Open Paleo

Alamosaurus

Gilmore, 1922 AL-a-mo-SAWR-us /ˌæləmoʊˈsɔːrəs/

Named after the Ojo Alamo rock formation combined with Greek 'sauros' meaning 'lizard'; meaning 'Ojo Alamo lizard'

72.2–66.0 million years ago Maastrichtian, Late Cretaceous
Late Cretaceous stages

Alamosaurus is a genus of titanosaurian sauropod dinosaurs containing a single known species, Alamosaurus sanjuanensis, from the Maastrichtian age of the Late Cretaceous period in what is now southwestern North America. It is one of the few known titanosaurs to have inhabited North America after the nearly 30-million-year absence of sauropods from the North American fossil record ("sauropod hiatus") and probably represents an immigrant from South America.

Under dispute

Placed within Saltasauridae as incertae sedis. Alamosaurus has long resisted precise placement: an early assessment (Lehman, 2002) found it a mosaic of titanosaur features and could not resolve it beyond an indeterminate derived titanosaur. More recent work disagrees over its affinities: Navarro and colleagues (2022) recovered it within Saltasaurinae as the sister taxon of Baurutitan, and it has been grouped among Laurasian titanosaurs close to Saltasaurinae (Santucci and Filippi, 2022), whereas Cerda and colleagues (2021) instead recovered a South American affinity close to Pellegrinisaurus. These placements fall within Saltasauridae under some definitions. It is placed at Saltasauridae pending resolution. The content of the genus is separately contested: Paul (2025) argued that the late Maastrichtian Utah material differs from the early Maastrichtian New Mexican type material, made USNM 15560 the holotype of a new genus Utetitan, and would restrict Alamosaurus sanjuanensis to its New Mexican type horizon. That split rests on qualitative scapular and ischial characters, is not supported by a cladistic analysis, and has not been taken up by subsequent work; USNM 15560 is retained as referred material of A. sanjuanensis, following its long-standing referral (Gilmore, 1946).

Diagnosis

  1. Scapula blade gradually widens from below upward to superior end with no expansion of anterior border and no rapid superior expansion of both borders as in other sauropods
  2. Scapular spine runs at approximately 90 degrees to the longitudinal axis of the bone, not at an acute angle as in Diplodocus and Amphicoelias
  3. No superior fossa on upper side of scapular ridge, unlike many sauropod scapulae
  4. Ischium extremely short, lacking the long slender posterior extension characteristic of all other known sauropod ischia
  5. Ischium with unusually great dorsoventral diameter at proximal end and pubic articular surface extending distalward more than half the total bone length

Species

Alamosaurus sanjuanensis Gilmore, 1922

Type species

Named for San Juan County, New Mexico, where the first remains were found.

Holotype
USNM 10486 , National Museum of Natural History, Smithsonian Institution
Material
Nearly complete left scapula (greatest preserved length 155 cm, estimated complete length 170 cm).
Horizon
Naashoibito Member, Ojo Alamo Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
Barrel Spring Arroyo, New Mexico, United States

Synonyms

  • Utetitan zellaguymondeweyae possible subjective

    Erected on USNM 15560, a specimen referred to Alamosaurus sanjuanensis since Gilmore (1946); the separation proposed by Paul (2025) has not been adopted by subsequent work.

Synonyms of the genus

Name Type Reason
Utetitan possible subjective Erected by Paul (2025) for USNM 15560, the North Horn Formation skeleton long referred to Alamosaurus sanjuanensis, on qualitative scapular and ischial characters and without a cladistic analysis. The separation has not been adopted by subsequent work.

Notable specimens

  • USNM 15560

    National Museum of Natural History, Smithsonian Institution complementary

    A partial postcranial skeleton from the lower North Horn Formation of Utah preserving caudals 1-30, both sternals, the right scapulocoracoid, forelimb, both ischia and an osteoderm, nearly all of it absent from the scapula-only holotype. Paul (2025) made it the holotype of a separate genus.

