Turritelline-dominated assemblages (TDAs) frequently occur in the middle-late Miocene Gatun Formation, and are not uncommon features in the broader fossil record. By gaining a better understanding of the paleoenvironment and taphonomic processes leading to their formation we can gain insight into the conditions in the Western Atlantic (WA) during the Miocene shoaling of the Central American Seaway, as well as the conditions which may lead to TDA formation generally. TDA and non-TDA beds within the Gatun were examined for shell orientation, sclerobiont coverage, drilling predation frequency, and peeling-repair frequency, in addition to sediment composition. The most abundant species, T. altilira, was also examined using oxygen isotopic sclerochronology to compare growth rate and conditions during the formation of TDA and non-TDA beds. Mean annual range of temperature (MART) was found to be 6.2°C, with a moderate associated negative O-C correlation. These data confirm the influence of Tropical Eastern Pacific (TEP) upwelling waters in the WA at this time. Upwelling conditions were found to be associated with all T. altilira, regardless of their source, indicating that Gatun TDAs are not the result of variation in nutrient supply. Orientation data from within a TDA, grain size, and sclerobiont coverage all suggest that TDAs in the Gatun are the result of variation in sediment supply/winnowing. T. altilira was found to live up to 3 years, growing between 50 and 60 mm in the first year of life with a subsequent decline in growth rate. We used the Theoretical Apex System and a calculated minimum number of individuals to determine location and frequency of drilling predation.

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