By Zion Thomas
Dinosaurs are a diverse group of animals of the clade Dinosauria. They first appeared during the Triassic period, 231.4 million years ago, and were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 201 million years ago) until the end of the Cretaceous (66 million years ago), when the Cretaceous-Paleogene extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic Era. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic Period and, consequently, they are considered a subgroup of dinosaurs. Some birds survived the extinction event that occurred 66 million years ago, and their descendants continue the dinosaur lineage to the present day.
Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints. Birds, at over 10,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs. Dinosaurs are represented on every continent by both extant species and fossil remains. Some are herbivorous, others carnivorous. While dinosaurs were ancestrally bipedal, many extinct groups included quadrupel species, and some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and Spines. Evidence suggests that egg laying and nest building are additional traits shared by all dinosaurs. While modern dinosaurs (birds) are generally small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest Sauropod dinosaurs may have achieved lengths of 58 meters (190 feet) and heights of 9.25 meters (30 feet 4 inches). Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based on preservation bias, as large, sturdy bones are more likely to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm (20 in) long.
Although the word dinosaur means "terrible lizard", the name is somewhat misleading, as dinosaurs are not lizards. Instead, they represent a separate group of reptiles that, like many extinct forms, did not exhibit characteristics traditionally seen as reptilian, such as a sprawling limb posture or ectothermy. Additionally, many prehistoric animals, including Mosasaurs, ichtyosaurs, Pterosaurs, Pleiosaurs, and Dimetrodon, are popularly conceived of as dinosaurs, but are not taxonomically classified as dinosaurs. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted in the 1790's, however, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction.Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of world culture. The large sizes of some groups, as well as their seemingly monstrous and fantastic nature, have ensured dinosaurs' regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, and new discoveries are regularly covered by the media.
Pterosaurs meaning "winged lizard") were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Creataceous Peroid (228 to 66 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger.Early species had long, fully toothed jaws and long tails, while later forms had a highly reduced tail, and some lacked teeth. Many sported furry coats made up of hair-like filaments known as pycnofibers, which covered their bodies and parts of their wings. Pterosaurs spanned a wide range of adult sizes, from the very small Nemicolopterus to the largest known flying creatures of all time, including Quatzelcoatlus and Hatzegopteryx.Pterosaurs are often referred to in the popular media and by the general public as flying dinosaurs, but this is incorrect. The term "dinosaur" is restricted to just those reptiles descended from the last common ancestor of the groups Saurischia and Ornithischia (clade Dinosauria, which includes birds), and current scientific consensus is that this group excludes the pterosaurs, as well as the various groups of extinct marine reptiles, such as icthyosaurs, plesiosaurs, and mosasaurs. Like the dinosaurs, and unlike these other reptiles, pterosaurs are more closely related to birds than to crocodiles or any other living reptile. Pterosaurs are also colloquially referred to as pterodactyls, particularly by journalists. Technically, "Pterodactyl" refers only to members of the genus Pterodactylus, and more broadly to members of the suborder Pterodactyloidea of the Pterosaurs.
Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semi-aquatic life in a marine environment.
The earliest marine reptiles arose in the Permian period during the Paleozoic era. During the Mesozoic era, many groups of reptiles became adapted to life in the seas, including such familiar clades as the icthyosaurs, plesiosaurs (these two orders were once thought united in the group "Enaliosauria, a classification now cladistically obsolete), mosasaurs, nothosaurs, placodonts, sea turtles, thalattosaurs and thalattosuchians.
After the mass extinction at the end of the Cretaceous period, marine reptiles were less numerous. Extant marine reptiles include marine iguanas, sea snakes, sea turtles and saltwater crocodiles. The Murua gharial was yet another example of a fully marine reptile, that became extinct rather recently.
Some marine reptiles, such as ichthyosaurs and mosasaurs, rarely ventured onto land and gave birth in the water. Others, such as sea turtles and saltwater crocodiles, return to shore to lay their eggs. Some marine reptiles also occasionally rest and bask on land.
The Cretaceous–Paleogene extinction event, was a mass extinction of some three-quarters of plant and animal species on Earth—including all non-avian dinosaurs—that occurred over a geologically short period of time, 66 million years ago. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.
In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids.
As originally proposed by a team of scientists led by Luis Alvarez, it is now generally believed that the K–Pg extinction was triggered by a massive comet/asteroid impact and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, or sea level change, separately or together.