Article 28: A most important event in the Cretaceous, at least for terrestrial life, is the first appearance of flowering plants. At the end of the period, volcanic eruptions are poisoning the atmosphere and an asteroid hits the Earth.
The Cretaceous Period of the Earth’s history spans 80 million years, from 145 to 65 million years ago.
No great extinction or burst of diversity separates the Cretaceous from the Jurassic Period that has preceded it. In some ways things go on as they did before. Dinosaurs, both great and small, f.e. Tyrannosaurus Rex, move through forests of ferns, cycads and conifers. Ammonites, belemnites, other molluscs and fish are hunted by great marine reptiles, while pterosaurs and birds flap and soar in the air above. Yet, the Cretaceous sees the first appearance of many life forms that will go on to play key roles in the coming Cenozoic world, the next 65 million years.
Tyrannosaurus rex. Credit: National Geographic Society
By the beginning of the Cretaceous, the supercontinent Pangea (Laurasia and Gondwana) is already drifting apart and by the mid-Cretaceous it is split into several smaller continents. The Tethys Ocean still separates the northern Laurasia continent from southern Gondwana. Laurasia splits into the continents of North America and Eurasia. This creates large-scale geographic isolation, causing a divergence in evolution of all land-based life for the two new land masses. The drifting apart also generates extensive new coastlines and a corresponding increase in the available near-shore habitat. Additionally, seasons begin to grow more pronounced as the global climate becomes cooler in the Early Cretaceous, the Berriasian.
After the end of the Berriasian, however, temperatures increase again towards the Cretaceous Thermal Maximum, and these conditions are almost constant until the end of the period. This trend is due to intense volcanic activity, which produces large quantities of carbon dioxide. The production of large quantities of magma further pushes sea levels up, so that large areas of the continental crust are covered with shallow seas.
During the Cretaceous the South Atlantic Ocean opens. India separates from Madagascar and races northward on a collision course with Eurasia. Notice that North America is connected to Europe and that Australia is still joined to Antarctica.
The Central Atlantic began to open up in the late Jurassic Period. By the end of this period the continents are much closer to modern configuration. Africa and South America assume their distinctive shapes, but India has not yet collided with Asia and Australia is still part of Antarctica.
Forests evolve to look similar to present day forests, with oaks and magnolias becoming common in North America by the end of the Cretaceous. Perhaps the most important event, at least for terrestrial life, is the first appearance of flowering plants. First appearing in the Lower Cretaceous, around 125 million years ago, the flowering plants first radiate in the middle Cretaceous, about 100 million years ago.
Flowering plants, called angiosperms by scientists, were believed to be already diverse and found in most locations by the middle of the Cretaceous period, about 100 million years ago.
Also during this time, we find the first fossils of many insect groups, modern mammal and bird groups. During the Cretaceous period, birds replace the pterosaurs in the air.
Shanweiniao cooperorum was a species of Enantiornithes that did not survive past the Cretaceous period. Credit: Nobu Tamura
The origin of flight is debated by many experts. In the “trees down” theory, it is thought that small reptiles may have evolved flight from gliding behaviors. In the “ground up” hypothesis, flight may have evolved from the ability of small theropod dinosaurs to leap high to grasp prey. Feathers probably evolved from early body coverings whose primary function, at least at first, was thermoregulation. At any rate, it is clear that Avians are highly successful and become widely diversified during the Cretaceous.
At the end of the Cretaceous Period, 65 million years ago, the Deccan Traps and other volcanic eruptions are poisoning the atmosphere.
The Deccan Traps. Credit: The Geological Society of America
An asteroid hits the Earth in the Yucatan Peninsula, Mexico, forming what is called today the Chicxulub impact crater. It has been estimated that half of the world’s species goes extinct at about this time, but no accurate species count exists for all groups of organisms.
Artist’s impression of a 6-mile-wide asteroid striking the Earth. Scientists now have fresh evidence that such a cosmic impact ended the age of dinosaurs near what is now the town of Chixculub in Mexico. Credit: Don Davis
As everyone knows, this is the great Cretaceous–Tertiary extinction, in which the dinosaurs die out, except for the birds, of course. The other lineages of marine reptiles, the ichthyosaurs, plesiosaurs and mosasaurs, are also extinct by the end of the Cretaceous, as are the flying pterosaurs, but some, like the ichthyosaurs, are probably extinct a little before the end of the Cretaceous.
Many species of foraminiferans are extinct at the end of the Cretaceous, as are the ammonites.
But many groups of organisms, such as flowering plants, snails and clams, amphibians, lizards and snakes, crocodilians and mammals, sail through the Cretaceous-Tertiary boundary, with few or no apparent extinctions at all.
Thanks to the University of California, Berkeley, Museum of Paleontology, to Christopher R. Scotese, and to LiveScience