In 1877, Othniel C. Marsh named a headless juvenile dinosaur skeleton collected near Morrison, Colorado, Apatosaurus and published a complete description of the material in 1878. Brontosaurus was named by Marsh in 1879 based on an adult skeleton collected at Como Bluff,Wyoming. Because Brontosaurus had no head, Marsh added one from a nearby site in 1883. Because the skeleton of Brontosaurus was more complete than that of Apatosaurus, its skeleton was mounted at a number of museums around the United States. In 1903, Elmer Riggs concluded that the specimens of Apatosaurus and Brontosaurus belonged to the same species and, using the rule of priority by which the first name assigned is the one used, combined them under the name Apatosaurus. In addition, he noted that the head that had been attached to Brontosaurus had a short snout and spoon-shaped teeth, and belonged to another sauropod, Camarasaurus. Many people still recognize this dinosaur as Brontosaurus, while Apatosaurus is thenameaccepted by most professionals.
Apatosaurus was confined to the Late Jurassic era (156 to 145 million years ago) of the western United States and, perhaps, adjacent Mexico. Adults were about 21 meters (70 feet) long and 4.5 meters (14.5 feet) tall, weighing 20 tons. The long, whiplike tail was lashed about in defense. Their maximum land speed is estimated to have been from twenty to thirty kilometers per hour. Since dry habitats predominated at this time in the western United States, Apatosaurus had to migrate long distances to find food. Its long, straight legs were highly suitable for walking. Apatosaurus was gregarious and traveled in herds. Their heads were quite small relative to their large bodies. Only one partial skull of Apatosaurus has been found. Because the joint connecting the skull to the spine was very weak, the skulls were often lost before burial. The eyes were located at the back of the skull, as were the nostrils. The teeth were peglike and confined to the front of an elongate snout. Because the small head lacked both the teeth and the musculature for effective chewing, Apatosaurus nipped off leaves that were swallowed nearly whole. Apatosaurus probably held its head horizontally while feeding on shrubs and other low-growth vegetation. Contemporary stegosaurs and ornithopods ate the vegetation up to a height of about three meters. Apatosaurus foraged at heights above three meters. The tallest trees were eaten by brachiosaurids. Gastroliths helped grind up coarse plant parts prior to their being passed on to the stomach. Bacteria were present in the digestive tract to aid in breaking down food. Ferns, the chief herbaceous plants of the Late Jurassic, would have comprised the major part of their diet, although conifers, ginkgoes, and cycads were also eaten. Flowering plants, which made up a large part of the diet of Cretaceous herbivores, such as Triceratops, had not yet evolved. Although Apatosaurus browsed on low-growing vegetation, it could also rear up on its hind legs and tail to feed in the trees, engage in combat with other members of its species, defend its young, and, at least in the males, mate.
Order: Saurischia (lizard-hipped dinosaurs)
Suborder: Sauropoda (long-necked herbivores)
Family: Diplodocidae (quadrupeds whose forelimbs were shorter than their hind limbs)
Genus and species: Apatosaurus ajax, A. excelsus, A. louisae
Note: A number of competing classification schemes exist and will probably continue to do so in the future.
Geographical location: Western United States, including Colorado, Utah, and Wyoming
Habitat: The dry habitats that predominated in the western United States during the Late Jurassic
Gestational period: Although no eggs have been found, Apatosaurus was undoubtedly an egg layer; the frequency at which the eggs were laid, the time it took for them to hatch, and the reproductive life span of the adults are unknown
Life span: Based on mammalian models, sexual maturity would be reached after ten years, and the life span was probably in excess of one hundred years
Special anatomy: All the diplodocids, including Apatosaurus, had Vshaped neural spines on the upper side of the cervical (neck) vertebrae; a taut ligament running between the arms of the V held the neck rigid so that constant muscular exertion was not necessary to hold the head up against the pull of gravity
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