Period: Early Cretaceous
Order, Suborder, and Family: Ornithischia, Ornithopoda, Hypsilophodontidae
Location: North America (United States)
Length: 22 feet (6.5 meters)
Tenontosaurus tilletti was a medium-size ornithopod dinosaur from Montana and Wyoming. Barnum Brown of the American Museum of Natural History discovered the first Tenontosaurus skeleton in Montana in 1903. Since then, several dozen partial to complete skeletons of Tenontosaurus have been found, as well as parts of many more. These skeletons range in size from very small juveniles to almost 22-foot-long adults.
Some of the skeletons of the younger animals were found jumbled together in groups-three in one group and four in another. The group of four was found with an adult Tenontosaurus. These Tenontosaurus juveniles may have gathered in groups or stayed in family groups after they hatched, possibly for protection from predators.
Tenontosaurus had an extremely long, deep tail that was stiffened by "ossified tendons"-tendons that turned to bone. Since Tenontosaurus walked mostly on its back legs, these ossified tendons probably helped hold its tail out straight to balance the front of its body. It is these stiffened tendons that gave the dinosaur its name: tenon is the Greek word for "sinew" or "tendon." About two-thirds of the entire length of Tenontosaurus was tail. Ossified tendons were also present along its back and over its hips. Even though it probably did most of its walking and running on its back legs, Tenontosaurus also had very strong front legs with short, wide front feet. It probably used them to walk quadrupedally (on all four limbs) to browse on low vegetation.
Tenontosaurus had a long, flexible neck. The front of its mouth had no teeth; it had a horny beak for biting off plants. Rows of strong, tightly fitted teeth ground up even tough plants.
At the time Tenontosaurus lived, the weather was quite warm and seasonal, with some rain. Plants like cycads, ferns, and conifers were common, and the flowering plants were just beginning to evolve. While browsing for food, Tenontosaurus probably had to keep watch for the small, fast carnivorous dinosaur Deinonychus. We know that Deinonychus preyed on Tenontosaurus because their broken teeth have been found with some Tenontosaurus skeletons. Some paleontologists believe that Deinonychus may have hunted Tenontosaurus in packs.
Other animals that lived alongside Tenontosaurus included the ankylosaur Sauropelta, the coelurid Microvenator, and the hypsilophodontid Zephyrosaurus. Tenontosaurus seems to have been most closely related to the iguanodontids Dryosaurus, Iguanodon, and Camptosaurus. Tenontosaurus was also distantly related to the smaller hypsilophodontids, such as Hypsilophodon.