RESEARCH ARTICLE
Lower jaws of two Late Cretaceous pachydiscid ammonites from Hokkaido, Japan with discussion on taxonomic and paleoecological implications of pachydiscid jaw apparatus
2025 年 29 巻 p. 300-315
詳細
2025 年 29 巻 p. 300-315
The lower jaws of two pachydiscid ammonites, Menuites japonicus (Matsumoto) and Eupachydiscus sp., are described based on two specimens from the Coniacian to Santonian (Upper Cretaceous) strata of the Haboro and Mikasa areas in Hokkaido, Japan. They are preserved in situ in the body chamber and exhibit features intermediate between the lower jaws of anaptychus and aptychus types, specifically, a broadly open outer lamella composed of inner “chitinous” and outer calcareous layers, a flat rostrum, and a median furrow along the hood ridge of the inner layer. These characteristics have also been documented in the lower jaws of other previously described pachydiscid species, suggesting they are diagnostic of the family. The unusually large, shovel-shaped lower jaws with flat rostral margins may have evolved in relation to a microphagous feeding strategy. Additionally, the ventral outline of the lower jaw roughly matches the cross-sectional outline of shell aperture, although the former is smaller than the latter. As previously supposed in some Jurassic ammonoids, Late Cretaceous pachydiscids were presumably able to withdraw their soft parts deep into the body chamber, thereby avoiding being eaten by predators. Under such circumstances, their widely open lower jaws could have an operculum-like role to protect the retracted soft body from predator’s attack.
As in both modern and other extinct cephalopod mollusks, ammonoids possessed a well-developed jaw apparatus consisting of upper and lower elements (synonymous with beaks or mandibles) and a radula as the primary feeding organ. This has been confirmed by occasional in situ occurrences of horny and calcareous jaw remains, along with a radula, within the body chambers of some late Paleozoic and Mesozoic ammonoid conchs whose taxonomic relationships are known (Kruta et al., 2015; Nixon, 2015; Tanabe et al., 2015a for a recent review). These exceptionally preserved specimens have allowed researchers to identify upper and lower jaws of the corresponding taxa by comparing them with those of modern cephalopods (Saunders et al., 1978; Tanabe and Fukuda, 1999; Tanabe, 2012; Kruta et al., 2015; Nixon, 2015). Specifically, the larger form, composed of a widely open, concave outer lamella and a shortly reduced inner lamella on the ventral side, is identified as the lower jaw, whereas the smaller form featuring paired wide inner lamellae and a short reduced outer one on the dorsal side is interpreted as the upper jaw (Tanabe and Landman, 2002, text-figs. 2, 3; Tanabe et al., 2015a, fig. 10.4a–d). When only a single jaw element is preserved inside the body chamber, its identification as either upper or lower jaw is based on a morphological comparison with previously co-occurring upper and lower jaws.
In Cretaceous ammonoids, jaw apparatuses have been described in 62 species across 42 genera, which are distributed among the suborders Phylloceratina, Lytoceratina, Ammonitina and Ancyloceratina (see Tanabe et al., 2015a, table 10.1 for the list of species and their sources described by 2014; Tanabe et al., 2015b, 2021; Tanabe and Shigeta, 2019; Košťák et al., 2024). Previous X-ray diffraction and Raman spectral analyses demonstrated that the dark-colored horny lamellae of ammonite jaws from the Upper Cretaceous Yezo Group in Hokkaido are composed of phosphate minerals such as apatite and wilkeite (Kanie, 1982; Tanabe and Fukuda, 1983; Tanabe et al., 2019). Tanabe et al. (2015a) suggested that the upper and lower jaws of Mesozoic ammonoids were primarily composed of a chitin-protein complex as in those of modern nautilid and coleoid cephalopods (Saunders et al., 1978; Hunt and Nixon, 1981) and that they were replaced by phosphate minerals during fossil diagenesis. We hereafter refer to the black phosphate lamellae of the jaws of Cretaceous ammonoids as “chitinous” lamellae.
The upper jaws of the Cretaceous ammonoids generally share similar overall morphology and structure, consisting of outer and inner “chitinous” lamellae that unite in the anterior portion to form a sharply pointed rostral tip (Tanabe and Landman, 2002, text-fig. 2; Tanabe et al., 2015a, fig. 10.4a–d). Both lamellae are composed of a black “chitinous” substance, with a calcified tip observed in the two Late Cretaceous phylloceratids Hypophylloceras subramosum and Phyllopacyceras ezoense (Tanabe et al., 2013, figs. 2, 4) and in the Early Cretaceous (Aptian) haploceratoid ammonite Aconeceras trautscholdi (Doguzhaeva and Mutvei, 1992, pl. 5).
