Damesites damesi ( Jimbo, 1894 )

Nishimura, Tomohiro & Maeda, Haruyoshi, 2025, Taxonomy and paleobiogeography of some Late Cretaceous desmoceratine ammonoids from the northwest Pacific province, Acta Palaeontologica Polonica 70 (2), pp. 259-283 : 262-271

Damesites damesi ( Jimbo, 1894)

Figs. 2A , 7–9 .

1890 Desmoceras sugata Forbes, 1846 ; Yokoyama 1890: 185, pl. 20: 11.

1894 Desmoceras Damesi sp. nov.; Jimbo 1894: 172, pl. 1: 2, 3.

1932 Desmoceras ( Kotôceras) semicostatus Yabe M.S. ; Nagao 1932: 178.

1942 Damesites damesi ( Jimbo, 1894) ; Matsumoto 1942a: 27, text­fig. 1e.

1942 Damesites semicostatus (Yabe M.S.) ; Matsumoto 1942a: 27, text­fig. 1h.

1954 Damesites damesi ( Jimbo, 1894) ; Matsumoto 1954: 267, pl. 5: 1–3.

1954 Damesites damesi intermedia subsp. nov.; Matsumoto 1954: 270, pl. 6: 4.

1955 Damesites semicostatus (Yabe M.S.) Matsumoto, 1942a ; Matsumoto and Obata 1955: 126, pl. 25: 1–5; pl. 26: 1–3; pl. 30: 6.

1955 Damesites damesi intermedius Matsumoto, 1955 ; Matsumoto and Obata 1955: 131, pl. 27: 1a–b.

1956 Damesites laticarinatus sp. nov.; Saito and Matsumoto 1956: 192–193, text­fig. 1a–c.

1958 Kotôceras frazierense sp. nov.; Anderson 1958: 217, pl. 40: 5, 5a.

1960 Damesites damesi ( Jimbo, 1894) ; Obata 1960: fig. 4b, pl. 15: 4.

1963 Damesites semicostatus (Yabe M.S.) Matsumoto, 1942a ; Tanaka 1963: pl. 1: 3a–b.

1978 Damesites semicostatus Matsumoto, 1942a ; Obata et al. 1978: fig. 3, pl. 2: 1a–c

1987 Damesites semicostatus Matsumoto, 1942a ; Tanabe and Shigeta 1987: figs. 2, 5A, B.

1989 Damesites sugata ( Forbes, 1846) ; Haggart 1989: 195–196, textfigs. 14–23, pl. 8.4.

1993 Damesites sugata ( Forbes, 1846) ; Alabushev and Wiedmann 1993: fig. 1A–E.

tongue-like elevation keel

?1996 Damesites damesi ( Jimbo, 1894) ; Cooper and Greying 1996: fig. 7I–L.

1996 Damesites semicostatus Matsumoto, 1942a ; Wright et al. 1996: fig. 64: 2c–e.

2000 Damesites damesi intermedius Matsumoto, 1954 ; Seki et al. 2000: pl. 1: 4.

2000 Damesites semicostatus Matsumoto, 1942a ; Seki et al. 2000: pl. 1: 5–6.

2000 Damesites sugata ( Forbes, 1846) ; Seki et al. 2000: pl. 1: 7–8.

2002 Damesites damesi ( Jimbo, 1894) ; Henderson et al. 2002: fig. 3A–C.

2003 Damesites damesi ( Jimbo, 1894) ; Nishimura 2003: fig. 2C–F.

2003 Damesites semicostatus Matsumoto, 1942a ; Nishimura 2003: fig. 2A, B.

?2005 Damesites cf. sugata ( Forbes, 1846) ; Maeda et al. 2005:102, 104, fig. 53: 1–6.

2009 Damesites sugata ( Forbes, 1846) ; Tsujino 2009: fig. 6A.

2010 “ Damesites damesi ( Jimbo, 1894) ”; Nishimura et al. 2010: figs. 4A–C, 8A–D, 9F, 11B, D, 12D, 13D–F, 15B, D, 16A–B.

