Sauropod dinosaurs were massive, long-necked, long-tailed creatures that have long been regarded as the largest land animals ever to have lived. They reached their most diverse in the Late Jurassic, with only two groups surviving into the Cretaceous, and only one of those groups and only one of those groups surviving till the end of the period. However the group that did, the Titanosaurs, grew to become the very largest of the group, with the largest known, Argentinosaurus reaching an estimated weight of 78 000 kg.
Argentinosaurus as depicted by Argentinean artist Jorge Antonio Gonzalez.
These super-sized sauropod dinosaurs were first recognized in South America, and have subsequently been found in Eurasia and Africa. No North American titanosaurs have been formally recorded, although scattered remains of therapod dinosaurs have been found from the Maastrichtian (Latest Cretaceous) of the Southwestern United States since at least the 1920s. These have been assigned to the species Alamosaurus sanjuanensis, but have generally been considered to fragmentary to allow detailed studies of their taxonomic affinities.
The latest edition of the journal Acta Palaeontologica Polonica contains a paper by Denver Fowler of the Museum of the Rockies and the Department of Earth Sciences at Montana State University and Robert Sullivan of The State Museum of Pennsylvania in which they re-examine A. sanjuanesis and publish a description of new material gathered from the Ojo Alamo Formation in New Mexico by field crews from the State Museum of Pennsylvania.
Like previous material assigned to A. sanjuanensis the Ojo Alamo material is extremely fragmentary. It clearly contains material from more than one animal, and this is either from parts of the animal that have not previously been discovered, or that are to badly damaged for easy comparison. This is not unusual when dealing with the bones of a large sauropod dinosaur; in order for the animal to be preserved well it needs to be buried fairly quickly, unlikely with a creature of this size.
The absence of a direct comparison between specimens prevents 100% confidence that the new material belongs to the same species as the previously discovered material, however in other parts of the world there is typically only one species of large sauropod dinosaur living at any one time, and there is no evidence to suggest that the Southwestern United States was any different in this respect, so Fowler and Sullivan have assigned the new material to A. sanjuanensis.
The femur of Alamosaurus sanjuanensis in the location where it was found.
Some of the new material is also considerably larger that previously discovered specimens assigned to A. sanjuanensis, suggesting that it came from a much bigger animal. If all the material is considered to come from members of the same species, then the only way that this can be explained is if all previous material has come from juvenile individuals. This is not as great a presumption as it sounds.
Earlier this year David Varricchio of Montana State University published a paper in the journal Historical Biology articulating the view, long held by many palaeontologists, that dinosaurs, and in particular vert large dinosaurs, may have had a life cycle in which a large number of young were produced and the overwhelming majority of individuals died before they reached full maturity. The implication of this is twofold. Firstly the majority of fossil dinosaurs discovered will be juveniles, smaller than fully grown adult individuals. Secondly, as in modern birds, these individuals are unlikely just to be smaller versions of the adult animals, they will almost certainly lack some morphological traits seen in the adults, such as sexual characteristics.
Comparison of the size of a new vertebra found by the Pennsylvania team allows a direct comparison of the size of A. sanjuanensis with other titanosauran sauropods. It has previously been suggested that A. sanjuanensis might weigh slightly over 32 500 kg, but examination of the new material suggests that it may well have grown to sizes rivaling Argentinosaurus. Fowler and Sullivan do not make a claim for the absolute size of A. sanjuanensis, but they do observe that it was almost certainly the largest dinosaur ever to roam North America.
See also New dinosaurs from old drawers, Nesting behavior and parental care in an Ornithischian Dinosaur? Dinosaur footprints discovered in Southwest Arkansas and Dinosaurs on Sciency Thoughts YouTube.