The prehistoric reptiles known as dinosaurs arose during the Middle to Late Triassic Period of the Mesozoic Era, some 230 million years ago. They were members of a subclass of reptiles called the arc
Scientists first began studying dinosaurs during the 1820s, when they discovered the bones of a large land reptile they dubbed a Megalosaurus (“big lizard”) buried in the English countryside. In 1842, Sir Richard Owen, Britain’s leading paleontologist, first coined the term “dinosaur.” Owen had examined bones from three different creatures–Megalosaurus, Iguanadon (“iguana tooth”) and Hylaeosaurus (“woodland lizard”). Each of them lived on land, was larger than any living reptile, walked with their legs directly beneath their bodies instead of out to the sides and had three more vertebrae in their hips than other known reptiles. Using this information, Owen determined that the three formed a special group of reptiles, which he named Dinosauria. The word comes from the ancient Greek word deinos (“terrible”) and sauros (“lizard” or “reptile”).
Since then, dinosaur fossils have been found all over the world and studied by paleontologists to find out more about the many different types of these creatures that existed. Scientists have traditionally divided the dinosaur group into two orders: the “bird-hipped” Ornithischia and the “lizard-hipped” Saurischia. From there, dinosaurs have been broken down into numerous genera (e.g. Tyrannosaurus or Triceratops) and each genus into one or more species. Some dinosaurs were bipedal, which means they walked on two legs. Some walked on four legs (quadrupedal), and some were able to switch between these two walking styles. Some dinosaurs were covered with a type of body armor, and some probably had feathers, like their modern bird relatives. Some moved quickly, while others were lumbering and slow. Most dinosaurs were herbivores, or plant-eaters, but some were carnivorous and hunted or scavenged other dinosaurs in order to survive.
At the time the dinosaurs arose, all of the Earth’s continents were connected together in one land mass, now known as Pangaea, and surrounded by one enormous ocean. Pangaea began to break apart into separate continents during the Early Jurassic Period (around 200 million years ago), and dinosaurs would have seen great changes in the world in which they lived over the course of their existence. Dinosaurs mysteriously disappeared at the end of the Cretaceous Period, around 65 million years ago. Many other types of animals, as well as many species of plants, died out around the same time, and numerous competing theories exist as to what caused this mass extinction. In addition to the great volcanic or tectonic activity that was occurring around that time, scientists have also discovered that a giant asteroid hit Earth about 65.5 million years ago, landing with the force of 180 trillion tons of TNT and spreading an enormous amount of ash all over the Earth’s surface. Deprived of water and sunlight, plants and algae would have died, killing off the planet’s herbivores; after a period of surviving on the carcasses of these herbivores, carnivores would have died out as well.
Despite the fact that dinosaurs no longer walk the Earth as they did during the Mesozoic Era, unmistakable traces of these enormous reptiles can be identified in their modern-day descendants: birds. Dinosaurs also live on in the study of paleontology, and new information about them is constantly being uncovered. Finally, judging from their frequent appearances in the movies and on television, dinosaurs have a firm hold in the popular imagination, one realm in which they show no danger of becoming extinct.
Cannibal Clues Worsen Tyrannosaurus Rex’s Bad Rap
As if its reputation weren't bad enough already, scientists have unearthed new evidence that Tyrannosaurus rex was one of fiercest predators the world has ever seen. It turns out the 7-ton, 13-foot-tall monster may have had a taste for its own kind.
Meals were a gruesome production for Tyrannosaurus rex, one of the largest—and toothiest—creatures to ever stalk the earth. For the most part, the king of dinosaurs probably subsisted on its slower, smaller, vegetarian brethren, such as duck-billed hadrosaurs like Edmontosaurus and horned ceratopsians like Triceratops. It used its incredible speed and massive jaw to hunt and slay its victims, but it also wasn’t above scavenging for dead animals. Some scientists have even theorized that T. rex poisoned its struggling prey with toxic saliva, much like the modern-day Komodo dragon.
