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Cretaceous: the story on HearLore | HearLore
Cretaceous
The name Cretaceous comes from the Latin word for chalk, a substance so abundant in the geological record of this period that it defines the era itself. Belgian geologist Jean d'Omalius d'Halloy first defined this period in 1822, naming it the Terrain Crétacé based on the extensive white deposits found in the Paris Basin. These deposits are not merely rock; they are the compressed skeletons of microscopic algae called coccolithophores, which thrived in the warm, shallow seas that covered much of the Earth. The Cretaceous lasted from approximately 143.1 million years ago until 66 million years ago, making it the longest period of the Mesozoic Era and the ninth longest geological period in the entire Phanerozoic Eon. While the name suggests a dry, dusty landscape, the reality was a world of high sea levels where one-third of Earth's present land area was submerged under warm, shallow waters. This unique geological setting created a global archive of fossils, from the white cliffs of Dover to the rich marine beds of Kansas, preserving a snapshot of life when the planet was in a state of extreme warmth and biological flux.
A World Without Ice
During the Mid-Cretaceous Hothouse, which lasted from the early Albian until the early Campanian, the Earth experienced temperatures that would be unrecognizable to modern life. Mean annual temperatures at the poles exceeded 14 degrees Celsius, allowing ectothermic reptiles to inhabit regions as far south as 82 degrees latitude. The temperature gradient from the equator to the poles was so gentle that there was no significant difference in climate between the tropics and the poles, creating a world without ice caps. This extreme warmth was driven by rapid seafloor spreading and massive releases of carbon dioxide from volcanic activity, such as the Paraná-Etendeka Large Igneous Province and the Deccan Traps. The oceans were stagnant, with deep water temperatures reaching as high as 20 degrees Celsius, and the atmosphere held carbon dioxide levels that varied by thousands of parts per million. Despite the heat, the climate was not static; it experienced dramatic thermal excursions, including the Leenhardt Thermal Event and the Cenomanian-Turonian Thermal Maximum, which was the most extreme hothouse interval of the Cretaceous. These fluctuations created a dynamic environment where tropical sea surface temperatures averaged 30 degrees Celsius, yet the water was not hypersaline, proving that the planet could sustain such heat without boiling away.
The Flowering Revolution
The Cretaceous witnessed the most significant botanical revolution in Earth's history: the rise of flowering plants, or angiosperms. Prior to this period, the world was dominated by gymnosperms like cycads, conifers, and ginkgophytes, but the first evidence of flowering plants appears in the late Valanginian, around 134 million years ago, as monosulcate pollen grains found in Israel and Italy. By the middle Cretaceous, these plants underwent a rapid radiation, becoming the dominant group of land plants by the end of the period. The oldest known fossils of grasses date back to the Albian, and the oldest large angiosperm trees, found in New Jersey, had trunks with a preserved diameter of 1.5 meters and an estimated height of 30 meters. This diversification coincided with the decline of previously dominant groups, reshaping ecosystems globally. The Cretaceous Resinous Interval, lasting from 125 to 75 million years ago, saw trees producing massive amounts of resin to combat damage from storms and arthropods, creating the amber that preserves countless insects today. Ferns in the order Polypodiales also began to diversify, eventually making up 80 percent of living fern species. This botanical shift was not merely a change in plant life; it fundamentally altered the food chains and habitats available to animals, setting the stage for the evolution of modern ecosystems.
Who defined the Cretaceous period and when was it named?
Belgian geologist Jean d'Omalius d'Halloy first defined the Cretaceous period in 1822. He named it the Terrain Crétacé based on the extensive white chalk deposits found in the Paris Basin.
When did the Cretaceous period last and how long was it?
The Cretaceous period lasted from approximately 143.1 million years ago until 66 million years ago. It was the longest period of the Mesozoic Era and the ninth longest geological period in the entire Phanerozoic Eon.
What caused the extreme warmth during the Mid-Cretaceous Hothouse?
Extreme warmth during the Mid-Cretaceous Hothouse was driven by rapid seafloor spreading and massive releases of carbon dioxide from volcanic activity. These volcanic events included the Paraná-Etendeka Large Igneous Province and the Deccan Traps.
When did flowering plants first appear and when did they become dominant?
The first evidence of flowering plants appears in the late Valanginian, around 134 million years ago. By the middle Cretaceous, these plants underwent a rapid radiation and became the dominant group of land plants by the end of the period.
What caused the Cretaceous, Paleogene extinction event?
The Cretaceous, Paleogene extinction event was caused by the impact of a large asteroid that formed the Chicxulub crater in the Gulf of Mexico. This impact created the K, Pg boundary, an iridium-rich layer found worldwide that marks the end of the Mesozoic Era.
