Introduction
The genus Diplomoceras Hyatt, Reference Hyatt and von Zittel1900 ranks amongst the more important, globally distributed latest Cretaceous heteromorph ammonites and has a convoluted history with regard to aspects of its taxonomy and palaeogeographical and biostratigraphical distribution. From the very beginning, the type species, Baculites cylindracea Defrance, Reference Defrance1816, was recorded as ‘foss. De Maestricht’, but, as noted by Kennedy (Reference Kennedy1987, p. 183), Defrance (Reference Defrance1816, p. 160) presented morphological characters of his Baculites cylindracea, while in the Supplement of volume 3 of the same publication (Defrance, Reference Defrance1816), this particular specimen is referred to as ‘Le Baculite vertebrae, foss. De Maestricht’. Kennedy (Reference Kennedy1987) also noted specimens of the associated fauna with Defrance’s Baculites vertebralis from Maastricht and that one of the illustrations in the volume of Dictionnaire plates confirmed the typical Maastricht preservation. Thus, there can be no doubt that the first specimen described by Defrance as Baculites cylindracea originated from the Maastricht stratotype area. Unfortunately, Defrance’s originals have never been traced, which greatly complicated the situation. For this reason, Kennedy (Reference Kennedy1987, p. 181, pl. 24, Figs 1–3) designated a specimen from the upper Maastrichtian Nekum or Meerssen members (Maastricht Formation) of the Sint-Pietersberg, Maastricht, the Netherlands, neotype.
Hyatt (Reference Hyatt and von Zittel1900) considered Baculites cylindracea not to be closely related to baculitids (Baculitidae) and erected a new genus, Diplomoceras, to accommodate it, noting the character of the suture line, in particular. However, Hyatt’s description is very brief and lacks any illustration of the specimen, yet does show the suture line; the age is indicated merely as Late Cretaceous. Wright et al. (Reference Wright, Calloman, Howarth and Moore1996) indicated the range of the genus Diplomoceras to be Campanian to upper Maastrichtian and illustrated D. notabile Whiteaves, Reference Whiteaves1903 from the Campanian of British Columbia, Canada (Usher, Reference Usher1952). However, Kennedy (Reference Kennedy1987) had previously revised the genus Diplomoceras and species referred to it and demonstrated that, in many cases, finds of D. notabile should in fact be considered as D. cylindraceum, including the specimen illustrated by Usher (Reference Usher1952). Following Kennedy (Reference Kennedy1987, p. 186), the stratigraphical range of the genus Diplomoceras and its type species, D. cylindraceum, is Maastrichtian ‘where these specimens are well dated’. Additionally, Kennedy (Reference Kennedy1987) noted that almost all other ‘species’ of Diplomoceras fell into the range of variation of D. cylindraceum. Later, Hancock & Kennedy (Reference Hancock and Kennedy1993, p. 165) concluded that ‘Diplomoceras cylindraceum ranges throughout the whole of the Maastrichtian, and may appear in the upper Campanian’. Klinger & Kennedy (Reference Klinger and Kennedy2003a) discussed the origination of D. cylindraceum and its geographical distribution, while Dochev & Metodiev (Reference Dochev and Metodiev2016, Fig. 2) have recently summarised the palaeogeographical distribution of Diplomoceras, adding new occurrences.
Thus, to date, there are questions surrounding both taxonomy and stratigraphical range of the genus and species. The sole conclusion that can be drawn is that D. cylindraceum has definitely been recorded from across the globe, but that D. notabile has never been found beyond the Pacific Realm.
Newly collected Diplomoceras: geological and stratigraphical settings
New finds of D. cylindraceum, described and illustrated herein, stem from the upper Maastrichtian Kokuy Unit (Fig. 1), as exposed in the Tesnaya River Basin, near Lake Pekulney in the south-eastern part of Koryak Upland. Three rather poorly preserved fragments of shafts and bends were collected in 2009 by one of us (LBG) from the right bank of the Kontaktniy Creek stream, locality 325 (Figs. 1, 2). At this site, the upper part of the Kakanaut Formation and the lower part of the Kokuy Formation are well exposed. The ammonites (Fig. 3) are kept in the collections of the CNIIGR Museum (Karpinsky Russian Geological Research Institute, Sankt-Peterburg, Russia), under registration number 13,417. The material consists of three fragmentary internal moulds of phragmocones (Fig 3). The largest specimen is a straight shaft fragment of 120 mm length and a whorl height of about 25 mm (Fig. 3 A-C). It is clearly ribbed with a rib index (following Kennedy, Reference Kennedy1987, p.160) of 8. Two other specimens are smaller fragments of internal moulds of phragmocones with probably similar whorl heights of about 20–25 mm showing 8–10 ribs (Figs. 3D-I). There is a probability that those three fragments belong to the same individual. However, it is not possible to establish with certainty. However, the whorl height and rib index generally correspond with those recorded for D. cylindraceum from many previous works (e.g. Birkelund, Reference Birkelund1993; Hancock & Kennedy, Reference Hancock and Kennedy1993).
