3D RECONSTRUCTION OF MELOSIRA HUEPACENSIS, A FRESHWATER CRETACEOUS DIATOM

Because pre-Mesozoic diatom findings are still not considered valid for the scientific community, the oldest undoubted records of freshwater diatoms are of Late Cretaceous age (~70 Ma) and comprise both centric and pennate morphologies. One type of well-preserved diatoms in the Huepac Chert of Sonora, Mexico, exhibit remarkable similarity with Melosira sp. modern centric diatoms. The reconstruction of fossil examples in 3D allows a better comparison of morphological traits, as these can be rotated and seen in planes that cannot be easily achieved by regular microscopy of petrographic sections. In general, cylindrical cells with convex edges show valve walls with little or no external ornamentation. Cells (2-3) are within hyaline cylinders (linking valves), are in contact through spines, and have a cingulum toward the center of each cell. Modern species of Melosira sp. grow in marine and freshwater habitats and have either planktonic and benthonic habits. Other similar fossils of Aulacoseira sp. can be distinguished from Melosira huepacensis based on the rounded epi- and hypovalves, which are angular in Aulacoseira, and on the arrangement in pairs or triplets of cells inside one linking valve as in M. nummuloides and M. roeseana.