Paradamesites sugata ( Forbes, 1846 ), 2025

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 : 272-279

Paradamesites sugata ( Forbes, 1846) comb. nov.

Figs. 2D , 3–5 .

1846 Ammonites Sugata sp. nov.; Forbes 1846: 113: pl. 10: 2a–c.

1865 Ammonites Sugata Forbes, 1846 ; Stoliczka 1865: 60, pl. 32: 4–6; pl. 33: 1, 2.

1898 Desmoceras Sugata Forbes, 1846 ; Kossmat 1895 –1898: 111 (176), pl. 18 (24): 11; pl. 19 (25): 1.

1921 Hauericeras? sugata ( Forbes, 1846) ; Spath 1921: 46, pl. 6: 3.

1921 Desmoceras compactum n. sp.; Hoepen 1921: 21–23, pl. 4: 5–7.

1931 Desmoceras Sugata ( Forbes 1846) ; Basse 1931: 21, pl. 2: 19, 20.

non 1955 Damesites sugata ( Forbes, 1846) ; Matsumoto and Obata 1955: 128, pl. 26: 4, pl. 27: 3, 4.

1957 Damesites sugata ( Forbes, 1846) ; Arkell et al. 1957: L370, fig. 482: 4.

1957 Damesites ainuanus sp. nov.; Matsumoto 1957: 86, pl. 15: 1, 2.

1958 Kotȏceras richardsoni sp. nov.; Anderson 1958: 217, pl. 36: 3, 3a.

1958 Kotȏceras subsugatum Anderson, 1902 ; Anderson 1958: 217– 218, pl. 35: 2, 2a.

1961 Desmoceras sugatus ( Forbes, 1846) ; Collignon 1961: 67, pl. 27: 1, 2, text­fig. 7.

1965 Desmoceras sugata ( Forbes, 1846) ; Collignon 1965: 20, pl. 421: 1751.

1968 Damesites ainuanus Matsumoto, 1957 ; Howarth 1968: 222, pl. 1: 3–5.

1968 Damesites sugata ( Forbes, 1846) ; Pauliuc 1968: 103, pl. 29: 2, 3.

1969 Damesites petrobrasia sp. nov.; Beurlen 1969: 149–150, pl. 2: 1, pl.4: 1a, 1b.

1980 Damesites ainuanus Matsumoto, 1957 ; Futakami et al. 1980: pl. 1: 3.

1980 Damesites compactus ( Hoepen 1921) ; Summesberger 1980: 278–280, pl. 1: 3, 4.

1981 Damesites spp. ; Szász 1981: 101, pl. 2: 3a, 3b.

1983 Damesites sugatus ( Forbes, 1846) ; Collignon 1983: 190, pl. 2: 4.

1987 Damesites damesi Jimbo, 1894 ; Poyarkova 1987: pl. 24, fig. 5a–c.

1991 Damesites sugata (Forbes. 1846) ; Kennedy and Henderson 1991: 471, figs. 1A–I, 2A–G.

1996 Damesites sugata ( Forbes, 1846) ; Wright et al. 1996: 84, fig. 64: 2a, b.

2003 Damesites ainuanus Matsumoto, 1957 ; Cooper 2003a: 109–110, fig. 3C, D

2003 Damesites sugata ( Forbes, 1846) ; Cooper 2003b: 155–157, fig. 6E, F

2010 “ Damesites ainuanus Matsumoto, 1957 ”; Nishimura et al. 2010: figs. 5A–B, 11A, 14D–F, 16H.

2010 “ Damesites sp. ”; Nishimura et al. 2010: figs. 5C–F, 7C–F, 15A, 16F–G.

2011 Damesites ainuanus Matsumoto, 1957 ; Jagt­Yazykova 2011: pl. 4: 2–6.

2012 Damesites aff. sugata ( Forbes, 1846) ; Tanabe et al. 2012: fig. 5A–C.

2013 Damesites sugata ( Forbes, 1846) ; Kennedy and Klinger 2013: fig. 7A–J.

2020 Damesites sugata ( Forbes, 1846) ; Kennedy and Gale 2020: fig. 9A–I.

Type material: Lectotype BMNH C22674 , the original of Forbes

(1846) is small­sized specimen from South India. Paralectotypes :

BMNH C24169 a, b and BMNH C22675 are small­sized specimens as lectotype. These type specimens are said to be collected from Ariyalur

Group of Vridachellum , South India. Lectotype and paralectotypes are designated by Matsumoto (1942a) .

