Paleontologists have long stressed the need to know how sampling the fossil record might influence our knowledge of the evolution of life. Here, we combine fossil occurrences of North American marine invertebrates from the Paleobiology Database with lithologic data from Macrostrat to identify sampling patterns in carbonate and siliciclastic rocks. We aim to quantify temporal trends in sampling effort within and between lithologies, focusing on the proportion of total available volume that has been sampled (sampled fossiliferous proportion, here called κ). Results indicate that the sampled fossiliferous proportion was stable during the Paleozoic, and variable during the post-Paleozoic, but showed no systematic increase through time. Fossiliferous carbonate rocks are proportionally more sampled than siliciclastic rocks, with intervals where the carbonate κ is double the siliciclastic κ. Among possible explanations for the apparent oversampling of fossiliferous carbonate rocks, analyses suggest that barren units, taphonomic dissolution, or data entry errors cannot completely explain sampling patterns. Our results suggest that one of the important drivers might be that paleontologists publish taxonomic descriptions from carbonate rocks more frequently. The higher diversity in carbonate rocks might account for an ease in the description of unknown species and therefore a higher rate of published fossils. Finally, a strong effect in favor of carbonate rocks might distort our perception of diversity through time, even under commonly used standardization methods. Our results also confirm that previous descriptions of an increase in the proportion of sampled fossiliferous rocks over time were driven by the sampling of the nonmarine fossil record.