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The jaw apparatus of the Late Cretaceous heteromorph ammonoid Pravitoceras

Published online by Cambridge University Press:  29 October 2015

Kazushige Tanabe,
Yasuyuki Tsujino,
Kosuke Okuhira and
Akihiro Misaki
Kazushige Tanabe
Affiliation:
Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan, 〈tanabe@um.u-tokyo.ac.jp〉
Yasuyuki Tsujino
Affiliation:
Tokushima Prefectural Museum, Tokushima City, Tokushima 770-8070, Japan, 〈tsujino-yasuyuki-1@mt.tokushima-ec.ed.jp〉
Kosuke Okuhira
Affiliation:
Tokushima Prefectural Museum, Tokushima City, Tokushima 770-8070, Japan, 〈tsujino-yasuyuki-1@mt.tokushima-ec.ed.jp〉
Akihiro Misaki
Affiliation:
Kitakyushu Museum of Natural History and Human History, Kitakyushu City, Fukuoka 805-0071, Japan, 〈misaki_a@kmnh.jp〉
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Abstract

Well-preserved upper and lower jaws of the aptychus-type found inside the body chambers of two specimens of the heteromorph ammonoid Pravitoceras sigmoidale Yabe, 1902 (Nostoceratidae, Ancyloceratina) are described from the Upper Cretaceous Izumi Group in Southwest Japan. They are similar in overall morphology to those of other nostoceratid and diplomoceratid ammonoids currently known, suggesting the morphological stability of the jaw features among these taxa. The equal size of the upper and lower jaws with beak-like rostral projection suggests that the jaw apparatus of this species might function to bite and cut up prey.

Information

Type
Articles
Information
Journal of Paleontology , Volume 89 , Issue 4 , July 2015, pp. 611 - 616
Copyright
Copyright © 2015, The Paleontological Society 

Introduction

The jaw apparatus is a primary feeding organ in all modern and fossil cephalopods. In modern taxa, it consists of upper and lower elements (synonymous with beaks or mandibles) that are joined anteriorly (Tanabe and Fukuda, Reference Tanabe and Fukuda1999). The upper and lower jaws are housed together with a radula in a globular muscular organ called the buccal mass located in the proximal portion of the digestive tract. They are used to bite and shear food with the aid of the surrounding strong jaw-radular muscular system (Clarke, Reference Clarke1962; Nixon, Reference Nixon1988a, Reference Nixon1988b; Tanabe and Fukuda, Reference Tanabe and Fukuda1999). Modern cephalopod jaws are composed mainly of a chitin-protein complex, which in nautilids is associated with an anterior calcareous covering (Saunders et al., Reference Saunders, Spinosa, Teichert and Banks1978; Hunt and Nixon, Reference Hunt and Nixon1981; Gupta et al., Reference Gupta, Briggs, Landman, Tanabe and Summons2008). The chitinous jaw lamellae are preserved as fossils when they have been diagenetically replaced by phosphatic material in calcareous concretions (Tanabe and Fukuda, Reference Tanabe and Fukuda1983; Tanabe et al., Reference Tanabe, Mapes and Kidder2001) or by carbonaceous remnants in shales and limestones (Schweigert, Reference Schweigert1998; Klug et al., Reference Klug, Riegraf and Lehmann2012). Indeed, calcified and chitinous jaw remains of the Ammonoidea have been found in Devonian to Cretaceous marine deposits (Lehmann, Reference Lehmann1980, Reference Lehmann1990; Tanabe and Fukuda, Reference Tanabe and Fukuda1999; Tanabe et al., Reference Tanabe, Kruta and Landman2015). Most occur as isolated specimens, but are rarely preserved in situ within the body chambers of ammonoid shells.

In Cretaceous heteromorph ammonoids of the suborder Ancyloceratina, in situ jaws have been reported from 32 species representing 20 genera that are distributed in seven families within three superfamilies (Meek and Hayden, Reference Meek and Hayden1864; Kennedy and Cobban, Reference Kennedy and Cobban1976; Kanie et al., Reference Kanie, Tanabe, Fukuda, Hirano and Obata1978; Tanabe et al., Reference Tanabe, Hirano and Kanie1980b, 2002, 2015; Landman and Waage, Reference Landman and Waage1993; Tanabe and Landman, Reference Tanabe and Landman2002; Kennedy et al., Reference Kennedy, Landmann, Cobban and Larson2002; Doguzhaeva and Mikhailova, 2002; Lukeneder and Tanabe, Reference Lukeneder and Tanabe2002; Engeser and Keupp, Reference Engeser and Keupp2002; Frerichs, Reference Frerichs2004; Kruta et al., Reference Kruta, Rouget, Landman, Tanabe and Cecca2009, Reference Kruta, Landman, Rouget, Cecca and Larson2010, Reference Kruta, Landman, Rouget, Cecca and Tafforeau2011, Reference Kruta, Landman, Rouget, Cecca and Tafforeau2013; Tanabe, Reference Tanabe2011). These previous studies reveal that the lower jaws of the Cretaceous Ancyloceratina exhibit wide taxonomic variation in their relative size as compared to the upper jaws, overall morphology, and the degree of development of the outer calcitic covering (Tanabe and Landman, Reference Tanabe and Landman2002; Tanabe et al., 2015).

