RESEARCH ARTICLE
Taxonomic re-evaluation of the genus Canadoceras (Ammonoida) from the middle Campanian of northern Hokkaido, Japan
2026 年 30 巻 p. 187-204
詳細
2026 年 30 巻 p. 187-204
A taxonomic re-evaluation of the genus Canadoceras Spath (Ammonoida, Pachydiscidae) was conducted based on a large sample collected from the Orannai Formation in the Soya area, northern Hokkaido, Japan. Morphometric analyses of 37 specimens from the middle Campanian (Upper Cretaceous) utilizing X-ray computed tomography and statistical tests revealed that four species described from the Sphenoceramus schmidti Zone in Hokkaido can be reduced to three valid taxa: C. yokoyamai (Jimbo), C. kossmati Matsumoto, and C. mysticum Matsumoto. Statistical analysis of relative umbilical size and relative whorl thickness identifies three distinct ontogenetic trajectories corresponding to these species. Notably, Canadoceras minimum Matsumoto and Miyauchi is herein regarded as a junior synonym of C. mysticum, as its holotype represents a juvenile stage within the variation range of the latter. The unique ontogenetic growth patterns observed in each species are established at very early stages, driven by an interplay between modifications in the whorl expansion rate and the underlying geometric constraints of shell construction. This suggests that modifications in these early growth modes were a primary factor in the diversification of the genus. These findings refine paleobiodiversity estimates for the Northwestern Pacific and provide a clearer framework for understanding the phylogenetic and paleobiogeographic history of the genus Canadoceras.
It is well documented that ammonoid shell morphology including coiling patterns, whorl sections, and ribbing intensity can exhibit significant intraspecific variation (e.g. Reeside and Cobban, 1960; Howarth, 1973; Dagys and Weitschat, 1993). Historically, subtle morphological differences have frequently been misinterpreted as diagnostic features of new species, resulting in an excessive proliferation of taxonomic names for what are likely phenotypic variants within a single species (e.g. Buckman, 1887–1907). Failure to account for such variation risks a substantial overestimation of paleobiodiversity (e.g. Westermann, 1966; Nardin et al., 2005; Korn and Klug, 2007). To resolve this critical issue between intraspecific variation and species estimation, approaches such as 1) statistical analysis of large samples to identify the range of variation and 2) morphometrics measurements to determine whether variation is continuous or discrete are required (De Baet et al., 2015).
Canadoceras Spath, 1922 (Ammonoida, Pachydiscidae) is a Cretaceous ammonoid genus distributed across South Africa, Madagascar, Brazil, and the North Pacific region, spanning the upper Santonian to the Campanian (e.g. Wright et al., 1996). The genus is particularly abundant in the middle Campanian of the Northwest Pacific region, where numerous large specimens exceeding 50 cm in diameter have been reported (e.g. Matsumoto, 1954; Maeda et al., 2010). To date, the following five species have been described from Japan and the Russian Far East: Canadoceras yokoyamai (Jimbo, 1894); C. kossmati Matsumoto, 1954; C. multicostatum Matsumoto, 1954; C. mysticum Matsumoto, 1954; and C. minimum Matsumoto and Miyauchi, 1984. With the exception of C. multicostatum, which occurs in the Metaplacenticeras subtilistriatum Zone, four of these species were originally described from the Sphenoceramus schmidti Zone (Figure 1). Notably, Canadoceras compressum Matsumoto, 1954, from the lower Maastrichtian of Hokkaido, was subsequently reassigned to the genus Patagiosites Spath, 1953 (Matsumoto, 1959).
The type specimens (holotypes and lectotype) of the four species described from the Sphenoceramus schmidti Zone in Hokkaido vary in size, ranging from approximately 30 to 100 mm in diameter. Consequently, it remains unclear whether these morphological differences represent interspecific, ontogenetic, or intraspecific variation. As these type specimens likely represent juvenile shells, specific identification of medium- to large-sized specimens has proven difficult. Although Shigeta (2019) examined the ontogeny of a well-preserved specimen (150 mm in diameter) and demonstrated that its inner whorls resembled the holotype of C. kossmati, thereby distinguishing it from the other three species, the overall range of intraspecific variation and the specific ontogenetic growth modes of these taxa remain poorly understood. In particular, it is crucial to determine whether the morphological differences observed in juvenile type specimens are maintained through ontogeny or represent divergent growth trajectories.
Here, I re-evaluate the taxonomy of Canadoceras based on morphometric measurements and statistical analyses of large samples. Specifically, I examine ontogenetic growth patterns and morphological variation using a large series of specimens collected near Cape Soya in northernmost Hokkaido from the Fukiyose Member (Sphenoceramus schmidti Zone) of the Orannai Formation (Yezo Group). The Fukiyose Member is renowned for yielding abundant, well-preserved ammonoids (Matsumoto and Miyauchi, 1984).
