Alamosaurus (ˌæləmoʊˈsɔːrəs; meaning "Ojo Alamo lizard") is a genus of opisthocoelicaudiine 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. Isolated vertebrae and limb bones indicate that it 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 spanning the Maastrichtian age. 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 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.
Alamosaurus was a gigantic quadrupedal herbivore with a long neck, long 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 known 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.