A. Position of the Kakanaut locality in Koryak Upland (Far East Russia). B. Map showing the locality (site 325) yielding Diplomoceras cylindraceum, as well as the main floral and faunal sites. Generalised stratigraphical section of the Kakanaut River Basin (modified after Golovneva & Shczepetov, Reference Golovneva and Shczepetov2010): 1 – marine deposits (Kokuy Unit); 2, 3 – terrestrial strata (Kakanaut Formation): 2 – effusive-pyroclastic deposits; 3 – volcanogenic-terrigenous deposits; 4 – marine Elginmyn Formation; 5 – fossil sites: (а) plant remains, (b) molluscan remains and (c) dinosaur remains.
Localities with finds of Diplomoceras in Koryak Upland: 1 – Central part of Koryak Upland, Impeveem River Basin, Impeveem Formation, Maastrichtian (Dundo, Reference Dundo and Vasilevskaya1971); 2 – interfluve of Opuka and Khatyrka rivers, Bistrorechensk Formation, upper Maastrichtian (Pochialaynen & Byalobzheskiy, Reference Pochialaynen and Byalobzheskiy1984); 3 – interfluve of Khatyrka and Svetlaya rivers, Vysokorechensk Formation, Maastrichtian (Volobueva & Krasniy, Reference Volobueva and Krasniy1979); 4 – right shore of Pekulney Lake, Rinotanmelgen River Basin, upper part of the Engilminskaya Formation, Maastrichtian (Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979); 5 – right shore of Pekulney Lake, Tesnaya River Basin, Kokuy unit, upper Maastrichtian (Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979; present study); 6 – Velkelveem River Basin, Gangut Formation, Maastrichtian (P ochialaynen, Reference Pochialaynen and Copanov1984).
Diplomoceras cf. cylindraceum (Defrance, Reference Defrance1816), locality 325, Kokuy Unit, lower upper Maastrichtian; Tesnaya River in Kakanaut River valley, Koryak Upland, North-West Pacific Province (Far East Russia). A–C: CNIGRI 1/13417; D–F: CNIGRI 2/13417; G–I: CNIGRI 3/13417. Scale bar equals 10 mm.
In the south-eastern part of Koryak Upland, Maastrichtian marine and non-marine strata are widely distributed (Terekhova, Reference Terekhova1970; Dundo, Reference Dundo and Vasilevskaya1971, Reference Dundo and Vasilevskaya1974; Volobueva & Terekhova, Reference Volobueva, Terekhova and Vasilevskaya1974; Volobueva & Krasniy, Reference Volobueva and Krasniy1979; Golovneva, Reference Golovneva1994; Herman, Reference Herman2013). One of the most interesting sections with a diverse marine fauna, plant fossils and dinosaur remains has been studied near Lake Pekulney (Fig. 1). The greatest contributions to the stratigraphy of Upper Cretaceous deposits in this region were already made several decades ago by Galina Terekhova (Terekhova, Reference Terekhova1965; Volobueva & Terekhova, Reference Volobueva, Terekhova and Vasilevskaya1974; Terekhova & Dundo, Reference Terekhova and Dundo1987) and additional stratigraphical studies were carried out by Golovneva & Shczepetov (Reference Golovneva and Shczepetov2010).
On the right shore of Lake Pekulney, Maastrichtian deposits (Fig. 1) are divided, in ascending order, into the Elginmyn Formation, Kakanaut Formation and the youngest level of the Kokuy Unit or Formation (Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979; Golovneva & Shczepetov, Reference Golovneva and Shczepetov2010).
The Coniacian–Maastrichtian Elginmyn Formation was first described by Gladenkov (Reference Gladenkov1963) from the Khatyrka River Basin. On the right shore of Lake Pekulney, this formation reaches a significant thickness of between 500 and 1,000 m. The upper part of this unit is characterised by finds of the early Maastrichtian inoceramid species, Cataceramus pilvoensis Sokolov, Reference Sokolov1914 and Schachmaticeramus shikotanensis (Nagao & Matsumoto, Reference Nagao and Matsumoto [Matumoto]1940). The upper part of this unit has yielded the Maastrichtian ammonites, Neophylloceras marshalli (Shimizu, Reference Shimizu1935), Pachydiscus subcompressus Matsumoto, Reference Matsumoto1959, plus the inoceramid, Schachmaticeramus kusiroensis (Nagao & Matsumoto, Reference Nagao and Matsumoto [Matumoto]1940), which is considered to be an early late Maastrichtian index species (Terekhova, Reference Terekhova1965; Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979; Zonova et al., Reference Zonova, Kasintsova and Yazykova1993; Toshimitsu et al., Reference Toshimitsu, Matsumoto, Noda, Nishida and Maiya1995).