2010 “ Damesites damesi intermedius Matsumoto, 1955 ”; Nishimura et al. 2010: figs. 1D, E, 16C, E.

2010 “ Damesites semicostatus Matsumoto, 1942a ”; Nishimura et al. 2010: figs. 4F–H, 8E–F, 9E, 12A, 13A–C.

2010 “ Damesites laticarinatus Saito and Matsumoto, 1956 ”; Nishimura et al. 2010: figs. 6E–G, 16D.

2011 Damesites damesi ( Jimbo, 1894) ; Honda et al. 2011: fig. 9B, C.

?2019 Damesites sp. ; Shigeta 2019 in Shigeta et al. 2019: fig. 27A–AN.

2023 Damesites aff. sugata ( Forbes, 1846) ; Tanabe and Misaki 2023: fig. 7.

2025 Damesites damesi ( Jimbo, 1894) , Nishimura 2025: fig. 20B–D.

Lectotype: UMUT MM 7500 (GT. I­91), originally described a middlesized specimen by Jimbo (1894) without designation of the type. Referred to as the holotype by Matsumoto (1954: 267) but should be treated as the lectotype.

Type locality: Float in Obirashibe­gawa River in Tappu area, Hokkaido, Japan.

Type horizon: Most likely Santonian.

Material.— In addition to the lectotype, more than 100 specimens in GK, UMUT from various Santonian–Campanian localities in Hokkaido and Sakhalin, as well as 134 specimens ( KUM MM TN 145–279 ) from the Haboro­Tappu and Nakagawa areas collected by us ( Table 1) .

Emended diagnosis.— Sigmoidal ribs vary from very weak to coarse, the w becoming large in the middle to late growth stages. Moderate to obtuse keel developing on adult venter. Faint furrows sometimes may appear on both sides of the keel. The w increasing with growth. U/D ratio decreasing with growth. Suture line complex, bifid external lobe and trifid lateral lobe and umbilical lobes.

Measurements. —See Table 2. Description. —In early growth stage (e.g., D 1 mm), whorl is depressed (B/H = 2.0) ( Fig. 10A, B, F ), and the whorl becomes compressed with shell growth. In middle to late growth stage, whorl is moderately compressed (B/H is about 0.75 at D 50 mm) ( Fig. 10A, B ; see Nishimura et al. 2010: fig. 17B, D). Umbilical shoulder appearing in 3–4π stage about D 1.5 mm) and a sharp umbilical break appears in 5–6π stage (about 2.5 mm in D) ( Fig. 10F ). U/D ratio decreasing with growth ( Fig. 10D ; see Nishimura et al. 2010: fig. 17C), changing from 0.3 (at 1 mm in D) to 0.08 (at 40 mm in D) ( Fig. 10D ). Moderate to obtuse keel appearing at 10 to

40 mm in D ( Figs. 8D 2 , 9D 2 ). Faint furrows sometimes appear on both sides of the keel particularly in individual with both shoulders inflated ( Figs. 7A 2 , 8A 1 ; lectotype figured in Nishimura et al. 2010: fig. 4C). The w increasing with growth, starting at about 2.0 in the early growth stage (about 20 mm in D), and increasing to 2.5 in the middle to late growth stage (about 50 mm in D) ( Fig. 10E ; see Nishimura et al. 2010: fig. 17F).

The curvature of growth line changing from single concave to sigmoidal at 15 mm to 50 mm in D ( Figs. 8C 2 , 9B 1 ; see Nishimura et al. 2010: fig. 13B, E). The single concave growth line runs nearly straight on both flanks and becomes gently projected forwards in the ventral periphery ( Figs. 8D 1, E 1 , 9C 2 , D 1 ). The succeeding sigmoidal growth line is slightly biconvex in lateral view, weakly or strongly flexuous on both flanks and strongly projected forward at the venter ( Figs. 7A 1, B 1 , 8A 2, B, C 2, F 2 , 9A 1, B 1 ; see Nishimura et al. 2010: figs. 4B, 13C, F). A rostrum appears at mid­venter in the middle to adult stage ( Fig. 9B 2 ).

