Tenontosaurus

Tenontosaurus is a genus of iguanodontian ornithopod dinosaur. It had an unusually long, broad tail, which like its back was stiffened with a network of bony tendons. 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. It contains two species, Tenontosaurus tilletti and Tenontosaurus dossi. Many specimens of T. tilletti have been collected from several geological formations throughout western North America. T. dossi is known from only a handful of specimens collected from the Twin Mountains Formation of Parker County, Texas.

History of discovery

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History 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 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. Following his expedition, in 1970, Ostrom published a review of the fauna and geology of the Cloverly Formation. In that paper, he scientifically described Brown's taxon, calling it Tenontosaurus tilletti. The genus name has the same etymology as the informal "Tenantosaurus" name; the species name refers to the Lloyd Tillett family, who provided field parties from Yale with assistance and hospitality throughout their expeditions. The type specimen of T. tilletti, as designated by Ostrom, was AMNH 3040.
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. In 1997, remains from the Twin Mountains Formation of Texas were assigned to a new species of Tenontosaurus, T. dossi, named after the Doss Ranch site. In 2025, Tenontosaurus were unearthed from the Yucca Formation in Texas.

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, but Nicolás E. Campione and David C. Evans in 2020 estimated that this species would have weighed up to. Paul also estimated that T. dossi would have been long and weighed. The original describers of T. dossi favoured a length estimate of, and the same estimate has been given elsewhere.

Skull

The skull of Tenontosaurus was originally described by Ostrom as being very long and deep compared to taxa such as Theiophytalia. While this is true, Ostrom's reconstruction of the skull exaggerated the overall depth of the skull, leading to one which more closely resembles that of Hypsilophodon. The external nares of Tenontosaurus were very large, and were almost entirely encircled by the premaxillae. The premaxillae flared inferiorly, forming a thick, U-shaped beak, characteristic of iguanodontians. The beak extended as far back as the anterior process of the maxilla. The single longest bone in Tenontosaurus' skull is the vomer, which stretches from the middle of the beak to the very back of the orbital cavity. The maxilla was larger in general, though, comprising the majority of the rostrum and housing all of the upper teeth. The maxilla was divided into two components: a facial lamina, which comprised its lateral surface, and a dental ridge. In more derived iguanodontians, and Theiophytalia, the maxilla was bordered by a lateral process of the jugal relatively far forward; in Tenontosaurus, however, the facial laminae of the maxillas overlapped with the jugals, and almost contacted the inferior part of the lacrimal bone. T. tilletti had a long and narrow antorbital fenestra, which extended down from the lacrimals to the facial laminae; T. dossi, meanwhile, had a relatively small one, surrounded by a shallow fossa. Similarly, the dorsal process of T. dossi's maxilla did not expand posteriorly above the antorbital fenestra, as it did in T. tilletti. The process connecting the quadratojugal to the jugal was structured in a way that formed an additional temporal fenestra, which Ostrom presumed related to jaw adductor muscles. A similar fenestra is also seen in Hypsilophodon. The orbit was roughly triangular, and was larger than the fenestrae anterior to or posterior to it. The majority of the occipital condyle in Tenontosaurus was made up of a large basioccipital, though this structure was almost entirely excluded from the occipital surface itself. Ostrom noted similarities between the shapes of the paroccipitals in Tenontosaurus and in hadrosaurs, namely their hooked shape and their orientation in relation to the foramen magnum. The ventral flange of the pterygoid was broad and deep, particularly enlarged dorsally, providing a larger attachment site for parts of the pterygoid muscles. The quadrate was narrow, and was essentially vertical.

Lower jaw

Tenontosaurus' mandible was described by Ostrom as being "of moderate length and robustness". It terminated in a predentary bone, a structure found uniquely among ornithischians which formed the lower 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 surface which overlapped with the mandibular symphyses.' The dentaries 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. 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 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, although subsequent authors challenged this classification and proposed it actually belonged to Hypsilophodontidae. Subsequent phylogenetic analyses have found Tenontosaurus to be intermediate in position between Hypsilophodon-like taxa and Iguanodon-like taxa, as a non-dryomorph iguanodont. The precise phylogenetic position of Tenontosaurus varies between studies, with some studies finding it to be more closely related to dryomorphs than rhabdodontids, some studies finding it to be the sister taxon of a clade uniting rhabdodontids and dryomorphs.
The cladogram below follows an analysis by Butler et al, 2011.
The discovery of Iani in 2023 lent support for a close relationship between Tenontosaurus and rhabdodontids, as Iani exhibits transitional characteristics between Tenontosaurus and other rhabdodontomorphs. This result was verified in two datasets.
Topology 1: Poole dataset
Topology 2: Dieudonné et al. dataset
A similar result was recovered by Fonseca et al., 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.