According to a recent study, T. rex’s rather brutal feeding patterns may also have included one of humanity’s most unspeakable taboos: cannibalism. Paleontologist Nicholas Longrich of Yale University discovered giant tooth marks half an inch deep in arm and foot bones from Tyrannosaurus rex fossils found in western North America and dated to the late Maastrichtian period. Given the size and shape of the gouges, another T. rex is the only possible culprit.
“They’re the kind of marks that any big carnivore could have made, but T. rex was the only big carnivore in western North America 65 million years ago,” Longrich explained in a statement.
Writing in the October 15 issue of the journal PLoS ONE, Longrich and his colleagues said it was unlikely that the gashes resulted from claw-to-claw combat, which scientists believe was common behavior for Tyrannosaurus rex. Battling dinosaurs would probably have lunged for each other’s more vulnerable areas, such as the head and flanks, rather than the feet and arms. Instead, the researchers suggested, a hungry Tyrannosaurus probably came across the carcass of another while scavenging for food. Alternatively, the victor of a violent intraspecies squabble may have celebrated by eating its dead opponent, toes and all.
The researchers found four examples of T. rex cannibalism within a small sample of tooth-marked bones, an indication that the practice was rampant rather than a last resort in desperate times. (In other words, there’s no evidence of a dinosaur Donner Party.) This finding is hardly surprising: In 2007, a team of scientists made a compelling case for cannibalism in another theropod, Majungasaurus, which existed several million years before Tyrannosaurus.
Today, cannibalism occurs in every corner of the animal kingdom, especially among large carnivores such as bears, hyenas and alligators. Smaller critters do it too, including scorpions, rattlesnakes and, perhaps most notoriously, female praying mantises, which decapitate their mates during sex. For some species, cannibalism is so instinctual it starts before birth: Sand tiger sharks, for instance, are known to devour their siblings while still in the womb.
The study is significant because it sheds new light on the diets and feeding habits not only of Tyrannosaurus rex but also of other gigantic predators that left their massive footprint in the fossil record. “Given that cannibalism is known in Tyrannosaurus, Majungatholus and many extant, large-bodied carnivores, this behavior is likely to have been widespread in large, carnivorous dinosaurs,” the researchers wrote.
Mass Extinctions
What Causes Animal Die-Offs?
More than 90 percent of all organisms that have ever lived on Earth are extinct. As new species evolve to fit ever changing ecological niches, older species fade away. But the rate of extinction is far from constant. At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye.
Though these mass extinctions are deadly events, they open up the planet for new life-forms to emerge. Dinosaurs appeared after one of the biggest mass extinction events on Earth, the Permian-Triassic extinction about 250 million years ago. The most studied mass extinction, between the Cretaceous and Paleogene periods about 65 million years ago, killed off the dinosaurs and made room for mammals to rapidly diversify and evolve.
The causes of these mass extinction events are unsolved mysteries, though volcanic eruptions and the impacts of large asteroids or comets are prime suspects in many of the cases. Both would eject tons of debris into the atmosphere, darkening the skies for at least months on end. Starved of sunlight, plants and plant-eating creatures would quickly die. Space rocks and volcanoes could also unleash toxic and heat-trapping gases that—once the dust settled—enable runaway global warming.
An extraterrestrial impact is most closely linked to the Cretaceous extinction event. A huge crater off Mexico's Yucatán Peninsula is dated to about 65 million years ago, coinciding with the extinction. Global warming fueled by volcanic eruptions at the Deccan Flats in India may also have aggravated the event. Whatever the cause, dinosaurs, as well as about half of all species on the planet, went extinct.
Massive floods of lava erupting from the central Atlantic magmatic province about 200 million years ago may explain the Triassic-Jurassic extinction. About 20 percent of all marine families went extinct, as well as most mammal-like creatures, many large amphibians, and all non-dinosaur archosaurs. An asteroid impact is another possible cause of the extinction, though a telltale crater has yet to be found.