The Cretaceous Yezo Group is widely distributed as a north-south-trending belt across central Hokkaido and the West Sakhalin Mountains (e.g. Matsumoto, 1954; Vereshchagin, 1977; Shigeta and Maeda, 2005; Figure 2). Around Cape Soya in northernmost Hokkaido, the group is characterized by complex folding and faulting (Osanai et al., 1959; Matsumoto and Ogasawara, 1971). Matsumoto and Miyauchi (1984) subdivided approximately 500 m of strata into ten lithologic units (A to J in ascending stratigraphic order: Figure 2D).
Unit A (at least 50 m thick) consists of mudstone containing Eupachydiscus haradai (Jimbo, 1894). Unit B (~50 m thick) comprises mudstone with intercalated sandstone beds and yields Sphenoceramus orientalis (Sokolov, 1914). Unit C (8 m thick) is composed of medium- to coarse-grained cross-bedded sandstone containing S. schmidti (Michael, 1899). Unit D (100–250 m thick) consists of mudstone yielding S. schmidti and Gigantocapulus giganteus (Schmidt, 1873) followed by Unit E (~50 m thick), which is also mudstone but only sparsely fossiliferous. Unit F (60 m thick) is a sandy mudstone containing several sandstone beds rich in ammonoids, inoceramid bivalves, wood fragments, and pumice; its characteristic fossils include Canadoceras mysticum, C. minimum and S. schmidti. This unit is also referred to as the Fukiyose Beds or Fukiyose Member (Matsumoto and Miyauchi, 1984). Unit G (20 m thick) consists of sandy mudstone, and Unit H (30 m thick) is a green silty fine-grained sandstone characterized by Menuites soyaensis (Matsumoto and Miyauchi, 1984), Schluterella kawadai Matsumoto and Miyauchi, 1984) and C. multicostatum. Unit I (40 m thick) comprises dark grey sandy mudstone and siltstone containing Metaplacenticeras subtilistriatum (Jimbo, 1894). Finally, Unit J (at least 70 m thick) consists of alternating beds of sandstone and sandy mudstone, also yielding M. subtilistriatum.
According to Osanai et al. (1959), Units A–E and Units F–J correspond to the Ohmisaki and Orannai formations, respectively. As S. orientalis, S. schmidti, and M. subtilistriatum are key index fossils for the middle Campanian in the Northwest Pacific region (Shigeta et al., 2019), the interval from Unit B to Unit J is correlated with the middle Campanian.
Eighty-eight fossil specimens collected from several beds within Unit F of the Orannai Formation at the Kiyohama-1 (also known as Daiichi Kiyohama) fishing harbor near Cape Soya in northern Hokkaido, Japan (45°29′47.8″N, 141°52′58.1″E; Figure 2B–D) are stored at the National Museum of Nature and Science in Tsukuba, Ibaraki, Japan (NMNS). Of these, 37 well-preserved specimens referable to Canadoceras were examined in this study (NMNS PM 15986, 45705–45740).
For the taxonomic re-evaluation and comparative morphometric analysis, the following primary type specimens were also examined: the lectotype of Canadoceras yokoyamai (UMUT MM 7511; Figure 1A–C), the holotype of C. kossmati (UMUT MM 7650; Figure 1G–I), the holotype of C. mysticum (KYUM GKH 5184; Figure 1J–L) and the holotype of C. minimum (KYUM GKH 5976; Figure 1D–F).
MethodsFor biometric analysis of conch morphology, all specimens were scanned using X-ray computed tomography (inspeXio SMX-225CT FPD HR, Shimadzu) at the National Museum of Nature and Science in Tsukuba, Ibaraki, Japan. Scanning parameters were set to a voxel size of 0.030–0.141 mm, 225 kV and 70 μA. Four standard geometric parameters of the shell, i.e., shell diameter (D), umbilical diameter (U), whorl height (H) and whorl width (W), were measured at half-whorl intervals using cross-sectional CT images (Figure 3). Based on these measurements, three morphological ratios were calculated: relative umbilical size (U/D), and relative whorl thickness (W/H) and the whorl expansion rate (WER). The WER was determined using the formula WER = (D/D’)2, where D’ represents the diameter at the preceding half-whorl (see Klug et al., 2015). Additionally, the values of these parameters at a shell diameter of 30 mm were calculated using the allometric equations relating U, H, and W to D.
To facilitate ontogenetic observations of whorl morphology and ornamentation, the outer whorls of nine specimens (NMNS PM 45705, 45710, 45711, 45716–45721) were removed in increments of approximately one-quarter to one-half whorl. This process allowed for the detailed examination of internal developmental stages.
The examined specimens exhibit a wide range of shell morphologies, with the relative umbilical size (U/D) varying from fairly narrow to moderate (0.2–0.4) and the relative whorl thickness (W/H) ranging from fairly depressed to fairly compressed (0.7–1.9) during early to middle ontogeny (D = 5–100 mm). Based on whorl cross-sections and umbilical features, these specimens were categorized into three distinct morphological groups: A, B, and C (Figures 4, 5, 6, 7, 8, 9, 10, 11, 12, 13).
