CRETACEOUS STROMATOLITIC MICROFOSSILS FROM SONORA, MEXICO

Ten types of microfossils preserved in chert layers that outcrop in the Huepac locality of the Tarahumara Formation in Sonora, are associated to stromatolites from the Upper Cretaceous (Turonian-Maastrichtian). They are identified based on their form, shape, size, and 3-D arrangement, as similar to extant taxa of the Cyanoprokaryota (e.g., Microcystis, Calothrix, Spirulina, Gloeocapsa, Eucapsis), Chlorophyta (e.g., Tetraedron, Chlorella), and Heterokontophyta-Bacillaryophyceae (e.g., Tabellaria, Fragilaria, Melosira). Particular ecological requirements of the extant species suggest that microfossils had either benthonic or planktonic habits. Therefore, a few of the microfossils from Huepac could participate directly in the build up of the associated Cretaceous stromatolites that apparently indicate a continental non-marine, most probably freshwater, environment. These microfossils document an important diversity of organisms living in fresh-water environments during the Upper Cretaceous in northern Mexico, and also some of the most ancient freshwater diatoms.