Alamosaurus

Alamosaurus Temporal range: Late Cretaceous, 70–65.5Ma PreЄ Є O S D C P T J K Pg N ↓
Restoration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Saurischia
Clade:	†Sauropodomorpha
Clade:	†Sauropoda
Clade:	†Titanosauria
Family:	†Saltasauridae
Subfamily:	†Opisthocoelicaudiinae
Genus:	†Alamosaurus Gilmore, 1922
Species:	†A. sanjuanensis Gilmore, 1922

Alamosaurus (pron.: / ˌ æ l əm ɵ ˈ s ɔr ə s /; meaning "Ojo Alamo lizard") is a genus of titanosaurian sauropod dinosaur from the Late Cretaceous Period of what is now North America. It was a large quadrupedal herbivore. 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. Alamosaurus, like other sauropods, had a long neck and a long tail.

Naming

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.

Anatomy

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.

Classification

Alamosaurus is undoubtedly a derived member of Titanosauria, but 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.

History of discovery

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. It used to be thought that these formations were deposited immediately prior to the end of the Cretaceous, 65 million years ago. However, recent research has revised this view: there is no evidence that the Cretaceous–Paleogene boundary (K–T boundary) is present in any of them, and they are more likely to be early-mid Maastrichtian in age (~69 million years old)

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.

No skull material is known, except for a few slender teeth, and no armor scutes have been reported, such as those found in other advanced titanosaurians like Saltasaurus.

Paleobiogeography

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. In the south of Late Cretaceous North America, the transition from the Edmontonian to the Lancian 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 contemporaneous dinosaurs from that part of the world include tyrannosaurs, smaller theropods, the hadrosaur Edmontosaurus sp., the ankylosaur Glyptodontopelta, and the ceratopsids Torosaurus utahensis and Ojoceratops fowleri.