Alamosaurus

''Alamosaurus'' (; meaning "Ojo Alamo lizard") is a genus of opisthocoelicaudiine titanosaurian sauropod dinosaurs, containing a single known species, ''Alamosaurus sanjuanensis'', from the late Cretaceous Period of what is now southern North America. Isolated vertebrae and limb bones indicate that it reached sizes comparable to ''Argentinosaurus'' and ''Puertasaurus'', which would make it the largest dinosaur known from North America. Its fossils have been recovered from a variety of rock formations spanning the Maastrichtian age of the late Cretaceous period. Specimens of a juvenile ''Alamosaurus sanjuanensis'' have been recovered from only a few meters below the Cretaceous-Paleogene boundary in Texas, making it among the last surviving non-avian dinosaur species.

Description

''Alamosaurus'' was a gigantic quadrupedal herbivore with a long neck and tail and relatively long limbs. Its body was at least partly covered in bony armor. In 2012 Thomas Holtz gave a total length of or more and an approximate weight of 72.5–80 tonnes (80–88 short tons) or more. 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 . Scott Hartman estimates ''Alamosaurus,'' based on a huge incomplete tibia that probably refers to it, being slightly shorter at and equal in weight to other massive titanosaurs such as ''Argentinosaurus'' and ''Puertasaurus''. It is currently the only titanosaur from North America. 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. In 2019 Gregory S. Paul estimated the (SMP VP−1625) specimen at 27 tonnes (30 short tons), and he also mentioned a large partial anterior caudal vertebra which suggests an Alamosaurus specimen that is 15 percent dimensionally larger with similar mass to his '' Dreadnoughtus'' estimation of 31 tonnes (34 short tons). 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).

Though no skull has ever been found, rod-shaped teeth have been found with ''Alamosaurus'' skeletons and probably belonged to this dinosaur. The vertebrae from the middle part of its tail had elongated centra. ''Alamosaurus'' had vertebral lateral fossae that resembled shallow depressions. Fossae that similarly resemble shallow depressions are known from ''Saltasaurus'', ''Malawisaurus'', ''Aeolosaurus'', and ''Gondwanatitan''. ''Venenosaurus'' also had depression-like fossae, but its "depressions" penetrated deeper into the vertebrae, were divided into two chambers, and extend farther into the vertebral columns. ''Alamosaurus'' had more robust radii than ''Venenosaurus''.

History

''Alamosaurus'' remains have been discovered throughout the southwestern United States. The holotype was discovered in June 1921 by Charles Whitney Gilmore, John Bernard Reeside 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 which was deposited during the Maastrichtian stage of the Late Cretaceous Period. 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.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. 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. These specimens have not been described.

Smithsonian paleontologist Gilmore originally described the holotype, USNM 10486, a left scapula (shoulder bone), and the paratype USNM 10487, a right ischium ( pelvic bone) in 1922, naming the type species ''Alamosaurus sanjuanensis''. Contrary to popular assertions, the dinosaur is not named after the Alamo in San Antonio, Texas, or the battle that was fought there. The holotype, the specimen the name was based on, was discovered in New Mexico and, at the time of its naming, ''Alamosaurus'' had not yet been found in Texas. Instead, the name ''Alamosaurus'' comes from Ojo Alamo, the geologic formation in which it was found and which was, in turn, named after the nearby Ojo Alamo trading post (since this time there has been some debate as to whether to reclassify the ''Alamosaurus''-bearing rocks as belonging to the Kirtland Formation or whether they should remain in the Ojo Alamo Formation). The term ''alamo'' itself is a Spanish word meaning "poplar" and is used for the local subspecies of cottonwood tree. The term ''saurus'' is derived from ''saura'' (σαυρα), Greek 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.

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 right forelimb complete except for the fingers — which later research showed do not ossify with the Titanosauridae — and both ischia. 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 recently discovered juvenile skeleton from Texas, which allowed educated estimates of length and mass.

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''.
No skull material is known, except for a few slender teeth.

