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.


The structure of the cervical vertebrae of Barosaurus allowed for a significant degree of lateral flexibility in the neck, but restricted vertical flexibility. This suggests a different feeding style for this genus when compared to other diplodocids. Barosaurus swept its neck in long arcs at ground level when feeding, which resembled the strategy that was first proposed by John Martin in 1987. The restriction in vertical flexibility suggests that Barosaurus could not feed on vegetation that was high off the ground.

Skull cast, Natural History Museum of Utah

The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Diplodocus, Apatosaurus and Brachiosaurus. Dinosaurs that lived alongside Barosaurus included the herbivorous ornithischians Camptosaurus, Dryosaurus, Stegosaurus and Othnielosaurus. Predators in this paleoenvironment included the theropods Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Stokesosaurus, Ornitholestes and Allosaurus accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web. Other vertebrates that shared this paleoenvironment included bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, and several species of pterosaur. Early mammals were present such as docodonts, multituberculates, symmetrodonts, and triconodonts. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns, and ferns (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.

Assistant Curator David Evans mounted the ROM specimen conservatively, with a relatively low head to give the dinosaur moderate blood pressure. The extremely long neck, 10 metres (30 feet) may have developed to enable Barosaurus to feed over a wide area without moving around; it may also have enabled the dinosaurs to radiate excess body heat. Evans suggests that sexual selection might have favoured those with longer necks. (See video "Neck Impossible" at reference.)