The lower jaws of Cretaceous ammonoids are typically composed of a widely open outer lamella and a shortly reduced inner lamella, which are united in the anterior portion. These lower jaws exhibit notable taxonomic variation in overall shape and structure, as well as in the presence or absence of a calcareous covering on the outer “chitinous” lamella. Based on these features, they can be classified into three morphotypes: (1) aptychus type, featuring thick bivalved calcitic plates on the “chitinous” lamella of the lower jaw, termed aptychi by Meyer (1829); (2) rhynchaptychus type, with a calcified rostral tip on both upper and lower jaws; and (3) intermediate type, exhibiting characteristics between anaptychus-type lower jaws, composed of a single-valved “chitinous” lamella, termed anaptychus by Oppel (1856), and the aptychus type lower jaws (Lehmann et al., 1980; Lehmann, 1990; Tanabe et al., 2015a). However, taxonomic variation in the shape and lamellar structure of the lower jaws within individual families of Cretaceous ammonoids remains largely unexplored, except for the Scaphitidae (e.g. Landman and Waage, 1993; Tanabe and Landman, 2002; Tanabe et al., 2015a; Machalski, 2021).
We previously described in situ jaw apparatuses in four species of the family Pachydiscidae –– namely, Menuites naumanni (Yokoyama, 1890), Menuites soyaensis (Matsumoto and Miyauchi, 1984), Menuites sp., and Pachydiscus kamishakensis Jones, 1963 –– from the Cretaceous fore-arc basin deposits called the Yezo Group in Hokkaido and South Sakhalin and the Matanusuka Formation in southern Alaska (Tanabe and Landman, 2002; Tanabe et al., 2012, 2015a; Tanabe and Shigeta, 2019). While searching for additional jaw material, one of the authors (AM) collected two specimens identified as Menuites japonicus (Matsumoto, 1955) and Eupachydiscus sp. from the Yezo Group in Hokkaido, each of which preserves a lower jaw in situ in the body chamber. In this study, we describe the lower jaws of these two species and compare their overall shape and lamellar structure with those of the four previously described pachydiscid species. We further discuss the morphological characteristics of the jaw apparatuses in this family from taxonomic and paleoecological perspectives.
Institutional abbreviations.—GK, Kyushu University Museum, Fukuoka, Japan; KMNH, Kitakyushu Museum of Natural History and Human History, Kitakyushu, Fukuoka, Japan; NMNS, National Museum of Nature and Science, Tsukuba, Ibaraki, Japan; UH, Hokkaido University Museum, Sapporo, Hokkaido, Japan; UMUT, University Museum, The University of Tokyo, Tokyo, Japan.
Of the two specimens examined, the Menuites japonicus (Matsumoto, 1955) specimen (KMNH IvP 902013) was recovered from a float calcareous nodule at a locality along the Nakafutamata River in the Haboro area, northwestern Hokkaido (lat. 44°18′5″N, long. 141°57′58″E; Figure 1A). The nodule, composed of silty mudstone, contained many adult shells of the heteromorph ammonite Polyptychoceras sp., together with KMNH IvP 902013. It was presumably derived from a silty mudstone outcrop of the Member Uf in the uppermost part of the lower Haborogawa Formation, exposed approximately 1.5 km upstream of the sampling point (see Okamoto et al., 2003, figs. 2 and 6). The Member Uf is biostratigraphically assigned to the Inoceramus (I.) amakusensis Zone of the Santonian Stage (Toshimitsu et al., 2007, fig. 2; Okamoto et al., 2003, fig. 9).
The Eupachydiscus sp. specimen (KMNH IvP 902014) was found in a float calcareous marly nodule that co-occurred with shells of the tetragonitid ammonite Tetragonites glabrus. This nodule was collected at a locality in Kumaoizawa Creek, eastern part of the Mikasa area (Ikushunbetsu district), central Hokkaido (lat. 43°15′6″N, long. 142°3′26″E; Figure 1B). The Upper Cretaceous Kashima Formation of the Yezo Group is exposed around Katsurazawa Lake and the surrounding creeks in this area (Futakami et al., 2008, fig. 1). The formation exposed along Kumaoizawa Creek is over 160 m thick and consists of bioturbated silty mudstone interbedded with fine-grained sandstone and acidic tuffaceous sandstone (Tanaka et al., 2017, fig. 2C), suggesting an outer shelf to continental slope depositional environment (Takashima et al., 2004; Futakami et al., 2008). The Kashima Formation exposed in Kumaoizawa Creek is rich in mollusk fossils throughout the sequence, including inoceramid bivalves and ammonoids. Tanaka et al. (2017, fig. 2C) reported the Coniacian index inoceramids Platyceramus mantelli and Inoceramus uwajimensis and various ammonoids such as Damesites damesi, Eupachydiscus sp., T. glabrus, Gaudryceras tenuiliratum, Neophylloceras sp., and Polyptychoceras pseudogaultinum from the Kashima Formation exposed in Kumaoizawa Creek. The float calcareous nodule that yielded KMNH IvP 902014 and T. glabrus shells is presumed to have originated from the Kashima Formation near the sampling point. Eupachydiscus is a long-ranging genus, spanning the Coniacian to Campanian stages (Wright et al., 1996), and T. glabrus occurs abundantly from the middle Turonian to the upper Campanian in the Yezo Group of Hokkaido and Sakhalin (Shigeta, 1989). Due to the absence of diagnostic Campanian index ammonoids and inoceramids in the Kumaoizawa Creek section, the age of KMNH IvP 902014 is provisionally regarded as the Coniacian to Santonian.
