Dinosaur

Dinosaurs are a diverse group of warmblooded reptiles in the clade Dinosauria. They lived for most of the Mesozoic era, first appearing early in the Triassic period. After the Triassic–Jurassic extinction event, about 201.3 mya, they became the main land vertebrates. They stayed in charge through the Jurassic and Cretaceous periods. Fossils show that birds are feathered dinosaurs. They evolved from earlier theropods during the Late Jurassic epoch. Birds are the only dinosaur group known to have survived the Cretaceous–Paleogene extinction event, about 66 mya. Dinosaurs can be split into avian dinosaurs, which are birds, and the non-avian dinosaurs, which are all the other dinosaurs and are now extinct.

Dinosaurs come in many different forms, sizes, and ways of living. Birds alone have over 11,000 living species, so they are one of the most varied groups of vertebrates. Using fossils, paleontologists have identified over 900 genera and more than 1,000 species of non-avian dinosaurs. Dinosaur fossils have been found on every continent. For most of the 20th century, scientists thought dinosaurs were slow and cold-blooded. However, most research conducted since the 1970s has shown that dinosaurs were active animals with high metabolisms. Many of them also had ways of interacting with each other in groups. All dinosaurs seem to have had legs held straight under their bodies, like birds and many mammals. Some were herbivorous and others were carnivorous. The evidence suggests that all dinosaurs laid eggs, and that many of them, both avian and non-avian, built nests.

Dinosaurs first walked on two legs, but many later groups walked on four legs, and some could switch between the two. Showy features such as horns or crests are common to all dinosaur groups. Some extinct groups had features such as bony armor and spines. Modern birds are generally small because flight limits how big a body can be. Many non-avian dinosaurs, however, were very large. The largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters (130 feet) and heights of 18 m (59 ft), making them the largest land animals of all time.

The first dinosaur fossils were recognized in the early 19th century. In 1842, Sir Richard Owen named the "dinosaur," which means "terrible lizard," to describe these "great fossil lizards." Since then, mounted dinosaur skeletons have been major attractions at museums around the world, and dinosaurs have become an enduring part of popular culture. Their large sizes and fantastic looks have led to appearances in many popular books and films, such as the Jurassic Park franchise. Public interest in these animals has brought significant funding for dinosaur science, and new discoveries are often covered in the news.

Etymology

In 1842, the English scientist Sir Richard Owen created the word "dinosaur". He used it to describe a group of reptiles that were being discovered in England and around the world.

The word comes from Ancient Greek. Deinos means 'terrible, powerful or very great', and sauros means 'lizard or reptile'. People have often thought the name refers to dinosaurs' teeth, claws, and other strong features. Owen also meant it to show their size and grandeur.

Definition

Scientists usually define dinosaurs as the group made up of the most recent common ancestor (MRCA) of Triceratops and modern birds, along with all of its descendants. This definition uses phylogenetic nomenclature to name groups. Another idea is to use the MRCA of Megalosaurus and Iguanodon, two of the three genera that Richard Owen used when he first recognized dinosaurs. Both definitions cover the same known groups, Ornithischia and Saurischia. The major groups include ankylosaurians, which were armored plant-eaters on four legs, and stegosaurians, which had plates on their backs. Ceratopsians were plant-eaters with neck frills. Pachycephalosaurians walked on two legs and had thick skulls. Ornithopods, including the "duck-bills," were also plant-eaters. Theropods were mostly two-legged meat-eaters, and they include birds. Sauropodomorphs were large plant-eaters with long necks and tails.

Birds are the only dinosaurs alive today. In older classification, birds were put in a separate class that was thought to have evolved from dinosaurs. Most scientists now group animals by their ancestry rather than by how much they look alike. Birds belong to Maniraptora, which are coelurosaurs, which are theropods, which are saurischians.

In 2017, Matthew G. Baron, David B. Norman, and Paul M. Barrett suggested a big change. Their study found that ornithischians were closer to theropods than to sauropodomorphs. This would leave sauropods outside traditional dinosaurs. So they defined Dinosauria using the last common ancestor of Triceratops horridus, Passer domesticus, and Diplodocus carnegii, and all of its descendants. This way, sauropods stay included. They also brought back the group Ornithoscelida, which includes Ornithischia and Theropoda.

Dinosaurs can be described in general as archosaurs with hind limbs held erect beneath the body. Other ancient animals, such as pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, are often thought of as dinosaurs, but they are not classified that way. Pterosaurs are distant relatives and belong to the group Ornithodira. The others belong to Sauropsida, the group of reptiles and birds. The exception is Dimetrodon, which is a synapsid.

Dinosaurs were the main land animals of the Mesozoic Era, especially during the Jurassic and Cretaceous periods. Other animals stayed small by comparison. Dinosaurs were very varied, with over 900 kinds of non-bird dinosaurs identified. Some ate plants and others ate meat, and many were about human-sized or smaller. Some, such as Anchisaurus, could walk on two or four legs. Dinosaurs today live on every continent, and by the Early Jurassic epoch they had spread across the world. Some, like Microraptor, could fly or at least glide. Spinosaurids lived partly in water, as semiaquatic animals. A detailed study by Sterling Nesbitt confirmed a set of shared bone features, called synapomorphies, that come from the ancestral archosaur skeleton.

