Cretaceous
Belgian geologist Jean d'Omalius d'Halloy stood in the Paris Basin during 1822 and defined a new geological period. He called it Terrain Crétacé, derived from the Latin word creta for chalk. The name reflected the extensive beds of calcium carbonate found in Western Europe. These deposits formed from the shells of marine invertebrates known as coccoliths. Scientists still use the abbreviation K to represent this era today. This shorthand comes from the German translation Kreide. The twofold division of the Cretaceous was implemented by Conybeare and Phillips later that same year. Alcide d'Orbigny expanded the system further in 1840 with five distinct stages. His work laid the foundation for how we categorize time within this massive epoch.
The lower boundary of the Cretaceous remains undefined by modern standards. No Global Boundary Stratotype Section and Point exists for the Jurassic, Cretaceous transition yet. Calpionellids serve as enigmatic planktonic protists with urn-shaped calcitic tests. Their first appearance offers the most promising candidate for fixing the base of the Cretaceous. The working definition often places the boundary at the first appearance of the ammonite Strambergella jacobi. This ammonite formerly belonged to the genus Berriasella but its correlation has been questioned. The International Commission on Stratigraphy officially considers the boundary approximately 145 million years ago. Other estimates based on U-Pb geochronology range as young as 140 Ma. The upper boundary is sharply defined by an iridium-rich layer found worldwide. This layer dates to 66.043 Ma and circumscribes parts of the Yucatán Peninsula. It extends into the Gulf of Mexico where the Chicxulub impact crater lies.
High sea levels during the Cretaceous covered large areas of continents with warm shallow seas. These waters provided habitat for many marine organisms including now-extinct reptiles. The climate featured three broad phases according to palynological evidence. A Berriasian, Barremian warm-dry phase preceded an Aptian, Santonian warm-wet phase. A Campanian, Maastrichtian cool-dry phase followed these earlier periods. Mid-ocean ridge activity enriched oceans in calcium making them more saturated. This saturation increased bioavailability for calcareous nanoplankton. Temperatures reached extreme highs during the Mid-Cretaceous Hothouse interval. Mean annual temperatures at poles exceeded 14 degrees Celsius during this time. Tropical sea surface temperatures likely averaged around 30 degrees Celsius. Deep ocean temperatures were as much as 12 to 20 degrees warmer than today's values. Atmospheric carbon dioxide levels varied by thousands of ppm throughout this hothouse period. Global winds weakened resulting in less upwelling and more stagnant oceans. Black shale deposition became widespread across the globe during these conditions.
Monosulcate pollen grains from the late Valanginian era appeared in Israel and Italy approximately 134 million years ago. These represented the earliest widely accepted evidence of flowering plants. Molecular clock estimates conflict with fossil records suggesting diversification occurred during the Late Triassic or Jurassic. Among the oldest macrofossils are Montsechia from Barremian aged Las Hoyas beds in Spain. Archaefructus emerged from the Yixian Formation in China near the Barremian-Aptian boundary. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian stage. Flowering plants underwent rapid radiation beginning during the middle Cretaceous. They became the dominant group of land plants by the end of the period. The oldest known fossils of grasses date back to the Albian age. Large angiosperm trees reached heights estimated at 50 meters with trunk diameters of 1 meter. Ferns in the order Polypodiales began to diversify during this time as well. Gymnosperm groups like cycads and conifers declined coincident with the rise of angiosperms.
Mammals were generally small sized but formed a very relevant component of terrestrial fauna. Cimolodont multituberculates outnumbered dinosaurs in some sites across North America. True marsupials and placentals did not exist until the very end of the epoch. Non-marsucial metatherians and non-placental eutherians had already begun to diversify greatly. Apex predators included archosaurian reptiles especially dinosaurs which were at their most diverse stage. Avians such as ancestors of modern-day birds diversified and inhabited every continent. Pterosaurs were common in early and middle Cretaceous periods before declining for poorly understood reasons. By the end of the period only three highly specialized families remained: Pteranodontidae, Nyctosauridae, and Azhdarchidae. The Liaoning lagerstätte in China preserved remains of numerous types of small dinosaurs and mammals. Coelurosaur dinosaurs found there represent types of the group Maniraptora including dromaeosaurs and oviraptorosaurs. Fossils from this site are notable for the presence of hair-like feathers on these creatures.
Rays modern sharks and teleosts became common inhabitants of Cretaceous seas. Marine reptiles included ichthyosaurs in the early and mid-Cretaceous becoming extinct during the late Cenomanian-Turonian anoxic event. Plesiosaurs persisted throughout the entire period while mosasaurs appeared in the Late Cretaceous. Sea turtles in the form of Cheloniidae and Panchelonioidea lived during the period and survived extinction events. Baculites ammonite genus with straight shells flourished alongside reef-building rudist clams. Inoceramids were particularly notable among Cretaceous bivalves used to identify major biotic turnovers. Globotruncanid foraminifera and echinoderms such as sea urchins thrived in these waters. Thylacocephala crustaceans went extinct in the Late Cretaceous. The first radiation of diatoms occurred during the Cretaceous though freshwater varieties did not appear until the Miocene. Calcareous nannoplankton served as important components of marine microbiota recording environmental changes.
A large body impacted Earth at the end of the Cretaceous creating a sharp break known as the K, Pg boundary. This event caused the extinction of three-quarters of Earth's plant and animal species. Non-avian dinosaurs pterosaurs and large marine reptiles died out completely due to this catastrophe. Atmospheric particles blocked solar energy causing photosynthesizing organisms like phytoplankton and land plants to decline. Herbivorous animals died out as their food sources became scarce. Top predators such as Tyrannosaurus rex also perished following the collapse of lower food chains. Only three major groups of tetrapods disappeared completely: non-avian dinosaurs plesiosaurs and pterosaurs. Omnivores insectivores and carrion-eaters survived perhaps because of increased availability of detritus. Scientists theorize these survivors fed on dead plant and animal matter washing into streams. Crocodilians and champsosaurs were largest air-breathing survivors able to live as scavengers for months without food.
Common questions
Who defined the Cretaceous period in 1822?
Belgian geologist Jean d'Omalius d'Halloy defined the Cretaceous period during 1822. He named it Terrain Crétacé based on extensive chalk beds found in Western Europe.
When did the Cretaceous period begin and end according to modern standards?
The International Commission on Stratigraphy officially dates the start of the Cretaceous to approximately 145 million years ago. The period ended sharply at 66.043 Ma with an iridium-rich layer linked to the Chicxulub impact crater.
What were the climate conditions like during the Mid-Cretaceous Hothouse interval?
Mean annual temperatures at poles exceeded 14 degrees Celsius while tropical sea surface temperatures averaged around 30 degrees Celsius. Deep ocean temperatures reached values as much as 12 to 20 degrees warmer than today's levels.
Which plants first appeared during the late Valanginian era of the Cretaceous?
Monosulcate pollen grains from flowering plants appeared in Israel and Italy approximately 134 million years ago. These fossils represent the earliest widely accepted evidence of angiosperms before they became dominant land plants by the end of the period.
How many major groups of tetrapods disappeared completely during the K, Pg boundary event?
Only three major groups of tetrapods disappeared completely including non-avian dinosaurs plesiosaurs and pterosaurs. Non-avian dinosaurs pterosaurs and large marine reptiles died out due to the catastrophic extinction event that blocked solar energy.