Image Caption: Head of Tenontosaurus, Institut de paléontologie humaine, Paris, France. Credit: Rémih/Wikipedia (CC BY-SA 3.0)

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 Cretaceious period sediments of western North America, dating roughly between 115 to 108 million years ago. It was formerly thought to be a ‘hypsilophodont’, but since Hypsilophodontia is no longer considered to be a clade, it’s now considered to be a very primitive iguanodont.

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 the western parts of North America. T. dossi is known from only a few specimens collected from the Twin Mountains Formation in Parker County, Texas.

It was about 21 to 26 feet long and 9.8 feet high in a bipedal stance, with a mass of somewhere between 1 to 2 short tons. It had an abnormally long, broad tail, which like its back was stiffened with a network of bony tendons.

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History expedition in the year 1903. Following 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 period of time, though Barnum Brown of the AMNH gave it the informal name “Tenantosaurus”, “sinew lizard”, referencing the extensive system of stiffening tendons in its back and tail.

During the 1960s, Yale University began and extensive, long-term dig in the Big Horn Basin area of Montana and Wyoming. The expedition was led by John Ostrom, whose team discovered over 40 new specimens. Following his expedition, Ostrom became the first person to describe and name the animal, calling Tenontosaurus, a slight variation in spelling of Brown’s informal name.

Since 1970, many more Tenontosaurus specimens have been documented, both from the Cloverly and other geological formations, including the Antlers Formation in Oklahoma, Paluxy Formation of Texas, Cedar Mountain Formation of Utah, Arundel Formation of Maryland and the Wayan Formation of Idaho.

At the time, Tenontosaurus initially appeared in Wyoming and Montana, the regions climate was arid to semi-arid, dry with seasonal periods of rainfall and occasional droughts. However, during a period of a few million years, the climate in the region shifted to one of increased rainfall, and the environment became tropical to subtropical, with flood plains, river deltas, and forests with swampy inlets reminiscent of modern Louisiana, though marked dry seasons persisted to create environments much like that of a savannah. The change in rainfall levels is likely because of the advancing shoreline of the Skull Creek Seaway, a cycle of the Western Interior Seaway, which, later in the Cretaceous period, would entirely divide North America.

This dramatic shift in climate coincided with an obvious increase, rather than decrease, in the abundance of Tenontosaurus. This shows Tenontosaurus to have been an extraordinary adaptable animal, which persisted for a long span of time in one area despite changes to its environment.

Throughout the Cloverly Formation, Tenontosaurus is by far the most common vertebrate, five times more plentiful than the next most common, the ankylosaur Sauropelta. 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 allosaurid theropod and goniopholid crocodile. After the major climate shift at the start 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. The ecological community in the tropical state included the small mammal Gobiconodon, turtles as Glyptops, and species of lungfish as well.

The ecological community was similar in other regions, with dinosaurs like Tenontosaurus and Deinonychus as the most common sizeable vertebrates. In the Antlers Formation, they were joined by the sauropods Astrodon and Sauroposeidon, and the allosauroid Acrocanthosaurus, in addition to different lizards, fish, and mammals.

Plant life in the Tenontosaurus ecosystem was likely dominated with ferns and tree ferns, cycads and possibly primitive flowering plants. Larger plants and trees were represented by gymnosperm, such as ginkgo and conifer trees.  Tenontosaurus was a low browser, and an adult would have had a maximum browsing height of about 9 feet if it took on a bipedal stance. This restricted Tenontosaurus, particularly juveniles, to eating low-growing ferns and shrubs. The beak is U-shaped and very powerful and the angled cutting surfaces of its teeth, however, meant it was not limited to which part of the plant it. Wood, leaves, and even fruit may have formed part of its diet.

Teeth and a number of skeletons belonging to the bird-like carnivorous theropod Deinonychus have frequently been discovered associated with Tenontosaurus tilletti remains. Tenontosaurus specimens have been found at more than 50 sites, and 14 of those also contain remains of Deinonychus. According to one 1995 study, only six sites containing Deinonychus fossils contain no trace of Tenontosaurus, and Deinonychus remains are only infrequently found associated with other potential prey, for example, Sauropelta. In all, 20% of Tenontosaurus fossils are found in close proximity to Deinonychus, and several scientists have suggested that this implies Deinonychus was the major predator of Tenontosaurus. Adult Deinonychus, however, were much smaller in size than adult Tenontosaurus, and it’s unlikely a single Deinonychus must therefore have been capable of attacking a fully grown Tenontosaurus. While some scientists have suggested that Deinonychus must therefore had been a pack hunter, this view has been defied based on both lack of evidence for coordinated hunting as well as evidence that Deinonychus may have been cannibalizing each other, as well as the Tenontosaurus, in a feeding frenzy. It’s unlikely that Deinonychus favored juvenile Tenontosaurus, and that when Tenontosaurus reached a certain size, it passed out of range as a food source for the small theropods, though they may have scavenged larger individuals. The fact that most Tenontosaurus remains found with Deinonychus are half-grown individuals supports this theory. It also lived in the same area as the large carnivorous dinosaur called Acrocanthosaurus.

The presence of medullary bone tissue in the thigh bone and shin bone of one specimen indicates that tenontosaurs utilized this tissue, today only found in birds that are laying eggs in reproduction. 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 the age of 8 years. Because the theropod line of dinosaurs that includes Allosaurus and Tyrannosaurus departed from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue and reached reproductive maturity before reaching maximum size. A histological study illustrated that T. tilletti grew quickly early in their life and during sub-adult ontogeny, but grew very slowly in the years approaching maturity, unlike other iguanodonatians.