In the search for additional fossil jaw material from the Ancyloceratina, we discovered two specimens of Pravitoceras sigmoidale (Nostoceratidae) from Campanian (Upper Cretaceous) marine deposits in Japan with upper and lower jaws preserved within their body chambers. In this paper, we describe the overall morphology of the jaws of this species and compare it to those of other genera of Late Cretaceous heteromorph ammonoids, through a discussion of its taxonomic and paleoecological implications.

Notes on Pravitoceras sigmoidale

Pravitoceras sigmoidale, the type species of the monospecific genus Pravitoceras, was erected by Yabe (Reference Yabe1902) on the basis of several fragmentary specimens. This species has been known to occur in upper Campanian strata in Japan, namely within the Izumi Group in southwest Japan (Yabe, Reference Yabe1902, Reference Yabe1915; Matsumoto et al., Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981; Bando and Hashimoto, Reference Bando and Hashimoto1984; Morozumi, Reference Morozumi1985; Misaki et al., Reference Misaki, Maeda, Kumagae and Ichida2014) and the Hakobuchi Formation (Yezo Group) in central Hokkaido, northern Japan (Matsunaga et al., Reference Matsunaga, Maeda, Shigeta, Hasegawa, Nomura, Nishimura, Misaki and Tanaka2008). A doubtful specimen has also been documented from the Toyajo Formation (Sotoizumi Group) in southwest Japan (Yabe, Reference Yabe1915). Despite its restricted geographic distribution in Japan, a large number of well-preserved adult specimens of this species have been collected from the Izumi and Yezo groups since the pioneering work by Yabe (Reference Yabe1902, Reference Yabe1915), and they are housed in various museums. Based on these specimens, previous researchers described the peculiar shell features of this species (Fig. 1); namely, the phragmocone consists of helically coiled early whorls and subsequent planispirally coiled evolute whorls. Its relatively long body chamber is S-shaped and backward-twisted (retroversal hook), allowing for a shift in the position of the two rows of tubercles on the outer part of the hook reflecting the twisting of the body (Matsumoto et al., Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981; Morozumi, Reference Morozumi1985; Matsunaga et al., Reference Matsunaga, Maeda, Shigeta, Hasegawa, Nomura, Nishimura, Misaki and Tanaka2008; Misaki et al., Reference Misaki, Maeda, Kumagae and Ichida2014).

Figure 1

Adult specimens of Praviotoceras sigmoidale from the upper Campanian Seidan and Bandodani formations, Izumi Group. Left lateral views. Whitened with magnesium (1) and ammonium chloride (2). (1) KMNH IvP 900,014 from Seidan Formation at Nakano. (2) TKPM.GFI1443 from Bandodani Formation at Komori (same specimen as GLKU. IZ80003 figured by Matsumoto et al. Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981, pl. 26, fig. 1). Arrows point to the base of body chamber. See Figure 2 for these specimens’ localities.

Materials and geological settings

The upper part of the Izumi Group is considered to have been deposited during the Campanian and the Maastrichtian in a narrow and long strike-slip basin that was formed by the left-lateral, strike-slip movement of the Median Tectonic Line. The unit exhibits an upward-deepening sequence from shoreface to slope-basin settings with the latter dominated by turbidites (Tanaka, Reference Tanaka1989; Miyata, Reference Miyata1990).

