Microfossil algae associated with Cretaceous stromatolites in the Tarahumara Formation, Sonora, Mexico

Microfossils associated with stromatolites in the Upper Cretaceous (Campanian–Maastrichtian) are preserved in chert layers of the Tarahumara Formation that crop out at the Huepac Chert locality in Sonora, Mexico. A high diversity of microfossils was reported in a previous publication and ten of these are identified here as Cyanophyta (Calothrix estromatolitica, Eucapsis jacquesii, Gloeocapsa taverae, Microcystis cretacica, and Spirulina noveloi), Chlorophyta (Chlorella godinezii, and Tetraedron roldanii), and Bacillariophyta (Fragilaria tarahumara, Melosira huepacensis, and Tabellaria sonorensis), based on their similarities in form, shape, size, and three dimensional arrangement when compared to extant algae. Benthonic or planktonic habits of the extant species are thought to be similar to those of the fossils; therefore, a few could have participated in the build up of the stromatolites in non-marine, most probably freshwater environments. These fossils document the diversity of microorganisms in freshwater environments during the Late Cretaceous in northern Mexico, which include some of the most ancient freshwater diatoms.