Cretaceous Period: Animals, Plants & Extinction Event
Tyrannosaurus rex is part of the carnivorous groups of dinosaurs that, according to new research, maintained a stable level of biodiversity leading up to the mass extinction at the end of the Cretaceous.
Credit: AMNH/J. Brougham

The Cretaceous Period was the last and longest segment of the Mesozoic Era. It lasted approximately 79 million years, from the minor extinction event that closed the Jurassic Period about 145.5 million years ago to the Cretaceous-Paleogene (K-Pg) extinction event dated at 65.5 million years ago.

In the early Cretaceous, the continents were in very different positions than they are today. Sections of the supercontinent Pangaea were drifting apart. The Tethys Ocean still separated the northern Laurasia continent from southern Gondwana. The North and South Atlantic were still closed, although the Central Atlantic had begun to open up in the late Jurassic Period. By the middle of the period, ocean levels were much higher; most of the landmass we are familiar with was underwater. By the end of the period, the continents were much closer to modern configuration. Africa and South America had assumed their distinctive shapes; but India had not yet collided with Asia and Australia was still part of Antarctica.

Parts of supercontinent Pangaea eventually drifted apart to become the continents we know today.
Parts of supercontinent Pangaea eventually drifted apart to become the continents we know today.
Credit: USGS

One of the hallmarks of the Cretaceous Period was the development and radiation of the flowering plants. The oldest angiosperm fossil that has been found to date is Archaefructus liaoningensis, found by Ge Sun and David Dilcher in China. It seems to have been most similar to the modern black pepper plant and is thought to be at least 122 million years old.

It used to be thought that the pollinating insects, such as bees and wasps, evolved at about the same time as the angiosperms. It was frequently cited as an example of co-evolution. New research, however, indicates that insect pollination was probably well established before the first flowers. While the oldest bee fossil was trapped in its amber prison only about 80 million years ago, evidence has been found that bee- or wasp-like insects built hive-like nests in what is now called the Petrified Forest in Arizona.

These nests, found by Stephen Hasiotis and his team from the University of Colorado, are at least 207 million years old. It is now thought that competition for insect attention probably facilitated the relatively rapid success and diversification of the flowering plants. As diverse flower forms lured insects to pollinate them, insects adapted to differing ways of gathering nectar and moving pollen thus setting up the intricate co-evolutionary systems we are familiar with today.

There is limited evidence that dinosaurs ate angiosperms. Two dinosaur coprolites (fossilized excrements) discovered in Utah contain fragments of angiosperm wood, according to an unpublished study presented at the 2015 Society of Vertebrate Paleontology annual meeting. This finding, as well as others, including an Early Cretaceous ankylosaur that had fossilized angiosperm fruit in its gut, suggests that some paleo-beasts ate flowering plants. 

Moreover, the shape of some teeth from Cretaceous animals suggests that the herbivores grazed on leaves and twigs, said Betsy Kruk, a volunteer researcher at the Field Museum of Natural History in Chicago. 

During the Cretaceous Period, more ancient birds took flight, joining the pterosaurs in the air. 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 theropods to leap high to grasp prey. Feathers probably evolved from early body coverings whose primary function, at least at first, was thermoregulation.

 

About the size of a crow, Confuciusornis is the earliest known bird to have a true beak. It lived about 10 to 15 million years after Archaeopteryx, but like its early ancestor, it still had clawed fingers. Males were typically larger than females and sported long, narrow tail feathers that they may have used to attract mates. Some scientists question whether Confuciusornis was a direct ancestor of modern birds. They propose instead that it was a cousin that early on went its own separate way.
About the size of a crow, Confuciusornis is the earliest known bird to have a true beak. It lived about 10 to 15 million years after Archaeopteryx, but like its early ancestor, it still had clawed fingers. Males were typically larger than females and sported long, narrow tail feathers that they may have used to attract mates. Some scientists question whether Confuciusornis was a direct ancestor of modern birds. They propose instead that it was a cousin that early on went its own separate way.
Credit: Eduard Solà Vázquez

At any rate it is clear that avians were highly successful and became widely diversified during the Cretaceous. Confuciusornis (125 million to 140 million years ago) was a crow-size bird with a modern beak, but enormous claws at the tips of the wings. Iberomesornis, a contemporary, only the size of a sparrow, was capable of flight and was probably an insectivore. [Image Gallery: Avian Ancestors: Dinosaurs That Learned to Fly]

 

By the end of the Jurassic, some of the large sauropods, such as Apatosaurus and Diplodocus, went extinct. But other giant sauropods, including the titanosaurs, flourished, especially toward the end of the Cretaceous, Kruk said. 

