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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The closure of the late Neogene interoceanic seaways between the Western Atlantic (WA) and Tropical Eastern Pacific (TEP)—commonly referred to as the Central American Seaway—significantly decreased nutrient supply in the WA compared to the TEP. In marine invertebrates, an increase in parental investment is expected to be selectively favored in nutrient‐poor marine environments as prolonged feeding in the plankton becomes less reliable. Here, we examine turritelline gastropods, which were abundant and diverse across this region during the Neogene and serve as important paleoenvironmental proxies, and test whether species exhibit decreased planktotrophy in the WA postclosure as compared to preclosure fossils and extant TEP species. We also test for differences in degree of planktotrophy in extant sister species pairs. Degree of planktotrophy was inferred by measuring the size of protoconchs, the species' larval shell that represents egg size. Protoconch size was compared between extant postclosure WA and TEP species and preclosure fossil species. To compare extant sister species, we reconstructed the phylogeny of available WA and TEP species using one nuclear (H3) and three mitochondrial markers (12S, 16S, and COI). Compared to the preclosure fossils, protoconch size increased in WA species but remained the same in the TEP species. In the two extant sister species pairs recovered in the phylogenetic analysis, the WA species are inferred to be nonplanktotrophic while the TEP species are planktotrophic. This suggests that decreased nutrient availability and primary productivity in the WA may have driven this change in developmental mode, and was the primary selective force resulting in postclosure turritelline extinctions.

During the Mid-Miocene Climatic Optimum (MMCO, 14.7–17.3 Ma), global temperatures were warmer than present, and similar to predicted temperatures for the coming century. Limited paleoclimate data exist from the tropics during this period, despite its potential as an analog for future climate conditions. This study presents new subannual stable isotope data (δ18O and δ13C) from a large population of Miocene Turritella gastropods from the Jimol and Castilletes Formations of the Guajira Peninsula, Colombia. Turritellids are aragonitic marine mollusks that live in shallow coastal waters, and their rapid growth rates allow for high-resolution subannual records. We compare these fossils to modern Turritella gastropods from multiple tropical localities to reconstruct subannual climate conditions. The seasonal range in δ18O in the modern shells correlates with the seasonal variance of local precipitation, once temperature seasonality is accounted for. The Miocene fossils show larger (in some cases >2‰) seasonal variation in δ18O than modern Turritella from the same location, suggesting increased seasonality of precipitation in Miocene northern Colombia relative to today. We propose that this increased seasonality of precipitation was due to a more northerly position of the Intertropical Convergence Zone during the mid-Miocene. The resulting wet Miocene paleoenvironment is in stark contrast to semiarid conditions on the Guajira Peninsula today, indicating that this area of tropical South America has undergone a drastic environmental change since the Miocene.

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We use scanning electron microscopy imaging to examine the shell microstructure of fossil and living species in five families of caenogastropods (Strombidae, Volutidae, Olividae, Pseudolividae, and Ancillariidae) to determine whether parallel or convergent evolution is responsible for the development of a unique caenogastropod trait, the extreme parietal callus (EPC). The EPC is defined as a substantial thickening of both the spire callus and the callus on the ventral shell surface such that it covers 50% or more of the surface. Caenogastropods as a whole construct the EPC convergently, using a variety of low-density, poorly organized microstructures that are otherwise uncommon in caenogastropod non-callus shell construction. Within clades, however, we see evidence for parallelism in decreased regulation in both the shell and callus microstructure. Low-density and poorly ordered microstructure—such as used for the EPC—uses less organic scaffolding and is less energetically expensive than normal shell microstructure. This suggests the EPC functions to rapidly and inexpensively increase shell thickness and overall body size. Tests of functional ecology suggest that the EPC might function both to defend against crushing predation through increased body size and dissipation of forces while aiding in shell orientation of highly mobile gastropods. These interpretations hinge on the current phylogenetic placement of caenogastropod families, emphasizing the essential contribution of phylogeny when interpreting homoplasy.

Abundant species are typically also viewed as ecologically dominant, and are frequently used to characterize the communities in which they live. Such characteristic assemblages may also be used as indicators of environmental conditions, such as relative stability. Fossil and modern turritelline gastropods are often the most abundant species in the marine assemblages and communities in which they occur, forming ‘turritelline-dominated assemblages’ (TDAs). We use data on modern Turritella bacillum from waters around Hong Kong as a case study to analyse fluctuations in abundance over 25 years. While turritellines were not always dominant in the area surveyed (~1650 km2), populations were notably persistent, and rebound after decline of abundances occurred within ~5 years at some sites. δ18O sclerochronology suggests that individuals were ~1–2 years old. It is also notable that T. bacillum was found to be abundant at salinities as low as 10–15 psu, despite the general characterization of turritellines as fully marine. Comparison with data on modern T. communis in the western English Channel corroborates this pattern, as localized sites of high abundance also appear transient. These results have implications for the interpretation of TDAs in the fossil record: they may signify the cumulative result of short-lived, spatially restricted populations, possibly resulting from essentially stochastic larval settlement. This suggests that the palaeoenvironmental setting of fossil TDAs does not always control their occurrence on short temporal scales.