The latest Cenomanian to early Turonian (Late Cretaceous) section at Vallecillo, Mexico, contains a continuous and highly fossiliferous sedimentary record across the late Oceanic Anoxic Event (OAE) 2, and shows similar distributions of benthic inoceramids and planktic foraminifers during this period of an expanded oxygen minimum zone (OMZ). We suggest that these synchronous abundance changes result from similar environmental vulnerability of planktotrophic larval stages of inoceramids and deeper-dwelling planktic foraminifers, and that these synchronous changes reflect the expansion of the OMZ. The abundances of ammonites and fishes, used as independent control groups, do not correlate and rule out a preservational bias. Ammonites, fishes, and their hatchlings thus populated shallower water depths than inoceramid gametes and planktic foramininifers. The improving conditions at the end of OAE 2 resulted in the dispersal of new inoceramid and foraminiferal morphotypes.