CRETACEOUS NERINEOID GASTROPOD PALEOECOLOGY FROM PUNTA CHINA, BAJA CALIFORNIA, MEXCIO

The gastropod superfamily Nerineoidea was widespread in the Jurassic and the Cretaceous and dominated many carbonate platform environments. Nerineoids were especially common in Europe and the Mediterranean region but had a worldwide distribution. They are known from many different types of carbonate rocks and are often associated with rudist-coral assemblages. Occurrences in clastic rocks are comparatively rare. Typical mass occurrences of nerineoids are found in transgressive deposits overlying levels showing signs of emergence. Nerineoids have previously been suggested to have been semi-sessile suspension-feeders and such occurrences have been correlated with enhanced food availability due to run-off or coastal erosion during transgression. The Aptian-Albian Alisitos formation in Baja California, Mexico, comprises several thousand meters of andesitic volcaniclastic rocks deposited in a fringing volcanic island arc complex. At Punta China the succession contains recurrent massive caprinid limestone bioherms, which accumulated under tropical shallow-water conditions. In these limestones, Nerineoids can occasionally be found but do not occur in rock-forming abundance. However, mass accumulations of Nerineoids do occur in the volcaniclastic rocks where they are the only major macrofossil constituents. These unusual occurrences suggest that the nerineoids were acting as opportunists colonising less favourable environments possibly due to enhanced competition from other taxa. Besides the caprinids, potential competitors are turritellid gastropods, which also occur at Punta China and have been suggested to occupy a similar ecological niche as the nerineoids. In the late Cretaceous nerineoids began to decline and, before the end of the Maastrichtian, the group became extinct while turritellids began to rise and colonise both carbonate and siliciclastic environments. The Punta China outcrops may provide valuable insights into not only the autoecology of nerineoids but also the evolutionary “handoff” from one high-spired suspension feeding gastropod taxon to another.