To statistically test the validity of this grouping, a one-way analysis of variance (ANOVA) was performed on the U/D and W/H ratios at a shell diameter of approximately 30 mm, where morphological differentiation is most pronounced (Figure 10). The analysis revealed highly significant differences among the three groups for both parameters: U/D [F (2, 24) = 166.55, p < 0.001] and W/H [F (2, 24) = 126.05, p < 0.001]. These results demonstrate that the morphological differentiation among Groups A, B, and C is statistically robust.
Characterization of morphological groupsGroup A.—At shell diameters of approximately 5–10 mm, specimens exhibit a moderate umbilicus (U/D = 0.30–0.33) and a fairly depressed whorl section (W/H = 1.4–1.8), and a whorl expansion rate (WER) of approximately 1.8–1.9. As the shell grows up to approximately 25 mm in diameter, the U/D ratio increases rapidly to 0.35, while the W/H ratio decreases significantly, resulting in a more compressed whorl section. During this stage, the WER remains stable at 1.8–1.9. Beyond a diameter of 25 mm, the U/D ratio shows high variability, with individual trends ranging from decreasing to increasing; at a diameter of 50 mm, it shows a wide range between 0.32 and 0.39. In contrast, the W/H ratio continues to decline, ranging between 1.06 and 1.23 at 50 mm, while the WER gradually increases to 1.94 (Figures 4, 5, 6).
The whorl cross-section is subquadrate to subcircular, with an arched venter, indistinct ventral shoulders, and convex flanks (Figure 11). The maximum whorl width occurs at the mid-flank or slightly below, and the umbilical shoulders are rounded. Ornamentation comprises numerous prorsiradiate ribs and prominent major ribs; the latter originate from distinct tubercles on the umbilical shoulder and are preceded by shallow constrictions. Rib intercalation typically occurs between the inner and mid-flank. Notably, during juvenile stages (up to 30–40 mm in diameter), bifurcated ribs occasionally arise from the umbilical tubercle. The ribs cross the flank, bend gently forward at the ventral shoulders, and traverse the venter in a convex arch (Figure 11).
Group B.—At shell diameters of approximately 10–20 mm, specimens exhibit a whorl section that is as high as broad (W/H = 1.1–1.2) and a fairly narrow umbilicus (U/D = 0.25–0.30), and a WER of 2.0. As the shell grows to a diameter of approximately 50 mm, the U/D ratio gradually increases to around 0.30, while the W/H ratio decreases to approximately 1.0, resulting in a more compressed whorl section. During this ontogenetic stage, the WER remains stable at approximately 1.95–2.05. Beyond a diameter of 50 mm, the U/D ratio remains nearly constant or increases slightly. In contrast, the W/H ratio continues to decline, reaching 0.93 at approximately 100 mm, while the WER gradually decreases to 1.92 (Figures 4, 5, 6).
The whorl cross-section ranges from subcircular to elliptical, characterized by an arched venter, indistinct ventral shoulders, and gently convex flanks (Figure 12). The maximum whorl width is situated slightly below the mid-flank to near umbilical shoulder, and the umbilical shoulders are rounded. Ornamentation comprises numerous prorsiradiate ribs and prominent major ribs; the latter originate from distinct tubercles on the umbilical shoulder and are preceded by shallow constrictions. Rib intercalation typically occurs between the inner and mid-flank. The ribs cross the flank, bend gently forward at the ventral shoulders, and traverse the venter in a convex arch (Figure 12).
Group C.—At shell diameters of approximately 5–10 mm, specimens exhibit a whorl section that is as high as broad (W/H = 1.1–1.5) and a fairly narrow umbilicus (U/D = 0.20–0.25), and a WER of 2.0–2.2. As the shell grows to 25–30 mm in diameter, the U/D ratio remains nearly constant or increases slightly, while the W/H ratio gradually decreases to 0.85–1.0, resulting in a more compressed whorl section. During this stage, the WER increases to 2.1–2.3. Beyond a diameter of 30 mm, the U/D ratio increases rapidly to 0.28–0.31. In contrast, the W/H ratio continues to decline, reaching 0.8 at approximately 100 mm, while the WER rapidly decreases to 2.0 (Figures 4, 5, 6).
The whorl cross-section is elliptical, characterized by an arched venter, indistinct ventral shoulders, and nearly flat flanks (Figure 13). The maximum whorl width is situated slightly below the mid-flank to near umbilical shoulder, and the umbilical shoulders are rounded. Ornamentation comprises numerous prorsiradiate ribs and prominent major ribs; the latter originate from umbilical bullae on the umbilical shoulder and are preceded by shallow constrictions. Rib intercalation typically occurs between the inner and mid-flank. The ribs cross the flank, bend gently forward at the ventral shoulders, and traverse the venter in a convex arch (Figure 13).
Correlations between morphological ratiosThe correlations among the morphological ratios (U/D, W/H, and WER) are summarized as follows:
U/D versus WER.—Based on 107 data points with a diameter range of D = 5.09–98.60 mm, the WER ranges from 1.76 to 2.32 and the U/D from 0.20 to 0.39 (Figure 7). A strong negative linear correlation was observed between these parameters (r = −0.854), which is statistically significant (p < 0.01). When analyzed by individual groups, Group A (22 data points) and Group B (14 data points) showed no significant correlation (Group A: r = −0.076, p = 0.7364; Group B: r = −0.394, p = 0.1632). In contrast, Group C (71 data points) demonstrated a strong negative linear correlation (r = −0.724, p < 0.01).