Which animals survived the Cretaceous, Paleogene extinction event and why?
Omnivores, insectivores, and carrion-eaters survived the Cretaceous, Paleogene extinction event because they fed on detritus and dead matter. Crocodilians and champsosaurs were the largest air-breathing survivors due to their ability to live as scavengers and feed on invertebrates and dead organisms.
On land, the Cretaceous was the golden age of dinosaurs, which reached their peak diversity during this time. The apex predators were archosaurian reptiles, including the famous Tyrannosaurus rex, which perished at the end of the period, but the story of the Cretaceous is also one of feathers and flight. The Liaoning lagerstätte in China has preserved the remains of numerous small dinosaurs, birds, and mammals, revealing that many coelurosaur dinosaurs had hair-like feathers. These fossils include dromaeosaurs, oviraptorosaurs, and troodontids, all part of the Maniraptora group, which includes modern birds and their closest non-avian relatives. Birds themselves diversified during the Cretaceous, inhabiting every continent and even cold polar latitudes. Pterosaurs, the flying reptiles, were common in the early and middle Cretaceous but declined as the period progressed, leaving only three highly specialized families by the end. Mammals, though generally small, were a relevant component of the fauna, with cimolodont multituberculates outnumbering dinosaurs in some sites. The earliest relatives of placentals and marsupials, known as Eutheria and Metatheria respectively, appeared during this time, along with the earliest crown group birds. This era was not just about giant lizards; it was a complex web of life where small, feathered creatures and tiny mammals coexisted with the giants.
The Shallow Seas and Their Inhabitants
The high sea levels of the Cretaceous created vast shallow inland seas that teemed with life, from the Western Interior Seaway that split North America to the Tethys Sea connecting the tropics. These waters were home to a diverse array of marine reptiles, including ichthyosaurs in the early and mid-Cretaceous, plesiosaurs throughout the entire period, and mosasaurs, which appeared in the Late Cretaceous. The seas were also populated by ammonites, such as the straight-shelled Baculites, and reef-building rudist clams. Teleost fish, the most diverse group of modern vertebrates, continued to diversify, with the appearance of their most diverse subgroup, Acanthomorpha, during this period. The oceans were also home to sea turtles, including the ancestors of the leatherback sea turtle, and flightless marine diving birds known as Hesperornithiformes. The Cretaceous was an important interval in the evolution of bioerosion, with organisms producing borings and scrapings in rocks and shells. The marine environment was so rich that half of the world's petroleum reserves were laid down during this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. These dark shales, such as the Mancos Shale of western North America, serve as a testament to the stagnation of deep-sea currents and the abundance of organic matter that was preserved rather than decomposed.
The Great Cooling and the Final Extinction
Beginning in the Santonian, near the end of the Mid-Cretaceous Hothouse, the global climate began to cool, marking the transition into the Late Cretaceous-Early Palaeogene Cool Interval. Tropical sea surface temperatures declined from around 35 degrees Celsius in the early Campanian to around 28 degrees Celsius in the Maastrichtian, and deep ocean temperatures dropped to 9 to 12 degrees Celsius. This cooling was driven by falling levels of atmospheric carbon dioxide, yet the Maastrichtian was a time of chaotic, highly variable climate, with two upticks in global temperatures occurring between 70 and 69 million years ago and 66 to 65 million years ago. The end of the Cretaceous was punctuated by the Cretaceous, Paleogene extinction event, a large mass extinction that wiped out three-quarters of Earth's plant and animal species. The impact of a large asteroid, which formed the Chicxulub crater in the Gulf of Mexico, is widely accepted as the main cause of this event. The extinction created a sharp break known as the K, Pg boundary, an iridium-rich layer found worldwide that marks the end of the Mesozoic Era. Species that depended on photosynthesis, such as phytoplankton and land plants, declined or became extinct as atmospheric particles blocked solar energy, causing a collapse of the food chain that led to the demise of herbivorous and carnivorous animals alike.
Survivors of the Catastrophe
Despite the severity of the Cretaceous, Paleogene extinction event, some groups managed to survive the collapse of the food chain. Omnivores, insectivores, and carrion-eaters survived, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or carnivorous mammals; instead, the survivors fed on insects, larvae, worms, and snails, which in turn fed on dead plant and animal matter. Scientists theorize that these organisms survived the collapse of plant-based food chains because they fed on detritus. In stream communities, few groups of animals became extinct, as these communities rely less on food from living plants and more on detritus that washes in from land. The largest air-breathing survivors of the event were crocodilians and champsosaurs, which were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous. The extinction event did not wipe out all life; it simply reset the stage for the rise of the Cenozoic Era, where mammals and birds would eventually dominate the Earth.