The shells of Menuites japonicus (KMNH IvP 902013) and Eupachydiscus sp. (KMNH IvP 902014), both preserving a lower jaw in situ within the body chamber, exhibit a dark brown to gray coloration and lack pearlescent luster. This suggests that the original aragonitic shell material has been altered to calcite during fossil diagenesis.
Terminology and measurements.—Descriptive terms and measurements of the lower jaws of the two pachydiscid specimens examined herein follow the terminology for modern coleoid and nautilid cephalopods proposed by Clarke (1962, 1986) and Clarke and Maddock (1988) (Figure 2). These standards are based on comparative morphological analysis of the lower jaw structures in both ammonoids and extant cephalopods (Tanabe et al., 2015a, fig. 10.4). The following abbreviations are used for measurements: D = maximum shell diameter; H = maximum whorl height; B = maximum whorl breadth; MW = maximum length of wing; WW = width of paired wings; HH = height of hood; OAW = open angle of paired wings. Measurement data for the conch and lower jaw elements of the two pachydiscid specimens along with data from four previously described pachydiscid specimens are provided in Table 1.
| Species | Specimen | D (mm) | H (mm) | B (mm) | B/H | MW (mm) | WW (mm) | HH (mm) | OAW (deg.) | WW/MW | Sources |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Menuites sp. | NMNS PM 14321 | 152.4 | 71.5* | 86.9* | 1.22 | 57.6 | 68.0 | 57.0 | ca. 120 | 1.18 | Tanabe and Shigeta (2019) |
| Menuites soyaensis (Matsumoto and Miyauchi, 1984) | NMNS PM 14316 | 196.0 | 107.0 | > 71.0* | > 0.66 | 79.3 | 91.4 | 72.8 | ca. 140 | 1.15 | Tanabe and Shigeta (2019) |
| Menuites naumanni (Yokoyama, 1890) | UMUT MM 27835 | 131.2 | 61.6 | 103.2 | 1.68 | 49.0 | 77.6 | 46.0 | ca. 110 | 1.58 | Tanabe and Landman (2002); Tanabe et al. (2015a) |
| Menuites japonicus Matsumoto, 1955 | KMNH IvP 902013 | 77.1 | 32.8 | 40.0* | ca.1.22 | ca. 20.1 | 18.8 | ca. 18.5 | ca. 100 | ca. 0.94 | This study |
| Pachydiscus kamishakensis Jones, 1963 | UMUT MM 30876 | 305.0 | 175.7 | 93.8 | 0.53 | 102.9 | 58.8 | 111.2 | ca. 60 | 0.57 | Tanabe et al. (2012, 2015a) |
| Eupachydiscus sp. | KMNH IvP 902014 | 191.6 | 82.6 | ca. 75.0 | ca. 0.91 | ca. 43.8 | ca. 51.0 | > 30.3 | ca. 125 | ca. 1.16 | This study |
This specimen measures 77.1 mm in maximum shell diameter, though it is slightly dorsoventrally deformed (Figure 3A, B). The body chamber, approximately two thirds of a whorl in spiral length, is ornamented with broad, low, and distinct radial ribs. Each rib exhibits a weaker inner lateral tubercle and a stronger outer lateral spinose tubercle. This bituberculate ornamentation is diagnostic of adult shells of this species, as also seen in the holotype (MM 05626 = GT. I.-34629) (Matsumoto, 1955, figs. 4, 5, pl. 31, figs. 1, 2), which was collected from the lower Campanian Osoushinai Formation in the middle course of the Abeshinai River near the junction with Tannosawa Creek, Nakagawa area, northern Hokkaido (loc. T277c of Matsumoto, 1942a, pl. 12) (Takahashi et al., 2007, fig. 14).