W/H versus WER.—For the same 107 data points (D = 5.09–98.60 mm), the W/H values range from 0.73 to 1.69 (Figure 8). This relationship shows a moderate negative linear correlation (r = −0.528), which is also statistically significant (p < 0.01). However, intraspecific correlations varied significantly: Group A showed no significant correlation (r = −0.169, p = 0.4518), Group B exhibited a moderate but non-significant positive trend (r = 0.426, p = 0.1296), and Group C showed no discernible correlation (r = 0.174, p = 0.1431).
W/H versus U/D.—For 144 data points (D = 3.58–105.64 mm), where W/H ranges from 0.73 to 1.81 and U/D from 0.20 to 0.39, a weak positive linear correlation was found (r = 0.399, p < 0.01) (Figure 9). Notably, when the analysis was restricted to specimens with a diameter of approximately 30 mm, a remarkably strong positive linear correlation emerged (r = 0.959, p < 0.01) (Figure 10). Conversely, ontogenetic trends within each group showed different patterns: Group A (27 data points) exhibited a moderate negative linear correlation (r = −0.499, p < 0.01), while Group B (20 data points) showed no significant correlation (r = −0.437, p = 0.054). Group C (97 data points) showed a strong negative linear correlation (r = −0.602, p < 0.01).
To clarify the relationships between Groups A–C and previously established species, the relative umbilical size (U/D) and relative whorl thickness (W/H) of the four type specimens of Canadoceras and 27 specimens assigned to Groups A–C were plotted in Figure 10. These parameters were measured at a standardized shell diameter of 30 mm, where morphological differentiation is most pronounced. The morphospace of Group A encompasses the holotype of C. yokoyamai, while Group B includes C. kossmati, and Group C contains both C. mysticum and C. minimum. The ontogenetic patterns of these type specimens are consistent with those observed in the specimens of each respective group (Figures 4, 5, 6). These findings indicate that Group A is identifiable as C. yokoyamai, Group B as C. kossmati, and Group C as either C. mysticum or C. minimum.
The holotype of C. minimum measures approximately 30 mm in diameter, whereas that of C. mysticum is approximately 100 mm. As noted by Matsumoto and Miyauchi (1984, p. 50), at diameters of 20–30 mm, the umbilicus of the C. minimum holotype is slightly wider than that of the C. mysticum holotype; however, both fall within the range of variation observed in Group C (Figure 10). Furthermore, the frequency of major ribs and the density of intervening ribs in the C. minimum holotype also fall within the variation of Group C (Figure 13). Consequently, the two holotypes are considered to represent different growth stages of Group C, and C. minimum is herein regarded as a junior synonym of C. mysticum. Although some specimens among the three species exhibit remarkably similar morphologies at shell diameters exceeding 50 mm, they remain clearly distinguishable from one another at growth stages between 30 mm and 40 mm (Figures 4, 5).
Phylogeny and paleogeographyCanadoceras yokoyamai and C. kossmati both possess distinct umbilical tubercles and share closer morphological affinities with each other than with C. mysticum, suggesting a close phylogenetic relationship between the two. Although C. mysticum is characterized by umbilical bullae rather than tubercles, its overall morphology more closely resembles C. kossmati than C. yokoyamai, implying a relatively close relationship between C. mysticum and C. kossmati. Based on these observations, three plausible evolutionary scenarios can be proposed: (1) an independent divergence model in which both C. yokoyamai and C. mysticum branched off separately from a common C. kossmati lineage; (2) a stepwise evolutionary sequence where C. kossmati was derived from C. yokoyamai and subsequently gave rise to C. mysticum; and (3) an alternative stepwise progression in which C. kossmati originated from C. mysticum, eventually leading to the evolution of C. yokoyamai.
From a paleobiogeographical perspective, Canadoceras kossmati has been recorded exclusively from the Northwestern Pacific region (e.g. Matsumoto, 1954), including Japan and Sakhalin. In contrast, C. yokoyamai and C. mysticum occur not only in the Northwestern Pacific but also along the West Coast of North America (e.g. Matsumoto, 1959; Jones, 1963). If the latter two species were derived from C. kossmati, it would imply that these lineages originated in the Northwestern Pacific and subsequently expanded their distributions to the West Coast of North America. Alternatively, if the evolutionary sequence progressed from C. yokoyamai through C. kossmati to C. mysticum (or vice versa), the following model is proposed: first, C. kossmati diverged in the Northwestern Pacific from a broadly distributed C. yokoyamai (or C. mysticum) stock; subsequently, C. mysticum (or C. yokoyamai) branched off from C. kossmati and eventually expanded its range back to the West Coast of North America.