Largest Ever Die-Off
The Permian-Triassic extinction event about 250 million years ago was the deadliest: More than 90 percent of all species perished. Many scientists believe an asteroid or comet triggered the massive die-off, but, again, no crater has been found. Another strong contender is flood volcanism from the Siberian Traps, a large igneous province in Russia. Impact-triggered volcanism is yet another possibility.
Starting about 360 million years ago, a drawn-out event eliminated about 70 percent of all marine species from Earth over a span of perhaps 20 million years. Pulses, each lasting 100,000 to 300,000 years, are noted within the larger lateDevonian extinction. Insects, plants, and the first proto-amphibians were on land by then, though the extinctions dealt landlubbers a severe setback.
The Ordovician-Silurian extinction, about 440 million years ago, involved massive glaciations that locked up much of the world's water as ice and caused sea levels to drop precipitously. The event took its hardest toll on marine organisms such as shelled brachiopods, eel-like conodonts, and the trilobites.
Happening Now?
Today, many scientists think the evidence indicates a sixth mass extinction is under way. The blame for this one, perhaps the fastest in Earth's history, falls firmly on the shoulders of humans. By the year 2100, human activities such as pollution, land clearing, and overfishing may have driven more than half of the world's marine and land species to extinction.
http://science.nationalgeographic.com/science/prehistoric-world/mass-extinction/
Dinosaur Extinction
Sixty-five million years ago the last of the non-avian dinosaurs went extinct. So too did the giant mosasaurs and plesiosaurs in the seas and the pterosaurs in the skies. Plankton, the base of the ocean food chain, took a hard hit. Many families of brachiopods and sea sponges disappeared. The remaining hard-shelled ammonites vanished. Shark diversity shriveled. Most vegetation withered. In all, more than half of the world's species were obliterated.
What caused this mass extinction that marks the end of the Cretaceous and the beginning of the Paleogene? Scientists have yet to find an answer. The one that does must explain why these animals died while most mammals, turtles, crocodiles, salamanders, and frogs survived. Birds escaped. So did snails, bivalves, sea stars (starfish), and sea urchins. Even hardy plants able to weather climate extremes fared OK.
Scientists tend to huddle around one of two hypotheses that may explain the Cretaceous extinction: an extraterrestrial impact, such as an asteroid or comet, or a massive bout of volcanism. Either scenario would have choked the skies with debris that starved the Earth of the sun's energy, throwing a wrench in photosynthesis and sending destruction up and down the food chain. Once the dust settled, greenhouse gases locked in the atmosphere would have caused the temperature to soar, a swift climate swing to topple much of the life that survived the prolonged darkness.
Asteroid or Volcanoes?
The extraterrestrial impact theory stems from the discovery that a layer of rock dated precisely to the extinction event is rich in the metal iridium. This layer is found all over the world, on land and in the oceans. Iridium is rare on Earth but it's found in meteorites at the same concentration as in this layer. This led scientists to postulate that the iridium was scattered worldwide when a comet or asteroid struck somewhere on Earth and then vaporized. A 110-mile-wide (180-kilometer-wide) crater carved out of Mexico's Yucatán Peninsula, called Chicxulub, has since been found and dated to 65 million years ago. Many scientists believe the fallout from the impact killed the dinosaurs.
But Earth's core is also rich in iridium, and the core is the source of magma that some scientists say spewed out in vast, floodlike flows that piled up more than 1.5 miles (2.4 kilometers) thick over 1 million square miles (2.6 million square kilometers) of India. This bout of volcanism has also been dated to about 65 million years ago and would have spread the iridium around the world, along with sunlight-blocking dust and soot and greenhouse gases.
Both hypotheses have merit. Some scientists think both may have contributed to the extinction, and others suggest the real cause was a more gradual shift in climate and changing sea levels. Regardless of what caused the extinction, it marked the end of Tyrannosaurus rex's reign of terror and opened the door for mammals to rapidly diversify and evolve into newly opened niches.
http://science.nationalgeographic.com/science/prehistoric-world/dinosaur-extinction/
