TWO COLEOID JAWS FROM THE UPPER CRETACEOUS OF HOKKAIDO, JAPAN

Journal of Paleontology, Jan 2006 by Tanabe, Kazushige, Hikida, Yoshinori, Iba, Yasuhiro

ABSTRACT-

Two isolated cephalopod jaws recovered from the middle Turonian of the Obira area and the Campanian of the Nakagawa area, Hokkaido, Japan, consist of short outer, and large and posteriorly elongated inner "chitinous" lamellae, with a sharply pointed rostrum in the outer lamella. These features are common with the upper jaws of Recent cephalopods. Comparison with the upper jaws of ammonoids and Recent cephalopods indicates that the two Cretaceous upper jaws are attributed to the Coleoidea other than the Octopodida. This assignment is also suggested by the cladistic analysis of the Nakagawa specimen compared with five upper jaw characters on 22 Recent cephalopod species. The Obira specimen differs from the Nakagawa specimen in having a much smaller jaw and a larger jaw angle, but its order-level assignment could not be determined because of imperfect preservation. The Nakagawa specimen shares several common features with the upper jaws of Recent Oegopsina; thus we assigned its higher systematic position to this suborder. Based on the extremely large upper jaw (97 mm maximum length), a new genus and species (Yezoteuthis giganteus) is proposed. This new taxon would have been as large as the modern giant squid Architeuthis, which commonly exceeds more than 5 m in body length. Our study postulates that studies of jaws are important to reconstruct the phylogeny of the Coleoidea.

INTRODUCTION

JAWS (= BEAKS OR MANDIBLES) are the primary feeding apparatus of cephalopod mollusks. They consist of articulated upper and lower elements, and the upper jaw encloses the lower jaw. Each jaw is made up of outer and inner lamellae, which are joined anteriorly. In Recent cephalopods, upper and lower jaws and a radula are accommodated in the proximal portion of the digestive system (buccal mass) and their function is to bite and shear food by means of the strong jaw-radular muscular system (Kear, 1994; Tanabe and Fukuda, 1999). Cephalopod jaws are composed mainly of a hard chitinous material (Saunders et al., 1978; Hunt and Nixon, 1981), so that they have a preservation potential as fossils when they have been diagenetically replaced by phosphatic material (Tanabe and Fukuda, 1983; Tanabe et al., 2001). More commonly, calcified elements of jaws of nautilids (rhyncholites and conchorhynchs) and some Mesozoic ammonoids (aptychi) are preserved as fossils. Indeed, fossil remains of cephalopod jaws have been found in Silurian and younger marine deposits (Tanabe and Fukuda, 1999). The taxonomic value of the jaws of Recent cephalopods has been extensively studied by previous workers (Clarke, 1962, 1986; Saunders et al., 1978; Clarke and Maddock, 1988a; Neige and Dommergues, 2002), allowing a generic-level identification on the basis of their morphology.

In fossil cephalopods, the overall shape and architecture of jaws and their taxonomic variation are best known in the Ammonoidea, because they are occasionally found in situ within the body chambers of specimens whose taxonomic identities are known (e.g., Lehmann, 1980, 1990; Nixon, 1988, 1996; Tanabe and Fukuda, 1999; Tanabe and Landman, 2002). In contrast, jaws of the Nautiloidea and Coleoidea are rather rare as fossils.

We recently discovered two well-preserved cephalopod jaws that can be assigned to the Coleoidea, from the Upper Cretaceous marine deposits in Hokkaido, North Japan. Here, we describe morphological features of the two jaws and discuss their taxonomic relationships by comparing them with the jaws of Recent cephalopods and ammonoids.

MATERIALS

Two cephalopod jaws were found individually in calcareous nodules collected as float; one came from the middle course of Kamikinenbetsu Creek, a tributary of the Obirashibe River, Obira Town, northwestern Hokkaido (lat. 44�02'22''N; long. 142�00'18''E, and the other from the small southern tributary branch of the lower course of Wakkawenbetsu Creek, Nakagawa Town, northern Hokkaido (lat. 44�37'51''N; long. 142�03'04''E). The nodule that yielded the Obira specimen contains micro- and macroconchs of the scaphitid ammonite Yezoites puerculus (Jimbo, 1894) and the bivalve Inoceramus hobetsensis Nagao and Matsumoto, 1939. This nodule was presumably derived from the middle Turonian Saku Formation of the Middle Yezo Group, which yields the same molluscan assemblage. The nodule in which the Nakagawa specimen was found is rich in the bivalve Sphenoceramus naumanni (Yokoyama, 1890) and the heteromorph ammonite Polyptychoceras sp. These fossils, as well as the silty mudstone Iithology, suggest that the nodule came from the Campanian Osousyunai Formation of the Upper Yezo Group. The Obira and Nakagawa specimens are respectively housed in the University Museum, University of Tokyo (UMUT), and the Nakagawa Museum of Natural History (NMA).

For comparison, we examined the jaws of the following nine extant species: Sepia officinalis Linnaeus, 1758, S. esculenta Hoyle, 1885 (both Sepiidae, Sepiida), Loligo bleekeri Keferstein, 1866 (Loliginidae, Myopsina, Teuthida), Architeuthis sp. (Archteuthidae, Oegopsina, Teuthida), Todarodes pacificus pacificus (Steenstrup, 1880), Dosidicus gigas d'Orbigny, 1835 (both Ommastrephidae, Oegopsina, Teuthida), Octopus vulgaris Cuvier, 1797, Enteroctopus membranaceus Rochebrune and Mabille, 1889 (both Octopodidae, Incirrina, Octopodida), and Nautilus belauensis Saunders, 1981 (Nautilidae, Nautilida, Nautiloidea). The jaws of these extant species are deposited in the UMUT and the American Museum of Natural History, New York (AMNH).