The lower jaw was preserved within the body chamber, approximately one-quarter of a whorl from its base (Figure 3A, B). The upper jaw is not visible in this specimen. The mode of preservation suggests that the lower jaw rotated approximately 90° clockwise after detaching from the buccal mass during carcass decay. The lower jaw appears to consist of a widely open outer lamella and a reduced, short inner lamella, though the latter is obscured by host rock and is not externally visible. The outer lamella is convex anteroventrally and has an open angle of approximately 100° (Figure 3D). It comprises an inner black “chitinous” (i.e., horny) layer and an outer calcareous layer (‘ihl’ and ‘ocl’ in Figure 3C–E). The inner “chitinous” layer is clearly divided into paired wing portions by a median furrow (‘mf’ in Figure 3C, D), which begins at the rostral end and extends posteriorly along the hood ridge, although its posterior end is covered by the outer calcareous layer (Figure 3C). The anterior margin (shoulder) of the paired wings is weakly arched posteriorly, and the rostrum is nearly flat without a pointed tip (‘r’ in Figure 3C, D). The inner “chitinous” layer is approximately 0.5 mm thick and displays equally spaced, faint concentric growth lines. The outer calcareous layer, about 0.5 mm thick, is visible on the right lateral and posterior portions (Figure 3C, D), though it was secondarily exfoliated on the other portions during fossil preparation, suggesting it originally covered the entire inner “chitinous” layer. In contrast to the inner layer, the preserved portion of the outer calcareous layer lacks a median furrow (Figure 3C). It consists of columnar prisms of varying sizes with rounded outer surfaces, arranged in concentric rows. Each prism, measuring 0.1 to 0.4 mm in longer diameter, is enclosed by a dark intercrystalline organic matrix (Figure 3E).
Eupachydiscus sp. (KMNH IvP 902014)This specimen measures 191.6 mm in maximum shell diameter (D) and preserves a nearly complete body chamber, two-thirds of a whorl in spiral length (Figure 4). It exhibits diagnostic features of the genus, including an inflated and depressed whorl section and coarse, well-defined ribs associated with prominent umbilical tubercles in the mature stage. The preserved last one and a half whorls closely resemble shells of E. haradai (Jimbo, 1894) in the middle to late growth stages, as illustrated by Jimbo (1894, pl. 2 [18], fig. 2) and Matsumoto (1954, pl., 8, fig. 2a, b; pl. 9, figs. 1, 2; pl. 10, fig. 1). However, this specimen displays stronger major ribs and tubercles at the umbilical shoulder. It also shares similarities with the outer whorls of the holotype of E. keramasatoshii Matsumoto, 1990, though it features weaker major ribs. The early growth stages of this specimen remain unknown due to the absence of inner whorls. In this study, it is referred to as Eupachydiscus sp.
The lower jaw is preserved in the mid-ventral portion of the body chamber, about half a whorl from its base, and is oriented facing the anterior side of the shell aperture. The preservation indicates that the lower jaw rotated approximately 30° to the left before being buried in the unconsolidated sediment of the body chamber. The upper jaw is not visible in this specimen. The lower jaw appears to consist of a widely open outer lamella and a reduced inner lamella, the latter of which is obscured by host rock and not visible externally. The preserved outer lamella is incomplete, missing the posterior portion. It is gently convex anteroventrally, with an open angle of approximately 125°, and is composed of inner black “chitinous” and outer calcareous layers (‘ihl’ and ‘ocl’ in Figure 5A, B). The inner “chitinous” layer, about 51 mm in width, is distinctly divided into paired wing portions by a median furrow (‘mf’ in Figure 5A, B), which runs along the hood ridge. The furrow is widest anteriorly (approximately 2 mm wide) and becomes progressively narrower and less distinct posteriorly (Figure 5A). Due to the missing posterior margin, it remains unclear whether the median furrow originally extended to the rear of the inner layer. The paired wings are slightly elongated laterally, with a shoulder that is weakly arched posteriorly. The rostrum is flat and lacks a sharply pointed tip (‘r’ in Figure 5A, B). The outer surface of the inner “chitinous” layer appears smooth and lacks ornamentation. The outer calcareous layer, about 0.8 mm thick, is visible on the right lateral and left anterior sides of the inner layer (Figure 5A–C), though exfoliated elsewhere during fossil preparation, suggesting that it originally covered the entire surface of the inner layer. The outer calcareous layer is ornamented with concentric rows of evenly spaced, tile-like plates, each of which bears many tiny tubercles on it (Figure 5D). In the anterior region, these tile-like plate rows are sharply interrupted by the broad furrow along the hood ridge (Figure 5A–C). The plates range from oval to rectangular with rounded corners and are generally larger in the lateral and posterior regions (Figure 5C). In the mid-ventral portion of the preserved outer layer, they measure 200 μm to 500 μm in length (Figure 5D).
The lower jaws of Menuites japonicus and Eupachydiscus sp. described herein share a broadly open outer lamella composed of an inner “chitinous” layer and an outer calcareous layer with a nearly flat rostrum, and a median furrow along the hood ridge of the inner layer. However, whereas the outer calcareous layer in M. japonicus appears to lack a median furrow (Figure 3C), the corresponding layer in Eupachydiscus sp. is clearly divided by a median furrow in its anterior portion (Figure 5A–C).