The restored ''Alamosaurus'' skeletal mount at the Perot Museum (pictured right) was discovered when a member of an excavation team at a nearby site had to go around a hill to urinate.

Classification

Gilmore in 1922 was uncertain about the precise affinities of ''Alamosaurus'' and did not determine it any further than a general Sauropoda. In 1927 Friedrich von Huene placed it in the Titanosauridae.

''Alamosaurus'' was in any case an advanced, or derived, member of the group Titanosauria, but its relationships within that group are far from certain. The issue is further complicated by some researchers rejecting the name Titanosauridae and replacing it with Saltasauridae. One major analysis unites ''Alamosaurus'' with ''Opisthocoelicaudia'' in a subgroup Opisthocoelicaudiinae of the Saltasauridae. A major competing analysis finds ''Alamosaurus'' as a sister taxon to ''Pellegrinisaurus'', with both genera located just outside Saltasauridae. Other scientists have also noted particular similarities with the saltasaurid ''Neuquensaurus'' and the Brazilian'' Trigonosaurus'' (the "Peiropolis titanosaur") which is used in many cladistic and morphologic analyses of titanosaurians. A recent analysis published in 2016 by Anthony Fiorillo and Ron Tykoski indicates that ''Alamosaurus'' was a sister taxon to the Lognkosauria and therefore to species such as ''Futalognkosaurus'' and ''Mendozasaurus'' and lay outside the Saltasauridae (possibly being descended from close relations to the Saltasauridae), based on synapomorphies of cervical vertebral morphologies and two cladistic analyses. The same study also suggests that the ancestors of ''Alamosaurus'' hailed from South America, rather than Asia.

Age

''Alamosaurus'' fossils are most notably found in the Naashoibito member of the Ojo Alamo Formation (dated to between about 69–68 million years old) and in the Javelina Formation, though the exact age range of the latter has been difficult to determine.Sullivan, R.M., and 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. A juvenile specimen of ''Alamosaurus'' has been reported to come from the Black Peaks Formation, which overlies the Javelina in Big Bend, Texas, and which straddles 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. Only one geological site in the Javelina Formation has thus far yielded the correct rock types for radiometric dating. The outcrop, situated in the middle strata of the formation about below the K-Pg boundary and within the local range of ''Alamosaurus'' fossils, was dated to million years old in 2010. 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.

Habitat and geographic origin

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.

The appearance of ''Alamosaurus'' may have represented an immigration event from South America. ''Alamosaurus'' appears and achieves dominance in its environment very abruptly, which might support the idea that it originated following an immigration event. Other scientists speculated that ''Alamosaurus'' was an immigrant from Asia. However, critics of the immigration hypothesis note that inhabitants of an upland environment like ''Alamosaurus'' are more likely to be endemic than coastal species, and tend to have less of an ability to cross bodies of water. Further, Early Cretaceous titanosaurs are already known, so North American potential ancestors for ''Alamosaurus'' already existed.

Contemporaries of ''Alamosaurus'' in the American southwest include unidentified tyrannosaurids similar to ''Tyrannosaurus'', the oviraptorosaur ''Ojoraptorsaurus'', the hadrosaurid '' Kritosaurus'', the armored nodosaur ''Glyptodontopelta'' and the chasmosaurine ceratopsids cf. ''Torosaurus utahensis'', ''Bravoceratops'', and ''Ojoceratops''. Non-dinosaur taxa that had shared the same environment with ''Alamosaurus'' include the giant pterosaur ''Quetzalcoatlus'', various species of fishes and rays, amphibians, lizards, turtles like ''Adocus'', and multiple species of multituberculates like ''Cimexomys'' and ''Mesodma''.

References

Notes

{{Taxonbar, from=Q131583

Late Cretaceous dinosaurs of North America
Saltasaurids
Natural history of San Juan County, New Mexico
Fossil taxa described in 1922
Taxa named by Charles W. Gilmore
Paleontology in New Mexico
Paleontology in Utah
Paleontology in Texas
Ojo Alamo Formation
Maastrichtian genus first appearances
Maastrichtian genus extinctions