Type locality: Vridachellum (Verdachellum) in Madras state, South

India.

Type horizon: Coniacian or Santonian.

Material. — Five specimens ( BMNH C22674, C27169a, b, C22675, KUM MM TN 292) from South India including the type series of Forbes (1846). Five specimens ( GSI 145–148, 14870) from South India of Stoliczka (1865) and Kossmat (1895 –1898). One specimen was collected probably from Coniacian of the Varagur in Madras state, South India ( KUM MM TN 292; collected from 11°15’16.3”N, 79°02’51.5”E; Fig. 5B). About 10 registered specimens from the Yezo Group in various areas of Hokkaido and Sakhalin are deposited in the UMUT and GK. Additionally, 45 specimens from the uppermost Turonian–Coniacian in the Pombetsu and Haboro­Tappu areas in the Yezo Group are HMG­1623, 1794, 1795, and KUM MM TN 099–144 ( Table 1).

Emended diagnosis. —Narrowly umbilicate whorl with a moderate to acute high keel. Constriction is concave and becoming weakly flexuous on both flanks in later growth stage. The relative umbilical width versus shell D gradually increases during ontogeny (see Nishimura et al. 2010: fig. 17C). Suture line is complex, bifid external lobe and trifid lateral, umbilical, and internal lobes.

Measurements. —Shown in Table 2.

Description. —An acute, narrow, and high ventral keel develops after the early growth stage at < 30 mm in D in the lectotype and paralectotype collected from South India ( BMNH C22674, 22645 and 24196a, b). Both flanks of the whorl are flat and nearly parallel. The umbilicus is crater­like rounded in umbilical edge and slightly wider (U is 3.5 mm, 37 mm in D) than that in the species of the revised genus Damesites . The shell surface is almost smooth except for fine growth lines and periodic constrictions. The curvature of constriction is moderately prorsiradiate and single concave to the aperture.

Compared with the Indian types, specimens from the Yezo Group are better preserved in general and retain details of morphological change through growth as follows.

Whorls become more compressed during growth ( Fig. 10B, H ). In the early growth stage ( 1 mm in D), the whorl is depressed (B/H = 1.7), while it is moderately compressed in the middle to late growth stage (B/H is about 0.7 at 50 mm in D) ( Fig. 10B, H ; see Nishimura et al. 2010: fig. 17D).

The umbilical shoulder appears in the 3–4π stage and a desmoceratid umbilical break ( Maeda 1993) appears in the 5–6π stage ( Fig. 10H ). The U/D ratio decreases with growth, changing from 0.2 ( 1 mm in D) to 0.08 ( 30 mm in D) ( Fig. 10D ; see Nishimura et al. 2010: fig. 17C, H).

The w is small (about 2.2 at 50 mm in D) and is stable throughout growth ( Fig. 10E ; see Nishimura et al. 2010: fig. 17F).

The shell of the late growth stage attaining 80–90 mm in D consists of 6–7 whorl volutions ( Fig. 10H ). At venter, prorsiradiate and simple concave growth lines flex strongly forwards to form a projected rostrum ( Figs. 3A 1 , 4A 2 , 5A 2 ; see Nishimura et al. 2010: fig. 5E). Shell surface ornament is almost smooth except for periodic constrictions throughout growth (phase 1, Fig. 13 ). The constrictions become frequent in the last 30° of the outer whorl in some specimens GK. H 4198, KUM MM TN 129, Nishimura et al. 2010: fig. 5B, E). This may be a sign of maturity.

Growth lines are prorsiradiate and single concave Figs. 3–5 ). Only in the late growth stage weakly sigmoidal growth lines restrictedly appear on the periphery of constrictions although they retain a single concave pattern in most areas ( Nishimura et al. 2010: fig. 15A). The curvature of constriction changes from single concave to weakly sigmoid in the late growth stage (about 60 mm in D). Constrictions are well developed in the inner shell, and show a very weak rise in the outer shell. For example, the specimen with shell D of 80 mm; GSI 148 ( Stoliczka 1865: pl. 33: 1) has periodic constrictions showing weakly sigmoid curvature without growth lines, because of the weathered surface of the shell.

