JURASSIC-RECENT TURRITELLINE GASTROPOD-DOMINATED ASSEMBLAGES: A PATTERN OF PRODUCTIVITY AND GEOLOGIC CONTINGENCY

Turritelline gastropods (family Turritellidae, subfamilies Turritellinae and Protominae) originated in the late Jurassic and by the Cretaceous were diverse and widely distributed across most of the world’s oceans. They frequently form high-density, low diversity assemblages known as “turritelline dominated assemblages” (TDAs). Data from both modern and fossil TDAs indicate that they usually form in environments of high primary productivity. A new dataset consisting of more than 80 TDAs from the Lower Cretaceous to the Pliocene, including all continents, confirms and offers greater detail about the occurrence of these assemblages and the rises and falls of oceanic primary productivity.

TDAs occur widely in both carbonate and siliciclastic environments in the Cretaceous and Paleogene, but very rarely in carbonate environments in the Neogene, and not at all in such environments today. Neogene carbonate TDAs are restricted to the Miocene of India and Australia and Pliocene of Chile. The Australian and Chilean occurrences were likely associated with strong upwelling; environment of the Indian occurrence is still unclear. Peak TDA frequency occurs in the Lower Miocene (Burdigalian) with a secondary peak in the Lower Eocene (Ypresian). The Miocene peak approximately matches the peak in global turritelline species diversity. These patterns are consistent with other evidence for globally high primary productivity in the Miocene.

Although even higher levels of marine productivity likely occurred after the Miocene, turritelline diversity, abundance, and geographic range contracted in the late Neogene, likely because of the combined effects of regional productivity declines in the western Atlantic (associated with the closure of the Central American Isthmus) and destruction of the central Tethys (Mediterranean and Paratethys). Both the rise and fall of turritellines therefore appears to have been caused by the contingent effects of tectonics on ocean geometry and circulation: they rose in importance with the global rise in marine productivity associated with volcanism and the Marine Mesozoic Revolution, and fell with the reorganization of circum-tropical ocean circulation in the late Cenozoic.