Late Cretaceous Period
During the Late Cretaceous, land and seas continued to change and move. The climate of the world changed toward cooler and more seasonal weather. New types of plants and animals were appearing. An amazing collection of dinosaurs had evolved. Some had become advanced, even caring for their young. Dinosaurs, at first plentiful, disappeared from the earth by the end of the period.
Canadian Museum of Nature
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A shallow sea covered North America's middle. About halfway through the Late Cretaceous, Alaska butted up against Siberia. Eastern Asia and western North America were a single land mass called Beringia. The North American part of Beringia was tropical and swampy. The Asian section was dryer. The link from Eurasia to North America began to break. South America remained isolated throughout most of the Late Cretaceous. Australia and Antarctica remained joined around the South Pole, and Eurasia and Africa never drifted far apart.
Flowering plants spread rapidly throughout the northern hemisphere in the early Late Cretaceous, appearing first as small weeds. They quickly became the most important land plants. They provided a dense clutter of leaves, stems, and branches. In drier areas, flowering plants became underbrush. Able to grow rapidly after being eaten or trampled, flowering plants fed a larger number of animals.
During the early Late Cretaceous, the climate was warm. As the period came to a close, the average climate became cooler, but it was still much warmer than today. By the end of the period, the tropics were only in areas near the equator. The climate in the farther northern and southern hemispheres (and polar regions) became temperate and more seasonal, with cool winters and warm summers. Forests in the temperate zones became less tropical, with magnolias, sassafras, redwoods, and willow trees plentiful.
Late Cretaceous Dinosaurs in North America
Dinosaurs remained the main large land animals. Smaller land animals included turtles, crocodiles, snakes, lizards, frogs, and salamanders. Mammals remained small, but mammals that gave birth to live young appeared for the first time.
Almost all Late Cretaceous flying birds were tiny. But pterosaurs became the largest flying creatures ever known. Ichthyosaurs became extinct, and mosasaurs became the main marine predators. These evolved from small-to-medium-size monitor lizards. They shared the seas with plesiosaurs.
As many dinosaur species are known from the Late Cretaceous as are known from all the other periods together. In North America, the duckbilled hadrosaurs diversified into nearly two dozen known types, including Brachylophosaurus, Prosaurolophus, and Saurolophus. The horned ceratopsians divided into at least a dozen varieties, including the most famous, Triceratops.
Although the hadrosaurids are best known for their broad, horny, ducklike beaks, they are also famous for the different shaped crests on their heads. Perhaps used for vocalizing, the crests may also have identified male and female animals during mating season. Horns and frills evolved for the same reason in ceratopsians, with horns also used for combat and defense.
Plant-eaters were eaten by fierce tyrannosaurids. Most of those known from North America were 25 to 35 feet long, such as Albertosaurus. But the latest Late Cretaceous saw one of the smallest, Nanotyrannus, about 18 feet long, and the largest, Tyrannosaurus, 40 feet long. Ankylosaurs and nodosaurs were heavily armored plant-eaters that did not need the protection of a herd to avoid being eaten.
There were many small predators in North America. Among these were Aublysodon, Chirostenotes, and Troodon. They ate the small plant-eating dinosaurs, such the hypsilophodontids (Orodromeus, Parksosaurus, and Thescelosaurus), the small protoceratopsids (Leptoceratops and Montanoceratops), and the smaller dome-headed dinosaurs (Stegoceras and Stygimoloch). The swift, ostrichlike ornithomimids outran their predators.
Late Cretaceous Dinosaurs
Eastern Asia-especially Mongolia-was an abundant source of dinosaur species. Tyrannosaurids from Asia were smaller and more primitive than their North American relatives. Dome-headed dinosaurs from Mongolia were much different from those in North America. The only armored dinosaurs from Asia were ankylosaurids, such as Talarurus and Tarchia. At least three types of sauropods survived in Mongolia to the latest Late Cretaceous: Nemegtosaurus, Opisthocoelicaudia, and Quaesitosaurus.
The most interesting dinosaurs discovered in Mongolia and China are the segnosaurs. Wide-bodied herbivores with powerful claws, Erlikosaurus, Segnosaurus, and Therizinosaurus are known from nowhere else in the world. Another interesting Asian group was the oviraptorids, including Conchoraptor and Oviraptor.
Small predators closely related to those in western North America abounded in eastern Asia. These included the sickle-clawed dromaeosaurids Adasaurus, Hulsanpes, and Velociraptor. Other small predators were the troodontid Saurornithoides and Elmisaurus, which was found in both Mongolia and Canada. Shanshanosaurus from China may have been related to North America's Aublysodon. Ostrich-dinosaurs were quite unusual in Mongolia. Some were more primitive than their American relatives, while others were quite advanced, such as Anserimimus and the huge Gallimimus.
Europe was covered by a continental sea that divided it up into islands. This led to the evolution of dwarf dinosaurs known as "island endemics." Struthiosaurus was a miniature nodosaurid; Magyarosaurus and Hypselosaurus were small titanosaurids; and Rhabdodon and Craspedodon were small iguanodontids. Only Telmatosaurus, a primitive hadrosaurid, was about "normal" size.
The Gondwanaland continents had different Late Cretaceous dinosaurs. Almost all the Late Cretaceous Gondwanaland sauropods were titanosaurids. They were found mainly in South America, Africa, India, and Madagascar. Some titanosaurids were small, about 35 to 40 feet long, with ankylosaurlike armor scutes (plates) on their backs. Others were larger, such as Argyrosaurus.
The Late Cretaceous ended about 65 million years ago. All dinosaurs, mosasaurs, plesiosaurs, and pterosaurs vanished from the face of the earth. Furthermore, it seems that this extinction happened at almost the same time on all continents, in both northern and southern hemispheres. The rule of the dinosaurs ended with the end of the Mesozoic Era.