Temporal range: Late Cretaceous, 70–66 Ma
|Restored skeletons of Alamosaurus and Tyrannosaurus at Perot Museum|
Alamosaurus (//; meaning "Ojo Alamo lizard") is a genus of 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.
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. Though most of the complete remains come from juvenile or small adult specimens, one fragmentary specimen suggests that adult Alamosaurus could have grown to enormous sizes comparable to the largest known dinosaurs like Argentinosaurs, which has been estimated to weigh 73 tonnes (72 long tons; 80 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.
Alamosaurus was an advanced member of the group Titanosauria, but its relationships within that group are far from certain. One major analysis unites Alamosaurus with Opisthocoelicaudia in a subfamily Opisthocoelicaudinae of the family 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.
Alamosaurus remains have been discovered throughout the southwestern United States. The holotype was discovered 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.
Gilmore originally described a scapula (shoulder bone) and ischium (pelvic bone) in 1922. In 1946, he found a more complete specimen in Utah, consisting of a complete tail, a right forelimb complete except for the tips of the toes, and both ischia. Since then, many other bits and pieces from Texas, New Mexico, and Utah have been referred to Alamosaurus, often without much description. The most completely known specimen is a recently discovered juvenile skeleton from Texas, which allowed educated estimates of length and mass.
One specimen of Alamosaurus, "USNM 15660", was first uncovered in 1937. Although originally found by Gilmore, it was first recognized by Michael Brett-Surman. It has been catalogued under the same number as a more complete and well-known specimen of Alamosaurus, which was found in very close proximity based on bone impressions. In 2015 it was realized that the specimen, which had been previously described, also included osteoderms, the first confirmation they existed on Alamosaurus.
No skull material is known, except for a few slender teeth. Contrary to popular assertions, this dinosaur is not named after the Alamo in San Antonio, Texas, or the battle that was fought there. The holotype, or original specimen, 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 one species (A. sanjuanensis), which is named after San Juan County, New Mexico, where the first remains were found. Both genus and species were named by Smithsonian paleontologist Charles W. Gilmore in 1922.
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 later has been difficult to determine. 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 Ma 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 90 meters below the K-Pg boundary and within the local range of Alamosaurus fossils, was dated to 69.0 plus or minus 0.9 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. Some taxa may have co-occurred on both continents, including Kritosaurus and Avisaurus. 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.
Other contemporary dinosaurs from that part of the world include tyrannosaurs, smaller theropods, the hadrosaurs Edmontosaurus sp. and Kritosaurus sp., the ankylosaur Glyptodontopelta mimus, and the ceratopsids Torosaurus utahensis and Ojoceratops fowleri.
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