Alamosaurus (; meaning "Ojo Alamo lizard") 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 only known sauropods to have inhabited North America after the nearly 30-million-year absence of sauropods from the continent's fossil record ("sauropod hiatus") and probably represents an immigrant from South America or Asia.
Adults would have measured around long, tall at the shoulder and weighed up to , though some specimens indicate a larger body size. Isolated vertebrae and limb bones suggest that it could have reached sizes comparable to Argentinosaurus and Puertasaurus, which would make it the absolute largest dinosaur known from North America. Its fossils have been recovered from a variety of rock formations in the southwestern United States that were dated to the latest Maastrichtian age.
History of discovery
Alamosaurus remains have been discovered throughout the southwestern United States. The holotype was discovered in June 1921 by Charles Whitney Gilmore, John Bernard Reeside,, 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 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. 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 latest Maastrichtian age of the late Cretaceous period, making the Alamosaurus holotype specimen one of the geologically youngest specimens of this species. Fossils recovered from other Maastrichtian formations have also been referred to Alamosaurus, like the North Horn Formation of Utah, the Black Peaks and the 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. though these specimens may represent a distinct genus. 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 have been said to be housed at the Museum of Paleontology, University of California, Berkeley and have not been described. Fossils of an unknown titanosaur closely related to Alamosaurus have been collected from northeastern Chihuahua, Mexico. One of the large vertebrae resembles those of the latter. This specimen may be the first record of this genus from Mexico.
Smithsonian paleontologist Gilmore originally described 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.Anthony D. Fredericks, 2012, Desert Dinosaurs: Discovering Prehistoric Sites in the American Southwest, The Countryman Press, p. 102-103 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 if 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), 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.
In 1946, Gilmore posthumously described a more complete specimen, USNM 15560, 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. Since the description of USNM 15560, 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 titanosaurian material. The most completely known specimen, TMM 43621–1, is a juvenile skeleton from Texas which allowed educated estimates of length and mass. In 2015, Carrano and D'Emic described the presence of multiple osteoderms associated with USNM 15560. In 2025, Gregory S. Paul assigned USNM 15560 as the holotype of the new taxon Utetitan zellaguymondeweyae, and referred other specimens from the North Horn Formation, the Black Peaks Formation and possibly the Javelina Formation to it.
The restored Alamosaurus skeletal mount at the Perot Museum was based on a composite of several titanosaur specimens found in Late Cretaceous rocks of North America. One of these, an articulated series of nine cervical vertebrae, was discovered in 1997 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. Other bones included in this reconstruction are the forelimbs and tail of USNM 15560 (now the holotype of Utetitan zellaguymondeweyae), and the torso, pelvis, and hindlimbs of TMM 41541-1.
Description
Alamosaurus was a gigantic quadrupedal herbivore with a long neck, a long tail, relatively long limbs and a body partly covered with bony armor. It would have measured around long, tall at the shoulder and weighed up to based on known adult specimens including TMM 41541-1. Supplementary Information
Some scientists suggest larger size estimates for the largest adults. Thomas Holtz proposed a maximum length of around 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, though he states that scientists do not know whether the massive tibia belongs to an Alamosaurus or a completely new species of sauropod.
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. Uniquely among titanosaurs, the ribs of Alamosaurus are pneumatised for two-thirds of the rib length; other titanosaurs had proximal rib ends, but in no other taxon did the pneumaticity extend as far as it did in Alamosaurus. Alamosaurus had more robust radii than Venenosaurus. The complete femur that was assigned to Alamosaurus is long. A fibula in length from the North Horn Formation may belong to this genus, though it cannot be referred with certainty since it is now lost.
In 2008, Lehman and Woodward estimated that Alamosaurus would have reached a body mass of over within 45 years, growing up to per year. Lehman and Woodward estimated that the growth lines of Alamosaurus were drawn at 4–12 years, and the maximum age of Alamosaurus was 55 years for the age at 90% of the mass. The age shown in the growth increase table was estimated to be 60 years. Trackways show that Alamosaurus walked slowly at speeds of without dragging their tails.
Classification
In 1922, Gilmore 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 Titanosauridae.
Alamosaurus was, in any case, an advanced and 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 the subgroup Opisthocoelicaudiinae of Saltasauridae. A major competing analysis finds Alamosaurus as a sister taxon to Pellegrinisaurus, with both genera located just outside Saltasauridae. Studies finding a close relationship between Alamosaurus and Opisthocoelicaudia did not include Pellegrinisaurus in their analyses. Other scientists have also noted particular similarities with the saltasaurid Neuquensaurus and Trigonosaurus, which is used in many cladistic and morphologic analyses of titanosaurians. An 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. The same study also suggests that the ancestors of Alamosaurus hailed from South America instead of Asia. The position of Alamosaurus recovered by phylogenetic analyses varies. Alamosaurus has been recovered as an opisthocoelicaudiine, saltasaurine, or outside of Saltasauridae entirely.
In 2022, Navarro and colleagues included Alamosaurus in their phylogenetic analysis of titanosaurs. Their results placed BIBE 45854, a series of neck vertebrae traditionally assigned to Alamosaurus, in a position distantly related to Alamosaurus itself; the vertebrae were recovered within the Lognkosauria, while Alamosaurus was recovered as a member of the Saltasauridae, as the sister taxon to Baurutitan:
Paleogeography
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.