Sheltered preservation: a peculiar mode of ammonite occurrence in the Cretaceous Yezo Group, Hokkaido, north Japan

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Abstract

Maeda, H. 1991 01 15: Sheltered preservation: a peculiar mode of ammonitc occurrence in the Cretaceous Yeto Group, Hokkaido. north Japan. Lethaia, Vol. 24, pp. 69–82. Oslo. ISSN 0024–1164.

‘Sheltered preservation’, in which many small ammonites (‘refugees’) arc preserved in the inside and/or the lower umbilical void of a large ammonite shell (‘shelter’), is a common preseivational style in the Cretaceous Yezo Group in Hokkaido, north Japan. The best example is large Calycoceras of Middle Cenomanian age, attaining 300 to 500 mm diameter, whose shell usually hours more than 100 individuals of Desmoceras comprising mainly juveniles less than 12 mm in diameter. In contrdst with other preservational styles of Desmoceras from the same horizon. the shells of refugee ammonites have not suffered phragmoeone collapse, and the inner whorls and a protoconch are intact. Many refugee ammonites are allochthonous, as indicated by the incomplete preservation of their living chamber and the absence of a jaw apparatus. The sheltered preservation is, therefore, a kind of ‘Konzcntrat Lagerstatten’ completed by both pre- and post-burial processes. Acting as a ‘concentration trap’ on the sea floor, a large empty ammonite shell probably triggered off such accumulation of the small ammonite shells in post-mortem transport across the sea floor. This biostratinomical process biased the original thanatococnosis of ammonites, so that its immature specimens were selectively accumulated and preserved in the shelter. Ammonoidea, taphonomy, Konzentrat Lagerstätten'. differenrial preservation, Cretuceous. Hokkaido.

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