During the Late Cretaceous, Europe was an archipelago. Southern France and north-western Spain where its fossils are found was part of the large Ibero-Armorican island in the prehistoric Tethys Sea. The rock formations that have yielded "Gargantuavis" fossils have also produced abundant remains of fish, turtles, crocodylomorphs, pterosaurs, various titanosaurian sauropods (including "Ampelosaurus" and "Lirainosaurus"), ankylosaurians, ornithopods, and theropods, including other early avialans, like enantiornithes. The association of abundant fossils of the ornithopod "Rhabdodon", and the lack of any hadrosaurid fossils, have been used as index fossils to roughly date these formations to the late Campanian-early Maastrichtian interval. An age confirmed later by magnetostratigraphic evidence in two localities. The type locality of "Gargantuavis", the Bellevue site in the Marnes Rouges Inferieures Formation, is 71.5 million years old (earliest Maastrichtian). The Spanish site of Laño is slightly older with an age of 72 to 73.5 Ma (latest Campanian).