It is not too often that we are able to say that we have a fossil that comes from an area of the fossil record that basically butts up against the K-Pg boundary (Cretaceous - Paleogene Boundary; the site in time of the extinction event that claimed the non-avian dinosaurs). However, Alamosaurus, is one of those animals that appears to have been one of the very last of the non-avian dinosaurs that roamed the earth. Known largely from the Ojo Alamo Formation (as previously noted) and the Javelina Formation, many of the fossil remains of Alamosaurus are between 69 and 66.5 million years old. There is at least one juvenile skeleton known from the Black Peaks Formation, an area of the fossil record that envelops (contains) the K-Pg boundary. We can infer from this that Alamosaurus was alive (and well, probably) near, if not right up to, the extinction of the non-avian dinosaurs.
In its fossil range, Alamosaurus is one of the most commonly found fossils, along with Quetzalcoatlus, the enormous pterosaur. It has been hypothesized that Alamosaurus represents a reintroduction of enormous sauropod dinosaurs to North America. Opposing hypotheses point to ancestral sauropods originating further inland in western North America (and ultimately from Asia) and that Alamosaurus was essentially endemic (restricted in range to a certain area) to the North American inland plains. Lucas et al. (2016) noted that the dispersal of Alamosaurus fossil across the western inland plains provides evidence that Alamosaurus was actually quite mobile and not endemic, having a large demonstrated range. Their review did concede that Alamosaurus has only been found within inland plain environments and that no fossils of Alamosaurus have yet been found that would indicate that the animal made its way to the coastline or coastal plains of the Western Interior Seaway.
Lehman, T. M., (2001), Late Cretaceous dinosaur provinciality: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 310–328.
Lucas, S., Sullivan, R, Lichtig, A, Dalman, S., and Jasinski, S. (2016). Late Cretaceous dinosaur biogeography and endemism in the Western Interior basin, North America: A critical re-evaluation. New Mexico Museum of Natural History and Science Bulletin. 71. 195-213.
Sullivan, R.M., and Lucas, S.G. (2006). "The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America." New Mexico Museum of Natural History and Science, Bulletin 35:7-29.