Biomarkers, molecular fossils, are organic compounds in Holocene to Precambrian sedimentary deposits that can be related to specific chemical compounds produced in the biosphere. We demonstrate here that 24-norcholestane biomarkers, i.e., C26 steranes (saturated hydrocarbons having a steroid skeleton), can be useful to constrain the age and paleolatitude of geologic samples. The biological precursors of 24-norcholestanes remain unclear, but samples from more than 100 basins provide evidence that 24-norcholestanes may be derived directly or indirectly from diatoms. Ratios of 24-norcholestanes to 27-norcholestanes show an initial increase above background in Jurassic oils, but they increase dramatically in Cretaceous oils, coincident with diatom evolution. The highest ratios are found in oils and rock extracts from Oligocene or younger marine siliceous source rocks in which the sources were deposited at paleolatitudes >30°N.