Amphicoelias (/ˌæmfɪˈsiːliəs/, meaning "biconcave", from the Greek ἀμφί, amphi: "on both sides", and κοῖλος, koilos: "hollow, concave") is a genus of herbivorous sauropod dinosaur that lived approximately 150 million years ago during the Tithonian (Late Jurassic Period) of what is now Colorado, United States.

The namesake fossil of the type species Amphicoelias altus, American Museum of Natural History 5764, is uncertain in included material.

When described by Edward Drinker Cope shortly after its discovery in 1877, Amphicoelias was noted to include many back vertebrae, a single pubis, and a femur.

[1] Among the assortment of fossils discovered, a specimen including multiple dorsal vertebrae, a pubis, and a femur was found by Aaron Ripley, Lucas' brother-in-law, in Quarry XII.

[3] Cope determined that the taxon would have been a sauropod related to Camarasaurus found nearby, both being large animals with lightly built vertebrae and solid limbs.

Shortly before his death in 1897, he worked with the museum's artist Charles R. Knight to produce illustrations and paintings depicting some of his species in life, including Amphicoelias.

Based on notes accompanying his sketches provided to Knight as reference material, Cope evidently considered both Amphicoelias and Brontosaurus junior synonyms of Camarasaurus supremus at this time.

[5] Based on collection data, the specimen was determined to have come from the Late Jurassic Morrison Formation, in the youngest layers, separate from those that held the Camarasaurus fossils also described by Cope.

As well, Camarasaurus possessed pleurocoels (openings in the side of the centrum) that were taller than long-contrasting to the condition in Amphicoelias- and a more robust pubis and femur.

[4] The third named Amphicoelias species, A. fragillimus, was known only from a single, incomplete 1.5 m (4.9 ft) tall neural arch, either last or second to last in the series of back vertebrae.

[8] However, it has been argued that the scale bar in the published description contained a typographical error, and the fossil vertebra was in fact only 1.38 meters (4.5 ft) tall.

[10] In 2010, an article was made available, but not formally published, by Henry Galiano and Raimund Albersdorfer in which they referred to the new species "A. brontodiplodocus" to Amphicoelias, based on several complete specimens found in the Dana Quarry of Big Horn Basin, Wyoming and held in a private collection.

The specific name referred to their hypothesis based on these specimens that nearly all Morrison diplodocid species are either growth stages or represent sexual dimorphism among members of the genus Amphicoelias,[11] but this analysis has been met with skepticism and the publication itself has been disclaimed by its lead author, explaining that it is "obviously a drafted manuscript complete with typos, etc., and not a final paper.

From directly behind the prezygapophyses, the diapophyes (lateral processes for rib articulation) project slightly upwards and outwards, surrounded by shallow fossae and a large lamina extending up the neural spine.

[4] In 2007, John Foster suggested that the differences usually cited to differentiate Amphicoelias altus from the more well known Diplodocus are not significant and may be due to individual variation.

[9] This conclusion was disputed by Emanuel Tschopp et al. in 2015, where Amphicoelias was analysed as part of an extensive specimen-level phylogenetic analysis of Diplodocidae.

Carpenter cited several studies of giant mammalian herbivores, such as elephants and rhinoceros, which showed that larger size in plant-eating animals leads to greater efficiency in digesting food.

This is especially true of animals with a large number of 'fermentation chambers' along the intestine, which allow microbes to accumulate and ferment plant material, aiding digestion.

Carpenter argued that other benefits of large size, such as relative immunity from predators, lower energy expenditure, and longer life span, are probably secondary advantages.

Carpenter speculated that giant herbivores like Amphicoelias may have used the shade of the gallery forests to stay cool during the day, and done most of their feeding on the open savanna at night.