All three species co-occur in Unit F (Orannai Formation; upper Sphenoceramus schmidti Zone, lower Middle Campanian) in the Soya area. Although Matsumoto and Miyauchi (1984, p. 34) reported the occurrence of C. kossmati from the underlying Unit D (Omasaki Formation; lower S. schmidti Zone), this record remains unverified as the original specimens are currently unavailable for re-examination. To date, no confirmed records of Canadoceras exist from horizons below this co-occurrence level in Hokkaido and Sakhalin. Due to this lack of documented stratigraphical succession, inferring phylogenetic relationships based solely on chronostratigraphic data remains challenging. Further high-resolution stratigraphic studies are essential to elucidate the precise evolutionary history of the genus Canadoceras.
Morphological diversification and geometric constraintsThe three species of Canadoceras (C. yokoyamai, C. kossmati, and C. mysticum) exhibit significant morphological disparity in their relative umbilical size (U/D) and relative whorl thickness (W/H) throughout their ontogeny. Within each species, these parameters display moderate to strong negative linear correlations (Figure 9), indicating that each taxon follows a distinct allometric growth trajectory. As growth progresses, the whorl section generally becomes more compressed (decreasing W/H) while the relative umbilical size increases (increasing U/D). However, at a standardized diameter, C. yokoyamai possesses larger U/D and W/H values than C. mysticum, with C. kossmati consistently occupying an intermediate position (Figure 10).
In ammonoid shell morphology, a close correlation typically exists between whorl shape and umbilical size, a phenomenon recognized as part of Buckman’s first rule of covariation (e.g. Westermann, 1966; Monnet et al., 2015). Since whorl height (H) is a component of the shell diameter (D), the tendency for more evolute shells to possess more rounded whorl sections can be viewed as a logical geometric consequence of variations in the whorl expansion rate (WER). During early ontogeny, the WER of the three species increases in the order of C. yokoyamai, C. kossmati, and C. mysticum. Mathematically, a higher WER results in a relatively larger H and smaller W (leading to a lower W/H and more compressed shell), while simultaneously reducing U (leading to a narrower umbilicus). Thus, the initial morphological differences observed among these three species during early growth stages may be primarily attributed to such geometric constraints.
However, as growth continues, all three species exhibit a trend where W/H decreases as U/D increases, which contradicts the expected trend of Buckman’s first rule. Given that this rule is frequently subject to exceptions and is thus regarded more as a “rule of thumb” rather than a universal law (Monnet et al., 2015), the ontogenetic patterns of Canadoceras suggest that species-specific growth programs override simple geometric constraints during later development.
Consequently, the diversification of the genus Canadoceras was not the result of morphological divergence during late ontogenetic stages. Instead, it was likely driven by the interplay between modifications in the WER at the earliest ontogenetic stages and the underlying geometric constraints of shell construction, which collectively determined the subsequent species-specific growth trajectories.
Systematic descriptions follow the classification established by Wright et al. (1996) and Hoffmann et al. (2022), and morphological terminology follows Arkell (1957). Quantifiers describing the shape morphology follow the parameters proposed by Matsumoto (1954, p. 246) and subsequently modified by Haggart (1989, table 8.1).
Abbreviations for shell dimensions.—See Methods.
Institution abbreviations.—KYUM, the Kyushu University Museum, Fukuoka, Fukuoka, Japan; NMNS, National Museum of Nature and Science, Tsukuba, Ibaraki, Japan; UMUT, The University Museum, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
Superorder Ammonoida Haeckel, 1866
Order Ammonitida Haeckel, 1866
Suborder Ammonitina Hyatt, 1889
Superfamily Desmoceratoidea Zittel, 1895
Family Pachydiscidae Spath, 1922
Genus Canadoceras Spath, 1922
Type species.—Ammonites newberryanus Meek, 1876.
Canadoceras yokoyamai (Jimbo, 1894)
Figures 1A–C, 4, 5, 6, 7, 8, 9, 10, 11
Pachydiscus yokoyamai Jimbo, 1894, p. 31, pl. 2, fig. 3.
Pachydiscus multisulcatus Whiteaves, 1903, p. 349, pl. 50, fig. 1, text-fig. 24; Usher, 1952, p. 81, pl. 16, figs. 1–4, pl. 31, fig. 8.
Canadoceras yokoyamai (Jimbo). Matsumoto, 1954, p. 302, text-fig. 26; Jones, 1963, p. 43, pl. 36, fig. 6, pl. 37; Matsumoto, 1959, p. 56, pl. 12, fig. 1, pl. 13, fig. 1, pl. 15, fig. 1.
non Canadoceras yokoyamai (Jimbo). Matsumoto, 1954, p. 302, pl. 29, fig. 2, pl. 33, figs. 1, 2, text-fig. 26 (= C. kossmati); Matsumoto, 1959, p. 56, pl. 14, fig. 1 (= probably C. newberryanum Meek, 1857); Zonova et al., 1993, p. 162, pl. 82, fig. 2, pl. 83, fig. 1, pl. 85, fig. 1 (= C. kossmati); Jagt-Yazykova, 2011, pl. 8, figs. 1–3 (= C. kossmati).
Canadoceras newberryanum Meek. Poyarkova, 1987, pl. 26, fig. 5.
? Canadoceras yokoyamai (Jimbo). Zonova et al., 1993, p. 162, pl. 80, fig. 1.
