Tenontosaurus (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 and 108 million years ago.
The genus contains two species, Tenontosaurus tilletti (described by John Ostrom in 1970) and Tenontosaurus dossi (described by Winkler, Murray, and Jacobs in 1997). 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.
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.
Tenontosaurus fed on plant matter, which it broke off with its horny beak and then chewed with its teeth, all of which were at the back of its mouth. The small, swift-moving Deinonychus was Tenontosaurus’s main enemy. The teeth of this predator have been found along with Tenontosaurus skeletons. Some scientists therefore believe that Deinonychus hunted the larger animal in packs, but there is no real evidence to support this. In spite of what would seem to be the advantages of having clawed feet and a huge tail, Tenontosaurus would have been easy catch for packs of sharp-fanged Deinonychus.
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.