The presence of a widely open and gently convex outer lamella with a nearly straight rostrum, along with a median furrow on the inner “chitinous” layer, aligns the lower jaws of M. japonicus and Eupachydiscus sp. with those of four previously described pachydiscid species: Menuites soyaensis (Matsumoto and Miyauchi, 1984) (Figure 6A, B; see also Tanabe and Shigeta, 2019, fig. 3A–C), Menuites naumanni (Yokoyama, 1890) (Figure 6E, F; see also Tanabe and Landman, 2002, text-fig. 1; Tanabe et al., 2015a, fig. 10.5f; Tanabe and Shigeta, 2019, fig. 5C, D), Menuites sp. (Tanabe and Shigeta, 2019, figs. 4A–E), and Pachydiscus kamishakensis Jones, 1963 (Figure 6C, D; see also Tanabe et al., 2012, figs. 4E, F, 6A–C; Tanabe and Shigeta, 2019, fig. 5A, B). These shared features suggest that such lower jaw morphology is diagnostic of the family Pachydiscidae. However, the degree of development of the median furrow varies among species. In P. kamishakensis and M. naumanni, the furrow is limited to the anterior portion (Figure 6C–F; see also Tanabe and Shigeta, 2019, figs. 3, 5), whereas it extends from the rostral end to near the ventral margin in M. soyaensis (Figure 6A, B; see also Tanabe and Shigeta, 2019, fig. 3A, C), M. japonicus (Figure 3C, D) and Eupachydiscus sp. (Figure 5A–C).
As with M. japonicus (KMNH IvP 902013) and Eupachydiscus sp. (KMNH IvP 902014), the thin outer calcareous layer of P. kamishakensis (Tanabe et al., 2012, fig. 4E, F) and Menuites sp. (Tanabe and Shigeta, 2019, fig. 4C–E) is composed of columnar prisms of varying sizes with rounded mounds arranged in concentric rows (Tanabe et al., 2019, fig. 7B). In previously described pachydiscid specimens, the inner lamella is reduced and usually obscured by host rock, preventing external observation. However, Tanabe et al. (2019, fig. 5) identified a coalesced attachment line of the inner lamella on the cast shoulder portions of the exceptionally large lower jaw specimen KMNH IvP 902012 (MW = 125 mm), which was recovered from the Maastrichtian Shimonada Formation of the Izumi Group, Awaji Island, western Japan. Due to the similarity to the lower jaw of the early Maastrichitian P. kamishakensis from southern Alaska (Figure 6C, D; see also Tanabe et al., 2012, fig. 6; Tanabe et al., 2019, fig. 7) in having the posteriorly elongated “chitinous” outer lamella with a median furrow and straight rostral margin, Tanabe et al. (2019) assigned this specimen to the Pachydiscidae. The occurrence of several large pachydiscid shells exceeding 300 mm in diameter near the locality of KMNH IvP 902012 supports their interpretation. These observations suggest that pachydiscid lower jaws consist of a widely open and ventrally convex outer lamella and an extremely short inner lamella, as in KMNH IvP 902012.
Jaw morphology also varies with body chamber shape. In P. kamishakensis specimen UMUT MM 30876, which has a compressed body chamber (B/H = 0.53), the lower jaw is anteroventrally elongated (Figure 6C, D; see also Tanabe et al., 2012, fig. 6B; Tanabe and Shigeta, 2019, fig. 5A, B). In contrast, the lower jaws are laterally expanded in Menuites sp. (NMNS PM 14321) (Tanabe and Shigeta, 2019, fig. 4A, B), M. soyaensis (NMNS PM 14316) (Figure 6A, B; see also Tanabe and Shigeta, 2019, fig. 3), M. japonicus (KMNH IvP 902013) (Figure 3), and Eupachydiscus sp. (KMNH IvP 902014) (Figures 4, 5). These specimens excluding NMNS PM 14316 possess a depressed body chamber (B/H = 0.91–1.22). The body chamber of NMNS PM 14316 has been laterally compressed due to post-depositional deformation, resulting in an anomalously low B/H ratio of 0.66 (Table 1). Based on measurement data from four large specimens (D > 100 mm) of the same species recorded by Matsumoto and Miyauchi (1984, p. 42), which showed B/H ratios of 0.90–0.95, we infer that NMNS PM 14316 originally had a B/H ratio of approximately 0.9. The lower jaw is also laterally expanded in M. naumanni specimen UMUT MM 27835, which has a very depressed body chamber (B/H = 1.68) (Figure 6E, F; see also Tanabe and Landman, 2002, text-fig. 1; Tanabe et al., 2015a, fig. 10.5f; Tanabe and Shigeta, 2019, fig. 5C, D). Notably, the ratio of the width of paired wings to maximum wing length (WW/MW) correlates positively with the B/H ratio of the body chamber in all examined specimens, except NMNS PM 14316 (Figure 7). This indicates that the overall shape of the outer lamella in the lower jaw is adapted to fit the cross-sectional morphology of the corresponding ammonite’s body chamber.