Two specimens of P. sigmoidale with preserved upper and lower jaws in situ were collected from mudstone units within sequences of alternating sandstone and mudstone beds in the upper Campanian Bandodani Formation (Yamasaki, Reference Yamasaki1986) at Komori, Naruto City, Tokushima (34°10'07"N, 134°33'34"E; the same locality documented by Mastumoto et al., Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981, text-fig. 2), and the Seidan Formation (Morozumi, Reference Morozumi1985) at Nakano, Minami-awaji City, Awaji Island (34°17'52"N, 134°41'18"E; locality N9 of Misaki et al., Reference Misaki, Maeda, Kumagae and Ichida2014, fig. 1) (Fig. 2). Both specimens are partial body chambers that may have been broken into pieces during the recovery process. It is also possible that their phragmocone portions were weathered away prior to recovery. The examined specimens are reposited in the Tokushima Prefectural Museum, Tokushima, Japan (TKPM) and the Kitakyushu Museum of Natural History and Human History, Kitakyushu, Japan (KMNH).

Figure 2

Map of southwest Japan (in part) showing the distribution of the Upper Cretaceous Izumi Group and the localities of the Pravitoceras sigmoidale specimens examined.

Description

Recognition of upper and lower jaws

Because the jaw apparatus is entirely preserved in situ inside the body chambers of the two specimens, recognition of the upper and lower jaws is possible through comparative morphologic examination with the jaw apparatuses of modern cephalopods (Fig. 3). Based on this uniformitarian approach, the slightly larger component, which consists of a slightly concave outer lamella with a prominent radial furrow, is considered to be a lower jaw. It falls into the aptychus-type lower jaw described by Lehmann (Reference Lehmann1990). The smaller element, which is comprised of posteriorly elongated, paired lamellae, is identified as an upper jaw because it is partly enveloped by the concave outer lamella of the larger form, i.e., the lower jaw (see Fig. 3.2–3.4, 3.8). In the Nakano specimen (TKPM. GFI5990), both the upper and lower jaws appear to retain their original orientation within a buccal mass (Fig. 3.1–3.4). In the Komori specimen (TKPM. GFI5989), the upper jaw occurs in the posterior portion of the lower jaw, facing its rostral portion to the inner side of the lower jaw, suggesting that the upper jaw has been slightly shifted from its original position during biostratinomic processes prior to final burial.

Figure 3

Praviotoceras sigmoidale with preserved, in situ upper and lower jaws within the body chamber from the upper Campanian Seidan and Bandodani formations, Izumi Group. (1–4) mode of occurrence of jaw apparatus within body chamber near aperture (1), and frontal (2), right lateral (3), and ventral (4) views of the jaw apparatus, TKPM. GFI5990 from Seidan Formation at Nakano. (5–8) ventral (5) and left lateral (6) view of lower jaw, outer calcitic layer covering the underlying chitnous layer of lower jaw (7), and mode of jaw apparatus occurrence inside body chamber (8), TKPM. GFI5989 from Bandodani Formation at Komori. Upper sides of (5, 6, 8) indicate adoral sides. Abbreviations. s: radial furrow (symphysis), ar: adharmonic ridge.

Paleoecology and biostratinomy

A large number of adult specimens of this species have been collected from the same localities as those of the two specimens described herein (Matsumoto et al., Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981, pl. 26, fig. 1; Misaki et al., Reference Misaki, Maeda, Kumagae and Ichida2014, figs. 1, 9, 10, table 1; and unpublished data, Kosuke Okuhira, 2015). Many of them were preserved parallel to the bedding planes and retain a complete body chamber (Fig. 1). Occasionally, their phragmocones and body chambers are encrusted by sessile anomiid bivalves (Fig. 3.1; Matsumoto et al., Reference Matsumoto, Morozumi, Bando, Hashimoto and Matsuoka1981; Misaki et al., Reference Misaki, Maeda, Kumagae and Ichida2014). Co-occurrence of upper and lower jaws within the body chambers of the two specimens examined (Fig. 3) strongly suggests that carcasses of the two specimens rapidly sank to the sea floor after death and were buried there without the detachment of their body tissue from the shells’ body chamber.

Terminology and measurements

Basic morphology and measurements of the reconstructed jaw apparatus of P. sigmoidale are shown in Figure 4, in which the descriptive terminology follows that of Clarke (Reference Clarke1962, Reference Clarke1986) developed for modern coleoids. Morphological terms for the aptychus-type lower jaw proposed by Arkell (Reference Arkell1957) are also given in parentheses.

