Amphicoelias fragillimus (now Maraapunisaurus fragillimus)

Amphicoelias fragillimus is a dinosaur that holds a legendary status among paleontologists due to its potentially colossal size and the mystery surrounding its scant remains. Unearthed from the Late Jurassic Morrison Formation, dating back approximately 149-145 million years ago, this dinosaur was first described by Edward Drinker Cope in 1878. The name Amphicoelias comes from Greek, meaning “hollow on both sides,” referring to the deeply concave structure of its vertebrae, while fragillimus highlights the delicate, brittle nature of the fossil material. Known only from a single, enormous vertebra that was tragically lost, Amphicoelias fragillimus has fueled speculation about its place among the largest creatures to ever walk the Earth.

However, modern studies (Carpenter, 2018) have reclassified Amphicoelias fragillimus into a new genus, Maraapunisaurus, as evidence suggests it was not a diplodocid but a member of the Rebbachisauridae, a group of sauropods with distinct vertebral structures. While its exact size remains uncertain due to the loss of the fossil, it is still considered one of the largest known dinosaurs.

Physical Characteristics

Maraapunisaurus fragillimus is known solely from a dorsal vertebra that Cope described as towering at nearly 2.4 meters (7.8 feet) in height—an astonishing size for a single bone. This vertebra, which disintegrated or was misplaced shortly after its discovery, suggested a sauropod of truly immense proportions. Early estimates based on Cope’s documentation speculated lengths of 30 to 60 meters (98 to 197 feet) and weights exceeding 120 metric tons (132 tons), though these figures are now viewed with skepticism due to the lack of corroborating fossils and the possibility of measurement errors. More recent reconstructions suggest a more conservative length of around 30-40 meters (98-131 feet) and a weight of approximately 50-80 metric tons (55-88 short tons), still placing it among the giants of the dinosaur world.

If these estimates hold, Maraapunisaurus fragillimus likely resembled other rebbachisaurids rather than diplodocids, with an elongate neck and tail, a relatively small head, and a lightweight yet robust skeletal structure. The vertebra’s pneumaticity—air-filled spaces within the bone—would have reduced its weight, a critical adaptation for supporting such a massive frame. Rebbachisaurids had tall, uniquely shaped neural spines that may have influenced posture or muscle attachments. Its limbs were probably sturdy and pillar-like, designed to bear its bulk, while the tail may have been used for balance or communication.

Diet and Feeding Habits

As a herbivorous sauropod, Maraapunisaurus fragillimus would have subsisted on the abundant plant life of its Jurassic environment, including ferns, cycads, and conifers. However, unlike diplodocids, which are often reconstructed as high browsers using their long necks to reach tall trees, rebbachisaurids may have fed at a lower level, possibly using their specialized jaws to crop vegetation from mid-height plants.

Like its relatives, it likely possessed simple, peg-shaped teeth for raking vegetation, swallowing it whole to be processed in a voluminous gut through fermentation. Gastroliths, or stomach stones, may have assisted in breaking down tough plant material, a trait inferred from other sauropods. However, due to the lack of direct fossil evidence for Maraapunisaurus fragillimus, its exact feeding strategy remains speculative.

Habitat and Distribution

Maraapunisaurus fragillimus roamed the prehistoric landscapes of North America, within the Morrison Formation—a sprawling region of rivers, floodplains, and lush forests during the Late Jurassic. This ecosystem teemed with life, supporting a variety of dinosaurs, from the predatory Allosaurus to fellow sauropods like Apatosaurus and Diplodocus. Seasonal shifts between wet and dry periods would have influenced the availability of food and water, prompting Maraapunisaurus to traverse vast distances in search of sustenance.

Behavior and Social Structure

Given its potential size, an adult Maraapunisaurus fragillimus would have been nearly impervious to predation, dwarfing even the fiercest theropods of its time. While direct fossil evidence for its social behavior is lacking, trackways of other sauropods suggest that many of these giants may have traveled in herds, providing safety for younger individuals and optimizing foraging efforts.

Its long tail could have been used for defensive displays, communication, or balance, although whether it functioned as a whip-like weapon, as some diplodocids are hypothesized to have used, remains unknown. With a slow growth rate and a lifespan potentially stretching over decades, it would have taken years to reach maturity before becoming one of the dominant herbivores of its time.

Discovery and Research

The story of Maraapunisaurus fragillimus begins with Edward Drinker Cope, who unearthed its lone vertebra near Garden Park, Colorado, in 1878. His detailed notes and sketches described a bone of unparalleled size, but the fossil’s fragility led to its loss—possibly crumbling to dust or being misplaced during transport.

Since the original fossil is missing, paleontologists have debated its authenticity and scale for over a century. Some researchers have suggested that Cope may have overestimated its size due to errors in measurement or misinterpretation of the fossil. Others maintain that the vertebra was legitimate but that its extreme dimensions require further scrutiny.

Why Amphicoelias fragillimus became Maraapunisaurus fragillimus

Originally classified as Amphicoelias fragillimus, this dinosaur was placed within the genus Amphicoelias, which also included Amphicoelias altus, a smaller and better-documented species. For decades, it was assumed that fragillimus belonged to the same genus, despite its extraordinary size and limited remains.

However, a 2018 study by paleontologist Kenneth Carpenter and colleagues re-evaluated Cope’s descriptions and illustrations, comparing them to other sauropods, particularly rebbachisaurids. The analysis suggested that Amphicoelias fragillimus differed significantly from Amphicoelias altus in vertebral structure and proportions, warranting its separation into a distinct genus.

The new name, Maraapunisaurus, was proposed, drawing from the Ute language word for “giant” and reflecting its Native American discovery context, while retaining fragillimus to honor the original description. This reclassification aimed to clarify its taxonomic position within the Rebbachisauridae family and distinguish it from the more typical Amphicoelias species. Despite the change, the loss of the original fossil means that both names remain tied to the same elusive, speculative giant, with Maraapunisaurus fragillimus now the accepted designation in modern paleontology.