Alamosaurus (/ˌæləmoʊˈsɔːrəs/;[1] 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.[2] 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.[3]
Alamosaurus was a gigantic quadrupedal herbivore with a long neck and tail and relatively long limbs.[3] Its body was at least partly covered in bony armor.[4] In 2012 Thomas Holtz gave a total length of 30 meters (98 ft) or more and an approximate weight of 72.5–80 tonnes (80–88 short tons) or more.[5][6] 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 73 metric tons (80 short tons).[2] Scott Hartman estimates Alamosaurus, based on a huge incomplete tibia that probably refers to it, being slightly shorter at 28–30 m (92–98 ft) and equal in weight to other massive titanosaurs such as Argentinosaurus and Puertasaurus.[7] 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.[8] 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).[9] 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).[10]
Though no skull has ever been found, rod-shaped teeth have been found with Alamosaurus skeletons and probably belonged to this dinosaur.[3] The vertebrae from the middle part of its tail had elongated centra.[11] Alamosaurus had vertebral lateral fossae that resembled shallow depressions.[11] Fossae that similarly resemble shallow depressions are known from Saltasaurus, Malawisaurus, Aeolosaurus, and Gondwanatitan.[11] 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.[11] Alamosaurus had more robust radii than Venenosaurus.[11]
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.[12] 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.[13] 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.[14]