The suture line is a typical Desmoceras pattern ( Matsumoto 1954: 248), with a formula of [E, L, U 2, U 4, U 5, U 6 (= S), U 3, U 2, I]. Trifid lateral, umbilical, and internal lobes are symmetrical. The U 2 lobe is slightly asymmetrical. Saddles show the bifid pattern. Lobes are deeply incised and make many lobules ( Fig. 11B ).

Remarks. —In profile, the keel becomes gradually higher in an upward sequence in the Yezo Group of the northwest Pacific province; i.e., the latest Turonian “ D. ainuanus ” ( Fig. 3 ) via early Coniacian forms (from Unit Ua, Ub–c; Fig. 4 ) to late Coniacian forms (= “ Damesites sp. ” from Unit Ud–e; Fig. 5A). The height of keel changes from 0.2 mm (latest Turonian) to 0.6 mm (late Coniacian) via 0.4 mm (early Coniacian) at 20 mm H. Late Coniacian forms are most similar to South Indian P. sugata , inclusive of the types of Forbes (1846).

The morphology of the keel changes stratigraphically within the single biospecies P. sugata . This trend is confirmed by biostratigraphic observations in the northwest Pacific and African­Indian provinces. “ Damesites ainuanus ”, an early form with a narrow and low keel is reported not only from the upper Turonian of Hokkaido ( Matsumoto 1957) and Sakhalin ( Jagt­Yazykova 2011) but also found from the middle and upper Turonian of Angola ( Howarth 1968; Cooper 2003a). Paradamesites sugata ­like specimens were also reported from the lower Coniacian of South India ( Kennedy and Gale 2020). These specimens are identified as “ Damesites sugata ,” however, they possess a narrow and low keel. One of the specimens, OUM KY 2218 possesses a slightly broader and lower keel. This specimen should be identified as “ Damesites ainuanus ” in the previous scheme.

Paradamesites sugata was first described by Forbes (1846) as Ammonites Sugata. Many specimens from various areas have been identified as the present species ( Stoliczka 1865; Kossmat 1895 –1898; Spath 1921; Collignon 1961, 1965, 1983) or listed without illustration ( Chiplonkar et al. 1985). Kennedy and Henderson (1991) first photographically documented the lectotype, paralectotypes, and topotypic adult shell of Paradamesites sugata . According to Kennedy and Henderson (1991), Forbes’ (1846) specimens ( lectotype and paralectotypes) are said to have been collected from the Ariyalur Group of Vridachellum (Verdachellum) in Madras state, South India. Kossmat (1895 –1898) reported this species from the upper part of the Trichinopoly Group and the lower part of the Ariyalur Group, both of which were correlated to the Coniacian–Santonian. In Madagascar, this species ranges from the Coniacian to Santonian ( Collignon 1961), whereas in Angola, it occurs throughout the Santonian ( Cooper 2003b).

UMUT MM 27833 ( Damesites sugata in Tanabe and Landman 2002; Damesites aff. sugata in Tanabe et al. 2012) with a narrow and moderate high keel is identified as the early Coniacian form of P. sugata (e.g., Fig. 4 ).

The holotype and the other examples of the northeast Pacific province’s species Coniacian “ Kotôceras subsugatum Anderson, 1902 ” (figured in Anderson 1958: pl. 35: 2, 2a for the holotype) were lost in the San Francisco Fire of 1906. Judging from illustrations of the holotype, however, due to its almost smooth shell surface, slightly wider umbilicus, moderate height, and slightly narrower keel it resembles those of P. sugata .

The monotypic species of “ Damesites pterobrasia Beurlen, 1969 ” was reported from the Coniacian strata of Brazil. Judging from Beurlen’s (1969) photograph and illustration, the large shell ( 100 mm D), small w, slightly wider umbilicus, almost smooth shell surface and concave growth line suggest this “species” to be conspecific to P. sugata .