Figure 4

Basic morphology, terms, and measurements of jaw apparatus of Pravitoceras sigmoidale. (1, 2) upper jaw, right lateral (1) and frontal (2) views. (3, 4) Lower jaw, right lateral (3) and dorsal (4) views. Abbreviations. LLU: maximum length of lateral wall of upper jaw, LHU: length of hood of upper jaw, WLU: maximum width of paired lateral walls of upper jaw, LHL: maximum length of hood (=length of symphysis) of lower jaw, LWL: maximum length of wing of lower jaw, WWL: maximum width of paired wings of lower jaw. Descriptive terms follow those of Clarke (Reference Clarke1962, Reference Clarke1986) based on modern coleoids.

Specimen TKPM. GFI5989

This specimen represents a partial hook portion of the body chamber, measuring 73.6 mm in the preserved whorl length and 36.6 mm in larger whorl diameter (Fig. 3.5, 3.6, and 3.8; note that a part of the body chamber was removed to expose the jaw apparatus). The upper jaw is composed of an undetermined, dark-colored material that is presumed to have been originally chitinous. It consists of a shorter hood portion and a pair of larger, wing-like lateral walls, with the latter displaying a maximum length and height of 26.4 mm and 8.1 mm, respectively; however, the left lateral wall is still mostly embedded in the matrix (Fig. 3.8). The short hood portion is joined with the paired lateral walls, so that the upper jaw is composed of a single lamella. The paired lateral walls and hood become narrower anteriorly and join together to form the sharply pointed rostrum situated at the anterior end. They are ornamented with a combination of evenly spaced, fine concentric ribs interpreted as growth lines, and less common, weak radial striations that extend from the rostal tip to the posterior margin and intersect concentric ribs.

The lower jaw occurs on the ventral side of the upper jaw, and the outer chitinous lamella, 23.4 mm wide and 31.5 mm long, is wholly preserved in this specimen (Fig. 3.5, 3.6). It consists of a dark-colored material as in the upper jaw. A short inner lamella, which has been commonly recognized in the lower jaws of other Mesozoic ammonoids (e.g., Lehmann, Reference Lehmann1975, Reference Lehmann1980; Tanabe and Landman Reference Tanabe and Landman2002), may be present on the dorsal side of the outer lamella, but it could not be observed from the outside. The outer lamella is divided into two wing portions by a prominent radial furrow (symphysis) that is 30.1 mm long (Fig. 3.5). The lateral sides of the furrow are distinctly elevated ventrally to form sharp ridges (adharmonic ridges). The paired wing portions are ornamented with evenly spaced concentric ribs that are interpreted as growth lines and with a total of eight radial ridges on each side that extend from the rostral end (apex) to the outer posterior margin and intersect the concentric ribs (Fig. 3.6). The outer chitinous lamella was originally covered by a 0.5 mm thick, paired calcitic layers (aptychus sensu stricto), though it was exfoliated from the specimen during fossil preparation and is now only visible on the fractured surface of the external cast (Fig. 3.7).

Specimen TKPM. GFI5990

This specimen represents a fragmentary body chamber from near the aperture, measuring 51.0 mm in preserved whorl length and 44.2 mm in larger whorl diameter at the aperture (Fig. 3.1). Both upper and lower jaws occur as steinkerns; hence, the lower jaw’s calcitic covering is not preserved in this specimen (Fig. 3.2–3.4). The upper jaw consists of a shorter hood portion and a pair of larger, wing-like lateral walls, both of which become narrower anteriorly and join together to form the rostrum, whose anterior end was eroded away before recovery (Fig. 3.2). The paired lateral walls, 24.5 mm and 17.7 mm in maximum length and width, respectively, are ornamented with a combination of evenly spaced concentric ribs and less common, weak radial striations. The hood is inverted and triangular-shaped in frontal view (Fig. 3.2) and is ornamented with equal-spaced, fine concentric striae and rarer, weak radial striations.

The lower jaw is incompletely preserved and is represented by the anterior half of the widely open outer lamella, 23.5 mm wide (Fig. 3.4). This lamella is distinctly divided into two wing portions by a prominent radial furrow (symphysis), and is ornamented by a combination of evenly spaced concentric ribs and fewer weak radial striations that extend from the apex to the posterior margin.

