Changes in lifestyle and habitat of Zoophycos-producing animals related to evolution of phytoplankton during the Late Mesozoic: geological evidence for the ‘benthic-pelagic coupling model’

The trace fossil Zoophycos characterized by complex, three-dimensional morphology with systematic internal structures occurs throughout the Phanerozoic marine sediments. The specimens of Zoophycos examined herein consist of a downward and helical spreite and are a product of the excretory behaviour of endobenthic detritus feeders. They are divided into two basic types: pre-Jurassic and post-Cretaceous types on the basis of whorls of spreiten in a single specimen. The pre-Jurassic type has fewer than four whorls. In contrast, most of the post-Cretaceous specimens exhibit spreite with multiple coils more than ten whorls. The abrupt increase in whorl number during the Cretaceous suggests that the sedentary lifestyle of the producer should change from a short-term stay to long-term or permanent occupation of the same burrow. Timing of the lifestyle change the Zoophycos producers seems to be closely related to the deep-seaward migration of their habitats. The change in lifestyle and migration of Zoophycos-producing animals during the Cretaceous might be attributable to the establishment of eutrophic bottom conditions in the deep sea. These changes seem to be associated with the flux of large amounts of phytodetrital food produced by phytoplankton, which experienced an explosive increase in species diversity during the Late Jurassic to the Late Cretaceous. The series of changes in lifestyle and habitat of the Zoophycos animals during the Late Mesozoic can serve as one piece of geological evidence for the ‘benthic-pelagic coupling model’.