“ Damesites sugatus ” from Santonian of Spain, as recorded by Collignon (1983) has a slightly wider umbilicus (U/D = 0.12 in 67 mm D) and a narrow keel. These characteristics match those of P. sugata . However, we treated them as Paradamesites cf. sugata , because, Collignon (1983) does not show a photograph of the lateral view.

“ Damesites sugata ” was also recorded from the upper Turonian of Romania ( Pauliuc 1968). Based on photographs of two specimens, the small w (w = 1.96 at 23.9 mm in D and w = 2.03 at 44.8 mm in D), moderate or slightly wider umbilicus, and almost smooth shell surface ornamentation match diagnostic features of Paradamesites gen. nov. It is, however, difficult to identify species because of the only lateral view of photographs ( Pauliuc 1968). Thus, these Romanian specimens should be treated as Paradamesites cf. sugata according to our present knowledge.

“ Damesites spp. ” reported from the Coniacian of Romania ( Szász 1981), have a siphonal keel and a moderately wide umbilicus, and are similar to Paradamesites sugata . However, precise identification is difficult because of the small size and fragmentary nature of these specimens.

The shape of the keel in “ Damesites sp. ” from the “upper Cenomanian” in Germany ( Immel et al. 1981: fig. 2.2a, b) resembles that of the Coniacian forms of P. sugata . This record is important to reconstruct the evolutionary processes of the Damesites group, although the specimen is fragmentary. In addition, there is inconsistency in the biostratigraphic correlation between ammonoids ( Subprionocyclus Zone suggesting the upper Turonian) and planktonic foraminifera ( Helventogrobotruncana helvetica suggesting the lower and middle Turonian) for this German section. A more detailed interpretation of the German record will be postponed until the stratigraphic correlation is determined.

As far as northeast Pacific province’s species, Coniacian or Santonian “ Kotôceras richardsoni Anderson, 1958 ” is characterized by a small w, a slightly large umbilicus, frequent sigmoidal constrictions, a siphonal rostrum, and a siphonal keel. Although shell surface ornamentations in the holotype of this “species” have been abraded (see Anderson 1958: pl. 36: 3, 3a), what can be seen is identical to that of the Coniacian form of P. sugata .

Lower Campanian “ Damesites damesi ” from South Africa ( Cooper and Greyling 1996) is conspecific with P. sugata , because of its almost smooth shell ornament and siphonal keel. Cooper and Greyling (1996) described that the South African “ Damesites damesi ” has a biconcave peristome in the internal mold of the adult shell (ca. 92 mm in D). Around the constrictions, the shape of the growth lines of P. sugata of C­group exceptionally appears to be biconcave.

Kennedy and Klinger (2013) regarded “ Damesites compactus ( Hoepen, 1921) ” as a probable synonym of P. sugata . The present study agrees with Kennedy and Klinger (2013), and the differences between “ Damesites compactus ” and P. sugata are slight.

Compared with the Indian types, a detailed examination of specimens from the Yezo Group has revealed that Damesites sp. ” from the upper Coniacian ( Fig. 5A; see Nishimura et al. 2010: fig. 5C–F) should be identified as true P. sugata . These specimens possess all diagnostic characteristics of Paradamesites sugata such as the development of an acute keel from an earlier growth stage onwards. In contrast, the identification of Santonian “ D. sugata ” from the Yezo Group ( Fig. 6 ; see Nishimura et al. 2010: fig. 6A–D) is erroneous; and these specimens should be reassigned to P. rectus gen. et sp. nov. The stratigraphic range of the true P. sugata seems to be the middle Turonian to Santonian in India, Madagascar, and Angola ( Forbes 1846; Collignon 1961, 1965; Cooper 2003b; Chiplonkar et al. 1985).

Paradamesites sugata superficially resembles Damesites damesi in features of whorls and keel, but differs from the latter in having a smaller w (2.2 in the former compared to 3.0 in the latter at 50 mm in D), a slightly wider umbilicus U/D is 0.09 in the former and 0.08 in the latter), frequently developed periodic constrictions in later growth stage and almost smooth shell surface ornament.

Stratigraphic and geographic range.— Uppermost Turonian– Coniacian of Hokkaido and Sakhalin; middle Turonian– Santonian of South India, Madagascar, South Africa, Angola, and Spain.