Dinosaurs stood with their hind limbs erect, like most modern mammals. Most other reptiles have limbs that sprawl out to the sides. This upright posture likely helped early dinosaurs breathe easily while moving. It helped them become more active and have more stamina than those of "sprawling" reptiles. It probably also helped the evolution of large body size.

History of study

Further information: History of paleontology

People have known about dinosaur fossils for thousands of years, but they did not know what the fossils were. Chinese writers long described old animal bones as "dragon bones", and people used them in traditional medicine. In Europe, people often thought dinosaur fossils were the remains of giants and other creatures from the Bible. In the late 1600s, scientists in England began writing about what we now call dinosaur bones. In 1824, the Rev William Buckland became the first person to describe a non-avian dinosaur in a scientific journal. He based his description on Megalosaurus bones. In 1858, William Parker Foulke found the first known American dinosaur, Hadrosaurus foulkii, in New Jersey. In the late 1800s, two scientists, Edward Drinker Cope and Othniel Charles Marsh, competed with each other. Their competition led to many new discoveries. However, their rough digging methods also damaged many valuable fossils.

Evolutionary history

Dinosaurs appeared about 20 million years after the Permian–Triassic extinction event. That event wiped out many kinds of living things about 252 million years ago. The oldest well-preserved dinosaur fossils come from the Triassic period. They were found in the Ischigualasto and Santa Maria Formations of Argentina and Brazil, and in the Pebbly Arkose Formation of Zimbabwe. The early dinosaur Eoraptor suggests that the first dinosaurs were small and walked on two legs. They were predators. At first, dinosaurs were not the most common land animals. Their main rivals, the pseudosuchians, were more successful. Many of those other animals died out during the Triassic. Dinosaurs then spread into the open spaces they left behind.

Dinosaurs changed over time as plants and the land changed. In the Late Triassic and Early Jurassic, all the continents were joined together as one big landmass, Pangaea. Dinosaurs lived all over the world. Plants called conifers were an important food source. In the Jurassic, many dinosaur groups looked alike in different places, and true birds first appeared. In the Early Cretaceous, Pangaea broke apart. Dinosaurs became more different from one region to another. Flowering plants also appeared. Some plant-eating dinosaurs developed better ways to chew their food. In the Late Cretaceous, there were three main dinosaur groups in different parts of the world. Around 66 million years ago, the Cretaceous–Paleogene extinction event ended all dinosaur groups except the birds. After this, birds spread into many new habitats during the Paleogene period.

Classification

Saurischian pelvis structure (left side)

Tyrannosaurus pelvis (showing saurischian structure – left side)

Ornithischian pelvis structure (left side)

Edmontosaurus pelvis (showing ornithischian structure – left side)

Dinosaurs belong to a large group called archosaurs, which also includes modern crocodiles and alligators. Dinosaurs are mostly set apart from other archosaurs by the way they stand and move. Their legs go straight down under their bodies. The legs of lizards and crocodiles stick out to the sides.

Dinosaurs are split into two main groups, Saurischia and Ornithischia. These groups are mostly told apart by their pelvic (hip) bones. The "lizard-hipped" saurischians got their name from the Greek words for "lizard" and "hip joint." They kept the hip shape of their ancestors, with a pubis bone pointing forward, or cranially. Saurischia includes the theropods, which walked only on two legs and ate many kinds of food. It also includes the long-necked plant-eaters. The "bird-hipped" ornithischians had hips that looked a bit like a bird's, with the pubis bone pointing backward. Most ornithischians ate plants. Even though their name suggests it, birds are not ornithischians. They belong to Saurischia.

Paleobiology

See also: Dinosaur egg

Main article: Physiology of dinosaurs

Scientists learn about dinosaurs from many kinds of clues. These include fossil bones, feces, trackways, gastroliths, feathers, skin prints, internal organs and other soft tissues. Many fields of science help, such as physics (especially biomechanics), chemistry, biology, and the Earth sciences. Paleontology is one part of the Earth sciences. Two topics are of special interest: dinosaur size and behavior. Dinosaurs changed in size over time. The sauropods were the largest and heaviest dinosaurs. Only modern whales come close to their size. Scientists will probably never know for certain which dinosaurs were the largest and smallest, because very few animals became fossils and most fossils are incomplete. The smallest dinosaur known is the bee hummingbird.

Many modern birds live in flocks. Fossils and trackways suggest that many dinosaur groups also moved in herds or flocks. Some dinosaurs seem to have used their crests and frills to show off. Head wounds show that some theropods fought each other. Dinosaurs also used sounds and visual signals. Fossil larynxes suggest that some could make calls like birds. Dinosaurs laid amniotic eggs in nests, and some looked after their young after they hatched. Scientists think most dinosaurs were endothermic ("warm-blooded"), though researchers still argue about how far back this trait goes. Many scientists now think most dinosaurs had body temperatures between those of other reptiles and living birds and mammals. This in-between state is called mesothermy.

