| Tenontosaurus | |
|---|---|
 | |
| T. dossi on exhibit in thePerot Museum of Nature and Science | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Clade: | †Tenontosauridae |
| Genus: | †Tenontosaurus Ostrom,1970 |
| Species | |
Tenontosaurus (/tɪˌnɒntəˈsɔːrəs/ti-NON-tə-SOR-əs;lit. 'sinew lizard') is agenus ofiguanodontianornithopoddinosaur. 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 lateAptian toAlbian ages of theEarly Cretaceous period sediments of westernNorth America, dating between 115 and 108 million years ago. It contains two species,Tenontosaurus tilletti (described byJohn Ostrom in 1970[1]) andTenontosaurus dossi (described by Winkler, Murry, and Jacobs in 1997).[2] Many specimens ofT. tilletti have been collected from several geological formations throughout western North America.T. dossi is known from only a handful ofspecimens collected from theTwin Mountains Formation ofParker County,Texas.
The firstTenontosaurusfossil was found inBig Horn County,Montana by anAmerican Museum of Natural History (AMNH) expedition in 1903. Subsequent digs in the same area during the 1930s, spearheaded byBarnum Brown andRoland 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.[3]
Starting in 1962,Yale University conducted an extensive, five-year-long dig in the Big Horn Basin area (Cloverly Formation) of Montana andWyoming. The expedition was led byJohn Ostrom, whose team discovered more than 40 new specimens of the taxon recovered by Brown.[3] Following his expedition, in 1970, Ostrom published a review of the fauna and geology of the Cloverly Formation.[1][3] In that paper, he scientifically described Brown's taxon, calling itTenontosaurus tilletti. Thegenus name has the same etymology as the informal "Tenantosaurus" name; thespecies name refers to the Lloyd Tillett family, who provided field parties from Yale with assistance and hospitality throughout their expeditions. The type specimen ofT. tilletti, as designated by Ostrom, was AMNH 3040.[1]
Since 1970, many moreTenontosaurus specimens have been reported, both from the Cloverly and other geological formations, including theAntlers Formation inOklahoma,Paluxy Formation ofTexas,Wayan Formation ofIdaho,Cedar Mountain Formation ofUtah, andArundel Formation ofMaryland.[3] In 1997, remains from theTwin Mountains Formation of Texas were assigned to a new species ofTenontosaurus,T. dossi, named after the Doss Ranch site.[2] In 2025,Tenontosaurus were unearthed from the Yucca Formation in Texas.[4][5]
Tenontosaurus was a medium-sized ornithopod, with both species weighing about 1,000 kilograms (2,200 lb).Gregory S. Paul in 2016 estimated thatT. tilletti would have been 6 metres (20 ft) and weighed 600 kilograms (1,300 lb),[6] but Nicolás E. Campione and David C. Evans in 2020 estimated that this species would have weighed up to 971–1,019 kilograms (2,141–2,247 lb).[7] Paul also estimated thatT. dossi would have been 7 meters (23 ft) long and weighed 1,000 kilograms (2,200 lb).[6] The original describers ofT. dossi favoured a length estimate of 7 and 8 metres (23 and 26 ft),[2] and the same estimate has been given elsewhere.[8]
The skull ofTenontosaurus was originally described by Ostrom as being very long and deep compared to taxa such asTheiophytalia (then considered a species ofCamptosaurus).[1] 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 ofHypsilophodon. Theexternal nares (nostril openings) ofTenontosaurus were very large, and were almost entirely encircled by thepremaxillae.[9] The premaxillae flaredinferiorly (towards the bottom), forming a thick, U-shapedbeak,[1] characteristic of iguanodontians. The beak extended as far back as theanterior (front) process of themaxilla. The single longest bone inTenontosaurus' skull is thevomer, which stretches from the middle of the beak to the very back of theorbital cavity (eye socket). The maxilla was larger in general, though, comprising the majority of therostrum and housing all of the upper teeth. The maxilla was divided into two components: a facial lamina, which comprised its lateral (outer) surface, and a dental ridge. In more derived iguanodontians, andTheiophytalia, the maxilla was bordered by a lateral process of thejugal relatively far forward; inTenontosaurus, however, the facial laminae of the maxillas overlapped with the jugals, and almost contacted the inferior part of thelacrimal bone.[9]T. tilletti had a long and narrowantorbital fenestra, which extended down from the lacrimals to the facial laminae;[1]T. dossi, meanwhile, had a relatively small one, surrounded by a shallowfossa. Similarly, the dorsal (upper) process ofT. dossi's maxilla did not expand posteriorly above the antorbital fenestra, as it did inT. tilletti.[2] The process connecting thequadratojugal to the jugal was structured in a way that formed an additional temporal fenestra, which Ostrom presumed related to jaw adductor muscles.[1] A similar fenestra is also seen inHypsilophodon. The orbit was roughly triangular, and was larger than thefenestrae anterior to (in front of) orposterior to (behind) it.[9] The majority of theoccipital condyle inTenontosaurus was made up of a largebasioccipital,[1][2] though this structure was almost entirely excluded from the occipital surface itself. Ostrom noted similarities between the shapes of the paroccipitals inTenontosaurus and in hadrosaurs, namely their hooked shape and their orientation in relation to theforamen magnum. The ventral flange of the pterygoid was broad and deep, particularly enlarged dorsally, providing a larger attachment site for parts of thepterygoid muscles. Thequadrate was narrow, and was essentially vertical.[1]