Frequent desmoceratid constrictions occur as sharp grooves on internal mould in the early to middle growth stages (< 30 mm in D), while these are almost indiscernible on the shell surface, while exception of the ventrolateral shoul­ der ( Figs. 8D 1, E 1 , 9C 2 , D 1, E 1 ). Constrictions retain a single concave or weakly sigmoidal curvature throughout growth ( Fig. 9A 1 ). Unlike ancestral Desmoceras ( Pseudouhligella) spp., the curvature of constrictions does not completely match that of growth lines, particularly in the middle to late growth stages. The growth lines obliquely converge to the constrictions in the present species ( Nishimura et al. 2010: fig. 11B, D).

The curvature pattern of constriction is single concave or weakly sigmoidal; in contrast, the growth line is strongly sigmoidal in the middle to late growth stages. On the ventrolateral shoulder, patterns of constriction and growth line become similar ( Nishimura et al. 2010: fig. 11B, D).

Longitudinal striation crossing ribs gradually appears on the shell surface in the middle to late growth stages ( Figs. 8A 2, B, C 2 , 9C 2 , D 1 ). It is easily discernible in individuals with widely interspaced ribs but is indiscernible in specimens with narrowly interspaced ribs ( Nishimura et al. 2010: fig. 12).

Phase 1 in surface of shell ornament is characterized by almost smooth with very fine growth lines developing in the early to middle growth stages, at about 2–30 mm H ( 5–50 mm in D) ( Figs. 7B 3 , 8D 1, E 1 , 9A 1, B 1 , C 2 , D 1, E 1 ). The concave or slightly sigmoidal fine growth line is weakly convex on both flanks and moderately projected on the venter ( Figs. 8D 1, E 1 , 9A 1 , C 2 , D 1 ). These appear very weak in relief on the shell surface, while the internal mold is smooth ( Fig. 9E 1 ). The growth line is very faint, particularly on the inner flanks ( Figs. 8D 1, E 1 , 9A 1 , C 2 , D 1 ).

Phase 2 is characterized by weak ribbing on the shell surface. This phase follows phase 1 in the middle to late growth stages, although, the timing of change of phases 1 to 2 is variable among specimens ( Figs. 7A 1, B 1 , 8C 2 , D 1 ). The change from phase 1 to phase 2 is rather abrupt. The weak ribs, running parallel to fine growth lines, are moderately interspaced, sigmoidal, weakly convex on both flanks and moderately or strongly projected forward on the venter as on the rostrum ( Figs. 7A 1, B 1 , 8C 2 , D 1 , F 2 ). The rib profile is rounded to acute and most elevated on the ventrolateral shoulders and venter ( Figs. 7A 1, B 1 , 8C 2 , D 1 , F 2 ). Rib height decreases at the umbilical seam ( Figs. 7B 1 , 8C 2 , D 1 , F 2 ). Phase 3 is characterized by sharp and coarse ribbing ( Fig. 8A 2 , B; see Nishimura et al. 2010: fig. 13C). This phase follows phase 2 (weak ribbing), and appears in the middle to late growth stages, at about 15–25 mm H ( 25–45 mm in D) ( Nishimura et al. 2010: fig. 13C). The transition from phase 2 is rather abrupt. The coarse ribs are narrowly interspaced, sigmoidal, weakly convex on both flanks and strongly projected forwards on the venter ( Fig. 8A 2 , B; see Nishimura et al. 2010: fig. 13C). The rib profile is rounded to acute ( Fig. 8A 2 , B; see Nishimura et al. 2010: figs. 4B, 13C). The coarse ribs run parallel to growth lines. They are most elevated on the ventrolateral shoulders ( Fig. 8A 2 , B; see Nishimura et al. 2010: figs. 4B, 13C).

The coarse ribs at phase 3 also differ from weak ribs at phase 2 in the rib­height/H ratio. In the coarse ribs of phase 3, this ratio exceeds 0.0005 at the ventrolateral point at which the ribs are most elevated. In contrast, the ratio is less than 0.0005 in the weak ribs of phase 2.

