Paleontological evidence of the Campanian to Early Paleocene paleogeography of Baja California

Fossil assemblages of Campanian to Maastrichtian dinoflagellates, invertebrates, mammals, reptilians, and planktonic foraminifera in Baja California are mainly temperate, similar to those found in the North Atlantic, Rocky Mountains and North Pacific regions. Early Paleocene assemblages are mostly subtropical ones. Danian dinoflagellates suggest deposition in a temperate to subtropical province, and benthonic foraminiferal assemblages (Midway type) are common in a region from the Gulf of Mexico to the northeastern coast of the U.S.A. The molluscan assemblages recovered from early Paleocene rocks indicate deposition mostly in subtropical environments. Although these invertebrate assemblages are related to the Caribbean province, dominant mollusks and solitary corals indicate subtropical to warm temperate paleoenvironments. Considering that the dinoflagellate, invertebrate, vertebrate and planktonic foraminiferal assemblages indicate temperate environments, together with paleogeographic models based on plate tectonics, we conclude that at least since the Campanian, Baja California was attached to western Mexico. The peninsula did not move much more than the 300 km needed to close the Gulf of California.