The richest deposits of dinosaur fossils have been found in the midwestern states of North America. Although recent excavations to Mongolia suggest that this region will provide the greatest number of new fossildinosaur species. Some of these dinosaurs were no bigger than a chicken called Compsognathus, whereas others represent the largest land animals that have existed on the earth such as Apatosaurus which measured 25 m long and weighed at least 30 tonnes. A fossildinosaur, Seismosaurus, unearthed in 1986 appeared to have been 43 metres long and weighed about 100 tonnes. Another dinosaur called Ultrasaurus, may have been heavier still with an estimated mass of 150 tonnes.
Apatosaurus meaning "deceptive lizard", because its chevron bones were like those of Mosasaurus (Greek apatelos = deceptive + sauros = lizard), often referred to as Brontosaurus, is a genus of sauropoddinosaurs that lived about 140 million years ago, during the Jurassicperiod. They were some of the largest land animals that ever existed, about 4.5 metres (15 feet) tall at the hips, with a length of up to 25m (80 feet) and a mass up to 35 metric tonnes (40 tons).