The superfamily Desmoceratoidea comprises the families Pachydiscidae and Desmoceratidae. Both families possess lower jaws that show intermediate characteristics between the anaptychus and aptychus types, specifically, the presence of a distinct median groove or furrow on the outer “chitinous” lamella and a very thin calcareous layer overlaying it. Accordingly, their jaw apparatuses have been classified as the intermediate type (Tanabe et al., 2015a). Anaptychus-type lower jaws are known from Early Jurassic superfamilies such as Psiloceratoidea, Arietoidea, and Eoderoceratoidea, whereas aptychus-type lower jaws are common in many Jurassic and Cretaceous Ammonitina (excluding Desmoceratoidea) and in the Cretaceous Ancyloceratina (Tanabe et al., 2015a, fig. 10.12; Keupp et al., 2016, fig. 5).
However, the lower jaws of desmoceratids such as Tragodesmoceroides subcostatus Matsumoto, 1942b, Damesites semicostatus Matsumoto, 1955 in Matsumoto and Obata, 1955, D. damesi (Jimbo, 1894), and D. aff. sugata (Forbes, 1846) differ from those of pachydiscids. Desmoceratid lower jaws exhibit a pointed rostral tip and a more pronounced median depression on the entire hood of the “chitinous” outer lamella (Tanabe, 1983, text-figs. 3C, D, 4A, B; pl. 71, figs. 1d, 3b, c; Tanabe et al., 2012, fig. 5; Tanabe and Misaki, unpublished observations). Incidentally, Nishimura and Maeda (2025) reassigned D. semicostatus and D. aff. sugata to D. damesi and Paradamesites sugata (Forbes, 1846) (comb. nov.) respectively, based on examination of large population samples of desmoceratine ammonites from the uppermost Turonian to the lower Campanian strata of the Yezo Group in Hokkaido, together with type and figured specimens of previously described species.
In “D. aff. sugata” (i.e. P. sugata by Nishimura and Maeda, 2025) specimen UMUT MM 27833 from the Coniacian of Haboro area, the original aragonitic shell mineralogy is preserved, and a thin outer calcareous layer of the lower jaw is composed of aragonite and features a spherulitic prismatic microstructure (Tanabe et al., 2012, fig. 4C, D). By contrast, in P. kamishakensis specimen UMUT MM 30876 from the lower Maastrichitian of southern Alaska, which also retains an aragonitic shell wall, the thin outer calcareous layer of the lower jaw is composed of calcite and exhibits a columnar prismatic microstructure (Tanabe et al., 2012, fig. 4E, F). The outer calcareous layers of the lower jaws of Menuites sp. (Tanabe and Shigeta, 2019, fig. 4D, E) and M. japonicus (Figure 3E) similarly consist of columnar prisms, though the primary mineralogy of these layers could not be confirmed due to poor preservation.
As the jaw apparatus serves as the primary feeding organ in both modern and extinct cephalopods, the wide morphological diversity observed in the lower jaws of Cretaceous ammonoids likely reflects differences in feeding behavior and diet (Tanabe and Shigeta, 2019). In the Pachydiscidae, both upper and lower jaws were found in situ in specimen UMUT MM 27835 of Menuites naummani from the Campanian of Naiba area, South Sakhalin (Figure 6E, F; see also Tanabe and Landman, 2002, text-fig. 1; Tanabe and Shigeta, 2019, fig. 5C, D). Notably, the lower jaw of this species is approximately three times larger than the upper jaw.
Aptychus-type jaw apparatuses, which comprise a larger lower jaw with an almost flat rostral margin, thick bivalved calcitic plates, and a smaller upper jaw, are known in some Jurassic Ammonitina (e.g. Hildoceras levisoni, Lehmann, 1975, fig. 4; Physodoceras nattheimense, Schweigert and Dietl, 1999, pl. 4, fig. 4; Fontannesiella prolithographica and Metahaploceras sp., Schweigert, 2009, figs. 2, 3; Kepplerites sp., Keupp and Mitta, 2013, figs. 15, 16) and in Cretaceous Ancyloceratina (e.g. Hoploscaphites nebrascensis, Meek and Hayden, 1864, figs. 1–4; Landman and Waage, 1993, fig. 37; Baculites sp., Kruta et al., 2011, fig. 1). Previous researchers have identified small organismic remains, such as decapod crustaceans, foraminifers, ostracods, fragmented arms and calices of the stalkless crinoid Saccocoma, and fragments of a calcitic lower jaw element (aptychus sensu stricto) of ammonites, within the body chambers of some Jurassic ammonites that possessed unusually large lower jaws and small upper jaws of the aptychus type (Lehmann, 1971, 1972, 1975; Lehmann and Weitschat, 1973; Riegraf et al., 1984; Jäger and Fraaye, 1997; Schweigert and Dietl, 1999; Klug and Lehmann, 2015).
Assuming these remains represent stomach or crop contents, Lehmann (1975, 1980) and Morton and Nixon (1987) proposed that the aptychus-type lower jaws of these ammonites were not adapted for biting or cutting prey. Instead, they likely functioned as scooping devices, suited to feeding primarily on small benthic organisms. More recently, Kruta et al. (2011) used synchrotron X-ray microtomographic techniques to produce detailed three-dimensional images that showed isopod remains and a gastropod larval shell preserved along with a radula and an aptychus-type jaw apparatus within the buccal mass region of a Late Cretaceous (Campanian) Baculites specimen from the Western Interior Basin, U.S.A. Based on this evidence, Kruta et al. (2011) concluded that Baculites lived in the water column and fed on small zooplankton.
