Alamosaurus

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Alamosaurus
Temporal range:
Ma
↓
Restored skeletons of Alamosaurus and Tyrannosaurus at Perot Museum
Life reconstruction of Alamosaurus sanjuanensis
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Sauropoda
Clade: †Macronaria
Clade: †Titanosauria
Family: †Saltasauridae
Subfamily: †Opisthocoelicaudiinae
Genus: †Alamosaurus
Gilmore, 1922
Type species
†Alamosaurus sanjuanensis
Gilmore, 1922

Alamosaurus (

Cretaceous-Paleogene boundary in Texas, making it among the last surviving non-avian dinosaur species.[3] Alamosaurus is the only known sauropod to have inhabited North America after their nearly 30-million year absence from the North American fossil record and probably represents an immigrant from South America
.

Description

Size comparison, showing the scale of three Alamosaurus specimens.

Alamosaurus was a gigantic

quadrupedal herbivore with a long neck, long tail, and relatively long limbs.[3] Its body was at least partly covered in bony armor.[4] In 2012, Thomas Holtz gave a total length of 30 meters (98 ft) or more and an approximate weight of 72.5–80 tonnes (80–88 short tons) or more.[5][6] Though most of the complete remains come from juvenile or small adult specimens, three fragmentary specimens (SMP VP−1625, SMP VP−1850, and SMP VP−2104) suggest that adult Alamosaurus could have grown to enormous sizes comparable to the largest known dinosaurs, like Argentinosaurus, which has been estimated to weigh 73 metric tons (80 short tons).[2] Scott Hartman estimates Alamosaurus, based on a huge incomplete tibia that probably refers to it, being slightly shorter at 28–30 m (92–98 ft) and equal in weight to other massive titanosaurs, such as Argentinosaurus and Puertasaurus. It is currently the only titanosaur known from North America.[7] However, he says that, at the moment, scientists do not know whether the massive tibia belongs to an Alamosaurus or a completely new species of sauropod.[8]

Isolated caudal vertebra of Alamosaurus sanjuanensis from the Naashoibito member of the Kirtland Formation, New Mexico

In 2019, Gregory S. Paul estimated SMP VP−1625 at 27 tonnes (30 short tons) and he also mentioned a large partial anterior caudal vertebra that suggests an Alamosaurus specimen that is 15 percent dimensionally larger and with similar mass to his Dreadnoughtus estimation of 31 tonnes (34 short tons).[9] In 2020, Molina-Perez and Larramendi estimated the size of the largest individual at 26 meters (85.3 ft) and 38 tonnes (42 short tons).[10] In 2023, the largest individual was stated to be around 30 meters (98 ft) and 72.5 tonnes (80 short tons).[11]

Hypothetical restoration

Though no skull has ever been found, rod-shaped teeth have been found with Alamosaurus skeletons and probably belonged to this dinosaur.

radii than Venenosaurus.[13]

History

Holotype scapula and paratype ischium
Alamosaurus quarry in 2013 compared with 1937, North Horn Formation, North Horn Mountain.

Alamosaurus remains have been discovered throughout the

Charles Whitney Gilmore, John Bernard Reeside,[14]
and Charles Hazelius Sternberg at the Barrel Springs Arroyo in the Naashoibito Member of the Ojo Alamo Formation (or Kirtland Formation under a different definition) of New Mexico. This formation was deposited during the Maastrichtian age of the Late Cretaceous period.[15] Bones have also been recovered from other Maastrichtian formations, like the North Horn Formation of Utah and the Black Peaks, El Picacho, and Javelina Formations of Texas.[12] Undescribed titanosaur fossils closely associated with Alamosaurus have been found in the Evanston Formation in Wyoming. Three articulated caudal vertebrae were collected above Hams Fork and are housed at the Museum of Paleontology, University of California, Berkeley. However, these specimens have not been described.[16]

Perot Museum
.

cottonwood tree. The term saurus is derived from saura (σαυρα), the Greek word for "lizard", and is the most common suffix used in dinosaur names. There is only one species in the genus, Alamosaurus sanjuanensis, which is named after San Juan County, New Mexico, where the first remains were found.[15]

