Description and Classification

Amphicoelias altus, the primary species, was a large herbivorous dinosaur with the classic sauropod body plan: a very long neck, a massive body, four sturdy, pillar-like legs, and a long, tapering tail. It is estimated to have reached lengths comparable to other large diplodocoids like Diplodocus, perhaps around 20-25 meters (65-82 feet). Its vertebrae, being biconcave (hollow on both ends), are a key characteristic that gave the genus its name, although this feature is not entirely unique among sauropods.

Amphicoelias belongs to the group Sauropoda, which includes the largest land animals ever to have lived. More specifically, it is classified within the Diplodocoidea, a superfamily of slender-necked, long-tailed sauropods. This makes it a relative of well-known dinosaurs such as Diplodocus, Apatosaurus, and Barosaurus. The controversial species “Amphicoelias” fragillimus, if its original description was accurate, would have been one of the largest animals ever known. However, due to the loss of its only fossil and the limited information, many paleontologists now consider it a doubtful name (nomen dubium) or have reclassified it into a separate genus, Maraapunisaurus. Therefore, discussions about Amphicoelias typically focus on the better-understood A. altus.

Distinguishing Features

Amphicoelias possessed several features that, while some are shared with its relatives, help define it within the context of Late Jurassic sauropods:

• Vertebrae that are distinctly concave (hollowed out) on both their front and back articular surfaces.
• A typical diplodocoid body shape, characterized by an exceptionally long neck and an even longer, whip-like tail that may have been used for defense or communication.
• Relatively slender build for a sauropod of its size, especially when compared to more robust sauropods like brachiosaurids or titanosaurs.
• Peg-like teeth, common in diplodocoids, designed for stripping foliage from trees rather than extensive chewing.

Paleoenvironment and Diet

Amphicoelias lived in what is now western North America during the Late Jurassic period. Its remains are found in the Morrison Formation, a famous sequence of rocks known for its rich dinosaur fossil record. The environment at that time was a vast floodplain with a semi-arid climate, characterized by distinct wet and dry seasons. Rivers flowed across this plain, supporting gallery forests of conifers, cycads, tree ferns, and ginkgoes, while open areas were likely covered with ferns and horsetails.

As a herbivore, Amphicoelias would have used its long neck to browse on vegetation, possibly feeding at different heights than other sauropods. Its diet likely consisted of the available plant material, such as conifers and ferns. It shared its world with a diverse range of other dinosaurs, including other sauropods like Camarasaurus, Brachiosaurus, and Diplodocus; large predatory theropods such as Allosaurus and Torvosaurus; and ornithischian dinosaurs like Stegosaurus and Camptosaurus.

Significance and Ongoing Research

Amphicoelias holds historical significance as one of the sauropod genera named during the “Bone Wars,” a period of intense fossil hunting and scientific rivalry in the late 19th century. While Amphicoelias altus provides insights into diplodocoid diversity, the genus is most widely known due to the mystery and speculation surrounding “Amphicoelias” fragillimus. If Cope’s measurements of its single vertebra were accurate, it would have represented an animal of truly colossal proportions, far exceeding even the blue whale in length.

Ongoing research primarily focuses on clarifying the taxonomic status and relationships of Amphicoelias altus within the Diplodocoidea. Paleontologists continue to re-examine the existing fossil material and compare it with other sauropods to better understand its anatomy and ecology. The debate around “A.” fragillimus also continues, with some researchers attempting to re-evaluate its potential size and validity based on historical accounts and comparisons with related species, leading to its proposed reclassification as Maraapunisaurus. Studies of Amphicoelias contribute to our broader understanding of sauropod evolution, gigantism, and the ecosystems of the Late Jurassic Morrison Formation.