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Description

Barosaurus was an enormous animal, with some adults measuring more than 26 meters (86 feet) in length and weighing more than 20 metric tons (22 short tons). Barosaurus was differently proportioned than its close relative Diplodocus, with a longer neck and shorter tail, but was about the same length overall. It was longer than Apatosaurus, but its skeleton was less robust.

Sauropod skulls are rarely preserved, and scientists have yet to discover a Barosaurus skull. Related diplodocids like Apatosaurus and Diplodocus had long, low skulls with peg-like teeth confined to the front of the jaws.

Most of the distinguishing skeletal features of Barosaurus were in the vertebrae, although a complete vertebral column has never been found. Diplodocus and Apatosaurus both had 15 cervical (neck) and 10 dorsal (trunk) vertebrae, while Barosaurus had only 9 dorsals. A dorsal may have been converted into a cervical vertebra, for a total of 16 vertebrae in the neck. Barosaurus cervicals were similar to those of Diplodocus, but some were up to 50% longer. The neural spines protruding from the top of the vertebrae were neither as tall or as complex in Barosaurus as they were in Diplodocus. In contrast to its neck vertebrae, Barosaurus had shorter caudal (tail) vertebrae than Diplodocus, resulting in a shorter tail. The chevron bones lining the underside of the tail were forked and had a prominent forward spike, much like the closely-related Diplodocus. The tail probably ended in a long whiplash, much like Apatosaurus, Diplodocus and other diplodocids, some of which had up to 80 tail vertebrae.

The limb bones of Barosaurus were virtually indistinguishable from those of Diplodocus. Both were quadrupedal, with columnar limbs adapted to support the enormous bulk of the animals. Barosaurus had proportionately longer forelimbs than other diplodocids, although they were still shorter than most other groups of sauropods. There was a single carpal bone in the wrist, and the metacarpals were more slender than those of Diplodocus. Barosaurus feet have never been discovered, but like other sauropods, it would have been digitigrade, with all four feet each bearing five small toes. A large claw adorned the inside digit on the manus (forefoot) while smaller claws tipped the inside three digits of the pes (hindfoot).

Classification and systematics

Cladogram of Diplodocidae highlighting the position of Barosaurus

Diplodocidae	Apatosaurinae  Diplodocinae  Barosaurus Diplodocus

Barosaurus is a member of the sauropod family Diplodocidae, and sometimes placed with Diplodocus in the subfamily Diplodocinae. Diplodocids are characterized by long tails with over 70 vertebrae, shorter forelimbs than other sauropods, and numerous features of the skull. Diplodocines like Barosaurus and Diplodocus have more slender builds and longer necks and tails than apatosaurines, the other subfamily of diplodocids.

The systematics (evolutionary relationships) of Diplodocidae are becoming better established. Diplodocus has long been regarded as the closest relative of Barosaurus. Barosaurus is monospecific, containing only the type species, B. lentus, while at least three species belong to the species Diplodocus. Another diplodocid genus, Seismosaurus, is considered by many paleontologists to be a junior synonym of Diplodocus as a possible fourth species. Tornieria (formerly "Barosaurus" africanus) and Australodocus from the famous Tendaguru Beds of Tanzania in eastern Africa have also been classified as diplodocines. With its elongated neck vertebrae, Tornieria may have been particularly closely related to Barosaurus. The other subfamily of diplodocids is Apatosaurinae, which includes Apatosaurus and Supersaurus. The early genus Suuwassea is considered by some to be an apatosaurine, while others regard it as a basal member of the superfamily Diplodocoidea. Diplodocid fossils are found in North America, Europe, and Africa. More distantly related within Diplodocoidea are the families Dicraeosauridae and Rebbachisauridae, found only on the southern continents.

Discovery and naming

The first Barosaurus remains were discovered in the Morrison Formation of South Dakota by Othniel Charles Marsh and John Bell Hatcher of Yale University in 1889. Only six tail vertebrae were recovered at that time, forming the type specimen (YPM 429) of a new species, which Marsh named Barosaurus lentus, from the Classical Greek words βαρυς/barus ("heavy") and σαυρος/sauros ("lizard"), and the Latin word lentus ("slow"). The rest of the type specimen was left in the ground under the protection of the landowners until it was collected nine years later, in 1898, by Marsh's assistant, George Wieland. Marsh described these new remains, consisting of vertebrae, ribs and limb bones, and classified Barosaurus as a diplodocid for the first time. In his last published paper before his death, Marsh named two smaller metatarsals found by Wieland as a second species, B. affinis, but this has long been considered a junior synonym of B. lentus.