The suture line is a typical Desmoceras pattern ( Matsumoto 1954: figs. 3–6), with a consisting of E, L, U2, U4, U5, U6 (= S), U3, U2, and I lobes and corresponding saddles. Trifid lateral and umbilical lobes are symmetrical. The U 2 lobe is slightly asymmetrical. Saddles show the bifid pattern. Lobes are deeply incised and make lobules ( Fig. 11A ; see Matsumoto 1954: figs. 10, 11).

Remarks. —Similar to Turonian Tragodesmoceroides subcostatus Matsumoto, 1942a , from Hokkaido (see Nishimura et al. 2006), the shell ornament of this species changes during growth. Shell surface ornament changes from phase 1 (= stage 1 in Nishimura et al. 2010) (i.e., almost smooth with very fine growth lines) through phase 2 (weak rib) to phase 3 (coarse rib) as growth progresses ( Fig. 12 ; see Nishimura et al. 2010: fig. 13D–F).

Unlike ancestral T. subcostatus ( Nishimura et al. 2006) , the constriction exists even in phase 2 (< 15 mm in D) in Damesites damesi ( Fig. 12 ; see Nishimura et al. 2006: fig. 11), although frequent constrictions in the early growth stage are a common feature of D. damesi and T. subcostatus ( Fig. 12 ; see Nishimura et al. 2006: fig. 11). Three phases of shell surface ornament appear in a common order, which seems to be ontogenetically fixed in D. damesi as well as in

NISHIMURA AND MAEDA — LATE CRETACEOUS DESMOCERATINE AMMONOIDS 267

A1 A2 10 mm A 3 A 4 A 5 A 6 B1 B2 C1 C 2

T. subcostatus ( Fig. 12 ; see Nishimura et al. 2006: figs. 8, 11; 2010: fig. 13A–C).

The interspacing of ribs becomes narrower in the last 10° whorl volution of the outer whorl of some large­sized specimens (KUM MM TN 193, Nishimura 2003: fig. 1D). This might be a sign of maturity. In the last growth stage, constrictions almost disappear, while a strong forwardly projected rostrum and strongly sigmoidal growth line develop.

In a smooth­surfaced specimen (KUM MM TN 218, Fig. 9A ), exceptionally weak ribs are developed on the inner shell.

Shell form and shell ornament of the present species change stratigraphically ( Figs. 12 , 14 ). Specimens from lower Santonian (units Ud–e and Uf in the Haboro­Tappu area; see Figs. 7 , 8 ) are large when adult ( 100 mm in D) ( Figs. 7A 1 , 8B ; see Nishimura 2003: fig. 2D) and have a great whorl breadth (B/H = 0.85 in 30 mm in D) (e.g., Fig. 7B 2 ), an earlier appearance of a keel (1.0–1.5 mm in D) ( Fig. 8D 2 ), and an elevated keel ( Figs. 7A 2 , 8A 1 ). The adult shell ornament reaches phase 3 ( Figs. 8A 2 , B, 12 ). On the other hand, specimens from upper Santonian (units Ug and Uh in the Haboro­Tappu area, Fig. 9 ) and the lower Campanian (units Ui–j and Uk in the Haboro­Tappu area) have a smaller shell size ( 40–60 mm in D), moderately compressed whorl (B/H = 0.8 in 30 mm in D; e.g., Fig. 9C 2 ), retarded appearance of a keel (at 20–30 mm in D; Fig. 9C 1 ) and a low, obtuse keel ( Fig. 9B 2 , C 1 ). The shell ornament remains in phase 1 even in adulthood ( Fig. 9 ). However, this morphological trend appears to be intraspecific, judging from the presence of intermediate forms such as an upper Santonian individual from unit Ug with an intermediate adult shell size (ca. 60 mm D; Fig. 9A, B ). Most specimens from the middle Campanian ( Sphenoceramus orientalis Zone and S. schmidti Zone ) individuals from Osoushinai Formation in the Nakagawa area ( Fig. 9B, E ) are small size, less than 30 mm D. These specimens are similar to the lower Campanian individuals (units Ui–j and Uk in the Haboro­Tappu area). On the contrary to these juvenile specimens, middle sized specimen about 48 mm D ( holotype of “ Damesites damesi intermedius ”, Matsumoto 1954 : pl. 6: 4a, b; Nishimura et al. 2010: fig. 4D, E) from the middle Campanian ( S. orientalis Zone ) possess a slightly depressed whorl (B/H = 0.8 in 48 mm in D) ( Table 2). The shell ornament remains in phase 1 even in adulthood.