Tenontosaurus'mandible (lower jaw) was described by Ostrom as being "of moderate length and robustness". It terminated in apredentary bone,[1][9] a structure found uniquely among ornithischians which formed the lower beak.[10] The predentary inTenontosaurus was horseshoe-shaped and was lined with projecting serrations, referred to as pseudo-teeth,[1] or as denticles. They likely served, at least partly, as anchors for thekeratinous portion of the beak.[9] Such structures are present in many ornithischians, save for taxa such asDryosaurus orZalmoxes.[1] In at leastT. dossi, the predentary had a process on its ventral (bottom) surface which overlapped with themandibular symphyses.[2] Thedentaries (the tooth-bearing portions of the lower jaw) were robust.[2] There were two distinctcoronoid processes to the mandibles. The retroarticular processes, projections at the back of eachmandibular ramus, were long and curved. Ostrom noted that thesurangulars 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;[1] this was later confirmed. The surangulars also formed the lateral part of the retoarticular processes.[9]
All ofTenontosaurus' teeth wereenameled 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.[1]
Tenontosaurus was a facultative quadruped, capable of assuming either a bipedal or quadrupedal stance. It may have used a quadrupedal stance while feeding,[11][12] but was probably incapable of rapid quadrupedal movement.[11]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.[12] Themanus (the hand/front foot) shows a mixture of traits associated with bipeds and quadrupeds. It retains narrow claws, unlike the hoof-likeunguals of quadrupedal ornithischians,[12][13] has a shortmetacarpus, as in the bipedalHypsilophodon, and retains grasping adaptations. However, thephalanges are also shortened as an adaptation to weight bearing.[13] An analysis of the overall proportions and center of mass ofTenontosaurus found it to be a quadruped, although the analysis only distinguished bipeds from quadrupeds without including a facultatively bipedal category.[14]
Tenontosaurus was originally classified in the familyIguanodontidae,[1] although subsequent authors challenged this classification and proposed it actually belonged toHypsilophodontidae.[3] Subsequent phylogenetic analyses have foundTenontosaurus to be intermediate in position betweenHypsilophodon-like taxa andIguanodon-like taxa, as a non-dryomorphiguanodont. The precise phylogenetic position ofTenontosaurus varies between studies, with some studies finding it to be more closely related to dryomorphs thanrhabdodontids, some studies finding it to be the sister taxon of a clade uniting rhabdodontids and dryomorphs.
Thecladogram below follows an analysis by Butleret al, 2011.[15]
| Ornithopoda |
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The discovery ofIani in 2023 lent support for a close relationship betweenTenontosaurus and rhabdodontids, asIani exhibits transitional characteristics betweenTenontosaurus and other rhabdodontomorphs. This result was verified in two datasets.[16]
Topology 1: Poole (2022) dataset[17]
| Topology 2: Dieudonnéet al. (2021) dataset[18]
|
A similar result was recovered by Fonseca et al. (2024), who foundIani, Tenontosaurus, and alsoConvolosaurus to form the new familyTenontosauridae within theRhabdodontomorpha, defined in thePhyloCode as "the largest clade containingTenontosaurus tilletti, but notHypsilophodon foxii,Iguanodon bernissartensis, andRhabdodon priscus". This family may have represented an early North American radiation of the Rhabdodontomorpha.[19]
Plant life in theTenontosaurus ecosystem was likely dominated byferns andtree ferns,cycads, and possibly primitive flowering plants. Larger plants and trees were represented bygymnosperms, such asconifer andginkgo trees.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. This restrictedTenontosaurus, 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.[3]
Teeth and a number of skeletons belonging to the carnivorous theropodDeinonychus have often been discovered associated withTenontosaurus tilletti remains.Tenontosaurus specimens have been found at over 50 sites, and 14 of those also containDeinonychus remains. According to one 1995 study, only six sites containingDeinonychus fossils contain no trace ofTenontosaurus, andDeinonychus remains are only rarely found associated with other potential prey, likeSauropelta.[20] In all, 20% ofTenontosaurus fossils are found in close proximity toDeinonychus, and several scientists have suggested that this impliesDeinonychus was the major predator ofTenontosaurus. AdultDeinonychus, however, were much smaller than adultTenontosaurus, and it is unlikely a singleDeinonychus would have been capable of attacking a fully grownTenontosaurus. While some scientists have suggested thatDeinonychus 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[21]) as well as evidence thatDeinonychus may have been cannibalizing each other, as well as theTenontosaurus, in a feeding frenzy.[22] It is likely thatDeinonychus favored juvenileTenontosaurus, and that whenTenontosaurus 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 mostTenontosaurus remains found withDeinonychus are half-grown individuals supports this view.[3][23] It also lived in the same area as the large carnivorous dinosaurAcrocanthosaurus.[24]
