An artist’s reconstruction, released by the National Museum of Brazil, of the paleoecology inhabited by Futalognkosaurus (left). It is being menaced by Megaraptor, now known to be a tetanuran theropod.

For quite some time it was thought that after the Jurassic period the massive sauropods that roamed North America were all but extinct, a radiation of ankylosaurs, ceratopsians, hadrosaurs, and other ornithischians becoming the primary herbivores in place of the massive long-necked saurischians. At least one genus did hang on until the Late Cretaceous, the titanosaur Alamosaurus sanjuanensis, although it was a dinosaur that seemed to be out of place among the more famous Late Cretaceous North American dinosaur fauna, at least until now. Before plate tectonics the