Ikeda and Wani (2012) subdivided the Santonian desmoceratines into three related groups on the basis of “bathymetry” of an upward­shallowing succession in the Kotanbetsu area. Although their taxonomic treatment is out of date, their observation is generally concordant with our data. For example, the B of the Santonian and lower Campanian Damesites damesi becomes compressed in upward sequence. Such temporal change of D. damesi is not merely a local phenomenon because a similar pattern has been also observed in the other sections (e.g., the Naiba area, Sakhalin). In addition, the population sample of “ Damesites sugata ” in Ikeda and Wani (2012) includes not only specimens of P rectus gen. et sp. nov. ( Ikeda and Wani 2012: fig. 1.1) but also many specimens of D. damesi .

As described briefly in Shigeta et al. (2016), small desmoceratines previously identified as “ Desmophyllites diphilloides ( Forbes, 1846) ” from the Osoushinai Formation ( Sphenoceramus schmidti Zone ; middle Campanian), Nakagawa area (e.g., Matsumoto and Obata 1955), should be considered as juvenile individuals of Damesites damesi before the development of a keel because they have much wider umbilicus (U/D = 0.08 in 20 mm in D) than Desmophyllites diphylloides (U/D = 0.06 in 20 mm in D).

“ Damesites sp. ” in Shigeta et al. (2019) from the middle Campanian ( Sphenoceramus orientalis Zone ) of the Ribira area, Hokkaido, and “ Damesites cf. sugata ” in Maeda et al. (2005) from the middle Campanian ( Sphenoceramus orientalis Zone ) of the Makarov area, southern Sakhalin, should probably be included to Damesites damesi having sigmoid constriction in early growth stage. These specimens are quite similar to middle Campanian D. damesi from the Nakagawa area. However, it is difficult to accurately identify this specimen, because of early growth stage or outer shell is peeled off.

“ Desmoceras sugata ” in Yokoyama (1890: pl. 20: 11a–c) is here reassigned to the upper Santonian or lower to middle Campanian form of D. damesi .

Some individuals of “ Damesites sugata ” have been reported from the Santonian to the lower Campanian of British Columbia, Canada by Haggart (1989). These are here reassigned to Damesites damesi because of their sigmoid growth lines and shell form.

As for the northeast Pacific province’s species, Coniacian or Santonian (lower Senonian in Anderson 1958) Kotoceras frazierense Anderson, 1958 , is characterized by very compressed shell form, but this character is in fact the result of post­burial compaction. Sigmoidal weak ribs and shell shape show this “species” to belong to Damesites damesi .

A specimen of “ Damesites sp. ” from the upper Cenomanian (?) of the Western Interior, North America ( Saito and Matsumoto 1956, without illustration; a plaster cast, GK. H 20027 is deposited at GK.) is assignable to Tragodesmoceroides sp. on account of its much wider umbilicus and tongue­like elevation on the venter instead of keel.

As discussed in Nishimura et al. (2010), “ Damesites damesi ”,“ D. damesi intermedius ”,“ D. semicostatus ”, and “ D. laticarinatus ” should be lumped to a single biological species. For example, “ D. damesi ” and “ D. semicostatus ” co­occurring in the lower Santonian cannot be discriminated from each other because of the presence of numerous intermediate forms ( Nishimura et al. 2010: figs. 9, 10, 12).