After the turn of the 20th century, Pittsburgh's Carnegie Museum of Natural History sent fossil hunter Earl Douglass to Utah to excavate the Carnegie Quarry in what is now known as Dinosaur National Monument. Four neck vertebrae, each one meter (3 feet) long, were collected in 1912 near a specimen of Diplodocus, but a few years later, William Jacob Holland realized they belonged to a different species. Meanwhile, the type specimen of Barosaurus had finally been prepared at Yale and was fully described by Richard Swann Lull in 1919. Based on Lull's description, Holland referred the vertebrae (CM 1198), along with a second partial skeleton found by Douglass in 1918 (CM 11984), to Barosaurus. This second Carnegie specimen remains in the rock wall at Dinosaur National Monument and was not fully prepared until the 1980s.

The most complete specimen of Barosaurus was excavated from the Carnegie Quarry in 1923 by Douglass, now working for the University of Utah after the death of U.S. Steel founder Andrew Carnegie, who had been financing Douglass' earlier work in Pittsburgh. Material from this specimen was spread across three institutions. Most of the back vertebrae, ribs, pelvis, hindlimb and most of the tail stayed at the University of Utah, while the neck vertebrae, some back vertebrae, the shoulder girdle and forelimb were shipped to the National Museum of Natural History in Washington D.C., and a small section of tail vertebrae ended up in the Carnegie Museum in Pittsburgh. In 1929, Barnum Brown arranged for all of the material to be shipped to the American Museum of Natural History in New York City, where it remains today. A cast of this specimen (AMNH 6341) was controversially mounted in the lobby of the American Museum, rearing up to defend its young from a marauding Allosaurus.

More recently, more vertebrae and a pelvis were recovered in South Dakota. This material (SDSM 25210 and 25331) is stored in the collection of the South Dakota School of Mines and Technology in Rapid City. In 2007, paleontologist David Evans discovered a partial Barosaurus skeleton forgotten in the collection of the Royal Ontario Museum in Toronto, where he had recently become a curator. Earl Douglass excavated this specimen (ROM 3670) at the Carnegie Quarry in the early 20th century and the ROM acquired it in a 1962 trade with the Carnegie Museum. The specimen never made it out on exhibit and instead remained in storage until its rediscovery 45 years later. It is now one of the centerpieces of the dinosaur exhibit at the ROM. John McIntosh believes that this skeleton is the same individual represented by four neck vertebrae (CM 1198) in the collection of the Carnegie Museum.

Discoveries in Africa

In 1907, German paleontologist Eberhard Fraas discovered the skeletons of two sauropods on an expedition to the Tendaguru Beds in German East Africa (now Tanzania). He classified both specimens in the new genus Gigantosaurus, with each skeleton representing a new species (G. africanus and G. robustus). However, this genus name had already been given to the fragmentary remains of a sauropod from England. Both species were moved to a new genus, Tornieria, in 1911. Upon further study of these remains and many other sauropod fossils from the hugely productive Tendaguru Beds, Werner Janensch moved the species once again, this time to the North American genus Barosaurus. In 1991, "Gigantosaurus" robustus was recognized as a titanosaur and placed in a new genus, Janenschia, as J. robusta. Meanwhile, many paleontologists suspected "Barosaurus" africanus was also distinct from the North American genus, which was confirmed when the material was redescribed in 2006. The African species, although closely related to Barosaurus lentus and Diplodocus from North America, is now once again known as Tornieria africana.

Paleoecology

Barosaurus remains are limited to the Morrison Formation, which is widespread in the western United States between the Great Plains and Rocky Mountains. Radiometric dating agrees with biostratigraphic and paleomagnetic studies, indicating that the Morrison was deposited during the Kimmeridgian and early Tithonian stages of the Late Jurassic Period, or approximately 155 to 148 million years ago. Barosaurus fossils are found in late Kimmeridgian sediments, around 150 million years old.

The Morrison Formation was deposited in floodplains along the edge of the ancient Sundance Sea, an arm of the Arctic Ocean which extended southward to cover the middle of North America as far south as the modern state of Colorado. Due to tectonic uplift to the west, the sea was receding to the north, and had retreated into what is now Canada by the time Barosaurus evolved. The sediments of the Morrison were washed down out of the western highlands, which had been uplifted during the earlier Nevadan orogeny and were now eroding. Very high atmospheric concentrations of carbon dioxide in the Late Jurassic led to high temperatures around the globe, due to the greenhouse effect. One study, estimating CO₂ concentrations of 1120 parts per million, predicted average winter temperatures in western North America of 20°C (68°F) and summer temperatures averaging 40-45°C (104-113°F). A more recent study suggested even higher CO₂ concentrations of up to 3180 parts per million. Warm temperatures that led to significant evaporation year-round, along with possible rain shadow effect from the mountains to the west, led to a semi-arid climate with only seasonal rainfall.