Paleontology and geologyIn the Cretaceous, the opening of the Gulf of Mexico ceased, but the eastern margin of Northeastern Mexico dropped down, allowing shallow seas to cover the area. Around islands, lagoons formed where sea water periodically evaporated and left deposits of salt and gypsum. Farther out to sea, rudist bivalves built up huge reefs, which were home to corals, stromatoporoids, brachiopods, echinoids, gastropods, bryozoans, sponges, and other bivalves. Ammonites, mosasaurs, sea turtles, various bony fish, and sharks also swam in these seas. In the west, subduction continued to push up the land and form huge volcanoes as it added the last islands and slices of oceanic crust to the continent. The land was covered in rivers, lakes, swamps, and forests that stretched down to coastal floodplains and deltas. These habitats were filled with ceratopsians, hadrosaurs, tyrannosaurs, ankylosaurs, dromaeosaurs, and titanosaurs. Hermit crabs, crayfish, brittle stars, crocodiles, turtles, lizards, pterosaurs, birds, freshwater fish, and early mammals called multituberculates lived here. Some of these animals also left behind footprints that have been preserved. |
