Paleoecology of gastropods preserved in turbiditic slope deposits from the Upper Pliocene of Ecuador

Upper Pliocene gastropods preserved in turbiditic deposits from the Upper Onzole Formation of northwestern Ecuador provide an excellent window into the past for understanding paleoecological dynamics in deep-water habitats, such as shells used as substrata by epi- and endobionts (encrusters and bioeroders), shell use by hermit crabs, and predation (shell repair and drilling) just prior to the closing of the Isthmus of Panama. Previously, paleoecological and taphonomic information was lacking for deep-water (mid-to-outer shelf) and bathyal fossil gastropods in contrast to shallow-water habitats that have been relatively well studied. This study showed that while epibionts were relatively rare on the fossil shells because of curatorial bias, endobionts (bioeroders) occurred on 51% of the 59 species examined. Of the bioeroders, the trace fossil Helicotaphrichnus commensalis was found on 27% of the species. The presence of Helicotaphrichnus indicates that hermit crabs were also a part of the deep-water assemblage prior to burial by the turbidity flow although there are no body fossils of hermit crabs yet reported from this area. Using epi- and endobiont criteria preserved on the shells, the hermit crabs appeared to prefer shells that had large last whorl volumes. Recycling and retention of Upper Pliocene shells by Recent hermit crabs (biological remanié) on the coast of Ecuador also occurs, which could lead to potential temporal anomalies in the fossil record. In addition, the trace fossil Helicotaphrichnus is now reported for the first time from the Upper Pliocene of Ecuador. This trace fossil also occurred on bathyal shells, suggesting that its bathymetric range may be much deeper than previously reported. Records of predation retained on the deep-water fossils were common (66% of the species had shell repair; 68% of the species had drilled shells) and represented a variety of molluscivorous predators. The durophagous predators (e.g. crabs) and drilling predators (naticids, octopods, and muricids) appeared to be selective in their choice of prey items. High-spired gastropods (especially of the family Turridae) were statistically more likely to be affected by predators than all other prey species and groups examined. Shell architecture for other shell types examined, however, did not appear to affect the frequency of shell repair occurrence. Although predation is thought to be a generalist activity in deep-water environments, it may be that in certain areas, specialist predators may be more common than previously considered. Thus, fossil deep-water communities may contain a rich legacy of paleoecological interactions that can then be used for evolutionary ecological questions pertaining to selective predation with depth, variation in encrusters and bioeroders with depth, and to compare differences in paleoecological structure of these molluscan communities before and after the closure of the Isthmus of Panama.