“ Damesites laticarinatus ” (see Saito and Matsumoto 1956: text­fig. 1a–c; Nishimura et al. 2010: fig. 6E–G) is a monotypic species defined by almost smooth shell surface and diagnostic broad keel. The fragmentary holotype (GK. H 20026) was recovered from a river float. Saito and Matsumoto (1956) presumed that the holotype came from the lower Cenomanian (?), but this stratigraphic assignment appears to be questionable by the following reason. Wide continuous variation in the profile of the keel of “ D. damesi ” suggests that it is difficult to discriminate “ D. laticarinatus ” from the other forms on the basis of the keel ( Nishimura et al. 2010: fig. 16). “ Damesites laticarinatus ” has sigmoidal growth lines and narrowly umbilicate (U/D is about 0.07 at 50 mm in D) compressed whorls (B/H is about 0.7) without constrictions in the late growth stage. These are common features in the late Santonian forms of D. damesi (e.g., KUM MM TN 218, 229, see Fig. 9A ; compare Nishimura et al. 2010: fig. 13D–F). The preservation of the shell casts doubt about a lower Cenomanian origin of “ D. laticarinatus ” holotype because dark brown color of both shell tests and sparry calcite in the camera are dissimilar to those of the other ammonoids from lower Cenomanian in the Ikushunbetsu area (white sparry calcite in camera). Therefore, the Cenomanian affinity for “ D. laticarinatus ” is apparently erroneous, and it should be assigned to the upper Santonian form of D. damesi ( Fig. 9 ).

“ Damesites damesi intermedius ” (see Matsumoto 1954: pl. 6: 4a, b; Nishimura et al. 2010: fig. 4D, E) recorded from Japan, Sakhalin, and California, was said to discriminated from “ D. damesi damesi ” by its weakly sigmoidal constrictions ( Matsumoto 1954, 1959a). “ Damesites damesi intermedius ” was also assumed as a morphologic mosaic for having “ D. damesi ”­like whorl shape and “ D. sugata ” (= P. rectus gen. et sp. nov. herein)­like nearly concave constrictions from the Yezo Group ( Matsumoto 1954; Matsumoto and Obata 1955). However, discrimination of ” D. damesi damesi ” and “ D. damesi intermedius ” on the basis of the constriction curvature is difficult because in the former it is also weakly sigmoidal ( Nishimura et al. 2010: fig. 11D, E). Constriction curvature of “ D. damesi intermedius ” is more closely similar to that of “ D. damesi damesi ” than that of “ D. sugata ” from the Yezo Group (= P. rectus gen. et sp. nov. herein) ( Nishimura et al. 2010: fig. 11). Matsumoto and Obata (1955) treat D. damesi intermedius as an intermediate form between D. damesi damesi and “ D. sugata ” from the Yezo Group by shape of constriction, however, shape of constriction, shape of growth line, w, etc. do not show an ancestor­descendant relationship between these two subspecies. Diagnostic feature of the holotype of “ D. damesi intermedius ” (GK. H 3269) from the middle Campanian of the Urakawa area ( Matsumoto 1954) is its greater B. Most similar specimens occur in the lower Santonian (KUM MM TN 145, Fig. 7B ). This, “ Damesites damesi intermedius ” occur from around first appearance datum (FAD) and last appearance datum (LAD) of “ Damesites damesi ”; lower Santonian and middle Campanian. In addition, the difference of B/H ratio between the two subspecies of D. damesi is slight, and an intermediate form exists between them as in other morphological characters ( Nishimura et al. 2010). Considering those evidences, “ D. damesi intermedius ” should be lumped with D. damesi . Another specimen, UMUT MM 6747 (previously listed as “ D. damesi intermedius ”), has a moderately compressed whorl, and resembles the lower Santonian– lower Campanian forms of D. damesi .

