Buccal mass structure of the Cretaceous ammonite Gaudryceras

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Abstract
A rhynchaptychus attributed to the lower jaw of Gaudryceras sp. from the Upper Santonian rocks of Hokkaido, Japan, has an interesting impression well-preserved on the inner surface of the outer lamella. The impression is regarded as the imprints of chitin-secreting cells (beccublast cells), because of similarity in the characteristics arrangement of polygonal pits to those of modern coleoids. Each unit cell impression of Gaudryceras sp. is .apprx. 5-10 times larger than in modern coleoids. In modern coleoids and Nautilis a layer of tall beccublast cells is intercalated between the buccal muscles and the outer side of the upper jaw and/or the inner side of the lower jaw. The other sides of the jaws are, in contrst, free from jaw muscles, and are covered directly with a thin connecting tissue. Based on these observations a possible buccal mass structure of Gaudryceras sp. is restored. The beccublast cell impressions of Gaudryceras and modern coleoids markedly differ from that of modern Nautilus in the absence of numerous micropores. This fact suggests weaker mechanical properties of the jaw muscles in Gaudryceras keep a firm attachment of the jaw muscles on the jaw plates.