Tenontosaurus

Tenontosaurus ("sinew lizard") is a genus of ornithopod dinosaur from the Early Cretaceous of North America.

Description
Tenontosaurus measured 6.5 to 8 meters long and weighed between 1 to 2 tonnes. It had a particularly long tail, which was stiffened by bony tendons much like its back was.

Classification
Tenontosaurus was originally considered a hypsilophodont, but it is now considered a primitive iguanodont. There are two known species, T. tilletti (the type species) and T. dossi (known only from a few specimens in Texas).

Discovery
The first fossils of Tenontosaurus were discovered in 1903 in Montana. During the next few decades, dozens more were unearthed in the same area. The dinosaur was not formally described at the time, but paleontologist Barnum Brown informally called it "Tenantosaurus", meaning "sinew lizard", referencing the extensive tendons in the back and tail.

In the 1960s, a team from Yale University led by John Ostrom uncovered over 40 new specimens of Tenontosaurus in the Cloverly Formation in Montana. This expedition led to the description of the famous dromaeosaurid Deinonychus, but Ostrom also formally described Tenontosaurus at this time, modifying Barnum Brown's original name slightly. Many more specimens have since been reported.

Predatory associations
Teeth and skeletons that came from the dromaeosaurid dinosaur Deinonychus are often associated with Tenontosaurus remains. In fact, only six sites that contain Deinonychus fossils are found with no Tenontosaurus fossils, and the raptor is rarely found with other potential prey items. Because of this, it is often perceived that Deinonychus was a frequent predator of this dinosaur, although there are other possible scenarios that explain this association, such as cannibalization.

Diet
Tenontosaurus was herbivorous, and its diet likely consisted of ferns, shrubs, cycads, and possibly fruits.

Reproduction
In one specimen of Tenontosaurus, medullary bone was found in the animal's thigh and shin. Today, medullary tissue is only found in birds that are laying or going to lay eggs. To date, only two other dinosaurs have been found with this tissue (Tyrannosaurus and Allosaurus), making Tenontosaurus the first ornithischian known to possess it. All three specimens were not fully grown when died, suggesting that all dinosaurs produced this tissue and reached sexual maturity before they reached full size.