Forty-seven sediment samples were taken from a borehole core at Oppama Park, Yokosuka City, Kanagawa Prefecture, central Japan. Fifty-eight ostracod species, representing 32 genera, were identified in the 32 samples. Three ostracod biofacies (I–III) were identified using Q-mode cluster analysis. On the basis of ostracod biofacies, the following paleoenvironmental changes were recognized in the study area: land mudflat (no ostracods) → muddy bottom environment of inner bay (biofacies III) → sandy bottom environment of bay near rocky shore (biofacies II) → shallow mudflat (biofacies I). Modern analog technique (MAT) showed that the paleoenvironment of the study area was a warm (maximum bottom temperature in summer around 30°C; minimum bottom temperature in winter around 7°C) and shallow bay, similar to that around Honshu Island today.

An ostracod specimen, Parakrithella pseudadonta, with exceptionally well preserved soft parts was discovered in one of the horizons, at a core depth of 20.4–20.5 m. In this horizon, an event deposit, namely, one indicating an increase in paleobathymetery, was identified from ostracod assemblages, which were identified as tsunami deposits on the basis of previous studies of neighboring areas. Similar soft part-preserved arthropod fossils have been discovered from the Upper Cambrian (500 Ma) of Sweden. Many fecal pellets have also been found in the Orsten limestone. Such “cesspools” were exceptionally phosphatized during early diagenesis owing to the high local phosphorus levels produced by the accumulated fecal pellets. The “cesspool preservation hypothesis” provides an explanation for this kind of exceptional fossilization, found in the marine sediment record from the Late Cambrian onward.