Large herds of herbivorous ornithischians also thrived during the Cretaceous, such as Iguanodon (a genus that includes duck-billed dinosaurs, also known as hadrosaurs), Ankylosaurus and the ceratopsians. Theropods, including Tyrannosaurus rex, continued as apex predators until the end of the Cretaceous.

About 65.5 million years ago, nearly all large vertebrates and many tropical invertebrates became extinct in what was clearly a geological, climatic and biological event with worldwide consequences. Geologists call it the K-Pg extinction event because it marks the boundary between the Cretaceous and Paleogene periods. The event was formally known as the Cretaceous-Tertiary (K-T) event, but the International Commission on Stratigraphy, which sets standards and boundaries for the geologic time scale, now discourages the use of the term Tertiary. The "K" is from the German word for Cretaceous, Kreide.

In 1979, a geologist who was studying rock layers between the Cretaceous and Paleogene periods spotted a thin layer of grey clay separating the two eras. Other scientists found this grey layer all over the world, and tests showed that it contained high concentrations of iridium, an element that is rare on Earth, but common in most meteorites, Kruk said in a class she co-taught on Coursera.org.

Also within this layer are indications of “shocked quartz” and tiny glass-like globes called tektites that form when rock is suddenly vaporized then immediately cooled, as happens when an extraterrestrial object strikes the Earth with great force.

The Chicxulub (CHEEK-sheh-loob) crater in the Yucatan dates precisely to this time. The crater site is more than 110 miles (180 kilometers) in diameter and chemical analysis shows that the sedimentary rock of the area was melted and mixed together by temperatures consistent with the blast impact of an asteroid about 6 miles (10 km) across striking the Earth at this point.

When the asteroid collided with Earth, its impact triggered shockwaves, massive tsunamis and sent a large cloud of hot rock and dust into the atmosphere, Kruk said. As the super-heated debris fell back to Earth, they started forest fires and increased temperatures.

"This rain of hot dust raised global temperatures for hours after the impact, and cooked alive animals that were too large to seek shelter," Kruk said in the class. "Small animals that could shelter underground, underwater, or perhaps in caves or large tree trunks, may have been able to survive this initial heat blast."

Tiny fragments likely stayed in the atmosphere, possibly blocking part of the sun's ray for months or years. With less sunlight, plants and the animals dependent on them would have died, Kruk said. Furthermore, the reduced sunlight would have lowered global temperatures, impairing large active animals with high-energy needs, she said.

"Smaller, omnivorous terrestrial animals, like mammals, lizards, turtles, or birds, may have been able to survive as scavengers feeding on the carcasses of dead dinosaurs, fungi, roots and decaying plant matter, while smaller animals with lower metabolisms were best able to wait the disaster out," Kruk said. 

There is also evidence that a series of huge volcanic eruptions at the Deccan traps, located along the tectonic border between India and Asia, began just before the K-Pg event boundary. It is likely that these regional catastrophes combined to precipitate a mass extinction.

The world was a warmer place during the Cretaceous period. The poles were cooler than the lower latitudes, but "overall things were warmer," Kruk told Live Science. Fossils of tropical plants and ferns support this idea, she said.

Animals lived all over, even in colder areas. For instance, Hadrosaurus fossils dating to the Late Cretaceous were uncovered in Alaska

When the asteroid hit, the world likely experienced so-called "nuclear winter," when particles blocked many of the sun's rays from hitting Earth.

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Additional reporting by Staff Writer Laura Geggel. Follow her on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+