How big is Maraapunisaurus fragillimus?

Maraapunisaurus fragillimus was a sauropod, one of the giant long-necked dinosaurs, that lived in what is now Colorado in the Late Jurassic period. It is known from a single piece of backbone that has since been lost, so its size is an estimate. A 2018 study puts it at about 30 meters (100 feet) long from head to tail.

Size

The fossil was part of one vertebra, one of the bones that make up the backbone. The piece that was found was mainly the neural spine, the blade of bone that sticks up from the top of a vertebra. It was 1.5 meters (4.9 feet) tall. When he described it in 1878, the paleontologist Edward D. Cope estimated that the complete vertebra had been at least six feet (1.8 meters) tall.

With no other bones, the animal's length can only be estimated by scaling up a relative known from a more complete skeleton. In 2018, the paleontologist Kenneth Carpenter reexamined Cope's drawing and description. He concluded that the bone came from a rebbachisaurid, a member of a group of sauropods that includes Limaysaurus.

Rebuilt on the pattern of other rebbachisaurids, the complete vertebra would have been 2.4 meters (7.9 feet) tall. Scaled from Limaysaurus, the whole animal would have been 30.3 meters (99 feet) long and 7.95 meters (26 feet) tall at the hips. Larger sauropods have longer necks for their size, and allowing for that, it might have reached 32 meters (105 feet).

Size comparisons

It was about as long as a large blue whale, 30 meters (98 feet) long.
The piece of bone that was found was nearly as tall as a person 1.7 meters (5 feet 7 inches) tall. The complete vertebra would have been about 1.4 times that person's height.
At the hips, it stood nearly five times as tall as that person.
A Boeing 747-8 airliner, 76.25 meters (250 feet) long, is about two and a half times as long as Maraapunisaurus.
About three and two-thirds Maraapunisaurus, nose to tail, would span an American football field, which is 110 meters (120 yards) long with its end zones.

A fossil that was lost

A schoolteacher named Oramel Lucas found the bone at Garden Park, north of Cañon City, Colorado, in rock of the Morrison Formation. That rock dates from the Late Jurassic period, which lasted from about 163.5 to 145 million years ago. In 1878 he shipped it to Cope in Philadelphia, and Cope named it Amphicoelias fragillimus that year, with a drawing and measurements. Cope noted how fragile the bone was and how much care it took to preserve it.

The bone was one of several specimens missing when the American Museum of Natural History acquired Cope's collection after his death, and it has never been found. Carpenter thinks it most likely crumbled and was thrown away, perhaps after rough handling of its crate. Everything known about it comes from Cope's short published description.

Why the estimate shrank

From 1921 on, the bone was treated as coming from a diplodocid, a relative of Diplodocus. Scaled that way, it gave enormous estimates: 40–60 meters (130–200 feet) in 1994, and 58 meters (190 feet) from Carpenter himself in 2006. Cope had assumed the thigh bone was twice as long as the vertebra was tall. In Limaysaurus, a back vertebra is 83 percent as tall as the thigh bone is long, which shrinks the thigh bone to about 2.9 meters (9.5 feet), and the animal with it.

Carpenter also found a scalloped ridge of bone on the neural spine, a feature that marks rebbachisaurids. Because the bone no longer fit the genus Amphicoelias, he gave it a new name, Maraapunisaurus, from a Southern Ute word meaning "huge". Garden Park was Ute land before settlers displaced the Ute in the mid-1800s. If Carpenter is right, it was, as of his study, both the oldest rebbachisaurid known and the first found in North America.