Apatosaurus

Apatosaurus, formerly known as Brontosaurus, huge plant-eating dinosaur that lived during the late Jurassic Period, about 150 million years ago. The name Apatosaurus (Greek apatao, “deceive”; sauros, “lizard”) may refer to an early mistaken identification of Apatosaurus fossil remains with those of other extinct reptiles. When the American paleontologist Othniel Charles Marsh uncovered the first Apatosaurus skeleton in 1879, he claimed that the Y-shaped arches on the underside of the Apatosaurus tail were deceptively similar to those found in some large marine lizards known as mosasaurs. Paleontologists debated the name and the animal’s characteristics for a century. Their arguments centered around a dozen specimens belonging to two varieties of dinosaurs excavated from sites in Colorado, Utah, and Wyoming.

Skeletal remains of Apatosaurus have been found in sediments deposited on Jurassic-era lowlands now located in the central Rocky Mountains. In Dinosaur National Monument, near Vernal, Utah, scientists have excavated many of these Apatosaurus and other dinosaur skeletons from sands deposited by streams. Apatosaurus and other giant reptiles, such as Diplodocus, Brachiosaurus, and Camarasaurus, belonged to a group of saurischian (lizard-hipped) dinosaurs of the suborder Sauropoda. These five-toed, long-necked dinosaurs were the largest animals ever to live on land.

Apatosaurus adults were about 25 m (82 ft) long and weighed up to 32 metric tons. The smallest known specimens of this dinosaur were those of young or juvenile Apatosaurus. These animals were 4.5 m (15 ft) long and weighed only 250 kg (550 lb). Like all known sauropods, the animal had a small head, a long slender neck, and a long tail that was almost one and one-half times longer than the rest of the body. Apatosaurus lacked speed and agility, so scientists believe that its tail was its main defense against attack by flesh-eating dinosaurs. Apatosaurus’s barrel-like body was supported by thick, heavy legs. The forelimbs were shorter than the hind limbs, and the animal’s long spine counterbalanced the front part of its body against its heavy, muscular tail. The vertebrae also lengthened its neck, which was 6 m (20 ft) long. Apatosaurus had an elongated skull with short, pointed teeth. The nostrils were located high on its head, directly above and in front of the eyes. Apatosaurus skeletons are usually found headless, however, because the skull and neck connections were fragile. When Marsh discovered the first Apatosaurus fossils, he mistakenly paired them with the skull of the Camarasaurus, a smaller relative. Consequently, Apatosaurus was depicted as having Camarasaurus’s flat-nosed head and peglike teeth until the late 1970s, when scientists corrected the error.

In North America, Apatosaurus inhabited vast, low plains dotted with lakes and crossed by tree-lined streams. The climate was semiarid, broken by a rainy season. The young dinosaurs grew rapidly, reaching maturity within about ten years. The enormous bulk of the adult animals suggests that they grazed in an environment with abundant vegetation. Long necks and powerful thumb claws (which allowed the dinosaurs to stand firmly in place) indicate that the animals could span a wide arc, reaping a broad swath of vegetation without walking much. These features also suggest that Apatosaurus grazed primarily on low plants, such as ferns. Apatosaurus’s size gave it other advantages. Adults may have had a low metabolism because they were so large. If so, they could maintain a constant body temperature, subsist on low-quality food, and better survive droughts and famines. Evidence also indicates that they tended to avoid Diplodocus but may have associated occasionally with Stegosaurus. Fossil evidence indicates that the natural life span of Apatosaurus averaged about 100 years.