Taphonomy of half and compressed ammonites from the lower jurassic black shales of the toyora area, west Japan

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The origin of half and compressed ammonites from the Lower Jurassic (Upper Pliensbachian to Lower Toarcian) black shales of the Nishinakayama Formation in the Toyora area, west Japan, is discussed. Ammonite shells horizontally embedded in the black shales reveal two types of fossil preservation; (1) “half” ammonites without an upper flank, and (2) complete ammonites with both upper and lower flanks. Specimens of the first type mostly lack a calcified shell wall, leaving only a wrinkled periostracum, while those of the second type retain a calcified shell wall. Most specimens have suffered from a lateral deformation during sediment compaction, but the ratio of deformation is less than 40% at many localities. The mode of ammonite preservation correlates well with the petrochemical properties of the fossil-bearing shales, and the shales comprising complete ammonites are rich in carbonate CaO and calcite as compared with the “half” ammonite-bearing shales. These lines of evidence strongly suggest that the upper flank of the “half” ammonites had been dissolved away in early diagenesis before its complete burial. Thus the upper flank might have been initially exposed to the seawater. As the “half” ammonites are confined to the shales associated with layers of tiny sedimentary pyrite crystals, dissolution of aragonitic ammonite shell walls and septa might have happened under anaerobic or oxygen-depleted bottom conditions.