Lectotype.—UMUT MM 7511, designated by Matsumoto (1954, p. 302), is the original specimen illustrated by Jimbo (1894, p. 31, pl. 2, fig. 3). It was collected from the Sphenoceramus schmidti Zone along the Tsiptaushbets River (currently known as the Chupitaushinai River) in the Nakatonbetsu area, Hokkaido.
Material examined.—Five specimens (NMNS PM 45705–45709) collected from several fossiliferous beds within Unit F of the Orannai Formation, exposed at the Kiyohama-1 fishing harbor near Cape Soya, northern Hokkaido.
Description.—Early whorls (up to 25 mm in diameter): Shell fairly involute and depressed. Whorl section subquadrate to subcircular with rounded venter and indistinct ventral shoulders; flanks convex with maximum width at or slightly below mid-flank. Umbilicus moderate; umbilical wall moderately high, gently convex and sub-vertical; umbilical shoulder rounded. Ornamentation consists of numerous prorsiradiate ribs and prominent major ribs. Major ribs originate from distinct tubercles on umbilical shoulder, preceded by shallow constrictions. Rib intercalation typically occurs between inner and mid-flank. Notably, bifurcated ribs occasionally arise from umbilical tubercle during juvenile stages. Ribs cross flank, bend gently forward at ventral shoulders, and traverse venter in a convex arch.
Middle to later whorls (over 25 mm in diameter): Whorl section becoming more compressed with growth. Umbilical size variable during ontogeny, ranging from becoming wider to remaining constant or becoming narrower. Ribs significantly stronger and more widely spaced. Major ribs, arising from prominent umbilical tubercles, increase in both strength and frequency.
Measurements.—See File S1.
Discussion.—At least up to a shell diameter of 100 mm, Canadoceras yokoyamai is distinguished from C. kossmati and C. mysticum by its more depressed shell with a subquadrate to subcircular whorl section, slightly convex flanks, wider umbilicus, more prominent ribs, and distinct umbilical tubercles.
Pachydiscus multisulcatus from Canada resembles Canadoceras kossmati in specimens exceeding 100 mm in diameter due to its fairly narrow umbilicus (Whiteaves, 1903; Usher, 1952). However, at a diameter of 50 mm, it exhibits a moderate umbilicus (U/D = 0.35), which is characteristic of Canadoceras yokoyamai. Therefore, P. multisulcatus is herein regarded as a junior synonym of C. yokoyamai.
Among the specimens previously reported as Canadoceras yokoyamai from Hokkaido and Sakhalin, several are re-identified as C. kossmati based on their umbilical width and whorl breadth. These include the specimens figured by Matsumoto (1954: pl. 29, fig. 2; pl. 33, figs. 1, 2), Zonova et al. (1993: pl. 82, fig. 2; pl. 83, fig. 1; pl. 85, fig. 1), and Jagt-Yazykova (2011: pl. 18, figs. 1–3). The specimen of C. newberryanum Meek from Sakhalin figured by Poyarkova (1987: pl. 26, fig. 5) is also assignable to C. yokoyamai due to its moderate umbilicus. Conversely, the specimen figured by Zonova et al. (1993: pl. 80, fig. 1) is too poorly preserved for a definitive specific identification.
Furthermore, regarding the specimens reported as Canadoceras yokoyamai from California, the specimen illustrated by Matsumoto (1959: pl. 14, fig. 1) is likely assignable to C. newberryanum Meek, 1857 based on its shell morphology and ornamentation.
Occurrence.—Middle Campanian (Sphenoceramus schmidti Zone) of Hokkaido and Sakhalin but also recorded from the Campanian of Alaska, Vancouver Island, and California.
Canadoceras kossmati Matsumoto, 1954
Canadoceras kossmati Matsumoto, 1954, p. 295, pl. 29, fig. 1, pl. 30, figs. 1, 2, pl. 31, fig. 1, pl. 32, figs. 1–3, text-figs. 64–71, 73; Poyarkova, 1987, pl. 26, fig. 2; Zonova et al., 1993, p. 161, pl. 77, fig. 1, pl. 81, fig. 1, pl. 83, fig. 2, pl. 84; Saito et al., 1998, pl. 1, figs. 1–7, 9, 10; Shigeta et al., 1999, pl. 3, fig. 1; Kodama et al., 2002, fig. 8A, B; Maeda et al., 2005, figs. 1, 2; Maeda et al., 2010, figs. 3, 5B; Shigeta, 2019, figs. 2–6, 8.
? Canadoceras fraternum (Gabb). Anderson, 1958, p. 233, pl. 50, fig. 1.
? Pachydiscus aff. egertoni (Forbes). Poyarkova, 1987, pl. 26, fig. 3; Zonova et al., 1993, p. 165, pl. 86, figs. 1, 2.
non Canadoceras kossmati Matsumoto. Misaki and Maeda, 2009, fig. 8O, P (= C. mysticum); Ando et al., 2001, pl. 1, figs. 17, 19 (= C. newberryanum).
