Amphicoelias, meaning “doubly hollow”, is derived from the Greek words amphi: “on both sides” and koilos: “hollow”. It is a genus of sauropod dinosaur. It was first collected by Oramel Lucas in Garden Park, Colorado in 1877. The vertebrae found was in poor condition, but it was still enormously large (56 to 105 inches in height). The findings were first publicized in August of 1878. Although the original findings by Lucas place the skeleton in the Dakota Formation (mid-Cretaceous Period), the presence of other dinosaurs in the same rocks indicate that it may have belonged to the Morrison Formation, which places the dinosaur in the Late Jurassic Period. A gigantic fossilized femur was also found in the same area indicates that A. fragillimus may be the largest dinosaur ever discovered.

Based on descriptions of a single fossil bone recovered, A. fragillimus may have been upwards of 131 to 196 feet in length. Its weight could have easily topped 135 tons, rivaling the heaviest known animal of today, The Blue Whale. However, because the single fossil record was lost some point after being studied in the 1870s, all evidence of its size exists only in drawings and filed notes. Edward Drinker Cope, in 1877, discovered remains of Amphicoelias which he named, A. altus. More bones were assigned to A. altus in 1921 including a scapula, ulna, shoulder bone, and tooth.

Producing a somewhat accurate depiction of the actual size of A. fragillimus requires scaling of bones of other dinosaur remains of common sauropods. This is based on theory that relative proportions of dinosaurs were similar. Cope studied that in other sauropods, the femur was always at least twice as tall as the tallest dorsal vertebrae, and estimated that the femur of A. fragillimus was about 12 feet tall. In 1994, using the same dinosaur as a reference, Gregory S. Paul estimated the femur to be between 10 and 13 feet tall. A re-evaluation by Ken Carpenter in 2004 places the femur at between 14 – 15 feet tall. Carpenter’s estimate of the total size of the specimen was nearly 190 feet long, which he noted fell within range of Paul’s findings in 1994. Based on scales, Carpenter also speculated that the neck length was about 55 feet, the body length nearly 30 feet, and the tail length was 105 feet. He estimated that the overall height of the beast may have been 30 feet. By comparison, today’s largest animal, the Blue Whale, is only 110 feet long.

The environment in which Amphicoelias lived would have resembled a modern day savanna; however, since grass did not appear until the Late Cretaceous Period, ferns were probably the main food source for large dinosaurs such as A. fragillimus. Some scientists dismiss the fern as a food source due to its relatively weak caloric content. However, Carpenter pointed out that the digestive systems of most sauropods were well-adapted to handling low-quality food, which allowed for consumption of ferns as a large part of their diet. Carpenter also noted that the occasional presence of petrified logs in bone beds indicate the presence of 65 to 100 foot tall trees, which seem to conflict with the comparisons made to savanna land. Although the presence of trees was rare at best, it is speculated that these trees existed in small tracts of gallery forests, rather than immense woodlands. Carpenter also pointed out that the sauropods may have used the cover of trees during the day for shade, and done most of their foraging at night on the open savanna.