Fossil Tenontosaurus, on display in the Museum of the Rockies in Bozeman, Montana. This specimen was collected in Carbon County, Montana.

Tenontosaurus (the "sinewy lizard") is a primitive Iguanodont that lived in western North America about 115 to 108 million years ago. Fossils were first discovered in Big Horn County, Montana, in 1903. Tenontosaurus was about 21 to 26 feet long and 10 feet high when it stood upright. (It usually traveled on all fours, however.) It weighed aboout one to two tons. Its tail was unusually long and broad, and the tail and the back was stiffened with a network of bony tendons (hence the name).