Amphicoelias

Amphicoelias was a large sauropod dinosaur that lived in the Late Jurassic of North America. One species, A. fragillimus, may be the largest dinosaur so far discovered.

Description
Both species of Amphicoelias are known from incomplete remains. In general, the genus is similar in body shape to Diplodocus, although Amphicoelias has proportionally longer forelimbs.

The type species, A. altus, is estimated to have measured about 25 meters in length, similar in size to Diplodocus. However, A. fragillimus, which was known only from an incomplete vertebra, may have reached lengths of almost 60 meters, larger than any other dinosaur by far.

Classification
Edward Drinker Cope placed the two Amphicoelias species in their own family, but it is now generally accepted that they are part of the family Diplodocidae. It was first suggested by Henry Fairfield Osborn that A. fragillimus was merely a large example of the type species A. altus. Currently, the two are treated as separate species.

In 2007, John Foster suggested that Amphicoelias and Diplodocus were actually synonymous genera. Since Amphicoelias was named earlier, Diplodocus would have to be abandoned in favor of the older name (a similar situation occurred with the genera Apatosaurus and "Brontosaurus"). This suggestion has not been adopted by other researchers.

In 2010, a monograph arose that suggested all diplodocid species from the Morrison Formation were actually Amphicoelias specimens in various stages of growth. Such a classification has not been widely accepted, and the paper has yet to be formally published.

History
The type species, A. altus, was named by Edward Drinker Cope in 1877 (though not published until 1878). He also named a second species, A. latus, in the same paper, but it is now generally regarded to be a synonym of A. altus.

An extremely large sauropod vertebra was uncovered by one of Cope's fossil collectors in Colorado during 1878. Although in poor condition, Cope described it as A. fragillimus and noted its unusual size. Though he believed that the rocks in which the bone was found were of Cretaceous age, it is now known that they are part of the Morrison Formation, a Jurassic-age site. Since then, however, the bone disappeared, and all attempts to locate it have failed. The bone itself was very fragile and the rock it was preserved in fragmented easily, so Cope may have simply discarded the fossil after he had described it.

Paleobiology
In his 2006 redescription of Amphicoelias, Ken Carpenter argued that ferns were likely a dominant source of nutrition for this animal. Though this suggestion had previously been refuted by earlier papers, Carpenter believed that the digestive system of sauropods was adapted for processing low-quality foods such as ferns.