In 1946, Gilmore posthumously described a more complete specimen, USNM 15660, found on June 15, 1937, on the North Horn Mountain of Utah by George B. Pearce. It consists of a complete tail, a complete right forelimb (except for the fingers, which later research showed do not ossify with Titanosauridae), and both ischia.[18] Since then, hundreds of other bits and pieces from Texas, New Mexico, and Utah have been referred to Alamosaurus, often without much description. Despite being fragmentary, until the second half of the twentieth century they, represented much of the globally known titanosaurid material. The most completely known specimen, TMM 43621–1, is a juvenile skeleton from Texas which allowed educated estimates of length and mass.[3]

Some blocks catalogued under the same accession number as the relatively complete and well-known Alamosaurus specimen USNM 15660 and found in very close proximity to it based on bone impressions were first investigated by Michael Brett-Surman in 2009. In 2015, he reported that the blocks contained osteoderms, the first confirmation of their existence on Alamosaurus.[4]

Reconstructed skeleton

The restored Alamosaurus skeletal mount at the Perot Museum [19][circular reference] (pictured right) was discovered when student Dana Biasatti, a member of an excavation team at a nearby site, went on a hike to search for more dinosaur bones in the area.

Classification

In 1922, Gilmore was uncertain about the precise affinities of Alamosaurus and did not determine it any further than a general

Titanosauridae.[20]

Alamosaurus was, in any case, an advanced and derived member of the group

cladistic and morphologic analyses of titanosaurians.[3] A recent analysis published in 2016 by Anthony Fiorillo and Ron Tykoski indicates that Alamosaurus was a sister taxon to Lognkosauria and therefore to species such as Futalognkosaurus and Mendozasaurus, laying outside Saltasauridae (possibly being descended from close relations to the Saltasauridae), based on synapomorphies of cervical vertebral morphologies and two cladistic analyses.[23] The same study also suggests that the ancestors of Alamosaurus hailed from South America instead of Asia.[24]

Phylogeny

Alamosaurus in a cladogram after Navarro et al., 2022:[25]

Saltasauridae

Age

Alamosaurus fossils are most notably found in the Naashoibito member of the

Cretaceous-Paleogene boundary. The Alamosaurus specimen was reported to come from a few meters below the boundary, dated to 66 million years ago, though the position of the boundary in this region is uncertain.[3] Only one geological site in the Javelina Formation has yielded the correct rock types for radiometric dating so far. The outcrop, situated in the middle strata of the formation about 90 meters (300 ft) below the K-Pg boundary and within the local range of Alamosaurus fossils, was dated to 69.0±0.9 million years old in 2010.[27] Using this date, in correlation with a measured age from the underlying Aguja Formation and the likely location of the K-Pg boundary in the overlying Black Peaks Formation, the Alamosaurus fauna seems to have lasted from about 70–66 million years ago, with the earliest records of Alamosaurus near the base of the Javelina formation and the latest just below the K-Pg boundary in the Black Peaks Formation.[27]

Biogeography

Alamosaurus is the only known sauropod to have lived in North America after the sauropod hiatus, a nearly 30-million-year interval for which no definite sauropod fossils are known from the continent. The earliest fossils of Alamosaurus date to the Maastrichtian age, around 70 million years ago, and it rapidly became the dominant large herbivore of southern Laramidia.[28]