Canadoceras yokoyamai (Jimbo). Zonova et al., 1993, p. 161, pl. 82, fig. 2, pl. 83, fig. 1, pl. 85, fig. 1; Jagt-Yazykova, 2011, pl. 8, figs. 1–3.
Holotype.—UMUT MM 7650, illustrated by Matsumoto (1954, p. 295, pl. 13, fig. 1), originating from the Sphenoceramus schmidti Zone along the Chupitaushinai River in the Nakatonbetsu area, Hokkaido.
Material examined.—Six specimens (NMNS PM 45710–45715) collected from several fossiliferous beds within Unit F of the Orannai Formation, exposed at the Kiyohama-1 fishing harbor near Cape Soya, northern Hokkaido.
Description.—Early to middle whorls (up to 50 mm in diameter): Shell fairly involute and compressed. Whorl section elliptical; venter rounded with indistinct ventral shoulders. Flanks gently convex; maximum width slightly below mid-flank to near umbilical shoulder. Umbilicus fairly narrow to moderate; umbilical wall moderately high and vertical; umbilical shoulder rounded. Ornamentation consists of numerous prorsiradiate ribs and prominent major ribs. Major ribs originate from distinct tubercles on umbilical shoulder, preceded by shallow constrictions. Rib intercalation typically occurs between inner and mid-flank. Ribs cross flank, bend gently forward at ventral shoulders, and traverse venter in a convex arch.
Later whorls (over 50 mm in diameter): Whorl section becoming more compressed with growth. Umbilical size remaining constant. Ribs significantly stronger and more widely spaced. Major ribs, arising from prominent umbilical tubercles, increase in both strength and frequency.
Measurements.—See File S1.
Discussion.— At least up to a shell diameter of 100 mm, the present species is distinguished from Canadoceras yokoyamai by its more compressed shell with an elliptical whorl section, gently convex flanks, slightly weaker ribs, and narrower umbilicus. It differs from C. mysticum in possessing distinct umbilical tubercles rather than weak bullae, gently convex rather than nearly flat flanks, and more pronounced ribs.
The specimen from California reported as Canadoceras fraternum (Gabb, 1869) by Anderson (1958) reaches a diameter of approximately 200 mm and resembles C. kossmati in its moderate umbilicus (U/D = 0.31). However, morphological data from early to middle growth stages are required for a definitive identification. Similarly, the specimens from Sakhalin reported as Pachydiscus aff. egertoni (Forbes) by Poyarkova (1987, pl. 26, fig. 3) and Zonova et al. (1993, pl. 86, figs. 1, 2) are badly deformed. Although their ornamentation resembles that of C. kossmati, precise identification remains difficult.
In contrast, the specimen reported as Canadoceras kossmati from Wakayama, southwestern Japan, by Misaki and Maeda (2009) is here re-identified as C. mysticum based on its umbilical width and whorl breadth. Similarly, the specimen reported by Ando et al. (2001) as C. kossmati is re-identified as C. newberryanum based on its subtriangular whorl section. Furthermore, the specimens from Sakhalin previously assigned to C. yokoyamai by Zonova et al. (1993: pl. 82, fig. 2; pl. 83, fig. 1; pl. 85, fig. 1) and Jagt-Yazykova (2011: pl. 8, figs. 1–3) are herein re-identified as C. kossmati based on their shell morphology and ornamentation.
Occurrence.—Middle Campanian (Sphenoceramus schmidti Zone) of Hokakido and Sakhakin.
Canadoceras mysticum Matsumoto, 1954
Canadoceras mysticum Matsumoto, 1954, p. 307, pl. 31, fig. 2, pl. 35, figs. 1, 2, text-figs. 75, 76; Matsumoto, 1959, p. 59, pl. 15, figs. 2, 3; Matsumoto and Miyauchi, 1984, p. 48, pl. 19, figs. 2, 3, pl. 20, fig. 3; Shigeta et al., 2019, p. 45, figs. 31E–X, 32; Poyarkova, 1987, pl. 26, fig. 6; Zonova et al., 1993, p. 161, pl. 73, figs. 1, 2; pl. 81, fig. 2; Ando and Ando, 2002, pl. 3, figs. 1, 2.
? Canadoceras mysticum Matsumoto. Zonova et al., 1993, p. 161, pl. 78, figs. 2, 3, pl. 83, fig. 1.
Canadoceras minimum Matsumoto and Miyauchi, 1984, p. 50, pl. 19, fig. 1, pl. 20, fig. 1, pl. 21, figs. 1, 2.
Canadoceras multicostatum Matsumoto. Matsumoto and Miyauchi, 1984, p. 47, pl. 20, fig. 2.
Canadoceras kossmati Matsumoto. Misaki and Maeda, 2009, fig. 8O, P.
Holotype.—KYUM GKH 5184, illustrated by Matsumoto (1954, p. 307, pl. 31, fig. 1), originating from the Sphenoceramus schmidti Zone in the Nakagawa area, Hokkaido. Its precise geographic and stratigraphic provenance remain unknown.
Material examined.—Twenty-six specimens (NMNS PM 15986, 45716–45740) collected from several fossiliferous beds within Unit F of the Orannai Formation exposed at the Kiyohama-1 fishing harbor near Cape Soya, northern Hokkaido.
