Apatosaurus
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[Photo] Apatosaurus sketch by Jim Robins. Source http://palaeo.gly.bris.ac.uk/dinobase/picturesJR/dinopicturesJR.html Date 2004

Apatosaurus (IPA: [/????pæ.t????s??ː.r??s/]), popularly (though incorrectly) known as Brontosaurus, is a genus of sauropod dinosaur that lived about 140 million years ago, during the Jurassic Period. It was one of the largest land animals that ever existed, with an average length of 23 meters and a mass of at least 23 metric tons. The name Apatosaurus means 'deceptive lizard', so-given because the chevron bones were like those of Mosasaurus (Greek ???πατ??λο?? or ???πατ??λιο?? meaning 'deceptive' and σα???ρο?? meaning 'lizard').

The cervical vertebrae were less elongated and more heavily constructed than those of Diplodocus and the bones of the leg were much stockier (despite being longer), implying a more robust animal. The tail was held above the ground during normal locomotion. Like most sauropods, Apatosaurus had only a single large claw on each forelimb, with the first three toes on the hindlimb possessing claws.

Discovery and species
Fossils of this animal have been found in Nine Mile Quarry and Bone Cabin Quarry in Wyoming and at sites in Colorado, Oklahoma and Utah, USA.

A. ajax is the type species of the genus, and was named by the paleontologist Othniel Charles Marsh in 1877 after Ajax, the hero from Greek mythology. It is the holotype for the genus and two partial skeletons have been found, including part of a skull.

A. excelsus (originally Brontosaurus) was named by Marsh in 1879. It is known from six partial skeletons, including part of a skull, which have been found in the United States, in Colorado, Oklahoma, Utah, and Wyoming.

A. louisae was named by William Holland in 1915 in honor of Mrs. Louise Carnegie, wife of Andrew Carnegie who funded field research to find complete dinosaur skeletons in the American West. A. louisae is known from one partial skeleton which was found in Colorado in the United States.

Robert T. Bakker made A. yahnahpin the type species of a new genus, Eobrontosaurus in 1998, so it is now properly Eobrontosaurus yahnahpin. It was named by Filla, James and Redman in 1994. One partial skeleton has been found in Wyoming.

Apatosaurus/Brontosaurus
In 1877, Othniel Charles Marsh published notes on his discovery of Apatosaurus ajax. He followed this in 1879 with a description of another, more complete, dinosaur specimen. He speculated that the latter specimen represented a new genus and named it Brontosaurus excelsus. In 1903, it was discovered that Brontosaurus excelsus was in fact an adult Apatosaurus and the name Apatosaurus, having been published first, was deemed to have priority as the official name; Brontosaurus was relegated to being a synonym. In the 1970s, it was proven that the traditional "Brontosaurus" image known to all was, in fact, an Apatosaurus excelsus with a Camarasaurus head incorrectly placed on its body.

Paleobiology
Early on, it was believed that Apatosaurus was too massive to support its own weight on dry land, so it was theorized that the sauropod must have lived partly submerged in water, perhaps in a swamp. Recent findings do not support this. In fact, like its relative Diplodocus, Apatosaurus was a grazing animal with a very long neck and a long tail that served as a counterweight. Fossilized footprints indicate that it probably lived in herds. To aid in processing food, Apatosaurus may have swallowed gizzard stones (gastroliths) in the same way that many birds do today, as its jaws lacked molars with which to chew tough plant fibers.

Neck
Apatosaurus browsed the tops of trees, on riverbanks. Scientists believe that these sauropods could not raise their necks to an angle of 90 degrees, as doing so would slow blood flow to the brain excessively; blood starting at the body proper would take two or more minutes to reach the brain. Furthermore, studies of the structure of the neck vertebrae have revealed that the neck was not as flexible as previously thought.

Physiology
With such a large body mass, combined with a long neck, physiologists encounter problems determining how these animals managed to breathe.

Beginning with the assumption that Apatosaurus, like crocodilians, did not have a diaphragm, the dead-space volume (the amount of unused air remaining in the mouth, trachea and air tubes after each breath) has been estimated at about 184 liters for a 30 ton specimen.

Its tidal volume (the amount of air moved in or out during a single breath) has been calculated based on the following respiratory systems:

904 liters if avian
225 liters if mammalian
19 liters if reptilian.

On this basis, its respiratory system could not have been reptilian, as its tidal volume would not have been able to replace its dead-space volume. Likewise, the mammalian system would only provide a fraction of new air on each breath. Therefore, it must have had either a system unknown in the modern world or one like birds, i.e. multiple air sacs and a flow-through lung. Furthermore, an avian system would only need a lung volume of about 600 liters compared to a mammalian requirement of 2,950 liters, which would exceed the available space. The overall thoracic volume of Apatosaurus has been estimated at 1,700 liters allowing for a 500-liter, four-chambered heart (like birds, not three-chambered like reptiles) and a 900-liter lung capacity. That would allow about 300 liters for the necessary tissue. Assuming Apatosaurus had an avian respiratory system and a reptilian resting-metabolism, it would need to consume only about 262 liters (69 gallons) of water per day.

It is not known how Apatosaurs ate enough food to satisfy their enormous bodies. It is likely that they ate constantly, pausing only to cool off, drink or to remove parasites. It is surmised that they slept standing upright. They likely relied on their enormous size and herd behavior to deter predators.

Tail
An interesting speculation was reported by the Discover Magazine in 1997 about "whipcracking" millions of years ago. Nathan Myhrvold, a computer scientist from Microsoft, carried out a computer simulation of the tail of Apatosaurus, a very long, tapering tail resembling a whip, and concluded that sauropods were capable of producing a crack of over 200 decibels, comparable to the sound of a cannon.

Classification
Apatosaurus is a member of the Diplodocidae, along with Diplodocus and Barosaurus, although it is not as closely related to these genera as they are to each other, and hence Apatosaurus is usually placed in its own subfamily, Apatosaurinae, along with its close relative Supersaurus.

http://en.wikipedia.org/wiki/Apatosaurus

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