Fossil of Stomatocysts in Upper Cretaceous SedimentaryPyrite from Northern Mexico

In this paper six fossil forms of Chrysophycean stomatocysts are described. The material was collected from sedimentary pyrite embedded in well-laminated organic-rich limestone of the Upper Cretaceous Agua Nueva Formation at the locality of Xilitla, Central Mexico. The stomatocysts are represented by two spherical specimens with smooth surfaces lacking of ornamentation, three ovoid forms with rugose textures, one of them exposing presence of pore without collar,and one spherical specimen showing rugose texture and the presence of two short and rounded projections. The specimenshere described showed affinity with some stomatocysts morphotypes described for brackish and fresh water, butnot for known marine specimens. The presence of these microfossils in the Agua Nueva Formation represents the first formal description of fossil stomatocysts in Upper Cretaceous sedimentary pyrite in Mexico. The occurrence of both micro (planktonic foraminifera, calcispheres, radiolarians) and macrobiota (ammonites, inoceramid bivalves and fishes) and the presence of the specimens in sedimentary pyrite suggest that the stomatocysts were preserved under oxygen- deficiency conditions in a low energy environment. This event could have occurred in open marine waters in the Tampico-Misantla basin (Central Mexico) during the late Cenomanian throughout the early Cenomanian.