Tenontosaurus

Tenontosaurus (/tɨˌnɒntɵˈsɔrəs/ te-NON-to-SAWR-əs; meaning "sinew lizard") is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago. It was formerly thought to be a 'hypsilophodont', but since Hypsilophodontia is no longer considered a clade, it is now considered to be a very primitive iguanodont.

The genus contains two species, Tenontosaurus tilletti (described by John Ostrom in 1970) and Tenontosaurus dossi (described by Winkler, Murray, and Jacobs in 1997).[1] Many specimens of T. tilletti have been collected from several geological formations throughout western North America. T. dossi is known from only a handful of specimens collected from the Twin Mountains Formation of Parker County, Texas.

Description
It was about 6.5 to 8 metres (21 to 26 ft) long and 3 metres (9.8 ft) high in a bipedal stance, with a mass of somewhere between 1 to 2 tonnes (1 to 2 short tons). It had an unusually long, broad tail, which like its back was stiffened with a network of bony tendons.

History of study
The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History (AMNH) expedition in 1903. Subsequent digs in the same area during the 1930s unearthed 18 more specimens, and four specimens were found during the 1940s. Despite the large number of fossil specimens, the animal was not named or scientifically described during this time, though Barnum Brown of the AMNH gave it the informal name "Tenantosaurus", "sinew lizard", in reference to the extensive system of stiffening tendons in its back and tail.[2]

During the 1960s, Yale University began an extensive, long-term dig in the Big Horn Basin area (Cloverly Formation) of Montana and Wyoming. The expedition was led by John Ostrom, whose team discovered more than 40 new specimens. Following his expedition, Ostrom became the first to describe and name the animal, calling it Tenontosaurus, a slight variation in spelling of Brown's informal name.

Since 1970, many more Tenontosaurus specimens have been reported, both from the Cloverly and other geological formations, including the Antlers Formation in Oklahoma, Paluxy Formation of Texas, Wayan Formation of Idaho, Cedar Mountain Formation of Utah, and Arundel Formation of Maryland.[2]

The cladogram below follows an analysis by Butler et al, 2011