In contrast, the intermediate-type jaw apparatuses of the Desmoceratidae including Tragodesmoceroides subcostatus, “Damesites semicostatus” (i.e., D. damesi by Nishimura and Maeda, 2025), “Damesites ainuanus Matsumoto, 1957” (i.e., junior synonym of Paradamesites sugata (comb. nov.) by Nishimura and Maeda, 2025), and “Damesites aff. sugata” (i.e., P. sugata by Nishimura and Maeda, 2025) (see Tanabe, 1983, text-figs. 2–4; pl. 71, figs. 1–3; Tanabe et al., 2012, fig. 5) along with some aptychus-type jaw apparatuses of heteromorph ammonites of the families Nostoceratidae (e.g. Didymoceras nebrascense, Kruta et al., 2010, figs. 1–9; Pravitoceras sigmoidale, Tanabe et al., 2015b, figs. 3, 4), Diplomoceratidae (Scalarites mihoensis, Tanabe et al., 1980, fig. 1, pl. 20, fig. a–f; Polyptychoceras sp., Tanabe, 2011; Subptychoceras sp., Tanabe and Landman, 2002, text-figs. 2.6a, b; pl. 1, fig. 7a–c), Turrilitidae (Turrilites costatus, Tanabe et al., 2021, figs. 2, 3), and Scaphitidae (Hoploscaphites nebrascensis, Meek and Hayden, 1864, figs. 1–4; Hoploscaphites dorfi, Landman and Waage, 1993, fig. 39; Hoploscaphites speedeni, Landman and Waage, 1993, fig. 41) display sharply pointed rostral tips on similarly sized upper and lower jaws. The pointed rostral tips likely allowed for biting and cutting prey, as seen in many modern coleoid and nautilid cephalopods with predatory or scavenging habits (Tanabe and Fukuda, 1999; Tanabe, 2012).
In summary, the unusually large, shovel-shaped lower jaws of pachydiscid ammonites may have evolved to support a microphagous feeding strategy, similar to that proposed for some Jurassic Ammonitina and Cretaceous Ancyloceratina with large aptychus-type lower jaws. Future verification of this hypothesis will require examination of preserved stomach or crop contents in exceptionally well-preserved specimens using advanced techniques such as synchrotron X-ray microtomography, as demonstrated by Kruta et al. (2011).
Did the lower jaw of the Pachydiscidae have a secondary function as an operculum?Our findings, along with previous studies, indicate that the ventral outline of the lower jaw in pachydiscids with its nearly flat rostral margin and prominent median furrow roughly matches the cross-sectional outline of the body chamber in which it is accommodated (Figures 6, 7, 8; see also Tanabe and Landman, 2002, text-fig. 1; Tanabe et al., 2012, fig. 6; Tanabe and Shigeta, 2019, figs. 3–5). Similar correspondences between lower jaw and shell aperture shapes have been observed in several Jurassic and Cretaceous ammonites possessing aptychus-type lower jaws (e.g. Schindewolf, 1958, pls. 1, 8; Lehmann and Kulicki, 1990, fig. 1 and table 1; Keupp and Mitta, 2013, fig. 5). To explain this phenomenon, Lehmann and Kulicki (1990) proposed that aptychus-type lower jaws in select Jurassic and Cretaceous ammonites such as the Callovian aspidoceratid Euaspidoceras subbabeanum and the Maastrichitian scaphitid Hoploscaphites cheyennensis may have acquired a secondary function as opercula, with the thick, bivalved calcitic plates (aptychi) functioning as a protective barrier, joined along a midline joint (symphysis termed by Arkell, 1957) on the outer chitinous lamella.
Anatomical studies of modern cephalopods reveal that the inner surface of the lower jaw and the outer surface of the upper jaw are connected by jaw muscles, with a thin layer of tall columnar cells, referred to as beccublasts by Dilly and Nixon (1976), serving as an interface (Tanabe and Fukuda, 1987, figs. 1, 4a, b; Nixon, 1988, fig. 3a; Tanabe, 2012, fig. 1B, C). Anchor-type attachment scars of beccublasts have been identified on the inner surface of the inner chitinous lamella in an isolated aptychus-type lower jaw attributed to a Late Jurassic aspidoceratid ammonite (Tanabe et al., 2015a, fig. 10.2d). These findings suggest that the semiflexible outer chitinous lamella with a median furrow (functioning as a “hinge”) of the aptychus-type lower jaw was likely connected to jaw muscles via beccublasts on the dorsal side, allowing for potential mobility. As illustrated by Seilacher (1993, fig. 1), this anatomical configuration may have enabled a living ammonite of some Jurassic and Cretaceous taxa to retract the outer connective tissue backward and expose the bivalved calcitic plates of the aptychus-type lower jaw as a defensive shield against predation.
