Seismosaurus


 * The dinosaur genus originally known as Seismosaurus is now officially designated Diplodocus. Please see Diplodocus for more information.

Seismosaurus (meaning "Earth-shaking lizard") is a deprecated genus of extinct sauropod dinosaurs. While originally thought to be a distinct genus of dinosaur, studies have since shown that it is actually a large species of the well-known genus Diplodocus, known either as D. hallorum or D. longus. Seismosaurus was named for a partial skeleton discovered in New Mexico in 1979 consisting of vertebrae, pelvis, and ribs. The supposed associated gastroliths appeared to be stream-deposited cobbles of a channel-lag deposit. The species was formally recognised in 1991.

Size
When first described in 1991, Gillete calculated that Seismosaurus may have been up to 54 m (177.05 ft) long, making it the longest known dinosaur (excluding those known from especially poor remains, such as Amphicoelias). Some weight estimates ranged as high as 113 (rather only 50) tonnes (125 US short tons). Since the initial description of Seismosaurus, the initial reports of gigantic size have turned out to be greatly exaggerated. The most recent studies show that Seismosaurus was in fact shorter than its relative Supersaurus at about 32 m (105 ft) long, and may have weighed about 22 to 27 tonnes (25 to 30 US short tons). This review was based on recent findings that show that the giant tail vertebrae were actually placed further forward on the tail than David Gillete originally calculated. The study shows that the complete Diplodocus skeleton at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania on which estimates of Seismosaurus were based had its 13th tail vertebra come from another dinosaur, throwing size estimates for Seismosaurus off by up to 30%.

Classification
A presentation at the annual conference of the Geological Society of America in 2004 has made a case for Seismosaurus to be reassigned as a species of Diplodocus, namely D. hallorum. The authors argue that many of the distinctive features of Seismosaurus are either pathological or as a result of misplacement of the vertebrae. This suggestion was further developed by the authors in 2006, in an article where they not only synonymized the two genera and officially named Diplodocus hallorum, but also speculated that it was the same as D. longus. They eliminated one of the most distinct features of its skeleton, namely a hook-like process at the end of the ischium, by showing it to have been a vertebral neural arch fragment adhering to the ischium. Without this, the bone has a very normal outline.