Taphonomy of large canadoceras (Ammonoid) shells in the upper cretaceous series in South Sakhalin, Russia

Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, taphonomic histories of a large Campanian ammonoid, Canadoceras kossmati Matsumoto, 1954, were closely investigated. Large Canadoceras shells exceeding 30 cm in diameter are usually embedded horizontally and solitarily in muddy sandstone. A thin, lenticular calcareous concretion envelopes the shell (= envelope concretion). Their body chambers are mostly lost. The inner whorls comprising the center of the umbilicus completely disappear without exception, and only two or three outer whorls are preserved. The body and air chambers are somewhat compressed by compaction and are filled with sediments. Phycosiphon burrows are common not only in open body chambers but also in inner air chambers, which were originally closed. These observations suggest that the thin-shelled inner whorls and organic-rich siphuncular tubes degraded before final burial of the shell, and sediment infilling to the inside of the chambers followed. The early loss of inner whorls and siphuncular tubes gave rise to "draft-through currents." The continuous supply of oxygen and nutrients by the draft-through currents supported the Phycosiphon producers in the inner air chambers. Compared with other calcareous concretions containing intact fossils, values of minus-cement porosity (MCP) remain relatively low (6374%) and vary by areas even in the same envelope concretion. This indicates that the envelope concretions were cemented under a progressive increase of compaction during the later diagenetic stage. The formation of the envelope concretion appears to be a long-term phenomenon. Various events at different stages have been overprinted in a single large ammonoid fossil.