Open Paleo

Amphicoelias

Cope, 1877 AM-fi-SEEL-ee-as /ˌæmfɪˈsiːliəs/

From Greek 'amphi' meaning 'on both sides' and 'koilos' meaning 'hollow' or 'concave'; meaning 'biconcave', referring to the shape of the vertebrae

154.8–143.1 million years ago Kimmeridgian–Tithonian, Late Jurassic
Late Jurassic stages

Amphicoelias is a genus of herbivorous sauropod dinosaur that lived approximately 150 million years ago during the Tithonian (Late Jurassic Period) of what is now Colorado, United States. Amphicoelias was moderately sized at about 18 meters (59 ft) in length and 15 metric tons (17 short tons) in body mass, shorter than its close relative Diplodocus. Its hindlimbs were very long and thin, and its forelimbs were proportionally longer than in relatives.

Under dispute

The position of Amphicoelias altus within Diplodocoidea is highly labile. Tschopp and colleagues (2015) recovered it as Diplodocidae incertae sedis, outside both Apatosaurinae and Diplodocinae. A dedicated redescription (Mannion and colleagues, 2021) found its position unstable across matrices and weighting schemes, recovering it variously as the sister to Diplodocimorpha (a non-diplodocimorph diplodocoid, like Haplocanthosaurus), as a basal diplodocine, or as an apatosaurine; the low scorability of the holotype drives the instability. It is consistently recovered as a valid diplodocoid distinct from Diplodocus.

Diagnosis

  1. Dorsal vertebral centra unequally amphicoelous: posterior face more deeply concave with prominent margins, anterior face less expanded and only slightly concave
  2. Pneumatic foramen situated at bottom of large lateral fossa extending nearly the entire length of the superior portion of the centrum; foramen longer than high (contrasting with Camarasaurus supremus where it is round or higher than long)
  3. Neural spine thin with thickened and doubled anterior and posterior borders, each side divided into two shallow wide grooves by a median keel; apex much thickened transversely
  4. Femur remarkably slender relative to its length; third trochanter a prominent obtuse ridge situated slightly above mid-shaft directed backwards; condyles separated by deep popliteal groove
  5. Anterior zygapophyses separated by deep notch extending to base of neural spine, with articular surfaces inclining toward each other; hyposphen present as in Camarasaurus

Species

Amphicoelias altus Cope, 1877

Type species

From Latin 'altus' meaning 'tall' or 'high', likely referring to the tall neural spines of the vertebrae.

Holotype
AMNH 5764 , American Museum of Natural History
Material
Dorsal vertebra and femur.
Horizon
upper Brushy Basin Member, Morrison Formation, Kimmeridgian–Tithonian, Late Jurassic , 154.8–143.1 million years ago
Locality
Cope Quarry XII, Colorado, United States Mannion and colleagues (2021) give the quarry as "150.44–149.21 Ma, lower Tithonian", and Tschopp and colleagues (2015) as "Kimmeridgian/Tithonian". On the ICS chart v2024/12 the dated range is Kimmeridgian, ending at the base of the Tithonian, so the stages take both. Barrett and Maidment (2025), recalibrating the same magnetostratigraphy for another Garden Park site in the C5 systems tract, give the tract as "150.8–149.2 Ma (uppermost Kimmeridgian)" (p. 31).

References

  1. Cope, E. D. (1877). On Amphicoelias, a genus of Saurians from the Dakota epoch of Colorado. Paleontological Bulletin 27: 2-5. cope1877c
  2. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Sauropods. Princeton University Press.

    Source of the hip height estimate.

    jones2026b
  3. Tschopp, E.; Mateus, O.; Benson, R. B. J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3: e857. PeerJ. doi:10.7717/peerj.857

    Recovered Amphicoelias as Diplodocidae incertae sedis, outside Apatosaurinae and Diplodocinae.

    tschopp2015a
  4. Mannion, P. D.; Tschopp, E.; Whitlock, J. A. (2021). Anatomy and systematics of the diplodocoid Amphicoelias altus supports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA. Royal Society Open Science 8: 210377. doi:10.1098/rsos.210377

    Redescription found Amphicoelias phylogenetic position unstable across matrices and character weighting schemes

    mannion2021a
  5. Barrett, P. M.; Maidment, S. C. R. (2025). A review of Nanosaurus agilis Marsh and other small-bodied Morrison Formation ornithopods. Bulletin of the Peabody Museum of Natural History 66(1): 25-50. Bulletin of the Peabody Museum of Natural History. doi:10.3374/014.066.0102

    Dates the C5 systems tract at Cope's Nipple, Garden Park, to "150.8–149.2 Ma (uppermost Kimmeridgian)" on a recalibrated magnetostratigraphy.

    barrett2025a