Dinosaur

Humans never co-existed with dinosaurs, yet a surprising amount is known about these remarkable reptiles. Evidence about the existence and nature of dinosaurs is entirely indirect; it has been gleaned from fossilized traces that these animals left in sediment deposits.

The first indications suggesting the existence of the huge, extinct creatures that we now know as dinosaurs were traces of their ancient footprints in sedimentary rocks. Dinosaurs left their footprints in soft mud as they moved along marine shores or riverbanks. That mud was subsequently covered over as a new layer of sediment accumulated, and it later solidified into rock. Under very rare circumstances, this process preserved traces of the footprints of dinosaurs. Interestingly, the footprints were initially attributed to giant birds because of their superficial resemblance to tracks made by the largest of the living birds, such as the ostrich and emu.

The first fossilized skeletal remains to be identified as those of giant, extinct reptiles were discovered by miners in western Europe. These first discoveries were initially presumed to be astonishingly gigantic, extinct lizards. Several naturalists recognized substantial anatomical differences between the fossil bones and those of living reptiles, however, and so the dinosaurs were "discovered." The first of these finds consisted of bones of a 35-50 ft (10-15 m) long carnivore named Megalosaurus; this was the first dinosaur to be named scientifically. A large herbivore named Iguanodon was found at about the same time in sedimentary rocks in mines in England, Belgium, and France.

Discoveries of fantastic, extinct mega-reptiles in Europe were soon followed by even more exciting finds of dinosaur fossils in North America and elsewhere. These events captured the fascination of both naturalists and the general public. Museums started to develop extraordinary displays of re-assembled dinosaur skeletons, and artists prepared equally extraordinary depictions of dinosaurs and their hypothesized appearances and habitats.

This initial hey-day of dinosaur fossil discoveries occurred in the late nineteenth and early twentieth centuries. During this period, many of the most important finds were made by North American paleontologists who discovered and began to mine rich deposits of fossils in the prairies. There was intense scientific interest in these American discoveries of fossilized bones of gargantuan, seemingly preposterous animals, such as the awesome predator Tyrannosaurus and the immense herbivore Apatosaurus (initially known as Brontosaurus). Unfortunately, the excitement and scientific frenzy led to competition among some of the paleontologists, who wanted to be known for discovering the biggest, fiercest, or weirdest dinosaurs. The most famous rivals were two American scientists, Othniel C. Marsh and Edwin Drinker Cope.

Other famous discoveries of fossilized dinosaur bones were made in the Gobi Desert of eastern Asia. Some of those finds include nests with eggs that contain fossilized embryos used to study dinosaur development. Some nests contain hatchlings, suggesting that dinosaur parents cared for their young. In addition, the clustering of the nests of some dinosaurs suggests social behavior including communal nesting, possibly for mutual protection against marauding predatory dinosaurs. In the valley of Ukhaa Tolgod, Mongolia, the skeleton of an adult oviraptor was found hunched over her nest of eggs, just like any incubating bird.

A find of dinosaur eggs in an Argentinian desert in 1998 is one of the largest collections ever discovered. It consists of hundreds of 6 in (15 cm) eggs of Titanosaurs, 45 ft (13.7 m) long relatives of the Brontosaurus. The eggs were laid 70-90 million years ago, and skeletons of about 36 15-in (38-cm) long babies were also found in the mudstone. The paleontologists named the site "Auca Maheuvo," after a local volcano and the Spanish words for "more eggs." They hope to assemble an "ontological series" of eggs and embryos from the fossils to show all the stages of baby dinosaur development. Other scientists have speculated that this type of dinosaur gave birth to live young, and the discovery of the egg bonanza resolves that question.

Fossilized dinosaur bones have been discovered on all continents. Discoveries of fossils in the Arctic and in Antarctica suggest that the climate was much warmer when dinosaurs roamed Earth. It is also likely that polar dinosaurs were migratory, probably traveling to high latitudes to feed and breed during the summer and returning Dinosaur bones being excavated near Kauchanaburi, Thailand. Photograph by Bill Wassman. Stock Market. Reproduced by permission. to lower latitudes during the winter. These migrations may have occurred mostly in response to the lack of sunlight during the long polar winters, rather than the cooler temperatures.

Although the most important fossil records of dinosaurs involve their bones, there is other evidence as well. In addition to footprints, eggs, and nests, there have also been finds of imprints of dinosaur skin, feces (known as coprolites), rounded gizzard stones (known as gastroliths), and even possible stomach contents. Fossilized imprints of feathers associated with dinosaurs called Sinosauropteryx and Protarchaeiopteryx found in the Liaoning Province of China show not only long flight and tail feathers but downy under feathers. In addition, fossilized plant remains are sometimes associated with deposits of dinosaur fossils, and these can be used to infer something about the habitats of these animals. Inferences can also be based on the geological context of the locations of fossils, such as their proximity to ocean shores or geographical position for polar dinosaurs. These types of information have been studied and used to infer the shape, physiology, behavior, and ecological relationships of extinct dinosaurs.