Mesozoic Era (250-65 mya)
"Age of the Dinosaurs"
Cenozoic Era in a Nutshell
1. Triassic Period (251-200 mya)
By the start of the Triassic, all the Earth's landmasses had coalesced to form Pangaea, a supercontinent shaped like a giant C that straddled the Equator and extended toward the Poles. Almost as soon as the supercontinent formed, it started to come undone. By the end of the period 199 million years ago, tectonic forces had slowly begun to split the supercontinent in two: Laurasia in the north and Gondwana in the south.
The giant ocean called Panthalassa surrounded Pangaea. Areas near the coast were pummeled by seasonal monsoons, but ocean-circulation patterns kept the isolated and vast interior warm and dry. Even the Poles were ice-free. The Tethys Ocean filled the C and was the zipper upon which Pangaea began to split apart. Earlier failed attempts at the split formed rift valleys in North America and Africa filled with red sediments that today contain the best preserved fossils of Triassic life.
The oceans teemed with the coiled-shelled ammonites, mollusks, and sea urchins that survived the Permian extinction and were quickly diversifying. The first corals appeared, though other reef-building organisms were already present. Giant reptiles such as the dolphin-shaped ichthyosaurs and the long-necked and paddle-finned plesiosaurs preyed on fish and ancient squid. The bottom rung of the food chain was filled with microscopic plants called phytoplankton; two of the major groups still in the oceans today first appeared.
Frogs, salamanders, crocodiles, turtles, and snakes slunk and slithered on and off the Triassic coast, lakes, and rivers. Pterosaurs, a group of flying reptiles, took to the air. On firm ground, moss, liverwort, and ferns carpeted forests of conifers, ginkgoes, and palm-like cycads. Spiders, scorpions, millipedes, and centipedes thrived. Grasshoppers appeared. But perhaps the biggest changes came with the evolution of dinosaurs and the first mammals in the late Triassic, starting around 230 million years ago.
One of the earliest true mammals was the three-foot-long (one-meter-long)Eozostrodon. The shrewlike creature laid eggs but fed its young mother's milk. Among the first dinosaurs was the two-footed carnivore Coelophysis, which grew up to 9 feet (2.7 meters) tall, weighed up to a hundred pounds (45 kilograms), and probably fed on small reptiles and amphibians. It showed up about 225 million years ago. A few million years later came the 27.5-foot-long (8-meter-long) herbivore called Plateosaurus.
The Triassic closed in the same way it began. Something—perhaps a volcanic belch or an asteroid collision—caused another mass extinction. Dinosaurs, however, survived and went on to dominate the Jurassic.
2. Jurassic Period (200-146 mya)
At the start of the period, the breakup of the supercontinent Pangaea continued and accelerated. Laurasia, the northern half, broke up into North America and Eurasia. Gondwana, the southern half, began to break up by the mid-Jurassic. The eastern portion—Antarctica, Madagascar, India, and Australia—split from the western half—Africa and South America. New oceans flooded the spaces in between. Mountains rose on the seafloor, pushing sea levels higher and onto the continents.
All this water gave the previously hot and dry climate a humid and drippy subtropical feel. Dry deserts slowly took on a greener hue.
Palm tree-like cycads were abundant, as were conifers such as araucaria and pines. Ginkgoes carpeted the mid- to high northern latitudes, and podocarps, a type of conifer, were particularly successful south of the Equator. Tree ferns were also present.
The oceans, especially the newly formed shallow interior seas, teemed with diverse and abundant life. At the top of the food chain were the long-necked and paddle-finned plesiosaurs, giant marine crocodiles, sharks, and rays. Fishlike ichthyosaurs, squidlike cephalopods, and coil-shelled ammonites were abundant. Coral reefs grew in the warm waters, and sponges, snails, and mollusks flourished. Microscopic, free-floating plankton proliferated and may have turned parts of the ocean red.