Paradamesites rectus sp. nov.

Figs. 2B , 6 .

1927 Kotôceras damesi ( Jimbo, 1894) ; Yabe 1927: pl. 7 (5): 9a, b.

1942 Damesites sugatus ( Forbes, 1846) ; Matsumoto 1942a: 27, text­fig. 1f.

1955 Damesites sugata ( Forbes, 1846) ; Matsumoto and Obata 1955: 128, pl. 26: 4, pl. 27: 3, 4.

2003 Damesites sugata ( Forbes, 1846) ; Nishimura 2003: fig. 2G, H.

2010 “ Damesites sugata ( Forbes, 1846) ”; Nishimura et al. 2010: figs. 6A–D, 7A, 7B, 11C, 14A–C, 15C.

2012 Damesites sugata ( Forbes, 1846) ; Ikeda and Wani 2012: fig 1.1. ZooBank LSID: urn:lsid:zoobank.org:act:.

Etymology: From Latin rectus , straight; after simple concave, nearly straight shape of growth line in mid flank.

Type material: Holotype KUM MM TN 281 ( Fig. 6A ) is a mediumsized specimen . Paratype KUM MM TN 281 ( Nishimura et al. 2010: fig. 6C, D) is associated with the holotype in the same calcareous concretion . Paratypes ( KUM MM. TN 280 , 282–291 ( Fig. 6 ), see Nishimura et al. 2010: figs. 6C, D, 7A, B, 11C, 14A–C, 15C) are variously sized shells collected from the Santonian–Campanian in the Haboro­Tappu and Nakagawa areas .

Type locality: Akanosawa Creek (locality OT5034), Tappu area, Hokkaido, Japan ( SOM: fig. S3) .

Type horizon: Santonian.

Material. — The type material and ten additional specimens from the Santonian–Campanian strata of the Yezo Group at various localities in Hokkaido, Japan and Sakhalin, Russia. These are deposited in the UMUT and the GK collections ( Table 1) .

Diagnosis. —Widely umbilicate shell for desmoceratines with an obtuse keel. Growth lines and constrictions are less flexuous and simple concave in shape. Suture line is complex, bifid external lobe and trifid lateral and umbilical lobes.

Measurements. —See Table 2.

Description. —Whorl becomes compressed as growth progresses. B/H ratio changes from 2.0 ( 1 mm in D) to 0.75 ( 60 mm in D) ( Fig. 10B ; see Nishimura et al. 2010: fig. 17B, D).

The umbilical shoulder appears in the 3–4π stage and a desmoceratid umbilical break appears in the 6–7π stage ( Fig. 10G ). U/D ratio decreases during 5π to 10π stage (0.2 to 1.0, about 0.6 mm to 10 mm in D), increases again after 10π stage (U/D = 0.15 in 50 mm in D) ( Fig. 10D ). The w is small (about 2.2 at 50 mm in D) ( Fig. 10E ; see also Nishimura et al. 2010: fig. 17E) and is stable throughout growth. An obtuse keel develops at the venter after 20 mm in D ( Fig. 6B 1 ; Nishimura et al. 2010: fig. 6C).

During the late growth stage, the shell attains 90–100 mm D and consists of 7–8 whorls ( Fig. 10G ; see also Nishimura et al. 2010: fig. 6A–B). The whorl is compressed (B/H ratio is 0.75 at 50 mm in D) ( Fig. 10B ; see also Nishimura et al. 2010: fig. 17D). The umbilicus is wide (U/D ratio is 0.15 in 50 mm in D) ( Fig. 10D ; see also Nishimura et al. 2010: fig. 17C). Curvature of growth line is prorsiradiate and single concave throughout growth ( Fig. 6 ). Periodic constrictions develop frequently throughout growth (the number per volution is 3 or 4) and become very frequent in the late growth stage in some specimens (KUM MM TN 285, see Nishimura et al. 2010: fig. 6A, B). This may represent a sign of maturity in the present species.

The suture line is a typical Desmoceras pattern ( Matsumoto 1954: 248), with a formula of (E, L, U 2, U 4, U 5, U 6 [= S]). 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. 11C ).

