Open Paleo

Tenontosaurus

Ostrom, 1970 ti-NON-tuh-SOR-uhs /tɪˌnɒntəˈsɔːrəs/

From Greek tenon ('sinew') and sauros ('lizard'), meaning 'sinew lizard', referring to the bony tendons stiffening its back and tail

113.2–93.9 million years ago Albian–Cenomanian, Early Cretaceous–Late Cretaceous
Early Cretaceous and Late Cretaceous stages

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 (described by John Ostrom in 1970) and Tenontosaurus dossi (described by Winkler, Murry, and Jacobs in 1997). 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.

Under dispute

Whether Tenontosaurus belongs to Rhabdodontomorpha depends on the analysis. Poole (2022), Zanno and colleagues (2023) and Fonseca and colleagues (2024) recover it within the clade, with Iani and Convolosaurus, as do Czepiński and Madzia (2025) under implied weights. Most analyses on the matrices of Butler and colleagues (2008) and Boyd (2015) instead recover it outside Rhabdodontomorpha, as an early iguanodontian near Dryomorpha (Bell and colleagues, 2018; Dieudonné and colleagues, 2020; Maidment and colleagues, 2023), and Dieudonné and colleagues (2026) place it with Iani among the thescelosaurids, outside Iguanodontia.

Diagnosis

  1. Large bipedal ornithopod with extremely long tail reinforced by ossified tendons on both neural spines and chevrons
  2. Caudal series with 59-60 vertebrae, measuring two or more times precaudal column length
  3. Ischium long and straight, not expanded distally
  4. Pubis with straight, shallow, horizontal prepubic blade and long, narrow postpubic rod
  5. Ilium with long, narrow, sharply downturned anterior process and conspicuously concave upper margin
  6. Manus phalangeal formula 2-3-3-2-2 with broad, flattened metacarpals
  7. Pes phalangeal formula 2-3-4-5-0 with unreduced metatarsal I participating in tarsometatarsal joint
  8. Skull with large external nares, long slit-like antorbital fenestra and supplementary lateral temporal fenestra
  9. Subsidiary temporal fenestra situated beneath usual lateral fenestra at jugal-quadratojugal junction
  10. Orbit subrectangular, larger than either lateral fenestra
  11. Quadrate long, very narrow transversely with convex posterior margin

Species (2)

Tenontosaurus tilletti Ostrom, 1970

Type species

Named for the Lloyd Tillett family, who provided field parties from Yale University with assistance and hospitality throughout their expeditions in the Big Horn Basin.

Holotype
AMNH 3040 , American Museum of Natural History
Material
Partial skeleton lacking skull and forelimbs.
Horizon
Himes Member, Cloverly Formation, Albian–Cenomanian, Early Cretaceous–Late Cretaceous , 113.2–93.9 million years ago
Locality
AMNH 33-1, Montana, United States

Tenontosaurus dossi Winkler et al., 1997

Honors James and Dorothy Doss, who donated the specimens from their ranch.

Holotype
FWMSH 93B1 , Fort Worth Museum of Science and History
Material
Skull and nearly complete postcranial skeleton, including cervical, dorsal, sacral and caudal vertebrae with ossified tendons, pectoral and pelvic girdles, and fore- and hindlimb elements.
Horizon
Twin Mountains Formation, Aptian, Early Cretaceous , 121.4–113.2 million years ago
Locality
Doss Ranch, Texas, United States
Diagnosis
  1. One or more small premaxillary teeth present (never observed in T. tilletti)
  2. Ulnare and intermedium very tightly interlocked, tighter than the fit seen in T. tilletti
  3. Ilium lacks a laterally expressed brevis shelf, unlike T. tilletti
  4. Anterior pubic blade smoothly convex ventrally, lacking the bulge or kink seen in most T. tilletti specimens
  5. Postpubis as long as the ischium, a primitive condition relative to the shortened postpubis of T. tilletti
  6. Metatarsal V more than 30% of metatarsal III length, versus less than 30% in adult T. tilletti

Synonyms of the genus

Name Type Reason
Eureodon nomen nudum Abandoned informal name; the material was formally described as Tenontosaurus.
Tenantosaurus nomen nudum Informally used by Brown in the 1930s; the species was not formally described as Tenontosaurus until Ostrom (1970).

Notable specimens

  • OMNH 34784

    Oklahoma Museum of Natural History reproductive

    A specimen preserving medullary bone tissue in the femur and tibia, indicating a reproductively active female that had not reached full adult size, dying at about age 8; evidence that dinosaurs reached reproductive maturity before maximum size.

    Lee & Werning (2008)

References

  1. Lee, A. H.; Werning, S. (2008). Sexual maturity in growing dinosaurs does not fit reptilian growth models. Proceedings of the National Academy of Sciences 105(2): 582-587. doi:10.1073/pnas.0708903105 lee2008a
  2. Ostrom, J. H. (1970). Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana. Bulletin of the Peabody Museum of Natural History 35: 1-234. ostrom1970a
  3. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Ornithischians. Princeton University Press.