On land, dinosaurs were making their mark in a big way—literally. The plant-eating sauropod Brachiosaurus stood up to 52 feet (16 meters) tall, stretched some 85 feet (26 meters) long, and weighed more than 80 tons. Diplodocus, another sauropod, was 90 feet (27 meters) long. These dinosaurs' sheer size may have deterred attack from Allosaurus, a bulky, meat-eating dinosaur that walked on two powerful legs. But Allosaurus and other fleet-footed carnivores, such as the coelurosaurs, must have had occasional success. Other prey included the heavily armored stegosaurs.
The earliest known bird, Archaeopteryx, took to the skies in the late Jurassic, most likely evolved from an early coelurosaurian dinosaur. Archaeopteryx had to compete for airspace with pterosaurs, flying reptiles that had been buzzing the skies since the late Triassic. Meanwhile, insects such as leafhoppers and beetles were abundant, and many of Earth's earliest mammals scurried around dinosaur feet—ignorant that their kind would come to dominate Earth once the dinosaurs were wiped out at the end of the Cretaceous.
Near the end of the Jurassic period several species died out giving rise to the Cretaceous period. This transition was not as violent as the mass extinction at the beginning of the Jurassic period nor as devastation as what would come near the end of the Cretaceous. However, it led to another era and bid farewell to one of the most dynamic epochs of earth's history.
3. Cretaceous Period (144-65mya)
Though Gondwana was still intact in the beginning of the Cretaceous, it broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds.
There was a temperature spike partly through the Cretaceous Period, but the shifted continents, expanded coasts, and widened oceans had cooled and moistened the planet's climate and set in motion dramatic changes to the flora and fauna.
Gigantic sauropods led parades of dinosaurs through the forests, over the plains, and along the coasts; long-necked and toothy marine reptiles terrorized fish, ammonites, and mollusks in the seas; pterosaurs and hairy-feathered birds filled the skies. But as the continents spread, the ocean currents churned with ever more vigor.
Though dinosaurs ruled throughout the Cretaceous, the dominant groups shifted and many new types evolved. Sauropods dominated the southern continents but became rare in the north. Herd-dwelling ornithischians like Iguanodon spread everywhere but Antarctica. Toward the close of the Cretaceous, vast herds of horned beasts such as Triceratops munched cycads and other low-lying plants on the northern continents. The carnivore Tyrannosaurus rex dominated the late Cretaceous in the north while monstrous meat-eaters like Spinosaurus, which had a huge sail-like fin on its back, thrived in the south. Smaller carnivores likely battled for the scraps.
Other creatures, such as frogs, salamanders, turtles, crocodiles, and snakes, proliferated on the expanded coasts. Shrewlike mammals scurried about the forests. The largest pterosaur known soared overhead though the group as a whole faced ever stiffening competition from fast diversifying birds: Ancestors to modern grebes, cormorants, pelicans, and sandpipers all show up in the Cretaceous.
In the warm, shallow seas that spilled onto the continents, the long-necked plesiosaurs gave way to the giant, snakelike mosasaurs. Rays and modern sharks became common. Sea urchins and sea stars (starfish) thrived; coral reefs continued to grow. Diatoms, a type of shelled plankton, made their first radiation into the ocean.
But it was the rapid dispersal of flowering plants that stole the show—a spread enhanced with the help of insects from bees and wasps to ants and beetles. Magnolia, ficus, and sassafras quickly outnumbered ferns, conifers, gingkoes, and cycads.
Perhaps the most notable event of the Cretaceous was its conclusion. About 65 million years ago the second greatest mass extinction in Earth history occurred, resulting in the loss of the dinosaurs as well as nearly 50% of all the world’s species. Though not nearly as severe as the end-Permian mass extinction, the end-Cretaceous extinction is the most famous mass extinction in Earth history. Other great animals also went extinct at that time, including flying reptiles (pterosaurs) and the last Mosasaurs and plesiosaurs.