    Gilmore (1946)Paul (2025)

  • TMM 43621-1

    Texas Memorial Museum, Austin, Texas, USA (used for collections now housed in the Vertebrate Paleontology Collections at the Jackson School Museum of Earth History, Austin, Texas, USA), now called the Texas Science and Natural History Museum most complete

    The most completely known specimen of Alamosaurus, a juvenile skeleton from Texas that allowed researchers to make educated estimates of the genus's length and mass.

    Lehman & Coulson (2002)

  • SMP VP-1625, SMP VP-1850, SMP VP-2104

    State Museum of Pennsylvania largest individual

    Three fragmentary specimens from exceptionally large individuals, indicating that adult Alamosaurus reached sizes comparable to the largest known dinosaurs such as Argentinosaurus, potentially the largest dinosaur known from North America.

    Fowler & Sullivan (2011)

References

  1. Gilmore, C. W. (1922). A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico. Smithsonian Miscellaneous Collections 72(34): 1-9. gilmore1922a
  2. Sullivan, R. M.; Lucas, S. G. (2006). The Kirtlandian land-vertebrate "age" — faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America. New Mexico Museum of Natural History and Science Bulletin 35: 7-29.

    Places the Naashoibito Member in the Ojo Alamo Formation.

    sullivan2006c
  3. Lehman, T. M.; Coulson, A. B. (2002). A juvenile specimen of the sauropod Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76(1): 156-172. doi:10.1017/s0022336000017431 lehman2002a
  4. Fowler, D. W.; Sullivan, R. M. (2011). The first giant titanosaurian sauropod from the Upper Cretaceous of North America. Acta Palaeontologica Polonica 56(4): 685. doi:10.4202/app.2010.0105 fowler2011a
  5. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Sauropods. Princeton University Press.

    Source of the hip height estimate.

    jones2026b
  6. Navarro, B. A.; Ghilardi, A. M.; Aureliano, T.; Díez Díaz, V.; Bandeira, K. L. N.; Cattaruzzi, A. G. S.; Iori, F. V.; Martine, A. M.; Carvalho, A. B.; Anelli, L. E.; Fernandes, M. A.; Zaher, H. (2022). A New Nanoid Titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of Brazil. Ameghiniana 59(5). Asociacion Paleontologica Argentina. doi:10.5710/amgh.25.08.2022.3477

    Recovered Alamosaurus within Saltasaurinae as the sister taxon of Baurutitan.

    navarro2022a
  7. Cerda, I.; Zurriaguz, V. L.; Carballido, J. L.; González, R.; Salgado, L. (2021). Osteology, paleohistology and phylogenetic relationships of Pellegrinisaurus powelli (Dinosauria: Sauropoda) from the Upper Cretaceous of Argentinean Patagonia. Cretaceous Research 128. doi:10.1016/j.cretres.2021.104957

    Recovered Alamosaurus with South American affinity near Pellegrinisaurus in phylogenetic analysis

    cerda2021a
  8. Santucci, R. M.; Filippi, L. S. (2022). Last titans: Titanosaurs from the Campanian–Maastrichtian age. South American Sauropodomorph Dinosaurs: Record, Diversity and Evolution 341-391. Springer International Publishing. doi:10.1007/978-3-030-95959-3_10

    Grouped Alamosaurus among Laurasian titanosaurs closely related to Saltasaurinae, in a biogeographic discussion.

    santucci2022a
  9. Gilmore, C. W. (1946). Reptilian fauna of the North Horn Formation of central Utah. United States Geological Survey Professional Paper 210-C: 29-53. United States Geological Survey. doi:10.3133/pp210C

    Describes USNM 15560 from the North Horn Formation and refers it to Alamosaurus sanjuanensis.

    gilmore1946b
  10. Paul, G. S. (2025). Stratigraphic and anatomical evidence for multiple titanosaurid dinosaur taxa in the Late Cretaceous (Campanian-Maastrichtian) of southwestern North America. Geology of the Intermountain West 12: 201-220. Utah Geological Association. doi:10.31711/giw.v12.pp201-220

    Makes USNM 15560 the holotype of a separate genus, Utetitan, and would restrict Alamosaurus sanjuanensis to its New Mexican type horizon.

    paul2025a