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Raindrop impressions?
These circular impressions, exposed on the underside surface of a layer of fine sandstone, have been interpreted as raindrop impressions. Sedimentologists have cautioned that similar impressions can form in muddy substrates by the ascent of bubbles through the sediment, or by interaction with air bubbles trapped in the overlying fluid. Scale in cm. Outcrop of the Maastrichtian Laramie Fm., exposed along the Triceratops trail Golden, CO, US.
Coccoliths
Calcareous nannofossils (coccoliths) are among the tiniest fossils preserved in the sedimentary record, measuring around 5 to 10 microns. The image (plate) shows petrographic microscope photos of some of the most characteristic coccoliths of the Cretaceous/Paleogene (K/Pg) boundary. This boundary is associated with one of the most studied mass extinctions in the geological record, which has been correlated to a major catastrophic event. Photos 1 to 4 show nannofossils of the Upper Cretaceous (Maastrichtian) that are thought to have gone extinct at the K-Pg catastrophic event. Photos 5 to 7 are of Cretaceous survivors that show continuous distribution into the Paleogene (Danian). These last species are considered adapted to cold water and eutrophic environments (r-selected taxa). Photos 8 and 9 show Paleogene taxa, not present below the boundary. (Photos by E. Belia)