The presence of medullary bone tissue in thethigh bone andshin bone of one specimen indicates thatTenontosaurus used this tissue, today only found inbirds that are laying eggs, in reproduction. Additionally, likeTyrannosaurus andAllosaurus, 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 thetheropod line of dinosaurs that includesAllosaurus andTyrannosaurus diverged from the line that led toTenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue and reached reproductive maturity before maximum size.[25] Ahistological study showed thatT. tilletti grew quickly early in life and during sub-adultontogeny, but grew very slowly in the years approaching maturity, unlike otheriguanodontians.[26]
Throughout the Cloverly Formation,Tenontosaurus is by far the most common vertebrate, five times more abundant than the next most common, the ankylosaurSauropelta.[27] In the arid Little Sheep Mudstone Member,Tenontosaurus is the only herbivorous dinosaur, and it shared its environment with the common predatorDeinonychus as well as an indeterminate species ofallosauroid theropod andgoniopholid 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 ornithopodZephyrosaurus, theoviraptorosaurMicrovenator, and an indeterminate species oftitanosauriformsauropod andornithomimid. The ecological community in the tropical stage also included the smallmammalGobiconodon,turtles such asGlyptops, and species oflungfish.[3]
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The ecological community was similar in other regions, with dinosaurs likeTenontosaurus andDeinonychus as the most common large vertebrates. TheAntlers Formation stretches from southwest Arkansas through southeastern Oklahoma and into northeastern Texas. This geological formation has not been dated radiometrically. 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.[28] The area preserved in this formation was a largefloodplain that drained into a shallow inland sea. Several million years later, this sea would expand to the north, becoming theWestern Interior Seaway and dividing North America in two for nearly the entireLate Cretaceous period. 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-dayLouisiana.[28][3] In theAntlers Formation in what is now Oklahoma,Tenontosaurus andDeinonychus shared theirpaleoenvironment with other dinosaurs, such as thesauropodsAstrodon (Pleurocoelus) andSauroposeidon proteles, and thecarnosaurAcrocanthosaurus atokensis, which was likely theapex predator in this region.[29][23] The most common dinosaur in the paleoenvironment preserved in this formation isTenontosaurus. Other vertebrates present at the time ofTenontosaurus included theamphibianAlbanerpeton arthridion, the reptilesAtokasaurus metarsiodon andPtilotodon wilsoni, thecrurotarsan reptileBernissartia, thecartilaginous fishHybodus buderi andLissodus anitae, theray-finned fishGyronchus dumblei, the crocodilianGoniopholis, and the turtlesGlyptops andNaomichelys.[30][31] Possible indeterminate bird remains are also known from the Antlers Formation. The fossil evidence suggests that thegarLepisosteus was the most common vertebrate in this region. The early mammals known from this region includeAtokatherium boreni andParacimexomys crossi.[32]
During the Albian, much of North America was blanketed by vast forests composed of diverse plant life. The canopy was dominated by trees such aspines,redwoods,araucarians,cheirolepidiacean conifers,yews,taxodioids,athrotaxidoids,podocarps,plane trees (sycamores),magnoliales,laurales,fabids,malvids,Tempskyaceae ferns, andCyatheales ferns.[33][34][35]The forest floor supported a rich understory ofcycads,cycadeoids,horsetails,forked ferns,spleenworts, andOsmundaceae ferns.[33][35] More open areas would have been home to the fern known asRuffordia.[35][36] This wide array of plants probably kept these herbivores well fed. Possible cycad seeds were thought to have been found in the gut ofTenontosaurus individual, however these were probably mineral concretions.[37]
In theCloverly 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 noTenontosaurus 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 ofTenontosaurus, used this as evidence to suggest thatTenontosaurus populations did not arrive in the Bighorn Basin area until the time of the late Little Sheep Mudstone Member.[3]
At the timeTenontosaurus first appeared in Wyoming and Montana (the early Albian age), the regions climate wasarid 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 becamesubtropical totropical, with river deltas, floodplains, and forests with swampy inlets reminiscent of modernLouisiana, though marked dry seasons persisted to create savannah-like environments as well.[28] The change in rainfall levels is likely due to the advancing shoreline of the Skull Creek Seaway, a cycle of theWestern Interior Seaway which, later in the Cretaceous period, would completely divide North America.[3]
This dramatic shift in climate coincided with an apparent increase, rather than decrease, in the abundance ofTenontosaurus. This showsTenontosaurus to have been a remarkably adaptable animal, which persisted for a long span of time in one area despite changes to its environment.[3]
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