Tenontosaurus

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History (AMNH) expedition in 1903. Subsequent digs in the same area during the 1930s, spearheaded by Barnum Brown and Roland T. Bird, unearthed 18 more specimens. Four more were recovered during the 1940s: two by a team from the University of Oklahoma, and two by a private collector, Al Silberling, operating on the behalf of Princeton University. Despite the large number of fossil specimens, the animal was not named or scientifically described during this time, though Barnum Brown gave it the informal name "Tenantosaurus", "sinew lizard", in reference to the extensive system of stiffening tendons in its back and tail. Starting in 1962, Yale University conducted an extensive, five-year-long dig in the Big Horn Basin area (Cloverly Formation) of Montana and Wyoming. The expedition was led by John Ostrom, whose team discovered more than 40 new specimens of the taxon recovered by Brown. Since 1970, many more Tenontosaurus specimens have been reported, both from the Cloverly and other geological formations, including the Antlers Formation in Oklahoma, Paluxy Formation of Texas, Wayan Formation of Idaho, Cedar Mountain Formation of Utah, and Arundel Formation of Maryland. ==Description==

Size Tenontosaurus was a medium-sized ornithopod, with both species weighing about . Gregory S. Paul in 2016 estimated that T. tilletti would have been and weighed , Paul also estimated that T. dossi would have been long and weighed . Skull The skull of Tenontosaurus was originally described by Ostrom as being very long and deep compared to taxa such as Theiophytalia (then considered a species of Camptosaurus). The premaxillae flared inferiorly (towards the bottom), forming a thick, U-shaped beak, The predentary in Tenontosaurus was horseshoe-shaped and was lined with projecting serrations, referred to as pseudo-teeth, or as denticles. They likely served, at least partly, as anchors for the keratinous portion of the beak. Such structures are present in many ornithischians, save for taxa such as Dryosaurus or Zalmoxes. In at least T. dossi, the predentary had a process on its ventral (bottom) surface which overlapped with the mandibular symphyses.' The dentaries (the tooth-bearing portions of the lower jaw) were robust.' There were two distinct coronoid processes to the mandibles. The retroarticular processes, projections at the back of each mandibular ramus, were long and curved. Ostrom noted that the surangulars likely formed most of the lateral surface of the mandible at the level of the coronoid process and beyond, though was not able to confirm this due to the condition of the material at his disposal; this was later confirmed. The surangulars also formed the lateral part of the retoarticular processes. Dentition All of Tenontosaurus' teeth were enameled unilaterally (on only one side, that being medially, or on the inside). The maxillary teeth bore a series of ridges, subequal in length, running along their surface in a non-parallel arrangement. Those of the dentary lacked these ridges, instead bearing prominent vertical keels on their inner surfaces. Postcranial skeleton Tenontosaurus was a facultative quadruped, capable of assuming either a bipedal or quadrupedal stance. It may have used a quadrupedal stance while feeding, but was probably incapable of rapid quadrupedal movement. Tenontosaurus resembles quadrupedal ornithischians in having a tibia that is shorter than the femur and an anterolateral process on the ulna, but it resembles bipedal ornithischians in having a relatively narrow pelvis and a pendant fourth trochanter. The manus (the hand/front foot) shows a mixture of traits associated with bipeds and quadrupeds. It retains narrow claws, unlike the hoof-like unguals of quadrupedal ornithischians, has a short metacarpus, as in the bipedal Hypsilophodon, and retains grasping adaptations. However, the phalanges are also shortened as an adaptation to weight bearing. An analysis of the overall proportions and center of mass of Tenontosaurus found it to be a quadruped, although the analysis only distinguished bipeds from quadrupeds without including a facultatively bipedal category. ==Classification==

