Saturday, November 19, 2016

Life reconstruction of an individual rearing up to defend itself against a pair of Allosaurus

Barosaurus was a giant, long-tailed, long-necked, plant-eating dinosaur closely related to the more familiar Diplodocus. Remains have been found in the Morrison Formation from the Upper Jurassic Period of Utah and South Dakota (and possibly Africa, as exemplified by the Kadsi Formation). It is present in stratigraphic zones 2-5.

The composite term Barosaurus comes from the Greek words barys (βαρυς) meaning “heavy” and sauros (σαυρος) meaning “lizard”; thus “heavy lizard”.

Visitors to the American Museum of Natural History are greeted by a startling sight. Towering above them is a skeleton of a female Barosaurus protecting its infant from the menacing approach of an Allosaurus. The reconstruction is, of course, based to some extent on guesswrok, but it is a striking depiction of how life might have been 150 million years ago.

The official postcard (of the American Museum of Natural History) says this is a Barosaurus, and that "this unique freestanding mount is the only Barosaurus on view in the world". This was true until the installation of another Barosaurus specimen at the Royal Ontario Museum. The adult specimen pictured is AMNH 6341, classified as Barosaurus lentus. The juvenile specimen (AMNH 7530), originally classified as a juvenile Barosaurus, has since been reclassified as a specimen of Kaatedocus siberi.

Barosaurus is very similar to Diplodocus, to which it is closely related. Both were very long animals with relatively compact bodies that supported long necks and tails. Like Diplodocus, Barosaurus had front legs shorter than the hind ones. As a result, its back sloped gently forward. The neck projected around 30 feet (9 m) in front of the shoulders. It was once thought that sauropods’ long necks allowed them to feed high in tree tops. However, as the neck vertebrae would not have allowed much up-and-down movement, but would have permitted a considerable sweep from side to side, we now think that these dinosaurs fed much close to the ground on ferns and cycads.

Despite more than a century of searching and the recovery of five partial skeletons, some of them almost complete, the head and the tip of the tail of Barosaurus have never been found. The only clues we have about the head are a few bones from the skull collected in Tanzania and comparisons with close relatives such as Diplodocus. These indicate that Barosaurus had a horselike skull with a long snout and teeth restricted to the very front of the mouth.

Skull cast, Natural History Museum of Utah