    Source of the hip height estimate.

    jones2026a
  4. Fonseca, A. O.; Reid, I. J.; Venner, A.; Duncan, R. J.; Garcia, M. S.; Müller, R. T. (2024). A comprehensive phylogenetic analysis on early ornithischian evolution. Journal of Systematic Palaeontology 22: 2346577. doi:10.1080/14772019.2024.2346577

    Recovered Tenontosaurus within Rhabdodontomorpha, in Tenontosauridae with Iani and Convolosaurus.

    fonseca2024a
  5. Czepiński, Ł.; Madzia, D. (2025). Exploring the diversity and disparity of rhabdodontomorph ornithopods from the Late Cretaceous European archipelago. Scientific Reports 15(1). Springer Science and Business Media LLC. doi:10.1038/s41598-025-98083-z

    Recovered Tenontosaurus within Rhabdodontomorpha under implied weights; equal weights did not recover the clade.

    czepiński2025a
  6. Zanno, L. E.; Gates, T. A.; Avrahami, H. M.; Tucker, R. T.; Makovicky, P. J. (2023). An early-diverging iguanodontian (Dinosauria: Rhabdodontomorpha) from the Late Cretaceous of North America. PLOS ONE 18(6): e0286042. Public Library of Science (PLoS). doi:10.1371/journal.pone.0286042

    Recovered Tenontosaurus with Iani within Rhabdodontomorpha, noting weak and conflicting character support.

    zanno2023a
  7. Winkler, D. A.; Murry, P. A.; Jacobs, L. L. (1997). A new species of Tenontosaurus (Dinosauria: Ornithopoda) from the Early Cretaceous of Texas. Journal of Vertebrate Paleontology 17(2): 330-348. Informa UK Limited. doi:10.1080/02724634.1997.10010978 winkler1997a
  8. Poole, K. E. (2022). Phylogeny of iguanodontian dinosaurs and the evolution of quadrupedality. Palaeontologia Electronica 25(3): a30. doi:10.26879/702

    Recovered Tenontosaurus within Rhabdodontomorpha under parsimony, and as its sister in the Bayesian analysis.

    poole2022a
  9. Butler, R. J.; Upchurch, P.; Norman, D. B. (2008). The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6(1): 1-40. doi:10.1017/S1477201907002271

    Recovered Tenontosaurus as the sister of Dryosauridae plus Ankylopollexia, outside Rhabdodontidae.

    butler2008b
  10. Boyd, C. A. (2015). The systematic relationships and biogeographic history of ornithischian dinosaurs. PeerJ 3: e1523. PeerJ. doi:10.7717/peerj.1523

    Matrix from which several later analyses of Tenontosaurus derive.

    boyd2015a
  11. Bell, P. R.; Herne, M. C.; Brougham, T.; Smith, E. T. (2018). Ornithopod diversity in the Griman Creek Formation (Cenomanian), New South Wales, Australia. PeerJ 6: e6008. PeerJ. doi:10.7717/peerj.6008

    Recovered Tenontosaurus with Dryomorpha, apart from Rhabdodontomorpha.

    bell2018a
  12. Dieudonné, P. E.; Cruzado-Caballero, P.; Godefroit, P.; Tortosa, T. (2020). A new phylogeny of cerapodan dinosaurs. Historical Biology 33(10): 2335-2355. doi:10.1080/08912963.2020.1793979

    Recovered Tenontosaurus as the basalmost iguanodontian, outside Rhabdodontomorpha.

    dieudonné2020a
  13. Maidment, S. C. R.; Chapelle, K. E. J.; Bonsor, J. A.; Button, D.; Barrett, P. M. (2023). Osteology and relationships of Cumnoria prestwichii (Ornithischia: Ornithopoda) from the Late Jurassic of Oxfordshire, UK. Monographs of the Palaeontographical Society 176(664): 1-55. doi:10.1080/02693445.2022.2162669

    Recovered Tenontosaurus tilletti between Rhabdodontidae and Dryosauridae.

    maidment2023a
  14. Dieudonné, P.; Becerra, M. G.; Zanesco, T.; Tortosa, T.; Cruzado-Caballero, P.; Stein, K.; Torcida Fernández-Baldor, F. (2026). Foskeia pelendonum, a new rhabdodontomorph from the Lower Cretaceous of Salas de los Infantes (Burgos Province, Spain), and a new phylogeny of ornithischian dinosaurs. Papers in Palaeontology 12(1). Wiley. doi:10.1002/spp2.70057

    Recovered Tenontosaurus with Iani in Thescelosaurinae, outside Iguanodontia.

    dieudonné2026a