Damesites damesi resembles Tragodesmoceroides subcostatus Matsumoto, 1942a , in whorl shape (ca. 0.8 in B/H ratio at 30 mm in D), curvature of growth lines, threephased ontogenetic development of surface ornament and frequently developed constrictions in early growth stage (see Nishimura et al. 2006: figs. 9C, 10, 11). Damesites damesi possesses a keel from the juvenile to middle growth stages (i.e., 10–50 mm in D) ( Figs. 8D 2 , 9D 2 ). Tragodesmoceroides subcostatus , in contrast, possesses no keel except for a blunt tongue­like ventral elevation ( Fig. 2C 2 ; compare Nishimura et al. 2006: fig. 4B). The latter species has a wider umbilicus at the juvenile to middle growth stages (U/D ratio 0.11, at 30 mm in D) compared to the present species (U/D = 0.08, at 30 mm in D) ( Nishimura et al. 2010: fig. 17C).

Damesites damesi also resembles the lower Maastrichtian D. hetonaiensis Matsumoto, 1954 , in its sigmoidal growth lines, frequently developed constrictions in the early growth stage, and possession of a keel. The B is also similar in the two specimens (B/H = 0.80 in 30 mm in D). However, D. damesi differs from D. hetonaiensis in having a wider keel ( Matsumoto 1954: pl. 4, fig. 1c). The umbilicus of D. hetonaiensis is much narrower (U/D = 0.06 in 50 mm in D) than that of D. damesi (U/D = 0.08 in 50 mm in D).

There are similarities among D. damesi , P. sugata , and P. rectus gen. et sp. nov.; all have a keel. However, D. damesi is discriminated from Paradamesites spp. by having sigmoidal growth lines ( Figs. 7A 1, B 1 , 8A 2, B, F 2 ), a larger w ( Nishimura et al. 2010: fig. 17F) and appearance of ribs in the late growth stage (phase 3) ( Fig. 12 ).

Stratigraphic and geographic range. —Santonian–middle Campanian, north Pacific Realm.

Figures

Fig.2.Schematic illustrationof“Damesites”andTragodesmoceroidesshells.A.Damesitesdamesi.B.Paradamesitesrectussp.nov.C.Tragodesmoceroides subcostatus. D. Paradamesites sugata. In left lateral (A1–D1) and ventral (A2–D2) views.

Fig. 7. Desmoceratine ammonoid Damesites damesi (Jimbo, 1894) from the lowest Santonian (Unit Ud–e) of the Yezo Group, Hokkaido, Japan. A. KUM MM TN 146 (deformed and compressed laterally),loc.OT1601, in left lateral (A1) and ventral (A2) views.B. KUM MM TN 145, loc.TK4111, in left lateral (B1, B3) and apertural (B2) views.An open circle marks the position of constrictions. The bold arrow show the ontogenetic change of a single specimen.

Fig.8. Desmoceratine ammonoid Damesites damesi (Jimbo, 1894) from the lower Santonian (Unit Uf) of theYezo Group, Hokkaido,Japan.A. KUM MM TN 158, loc. OT7027, in ventral (A1) and right lateral (A2) views. B. KUM MM TN 193, loc. OT1429 (see Nishimura 2003: fig. 2D–F), in right lateral view. C. KUM MM TN 166, loc. OT7027, in ventral (C1) and right lateral (C2) views. D. KUM MM TN 189, loc. OT7028, in left lateral (D1) and ventral D2) views. E. KUM MM TN 183, loc. OT7028, in left lateral (E1) and ventral (E2) views. F. KUM MM TN 200, loc. OT1599, in ventral (F1) and right lateral (F2) views.An open circle marks the position of constrictions.

Fig. 9. Desmoceratine ammonoid Damesites damesi (Jimbo, 1894) from the upper Santonian (Unit Ug–h) (A, D) and middle Campanian, Osoushinai Formation, Nakagawa area (B, C, E) of the Yezo Group, Hokkaido, Japan. A. KUM MM TN 218, loc. TK1043 (Unit Ug), in right lateral (A1) and apertural (A2) views. B. KUM MM TN 264, in right lateral (B1) and ventral (B2) views. C. KUM MM TN 265, loc. NS1009 of Matsuda and Ubukata (1999), Sphenoceramus orientalis Zone, in ventral (C1) and right lateral (C2) views. D. KUM MM TN 239, loc. TK1019 (Unit Ug), in left lateral (D1) and ventral (D2) views. E. KUM MM TN 272, a float near loc. NS1007 of Matsuda and Ubukata (1999), in left lateral (E1) and ventral (E2) views. An open circle marks the position of constriction.

