DinosaursDinosaur is the name of large extinct reptiles of the Mesozoic Era,during which they were the dominant land animals on Earth. The term wasproposed as a formal zoologic name in 1842 by the British anatomist Sir RichardOwen, in reference to large fossil bones unearthed in southern England. Thevarious kinds of dinosaurs are classified in two formal categories, the ordersSaurischia and Ornithischia, within the subclass Archosauria. The first recorded dinosaur remains found consisted of a few teeth andbones. They were discovered in 1882 in Sussex, England, by an English doctor,Gideon Mantell, who named them iguanodon.
About the same time, other fossilteeth and bones were found near Oxford, England, by Rev. William Buckland. These were named Megalosaurus. Thousands of specimens have since beendiscovered nearly worldwide. Different types of dinosaurs varied greatly in form and size, and theywere adapted for diverse habitats.
Their means of survival can only beidentified from their fossil remains, and some identifications are in dispute. They ranged in weight from 4 to 6 lb. , in the case of the compsognathus, and upto 160,000 lb. , in the case of the brachiosaurus.Order now
Most dinosaurs were large,weighing more than 1,100 lb. , and few weighed less than 100 lb. Most wereherbivores, but some saurischians were carnivorous. The majority were four-footed but some ornithischians and all carnivores walked on their hind legs. Always classified as reptiles, dinosaurs have traditionally been assumedto have been reptilian in their physiology, cold-blooded, and ectothermic.
Inrecent years several different lines of evidence have been interpreted asindicating that dinosaurs may have had warm blood and high rates of metabolism,comparable to birds and mammals. Evidence supporting this view includes uprightposture and carriage; mammallike microscopical structure of bones; skeletalfeatures suggestive of high activity; and specialized food-processing dentitionsand low ratios of dinosaurian predators to prey animals, both suggesting highfood requirements. The evidence is not conclusive–all the facts can bealternatively explained–but some dinosaurs may have been endothermic. The reproductive means of most dinosaurs is as yet unknown. Fossil eggs,attributed to one of the horned dinosaurs and a sauropod, have been discoveredin Mongolia and France. Fragments that are presumed to be of dinosaur eggs havealso been found in Brazil, Portugal, Tanzania, and in the United States,Colorado, Montana, and Utah.
In Montana, Utah, and Alberta, Canada, fossils ofunhatched dinosaur eggs have been discovered. This evidence indicates egg-laying reproduction in dinosaurs, like most modern reptiles. A few scientistsbelieve that some dinosaurs may have given birth to living young, but noconclusive evidence has yet been found to support this. The two orders of dinosaurs are distinguished by numerous features, themost diagnostic being the arrangement of the three bones of the pelvious.
Insaurischians, these bones were arranged in a triradiate pattern similar to thatof modern crocodilians and lizards; the term Saurischia means lizard hip. Theornithischian pelvis was usually rectangular or tetraradiate; hence the name,which means bird hip. During the 140-million-year reign of the dinosaurs, many new varietiesevolved and older kinds died out. Not all kinds became extinct at once; butthe last of the dinosaurs disappeared at the end of the Cretaceous.
Many otheranimal kinds died out at about the same time, including the ichthyosaur,mosasour, plesiosaur, flying reptile, and a variety of lower organisms. Whatbrought about such widespread extinction among so many different kinds oforganisms is not known; it must, however, have involved major changes in theenvironment. Their extinction has been attributed to many causes, includingcosmic radiation, exploding supernova, world-wide fluctuations in sea level,acid rain caused by volcanic activity, climatic change, and continental drift. Independent evidence indicates that sea levels did fall and temperatures droppedat the end of the Mesozoic Era, a time when continents were drifting apart andnew mountain ranges were rising. Although none of these conditions is likely tohave been solely responsible for the extinction of the dinosaurs, collectivelythey may have been important. Whatever the cause, the dinosaurs are now gone.
In a way, however, theymay remain. That is, many paleontologists consider birds almost certainly tohave evolved from some small bipedal dinosaur during the Jurassic. If so, thechildren of the dinosaurs still exist today. English