Temporal trends of morphological variation are not observable in this species.

Remarks. —Specimens illustrated under the name of “ Damesites sugata ” in Ikeda and Wani (2012: fig. 1.1) and Santonian populations previously assigned to “ Damesites sugata ” by numerous authors should mostly be transferred to Paradamesites rectus gen. et sp. nov.

Paradamesites rectus gen. et sp. nov. ( Fig. 6 ) was described as “ Damesites sugata ” by Matsumoto (1942: 27, 28) and Matsumoto and Obata (1955: 128), on the basis of specimens from the Yezo Group. However, the FAD is apparently higher than that of the true P. sugata ( Forbes, 1846) , and these Santonian populations in the Yezo Group can be classified to P. rectus gen. et sp. nov.

“ Damesites sugata ” from the Yezo Group ( Nishimura et al. 2010) is here assigned to a new species of the new genus Paradamesites , P. rectus gen. et sp. nov. So far as we can determine at the moment, this species is endemic to the northwest Pacific province.

Paradamesites rectus gen. et sp. nov. resembles P. sugata in many morphological aspects. Shell surface ornament is almost smooth in both species except for frequent periodic constrictions ( Fig. 6 ). The number of constrictions similarly increases in their late growth stage ( Fig. 6A 1 ). Paradamesites rectus gen. et sp. nov. and P. sugata also share relatively compressed whorl shape (B/H ratio is about 0.75 at 50 mm in D; see Fig. 10B and compare Nishimura et al. 2010: fig. 17D) and a small w (about 2.0; see Fig. 10E , and compare Nishimura et al. 2010: fig. 17F). However, P. rectus gen. et sp. nov. has a much wider umbilicus (U/D is about 0.15 in 50 mm in D) than P. sugata (U/D ratio is about 0.09 at 50 mm in D) ( Fig. 10D ; Nishimura et al. 2010: fig. 17C). The curvature of constriction in P. rectus gen. et sp. nov. is single concave throughout growth ( Fig. 6 ) whereas that of P. sugata changes from single concave to weakly sigmoidal in late growth stage (< 60 mm in D) ( Nishimura et al. 2010: fig. 15A).

This species is similar to Tragodesmoceroides subcostatus in having a wide umbilicus in early growth stage (< 10 mm D; Nishimura et al. 2010: fig. 17C). Although relative umbilical width (U/D) changes ontogenetically, P. rectus gen. et sp. nov. possesses a much wider umbilicus, and its U/D ratio considerably increases during growth. In contrast, the U of T. subcostatus remains smaller in middle to late growth stages ( Nishimura et al. 2006: fig. 9D; Nishimura et al. 2010: fig. 17C).

Stratigraphic and geographic range. —Santonian–lower Campanian of Hokkaido, Japan, and Sakhalin, Russia.

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.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. 4. Desmoceratine ammonoid Paradamesites sugata (Forbes, 1846) from lower Coniacian of the Yezo Group in Hokkaido, Japan. A. KUM MM TN 103 from loc. OT1000, in ventral (A1), right lateral (A2), and apertural (A3) views. B. KUM MM TN 106 from loc. OT1402, in ventral (B1) and right lateral (B2) views. C. KUM MM TN 108 from loc. OT1402, in ventral (C1) and right lateral (C2) views.An open circle marks the position of constrictions.

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. 13. Schematic diagram showing ontogenetic development of shell ornament change in late Turonian–early Campanian desmoceratine ammonoids (C­group, Paradamesites spp.) from the northwest Pacific province. Paradamesites spp. exhibit only phase 1 (i.e., almost smooth shell surface) with frequent constrictions throughout growth. Detailed stratigraphic horizons of population samples (a–g) are shown in Fig. 12.

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. 6. Desmoceratine ammonoid Paradamesites rectus gen. et sp. nov. = “Damesites sugata” from the Yezo Group) from the Santonian–lower Campanian of the Yezo Group, Hokkaido,Japan.A. KUM MM TN 281 holotype, loc. OT5034, in left lateral (A1) and apertural (A2) views. B. KUM MM TN 287 paratype, loc. TK1019, in ventral (B1) and right lateral (B2) views.An open circle marks the position of constriction.