Description.—Early whorls (up to 30 mm in diameter): Shell fairly involute and compressed. Whorl section elliptical with rounded venter and indistinct ventral shoulders; flanks nearly flat with maximum width below mid-flank to near umbilical shoulder. Umbilicus fairly narrow; umbilical wall moderately high and vertical; umbilical shoulder rounded. Ornamentation consists of numerous prorsiradiate ribs and prominent major ribs. Major ribs originate from very weak umbilical bullae on umbilical shoulder, preceded by shallow constrictions. Rib intercalation typically occurs between inner and mid-flank. Ribs cross flank, bend gently forward at ventral shoulders, and traverse venter in a convex arch.
Middle to later whorls (over 30 mm in diameter): Whorl section becoming more compressed with growth. Ribs significantly stronger and more widely distant. Major ribs, arising from prominent umbilical bullae, increase in both strength and frequency.
Measurements.—See File S1.
Discussion.—At least up to a shell diameter of 100 mm, Canadoceras mysticum species is distinguished from C. yokoyamai and C. kossmati by its more compressed shell with an elliptical whorl section, nearly flat flanks, a narrower umbilicus, weaker ribs, and weak umbilical bullae.
Matsumoto and Miyauchi (1984, p. 50) distinguished C. minimum from C. mysticum by its slightly less involute shell and somewhat wider umbilicus. However, the present study demonstrates that the morphological characters of the holotype of C. minimum fall within the range of intraspecific variation observed in C. mysticum. Consequently, C. minimum is herein regarded as a junior synonym of C. mysticum.
The specimens reported as Canadoceras kossmati from Wakayama, southwestern Japan, by Misaki and Maeda (2009), as well as the specimen from Unit F of the Orannai Formation (Kiyohama-1 fishing harbor) previously identified as C. multicostatum by Matsumoto and Miyauchi (1984, pl. 20, fig. 2), are here re-identified as C. mysticum based on their umbilical width and whorl breadth. Additionally, the specimens of C. yokoyamai from Sakhalin illustrated by Zonova et al. (1993, pl. 78, figs. 2, 3; pl. 82, fig. 1) are too poorly preserved for a definitive specific identification.
Occurrence.—Middle Campanian (Sphenoceramus schmidti Zone) of Hokkaido, Sakhalin, and southwestern Japan and Campanian of California.
1. Morphometric and statistical analyses demonstrate that the four species previously described from the Sphenoceramus schmidti Zone in Hokkaido represent only three valid species: C. yokoyamai (Jimbo, 1894), C. kossmati Matsumoto, 1954, and C. mysticum Matsumoto, 1954.
2. Canadoceras minimum Matsumoto and Miyauchi, 1984, is confirmed as a junior synonym of C. mysticum. The diagnostic features originally used to distinguish the two species fall within the continuous range of intraspecific variation and merely represent the juvenile ontogenetic stages of C. mysticum.
3. Each of the three valid species follows a distinct ontogenetic trajectory in terms of relative umbilical size, relative whorl thickness, and whorl expansion rate. These unique trajectories are established at a remarkably early stage of development. My results suggest that the diversification of Canadoceras was not the result of late-stage morphological divergence but was primarily driven by the interplay between modifications in early growth modes and the underlying geometric constraints of shell construction.
File S1, Measurements (in mm) of specimens of Canadoceras yokoyamai (Jimbo, 1894), C. kossmati Matsumoto, 1954, and C. mysticum Matsumoto, 1954 included in this study. D, shell diameter; U, umbilical diameter; H, whorl height; W, whorl width; <H>, holotype; <L>, lectotype.
File S2, Calculated geometric parameters for Canadoceras yokoyamai (Jimbo, 1894), C. kossmati Matsumoto, 1954, and C. mysticum Matsumoto, 1954 at a shell diameter of 30 mm. These values were calculated from allometric equations established between each geometric parameter and shell diameter. D, shell diameter; U, umbilical diameter; H, whorl height; W, whorl width; <H>, holotype; <L>, lectotype.
I express my sincere gratitude to Daisuke Aiba (Fukada Geological Institute, Tokyo), René Hoffmann (Ruhr University, Bochum), and the associate editor Ryoji Wani (Yokohama National University, Yokohama) for their insightful comments and valuable suggestions on an earlier version of the manuscript. I am deeply indebted to the late Toshiya Miyauchi, as well as to Toshihiro Sakai (Asahikawa), Yuzo Tachibana (Monbetsu), and Hitoshi Izu (Wakkanai) for their generous donation of the specimens used in this study. My thanks also go to The University Museum, The University of Tokyo (Tokyo) and the Kyushu University Museum (Fukuoka) for kindly providing the opportunity to examine critical type specimens under their care. Special thanks are extended to Gento Shinohara, Shuhei Nomura, and Takahiro Kutsuna (National Museum of Nature and Science) for their technical assistance and dedicated maintenance of the micro-CT scanner and analytical software in the Research Wing of the National Museum of Nature and Science (NMNS, Tsukuba).