Tenontosaurus was originally classified in the family Iguanodontidae, }} }}|label1=Rhabdodontomorpha}} Topology 2: Dieudonné et al. (2021) dataset }} }} }}|label1=Rhabdodontomorpha}} A similar result was recovered by Fonseca et al. (2024), who found Iani, Tenontosaurus, and also Convolosaurus to form the new family Tenontosauridae within the Rhabdodontomorpha, defined in the PhyloCode as "the largest clade containing Tenontosaurus tilletti, but not Hypsilophodon foxii, Iguanodon bernissartensis, and Rhabdodon priscus". This family may have represented an early North American radiation of the Rhabdodontomorpha. ==Paleobiology==

Diet Plant life in the Tenontosaurus ecosystem was likely dominated by ferns and tree ferns, cycads, and possibly primitive flowering plants. Larger plants and trees were represented by gymnosperms, such as conifer and ginkgo trees. Tenontosaurus was a low browser, and an adult would have had a maximum browsing height of about if it adopted a bipedal stance. This restricted Tenontosaurus, especially juveniles, to eating low-growing ferns and shrubs. Its powerful, U-shaped beak and the angled cutting surfaces of its teeth, however, meant it was not limited to which part of the plant it consumed. Leaves, wood, and even fruit may have formed part of its diet. Predators Teeth and a number of skeletons belonging to the carnivorous theropod Deinonychus have often been discovered associated with Tenontosaurus tilletti remains. Tenontosaurus specimens have been found at over 50 sites, and 14 of those also contain Deinonychus remains. According to one 1995 study, only six sites containing Deinonychus fossils contain no trace of Tenontosaurus, and Deinonychus remains are only rarely found associated with other potential prey, like 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 than adult Tenontosaurus, and it is unlikely a single Deinonychus would have been capable of attacking a fully grown Tenontosaurus. 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) as well as evidence that Deinonychus may have been cannibalizing each other, as well as the Tenontosaurus, in a feeding frenzy. It is likely 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, or preyed on adults that were sick or injured. The fact that most Tenontosaurus remains found with Deinonychus are half-grown individuals supports this view. It also lived in the same area as the large carnivorous dinosaur Acrocanthosaurus. Reproduction The presence of medullary bone tissue in the thigh bone and shin bone of one specimen indicates that Tenontosaurus used 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 8 years old. Because the theropod line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged 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 maximum size. A histological study showed that T. tilletti grew quickly early in life and during sub-adult ontogeny, but grew very slowly in the years approaching maturity, unlike other iguanodontians. ==Paleoecology==

Throughout the Cloverly Formation, Tenontosaurus is by far the most common vertebrate, five times more abundant than the next most common, the ankylosaur Sauropelta.The forest floor supported a rich understory of cycads, cycadeoids, horsetails, forked ferns, spleenworts, and Osmundaceae ferns. This wide array of plants probably kept these herbivores well fed. Possible cycad seeds were thought to have been found in the gut of Tenontosaurus individual, however these were probably mineral concretions. Climate In the Cloverly Formation of Montana and Wyoming, Tenontosaurus remains are common in two distinct rock units: the more ancient Little Sheep Mudstone Member (Cloverly Formation unit V) and the more recent Himes Member (units VI and VII). The oldest part of the formation, the Pryor Conglomerate, contains no Tenontosaurus fossils, and they only appear in the uppermost, most recent part of the Little Sheep Mudstone Member. Catherine Forster, in a 1984 paper on the ecology of Tenontosaurus, used this as evidence to suggest that Tenontosaurus populations did not arrive in the Bighorn Basin area until the time of the late Little Sheep Mudstone Member. 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. However, during a period of a few million years, the climate in the region shifted to one of increased rainfall, and the environment became subtropical to tropical, with river deltas, floodplains, and forests with swampy inlets reminiscent of modern Louisiana, though marked dry seasons persisted to create savannah-like environments as well. The change in rainfall levels is likely due to the advancing shoreline of the Skull Creek Seaway, a cycle of the Western Interior Seaway which, later in the Cretaceous period, would completely divide North America. This dramatic shift in climate coincided with an apparent increase, rather than decrease, in the abundance of Tenontosaurus. This shows Tenontosaurus to have been a remarkably adaptable animal, which persisted for a long span of time in one area despite changes to its environment. ==References==