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New record of a rhyncholite (cephalopod upper jaw element) from the Maastrichtian of Hokkaido, Japan

Arrowheadand scallop-shaped calcified skeletal remains of organisms called rhyncholites (s. l.) and conchorhynchs, respectively (Teichert et al., 1964) have been found in late Paleozoic to Cenozoic marine deposits. These peculiar fossils have been interpreted as calcified jaw parts of cephalopod mollusks, since some of them (e.g., those identified as Rhyncolite Biguet, 1819) closely resemble the anterior calcareous tips of jaws of modern and fossil Nautilida (Owen, 1832; Foord, 1891; Teichert et al., 1964; Saunders et al., 1978; Klug, 2001). Similar calcified structures also occur in the anterior portion of jaws that are preserved in situ within the body chambers of some Mesozoic ammonoid genera of Lytoceratina (only lower jaws; Tetragonites, Gaudryceras, and Anagaudryceras; Tanabe et al., 1980; Kanie, 1982), Phylloceratina (only lower jaw; Phyllopachyceras; Tanabe and Landman, 2002), and Ammonitina (upper and lower jaws; Aconeceras; Doguzhaeva and Mutvei, 1992). These facts indicate that at least some rhyncholites and conchorhynchs were formed by the Nautiloidea and Ammonoidea. In spite of these previous observations, exact higher systematic relationships of individual rhyncholites and conchorhynchs are still unclear owing to their isolated mode of occurrence and marked morphological variation. One of us (TN) recently found a well preserved rhyncholite fossil from the Upper Cretaceous of northern Hokkaido, Japan. In this report, the overall morphology of the specimen is described, and its possible taxonomic relationship is discussed. This is the first report of a Cretaceous rhyncholite from the North Pacific region. Geological Setting