Discussion

Comparison with the jaws of other Late Cretaceous Ancyloceratina

The jaw apparatus of Pravitoceras sigmoidale is characterized by a posteriorly elongated outer lamella with a weakly pointed rostrum and a thin calcareous covering in the lower jaw, and the relatively large upper jaw whose longer paired lateral walls and shorter hood portion are joined together in the anterior portion. These features are shared with the aptychus-type jaw apparatuses of other Late Cretaceous Ancyloceratina belonging to the family Scaphitidae of the superfamily Scaphitoidea (Hoploscaphites: Meek and Hayden, Reference Meek and Hayden1864; Kennedy and Cobban, Reference Kennedy and Cobban1976; Landman and Waage, Reference Landman and Waage1993; Rhaeboceras: Kennedy et al., Reference Kennedy, Landmann, Cobban and Larson2002; Kruta et al., Reference Kruta, Landman, Rouget, Cecca and Tafforeau2013) and to the family Diplomoceratidae (Scalarites: Kanie et al., Reference Kanie, Tanabe, Fukuda, Hirano and Obata1978;Tanabe et al., Reference Tanabe, Hirano and Kanie1980b; Polyptychoceras: Tanabe and Landman, Reference Tanabe and Landman2002; Tanabe Reference Tanabe2011; Subptychoceras: Tanabe and Landman, Reference Tanabe and Landman2002; Tanabe et al., 2015) and the family Nostoceratidae (Didymoceras: Kruta et al., Reference Kruta, Landman, Rouget, Cecca and Larson2010) of the superfamily Turrilitoidea. The bivalved calcitic plates (aptychi) on the lower jaws of these taxa are ornamented with evenly spaced concentric ribs similar to those on the underlying bivalved chitinous lamella; they can be classified as Striaptychi according to the morphotypic classification of aptychi by Trauth (Reference Trauth1927). Therefore, the above-mentioned jaw features can be used as diagnostic taxonomic characteristics shared by the Scaphitidae, Diplomoceratidae, and Nostoceratidae.

The aptychus-type jaw apparatus is also known in the baculitids Sciponoceras and Baculites of the Turrilitoidea (Tanabe and Landman, Reference Tanabe and Landman2002; Landman et al., Reference Landman, Larson and Cobban2007; Kruta et al., Reference Kruta, Rouget, Landman, Tanabe and Cecca2009, Reference Kruta, Landman, Rouget, Cecca and Tafforeau2011), but these are distinguished from those of P. sigmoidale and other nostoceratids, diplomoceratids, and scaphitids by having a more anteroposteriorly elongated, bivalved calcitic covering (aptychus) without a rostral projection in the lower jaw, and an extremely reduced upper jaw compared to the lower jaw (see Kruta et al., Reference Kruta, Landman, Rouget, Cecca and Tafforeau2011, fig. 1C). The outer surface of the aptychus, 0.6–1 mm thick in Baculites is coarsely ornamented by strong, sharp ribs that are arranged with angulated bend; hence, the aptychi of the baculitids were classified as Rugaptychus by Trauth (Reference Trauth1927) and Arkell (Reference Arkell1957). The highly specialized jaw apparatus of the Baculitidae appears to have been developed independently in the Turrilitoidea.

Feeding habits of P. sigmoidale inferred from jaw apparatus

Because the jaw apparatus is a primary feeding organ of modern and extinct cephalopods, its morphological and structural features appear to reflect the feeding and dietary habits of living animals. Based on this hypothesis, previous authors compared the functional morphology of the jaw apparatuses found in the Ammonoidea to those of modern cephalopods whose feeding and dietary habits are known (Lehmann, Reference Lehmann1971, Reference Lehmann1975, Reference Lehmann1980, Reference Lehmann1988; Tanabe et al., Reference Tanabe, Fukuda, Kanie and Lehmann1980a, Reference Tanabe, Misaki, Landman and Kato2013; Seilacher, Reference Seilacher1993). Additionally, previous studies focused on the analysis of organismic remains preserved in the body chambers of ammonoids, under the assumption that they represent the diet preserved in the crop and/or stomach (see Jäger and Fraaye, Reference Jäger and Fraaye1997, for data on the gut contents in Triassic and Jurassic ammonoids) and in a buccal cavity (Kruta et al., Reference Kruta, Landman, Rouget, Cecca and Tafforeau2011).

