosaurs at some sites, and a variety of early therian and eutherian mammals had already spread into niches as carnivores, aquatic foragers, and herbivores. Insects diversified broadly, and the oldest known ants, termites, and early relatives of butterflies and moths appeared during this period.

  The Liaoning lagerstatte in China, from the Yixian Formation, preserved an extraordinary snapshot of Early Cretaceous life: small dinosaurs, early birds, and mammals are found together in exceptional detail. The coelurosaur dinosaurs found there belong to Maniraptora, the group that includes modern birds and close relatives such as dromaeosaurs and oviraptorosaurs, and their fossils are notable for the presence of hair-like feathers.

  In the seas, rays, modern sharks, and teleost fish became common. Ichthyosaurs persisted into the mid-Cretaceous before dying out during the Cenomanian-Turonian anoxic event. Mosasaurs, giant marine lizards related to snakes, appeared in the Late Cretaceous and became dominant ocean predators. Plesiosaurs swam throughout the entire period. Ammonites, particularly the straight-shelled genus Baculites, flourished alongside reef-building rudist clams and a rich community of echinoderms, foraminifera, and calcareous nannoplankton. Sea turtles of the family Cheloniidae were also present and survived into the modern era; today, one lineage is represented by a single surviving species, the leatherback sea turtle.

• 7. The End and Its Aftermath

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  The Cretaceous closed with a mass extinction that ended three-quarters of Earth's plant and animal species. A global iridium-rich layer, found worldwide and dated at 66.043 million years ago, marks the moment and is linked to the Chicxulub impact crater, whose boundaries encompass parts of the Yucatan Peninsula and extend into the Gulf of Mexico.

  The chain of collapse followed the food chain upward. Species dependent on photosynthesis declined first as atmospheric particles blocked sunlight. Herbivores starved as plants disappeared. Carnivores including Tyrannosaurus rex followed. Three major groups of tetrapods vanished completely: the non-avian dinosaurs, the plesiosaurs, and the pterosaurs. Some groups, including ichthyosaurs and the last temnospondyls and nonmammalian cynodonts, were already gone millions of years before the impact.

  Survival, it turned out, depended less on size or strength than on diet. Omnivores, insectivores, and scavengers pulled through, likely because the collapse of living plant-based food chains elevated the availability of detritus. Mammals and birds that survived fed on insects, larvae, worms, and snails, which themselves fed on dead matter. Stream communities fared better than open-water communities because they already relied on organic material washing in from land rather than on living phytoplankton. Crocodilians survived because they were semiaquatic, could scavenge, could go months without food, and produced young that fed largely on invertebrates and dead organisms in their early years. Those same traits that made crocodilians marginal opportunists in the rich Cretaceous world became precisely the characteristics that kept them alive after the lights went out.