The genus is known from the late Aptian to Albian ages of the Early Cretaceous period sediments of western North America, dating between 115 and 108 million years ago.

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History (AMNH) expedition in 1903.

[3] During the 1960s, Yale University began an extensive, long-term dig in the Big Horn Basin area (Cloverly Formation) of Montana and Wyoming.

[10] Tenontosaurus was originally classified in the family Iguanodontidae,[1] although subsequent authors challenged this classification and proposed it actually belonged to Hypsilophodontidae.

[12] Orodromeus Hypsilophodon Zephyrosaurus Yandusaurus Changchunsaurus Jeholosaurus Gasparinisaura Parksosaurus Bugenasaura Thescelosaurus Talenkauen Anabisetia Rhabdodontidae T. tilletti T. dossi Dryosauridae Ankylopollexia (including Iguanodon, hadrosaurids, and others) Plant life in the Tenontosaurus ecosystem was likely dominated by ferns and tree ferns, cycads, and possibly primitive flowering plants.

Tenontosaurus was a low browser, and an adult would have had a maximum browsing height of about 3 meters (10 ft) if it adopted a bipedal stance.

While some scientists have suggested that Deinonychus must therefore have been a pack hunter, this view has been challenged based on both a supposed lack of evidence for coordinated hunting (rather than mobbing behavior as in most modern birds and reptiles, though crocodilians have been documented to hunt cooperatively on occasion[14]) as well as evidence that Deinonychus may have been cannibalizing each other, as well as the Tenontosaurus, in a feeding frenzy.

Additionally, like Tyrannosaurus and Allosaurus, two other dinosaurs known to have produced medullary bone, the tenontosaur individual was not at full adult size upon her death at 8 years old.

[20] In the arid Little Sheep Mudstone Member, Tenontosaurus is the only herbivorous dinosaur, and it shared its environment with the common predator Deinonychus as well as an indeterminate species of allosauroid theropod and goniopholid crocodile.

After the major climate shift at the beginning of the Himes Member in the mid-Albian age, several more dinosaurs entered the region, including the less common ornithopod Zephyrosaurus, the oviraptorosaur Microvenator, and an indeterminate species of titanosauriform sauropod and ornithomimid.

Scientists have used biostratigraphic data and the fact that it shares several of the same genera as the Trinity Group of Texas, to surmise that this formation was laid down during the Albian stage of the Early Cretaceous Period, approximately 110 mya.

The paleoenvironment of the Antlers Formation consisted of tropical or sub-tropical forests, floodplains, river deltas, coastal swamps, bayous and lagoons, probably similar to that of modern-day Louisiana.

[21][3] In the Antlers Formation in what is now Oklahoma, Tenontosaurus and Deinonychus shared their paleoenvironment with other dinosaurs, such as the sauropods Astrodon (Pleurocoelus) and Sauroposeidon proteles, and the carnosaur Acrocanthosaurus atokensis, which was likely the apex predator in this region.

Other vertebrates present at the time of Tenontosaurus included the amphibian Albanerpeton arthridion, the reptiles Atokasaurus metarsiodon and Ptilotodon wilsoni, the crurotarsan reptile Bernissartia, the cartilaginous fish Hybodus buderi and Lissodus anitae, the ray-finned fish Gyronchus dumblei, the crocodilian Goniopholis, and the turtles Glyptops and Naomichelys.

[3] At the time Tenontosaurus first appeared in Wyoming and Montana (the early Albian age), the regions climate was arid to semi-arid, dry, with seasonal periods of rainfall and occasional droughts.