Among the Mesozoic Ammonoidea, the heteromorph ammonoids of the suborder Ancyloceratina display remarkable taxonomic and morphological diversity in various marine habitats during the Late Jurassic through the Cretaceous (Wiedmann, Reference Wiedmann1969). This fact, as well as the wide variety of jaw morphologies may suggest a spectrum of different life and feeding habits among the Ancyloceratina. Based on various paleoecological and functional morphologic evidence, Westermann (Reference Westermann1996) proposed that many Cretaceous heteromorph ammonoids were vertical migrants in the water column, some of which fed on small mesopelagic organisms including sluggish juvenile ammonoids, while others caught zooplankton in tentacles modified into umbrella-nets. Consistent with this hypothesis, Kruta et al. (Reference Kruta, Landman, Rouget, Cecca and Tafforeau2011), based on synchrotron X-ray microtomography, suggested that Baculites fed on zooplankton in the water column based on the discovery of isopods and larval shells of gastropods in the buccal cavity of the body chamber surrounded by a reduced upper jaw and a large lower jaw examined. In view of the shovel-like shape without a rostral tip, the lower jaw of the baculitids was presumably not used for biting and cutting up prey, and instead helped to convey prey into the buccal cavity with the aid of the small upper jaw. On the other hand, the almost equal-sized upper and lower jaws with beak-like rostral projection in the jaw apparatus of P. sigmoidale potentially function to bite and cut up prey, as suggested for the diplomoceratids (Tanabe et al., Reference Tanabe, Hirano and Kanie1980b; Tanabe and Landman, Reference Tanabe and Landman2002) and the nostoceratid Didymoceras (Kruta et al., Reference Kruta, Landman, Rouget, Cecca and Larson2010). An operculum-like secondary function of the aptychus-type lower jaw hypothesized by Lehmann and Kulicki (Reference Lehmann and Kulicki1990) and Seilacher (Reference Seilacher1993) can also be considered in this species in view of the presence of a distinct radial furrow on the chitinous outer lamella whose inner surface might be connected to the jaw muscle via chitin-secreting cells (Tanabe et al., 2015).

Although we could not obtain direct evidence to support the above interpretation, future analysis of organismic remains preserved as crop and/or stomach contents in well-preserved specimens including those used in this study by means of new techniques such as computed tomography and X-ray synchrotron analysis may elucidate the feeding and dietary habits of this species.

Acknowledgments

We thank I. Bando, T. Nishikawa, and H. Shiraishi (all in Tokushima Prefecture, Japan) for providing valuable information of the fossil localities. Special thanks are due to C. Klug (Universität Zülich), N. H. Landman (American Museum of Natural History), and P. J. Harries (University of South Florida) for critical comments and suggestions to improve this manuscript. This work was supported in part by the JSPS research grant (no. 26400498 to K.T.), (no. 24700942 to Y.T.), and (no. 25800290 to A.M.), and the Sasagawa Scientific Research Grant of the Japan Science Society (to A.M.).

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Figure 0

Figure 1Adult specimens of Praviotoceras sigmoidale from the upper Campanian Seidan and Bandodani formations, Izumi Group. Left lateral views. Whitened with magnesium (1) and ammonium chloride (2). (1) KMNH IvP 900,014 from Seidan Formation at Nakano. (2) TKPM.GFI1443 from Bandodani Formation at Komori (same specimen as GLKU. IZ80003 figured by Matsumoto et al. 1981, pl. 26, fig. 1). Arrows point to the base of body chamber. See Figure 2 for these specimens’ localities.

Figure 1

Figure 2Map of southwest Japan (in part) showing the distribution of the Upper Cretaceous Izumi Group and the localities of the Pravitoceras sigmoidale specimens examined.

Figure 2

Figure 3Praviotoceras sigmoidale with preserved, in situ upper and lower jaws within the body chamber from the upper Campanian Seidan and Bandodani formations, Izumi Group. (1–4) mode of occurrence of jaw apparatus within body chamber near aperture (1), and frontal (2), right lateral (3), and ventral (4) views of the jaw apparatus, TKPM. GFI5990 from Seidan Formation at Nakano. (5–8) ventral (5) and left lateral (6) view of lower jaw, outer calcitic layer covering the underlying chitnous layer of lower jaw (7), and mode of jaw apparatus occurrence inside body chamber (8), TKPM. GFI5989 from Bandodani Formation at Komori. Upper sides of (5, 6, 8) indicate adoral sides. Abbreviations. s: radial furrow (symphysis), ar: adharmonic ridge.

Figure 3

Figure 4Basic morphology, terms, and measurements of jaw apparatus of Pravitoceras sigmoidale. (1, 2) upper jaw, right lateral (1) and frontal (2) views. (3, 4) Lower jaw, right lateral (3) and dorsal (4) views. Abbreviations. LLU: maximum length of lateral wall of upper jaw, LHU: length of hood of upper jaw, WLU: maximum width of paired lateral walls of upper jaw, LHL: maximum length of hood (=length of symphysis) of lower jaw, LWL: maximum length of wing of lower jaw, WWL: maximum width of paired wings of lower jaw. Descriptive terms follow those of Clarke (1962, 1986) based on modern coleoids.