The origins of Alamosaurus are highly controversial, with three hypotheses that have been proposed. The first of these, which has been termed the "austral immigrant" scenario,[29] proposes that Alamosaurus is descended from South American titanosaurs. Alamosaurus is closely related to South American titanosaurs, such as Pellegrinisaurus.[30][31] Alamosaurus appears in North America at the same time that hadrosaurs closely related to North American species first appear in South America, suggesting that the Alamosaurus lineage crossed into North America on the same routes as hadrosaurs crossed into South America.[32] The austral immigrant hypothesis has been challenged on the grounds that the routes connecting North and South America during the Maastrichtian may have consisted of separate islands, which would have presented challenges to the dispersal of titanosaurs.[28][33] A second scenario, termed the "inland herbivore" scenario,[29] suggests that titanosaurs were present in North America throughout the Late Cretaceous and that their apparent absence reflects the relative rarity of fossil sites preserving the upland environments that titanosaurs favored, rather than their true absence from the continent.[28] However, there is no evidence for sauropods in North America between the mid-Cenomanian and the early Maastrichtian, even in strata that preserve more upland environments, and the sauropods that lived in North America before the hiatus are basal titanosauriforms, such as Sonorasaurus and Sauroposeidon, not lithostrotian titanosaurs.[32][34] A third option is that, as in the austral immigrant scenario, Alamosaurus is not native to North America, but originated in Asia instead of South America.[33] Alamosaurus is commonly considered to be closely related to the Asian titanosaur Opisthocoelicaudia, but this is based on analyses that did not take Alamosaurus's South American relative Pellegrinisaurus into account.[30] Though many dinosaurs crossed between Asia and North America across the Bering land bridge, sauropods were poorly adapted for high-latitude environments and Beringia would have been an inhospitable environment for titanosaurs.[35] Furthermore, in order to reach southern Laramidia from Asia, Alamosaurus would have had to cross through Northern Laramidia, which contains no known sauropod fossils of comparable age to Alamosaurus, despite containing the best-studied dinosaur faunas on the continent.[23] Overall, a South American origin has been favored by several studies[31][23][30][35] and was regarded as "the only viable origin" for Alamosaurus by Chiarenza et al.[35]

Paleoecology

Restoration

Skeletal elements of Alamosaurus are among the most common Late Cretaceous dinosaur fossils found in the United States Southwest and are now used to define the fauna of that time and place, known as the "Alamosaurus fauna". In the south of Late Cretaceous North America, the transition from the Edmontonian to the Lancian faunal stages is even more dramatic than it was in the north. Thomas M. Lehman describes it as "the abrupt reemergence of a fauna with a superficially 'Jurassic' aspect. These faunas are dominated by Alamosaurus and feature abundant Quetzalcoatlus in Texas. The Alamosaurus-Quetzalcoatlus association probably represent semi-arid inland plains.[28]

Contemporaries of Alamosaurus in the American southwest include the

multituberculates like Cimexomys and Mesodma.[citation needed
]

References
  1. ^ "Alamosaurus". Lexico UK English Dictionary. Oxford University Press. Archived from the original on March 3, 2021.
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  7. ^ "Assessing Alamosaurus". Skeletal Drawing.
  8. ^ "The biggest of the big". Skeletal Drawing.
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  11. ^ "Newest fossil display at NM Museum of Natural History & Science has deep ties to the Land of Enchantment". New Mexico Department of Cultural Affairs. Retrieved April 18, 2024.
  12. ^ a b Weishampel, D.B. et al.. (2004). "Dinosaur Distribution (Late Cretaceous, North America)". In Weishampel, D.B., Dodson, P., Oslmolska, H. (eds.). "The Dinosauria (Second ed.)". University of California Press.
  13. ^ a b c d e Tidwell, V., Carpenter, K. & Meyer, S. 2001. New Titanosauriform (Sauropoda) from the Poison Strip Member of the Cedar Mountain Formation (Lower Cretaceous), Utah. In: Mesozoic Vertebrate Life. D. H. Tanke & K. Carpenter (eds.). Indiana University Press, Eds. D.H. Tanke & K. Carpenter. Indiana University Press. 139–165.
  14. ^ [1], John Bernard Reeside (1889-1958) was a geologist specializing in the study of the Mesozoic stratigraphy and paleontology of the western United States. While receiving his education at The Johns Hopkins University (A.B., 1911; Ph.D., 1915), he joined the United States Geological Survey (USGS) as a part-time assistant... [and] remained with the USGS for his entire professional career... From 1932 to 1949, Reeside was Chief of the Paleontology and Stratigraphy Branch.
  15. ^ a b c Gilmore, C.W. (1922). "A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico" (PDF). Smithsonian Miscellaneous Collections. 72 (14): 1–9.
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  17. ^ Anthony D. Fredericks, 2012, Desert Dinosaurs: Discovering Prehistoric Sites in the American Southwest, The Countryman Press, p. 102-103
  18. ^ Gilmore, C.W. 1946. Reptilian fauna of the North Horn Formation of central Utah. U.S. Geological Survey Professional Paper. 210-C:29–51.
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  22. ^ Upchurch, P., Barrett, P.M. & Dodson, P. 2004. Sauropoda. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.) The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 259–322.
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Notes

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