Alamosaurus – late arrival…

With its long neck and whippy tail, Alamosaurus looks much like any other sauropod. However, it is rare because it is the only type of sauropod to be found in North America at this time.

In the Late Cretaceous, there is scant evidence for the titanosaurid sauropod fauna that dominated much of the world. It is postulated that Alamosaurus was a late migrant from South America, reintroducing sauropods which had been absent since the Early Cretaceous. The titanosaurids died out in the upper part of the Lower Cretaceous period in North America but were apparently reintroduced from South America late in the Upper Cretaceous. However, they only got as far north as Utah before the final dinosaurian extinction. Ironically, some of the best articulated titanosaur material is that of the large North American genus Alamosaurus. The sauropod family Titanosauridae has a broad distribution in the Late Cretaceous on southern continents. Titanosaurids also entered Europe (Magyarosaurus and Hypselosaurus) and North America (Alamosaurus), a continent from which sauropods had previously become extinct.

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.