Deposit feeding by the Pliocene deep-sea macrobenthos, synchronized with phytodetritus input: Micropaleontological and geochemical evidence recorded in the trace fossil Phymatoderma

• Echiuran fecal trace fossil Phymatoderma from Pliocene deep-sea strata was studied.
• Coccolith aggregates were recognized in the fecal pellets of Phymatoderma.
• Geochemically, Ca is significantly accumulated in the fecal pellets of Phymatoderma.
• Feeding by ancient echiurans was synchronized with coccolithophore bloom deposition.
• This study provides a geologic evidence for the ancient benthic-pelagic coupling.

The present study consists of the microscopic and geochemical analyses of specimens of the trace fossil Phymatoderma from the Pliocene deep-marine strata (Shiramazu Formation, Japan). The aim of this paper is to assess whether benthic-pelagic coupling functioned in the ancient ocean as it does in the modern ocean. Various types of microfossils (e.g., coccoliths, diatoms, planktonic foraminifera, and radiolaria) were detected by SEM observation of fecal pellets of Phymatoderma. Among these plankton, coccolithophores are considered to have been the most important dietary source for the Phymatoderma-producing echiuran worm, because excreted tuffaceous fecal pellets are occasionally found to be composed exclusively of coccoliths. Elemental analysis using the SEM-EDS system revealed that CaO, which is probably derived from the microfossils of calcareous plankton (i.e., coccoliths and planktonic foraminifera), was abundant in the tuffaceous fecal pellets. Considering the obtained microscopic and geochemical evidence, it is most likely that the echiurans’ deposit-feeding activity was synchronized with an episodic (probably seasonal) coccolithophore bloom deposition on the deep-sea floor. Such a mode of feeding may have facilitated the effective uptake of freshly deposited phytodetritus. The present study provides geologic evidence for benthic-pelagic coupling in the Pliocene Epoch. In addition, based on the integration of the obtained results and reinterpretations of the existing data, the present study further supports the possibility that the modern-type benthic-pelagic coupling had established during the Cretaceous Period.