Fig. 10. Ontogenetic changes of whorl dimensions (A, C) and whorl shape (B, D, and E) in relation to shell D, and diagrammatic drawings of median shell cross section (F–H) for representative specimens of desmoceratine ammonoid species. F. Damesites damesi (KUM MM TN 206, loc. OT1600), G. Paradamesites rectus gen. et sp. nov. (KUM MM TN 288, a float from Kotanbetsu River). H. Paradamesites sugata (KUM MM TN 134, loc. HB1028). Abbreviations: B/H, ratio of whorl breadth (B) to whorl height (H); U/D, ratio of width of umbilicus (U) to diameter (D).

Fig. 1. Maps of southern Sakhalin and Hokkaido. A. Map of southern Sakhalin and Hokkaido. B. Geological map showing Yezo Group deposits and location of the study area.

Fig.3. Desmoceratine ammonoid Paradamesites sugata (Forbes,1846) from the uppermost Turonian of theYezo Group in Hokkaido,Japan (= “Damesites ainuanus Matsumoto, 1957”). A. HMG­1623, topotype of “Damesites ainuanus” from the type locality (loc. IK 2013), in left lateral (A1) and ventral (A2) views. B. GK. H 4200, paratype of “Damesites ainuanus” (loc. IK 2013), in left lateral (B1) and apertural (B2) views. C. GK. H 4198, paratype of “Damesites ainuanus” (loc. IK 2013), in right lateral (C1) and ventral (C2) views.An open circle marks the position of constrictions.

Fig. 11. Suture line of desmoceratine ammonoids from Yezo Group, Hokkaido, Japan. A. Paradamesites rectus gen. et sp. nov. KUM MM TN 284, at the 19.50 mm H, Ui–j Unit of the tributary ofAkanosawa­Creek, Tappu area. B. Padradamesites sugata (Forbes, 1846). KUM MM TN 129, at the 19.65 mm H, the Nakafutamata­zawa Creek, Haboro (same specimen as Fig. 5A). C. Damesites damesi (Jimbo, 1894). KUM MM TN 164, at the 17.15 mm H, the Jugosen­zawa Creek, Tappu area. Broken line shows umbilical edge.Arrows show position of siphuncle and adoral direction.Abbreviations: E, L, U, external (E), lateral (L), and umbilical (U) lobes; us, umbilicus.

Fig. 12. Schematic diagram showing ontogenetic development of shell ornament change in late Turonian–early Campanian desmoceratine ammonoids (S­group, Damesites damesi) from the northwest Pacific province. Stratigraphic and morphological dates are collected from the Haboro­Tappu and Pombetsu areas, Hokkaido. Detailed ontogenetic development of Turonian Tragodesmoceroides subcostatus of S­group is shown in Nishimura et al. (2006). Each population sample and its horizon showing a–m. Ontogenetic development stages; phases 2 and 3 are recognized in Damesites damesi. Horizontal dashed lines show localities of molluscan fossil occurrence (see SOM: fig. S4).

Fig.14. Revised stratigraphic occurrence, morphological change in single species and phylogenetic relationship of the Turonian–early Campanian desmoceratine ammonoids in the northwest Pacific province. Desmoceratine ammonoids comprise two different lineages (S­ and C­groups).Stratigraphic occurrences of Tragodesmoceroides subcostatus are based on Nishimura et al. (2006). The last occurrence of Damesites damesi is confirmed the middle Campanian Sphenoceramus schmidti Inoceramid Zone, Osoushinai Formation,Nakagawa area.This inoceramid zone does notexist in the Haboro­Tappu area and is located in more upper horizon of Uk unit.The origin of C­group Paradamesites spp. is presumed in outside of the northwest Pacific province and Paradamesites sugata migrated in the latest Turonian to the Coniacian. Horizontal dashed lines show localities of molluscan fossil occurrence (see SOM: fig. S4).