Origin of birds

Main article: Origin of birds

Further information: Feathered dinosaurs

In 1868, Thomas Henry Huxley first suggested that dinosaurs might be the ancestors of birds. Later, after Gerhard Heilmann did his work, the theory that birds came from dinosaurs was set aside. Scientists thought birds came from simpler thecodonts instead. One main reason was that dinosaurs seemed to lack clavicles. Later discoveries showed that clavicles, or a single joined wishbone, were not really missing. They were found in 1924 in Oviraptor, but were wrongly named an interclavicle. In the 1970s, John Ostrom brought back the idea that birds came from dinosaurs, and many new fossils supported it. Birds share over a hundred features with other meat-eating dinosaurs. They are most closely related to a group called maniraptoran coelurosaurs. A few scientists, including Alan Feduccia and Larry Martin, have suggested other paths. They think birds may have come from meat-eating dinosaur-like animals that looked similar to dinosaurs but were not dinosaurs themselves.

Feathers are one of the most recognizable things about modern birds, and some non-bird dinosaurs had them too. Fossils show simple, branched, feather-like structures in heterodontosaurids, early neornithischians, theropods, and early ceratopsians. True feathers, like those of modern birds, have only been found in the Maniraptora group. Feather-like pycnofibres have also been found in pterosaurs. Archaeopteryx was the first fossil to show a possible link between dinosaurs and birds. It is a transitional fossil, which means it has features of both. It was found two years after Charles Darwin's On the Origin of Species (1859) and sparked debate between evolutionary biology and creationism. Bird and dinosaur skeletons share many features, including the neck, wrist, pectoral girdle, furcula, and breast bone. A 2005 study led by Patrick M. O'Connor found that large meat-eating dinosaurs had air sacs like those of modern birds. In 2008, the description of Aerosteon riocoloradensis used CT scanning to give strong evidence of a bird-like breathing system. Fossils of the troodonts Mei and Sinornithoides show that some dinosaurs slept with their heads tucked under their arms, just like many modern birds do. Some dinosaurs also swallowed gizzard stones to help them digest food, as modern birds do.

Extinction of major groups

Main article: Cretaceous–Paleogene extinction event

All non-avian dinosaurs and most bird lines died out in a mass extinction event at the end of the Cretaceous period. This event is called the Cretaceous–Paleogene (K-Pg) extinction event. Fossils of non-avian dinosaurs stop suddenly above the Cretaceous–Paleogene boundary, which is about 66 million years old. Two causes are thought to have been very important. One was an impact event in the Yucatán Peninsula. The other was volcanic eruptions of flood basalt in India. Many other animals also died out, including ammonites, rudists, and many kinds of tiny sea plankton. A large part of life on Earth was lost. Most dinosaurs were very big, and there were few small dinosaur kinds at the end of the Cretaceous. This may have made it harder for them to survive.

Scientists estimate that many different non-avian dinosaur kinds lived just before the extinction. It is still unclear whether dinosaurs were becoming fewer or doing well before it happened. In 1980, Walter Alvarez, Luis Alvarez, and their colleagues first suggested that a space rock hit Earth and caused the extinction. Later, scientists found the impact site in the Yucatán Peninsula. It is a crater about 180 kilometres (110 miles) wide. The Deccan Traps in India were also erupting at that time, and they may have played a part too. Scientists are still studying how much each cause contributed.

Cultural depictions

Main article: Cultural depictions of dinosaurs

Dinosaurs looked strange and were often huge, so people have loved them for a long time. The word "dinosaur" is used in everyday vernacular. In English, it can describe anything that is too big to be practical, out of date, or likely to disappear. People first became interested in dinosaurs in Victorian England. In 1854, about thirty years after the first scientific descriptions of dinosaur remains, a set of lifelike dinosaur sculptures was shown in London's Crystal Palace Park. They were so popular that small copies soon sold widely. In the following years, dinosaur displays opened at parks and museums around the world. Interest in dinosaurs also brought money for dinosaur science and often led to new discoveries. In the United States, for example, museums competed for attention in the Bone Wars of the 1880s and 1890s. Two paleontologists who did not get along made big scientific contributions during this time.

Dinosaurs appear often in literature, film, and other media. Starting in 1852 with a short mention in Charles Dickens' Bleak House, they have appeared in many fictional works. Examples include Jules Verne's 1864 novel Journey to the Center of the Earth, Sir Arthur Conan Doyle's 1912 book The Lost World, the 1914 animated film Gertie the Dinosaur (which featured the first animated dinosaur), the famous 1933 film King Kong, the 1954 Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton, and its 1993 film adaptation. Writers of general non-fiction books about dinosaurs, including some well-known paleontologists, have often used these animals to teach readers about science. Dinosaurs are also common in advertising. Many companies have shown dinosaurs in printed or TV ads. Sometimes they sell their own products this way, and sometimes they describe their rivals as slow, not very clever, or out of date.