Meanwhile the hypothesis of a secondary opercular function of aptychi was rejected by Machalski (2021) for the late Maastrichitian scaphitid Hoploscaphites constrictus crassus based on a significant misfit in size and shape between shell aperture and associated aptychus in specimens from the Kazimierz Dolny area in Poland representing successive stages of ontogeny. Larson and Landman (2017) also doubted the secondary opercular function of aptychi for the early Maastrichtian baculitid Baculites grandis, because those preserved in situ in two specimens from the Pierre Shale of Wyoming, U.S.A. were approximately 30% smaller than the corresponding apertural whorl section. The actual size of the lower jaw in the five pachydiscid specimens examined is also approximately 20 to 50% smaller than that of the apertural whorl cross section (Figure 8; Table 1). These observations indicate that, like H. constrictus crassus and B. grandis, pachydiscid ammonites were unable to completely seal their shell apertures with their lower jaws. In relation to this issue, Kröger (2002) analyzed healing patterns of sublethal injuries preserved on the shells of some Jurassic ammonoids, such as Toarcian dactylioceratids and Oxfordian perisphinctids, and suggested that they could withdraw the whole soft body onto the deepest parts of the observed injuries, approximately 1/3 to 1/2 of the body chamber length. Judging from the smaller lower jaw size than the apertural cross section and the preservation of the lower jaw in the body chamber far behind the shell aperture (see Figures 3, 4, 6C–F), Late Cretaceous pachydiscid ammonites were presumably able to withdraw their soft parts deep into the body chamber, thereby avoiding being eaten by predators. Under such circumstances, the widely open lower jaw could serve an operculum-like function to protect the soft body that was retracted deep in the body chamber.
In conclusion, the operculum-like secondary function of the aptychus-type lower jaw may have evolved independently in some Jurassic and Cretaceous ammonoid lineages belonging to the suborders Ammonitina and Ancyloceratina. This adaptation likely contributed to the development of an “aptychus-like” lower jaw, previously called the intermediate-type by Tanabe et al. (2015a), which exhibits the transitionary features from the anaptychus-type to the aptychus-type in the Late Cretaceous Pachydiscidae. Support for this interpretation is provided by the observed diversity in the microstructure and mineralogy of the calcified lower jaw elements among these groups (Farinacci et al., 1976; Kruta et al., 2009; Tanabe et al., 2012).
Two specimens of the pachydiscid ammonites Menuites japonicus (Matsumoto, 1955) and Eupachydiscus sp. with a lower jaw preserved in situ in the body chamber were described from the Upper Cretaceous (Coniacian to Santonian) strata of the Yezo Group in the Haboro and Mikasa areas of Hokkaido, Japan. The lower jaws of the two species exhibit intermediate features between the anaptychus- and aptychus-type lower jaws, including a widely open outer lamella composed of inner “chitinous” and outer calcareous layers, a flat rostrum, and a median furrow along the hood ridge of the inner layer. These characteristics have also been documented in the lower jaws of other previously described pachydiscid species, suggesting that they are diagnostic of the family.
The unusually large, shovel-shaped lower jaws of pachydiscid ammonites may have evolved in association with a microphagous feeding strategy, as proposed for certain Jurassic and Cretaceous ammonites possessing similarly shaped large aptychus-type lower jaws. Our current and prior studies indicate that in the Pachydiscidae, the ventral outline of the lower jaw with its nearly flat rostral margin and prominent median furrow roughly matches the cross-sectional outline of the shell aperture, although the former is smaller than the latter. Pachydiscid ammonites were presumably able to withdraw their soft parts deep into the body chamber, thereby avoiding being eaten by predators. Under such circumstances, their widely open lower jaw could have an operculum-like function to protect the retracted soft body from predator’s attack.
We thank Haruyoshi Maeda (Kyushu University Museum, Fukuoka) for his kind help during our field survey, Yasunari Shigeta (National Museum of Nature and Science, Tsukuba [NMNS]) for his useful discussion during the study on NMNS specimens, and the Hokkaido Regional Forest Office for the permission to survey in the national forest.
We are also grateful to Naoki Kohno (NMNS), editor-in-chief of the Bulletin of the National Museum of Nature and Science, Series C, for allowing us to reproduce figures published in the journal article in the present paper. Special thanks are due to Neil H. Landman (American Museum of Natural History, New York City) and Christian Klug (Universität Zürich, Zürich) for their helpful comments and suggestions to improve this manuscript. This study was supported in part by JSPS KAKENHI Grant Number JP19K04063 (to A. M.).
A. M. collected the specimens and studied their geological settings, and reviewed and edited the first draft. K.T. studied the specimens paleontologically and wrote the first draft of the manuscript. Both authors contributed to the writing of the paper.