The volcanogenic-terrigenous strata of the Kakanaut Formation attain a thickness of about 1,000 m (Fig 1) and comprise tuffaceous sandstones, siltstones, tuffs and basalts (Volobueva & Terekhova, Reference Volobueva, Terekhova and Vasilevskaya1974; Golovneva & Shczepetov, Reference Golovneva and Shczepetov2010). On the left shore of Lake Pekulney, deposits assigned to the Kakanaut Formation are predominantly non-marine and contain remains of plants and dinosaurs (Golovneva, Reference Golovneva1994; Godefroit et al., Reference Godefroit, Golovneva, Shczepetov, Garcia and Alekseev2009). The Kakanaut floral assemblage includes more than 40 species, representing such groups as Marchantiophyta (liverworts), Equisetidae (horsetails), Polypodiophyta (ferns), Cycadophyta (cycadophytes), Ginkgoaceae (ginkgos), Gymnospermae (conifers) and flowering plants (Angiospermae) (see Golovneva, Reference Golovneva1994; Gnilovskaya & Golovneva, Reference Gnilovskaya and Golovneva2016, Reference Gnilovskaya and Golovneva2018; Zolina et al., Reference Zolina, Golovneva and Spicer2020). The dinosaur fauna from the Kakanaut Formation comprises representatives of the families Hadrosauridae, Ankylosauridae, Ceratopsidae, Hypsilophodontidae, Troodontidae, Dromaeosauridae and Tyrannosauridae, inclusive of eggshell fragments, belonging to hadrosaurids and theropods (Godefroit et al., Reference Godefroit, Golovneva, Shczepetov, Garcia and Alekseev2009).
On the right shore of Lake Pekulney, deposits of the Kakanaut Formation are more volcanogenic than on the left shore and contain just a few plant remains. On some occasions, these levels have previously been mapped as Vysokorechensk Formation (Fig. 4), as established to the west of Lake Pekulney, in the Khatyrka River valley (Bogidaeva & Gladenkov, Reference Bogidaeva and Gladenkov1963). The Vysokorechensk Formation is coeval with the Kakanaut Formation and yields similar plant fossils, yet is distinguished by the presence of marine levels with a late Maastrichtian fauna, such as the inoceramid Schachmaticeramus kusiroensis and ammonites (Pachydiscus subcompressus and Diplomoceras sp.) (Volobueva & Krasniy, Reference Volobueva and Krasniy1979; Golovneva & Gnilovskaya, Reference Golovneva and Gnilovskaya2015).
Distribution of finds of Diplomoceras in Maastrichtian deposits of Koryak Upland, North-West Pacific Province (Far East Russia).
The Kakanaut Formation is overlain by coastal-marine deposits of the Kokuy Unit, which comprises siltstones with interlayers of sandstones (Golovneva & Shczepetov, Reference Golovneva and Shczepetov2010) and attains a thickness in the study area of over 700 m. This unit contains remains of molluscs, brachiopods, decapod crustaceans and corals, as well as poorly preserved wood and leaves. The fragments of D. cylindraceum described herein (Fig. 3) have been collected from locality 325 along the Kontaktniy Creek stream, Tesnaya River valley (Figs. 1, 2).
Brief review of previous finds of Diplomoceras
All previous mentions and descriptions, with or without illustrations, of Diplomoceras (Fig. 4) are listed below and briefly compared with those from Japan, Alaska and Canada.
Late Cretaceous ammonites of the Pacific Realm are highly endemic and provincial, as demonstrated in numerous papers (e.g. Toshimitsu et al., Reference Toshimitsu, Matsumoto, Noda, Nishida and Maiya1995; Jagt-Yazykova & Zonova, Reference Jagt-Yazykova and Zonova2012 and others). However, a number of extensive transgressive episodes have been documented from the region; these correspond to the appearance of several cosmopolitan ammonites. One of these episodes occurred in the late Campanian–early Maastrichtian. During the Campanian, ammonite diversity reached its maximum (acme) in the North-West Pacific Province (Yazykova, Reference Yazykova2002, Reference Yazykova2004; Jagt-Yazykova, Reference Jagt-Yazykova, Jagt-Yazykova and Jagt2012; Jagt-Yazykova & Zonova, Reference Jagt-Yazykova and Zonova2012) and the level of endemism decreased. One of cosmopolitan taxa that made its first appearance in this interval is the genus Diplomoceras.
The first find of Diplomoceras, identified as D. cf. notabile, was illustrated by Dundo (Reference Dundo and Vasilevskaya1971), who failed to provide a description. The specimen came from the lower part of the Maastrichtian Impenveem Formation (Figs. 2, 4), in the central part of Koryak Upland. However, Kennedy (Reference Kennedy1987, p. 182) included this record in the synonymy of D. cylindraceum. The Impenveem Formation has been divided into three subformations; Dundo (Reference Dundo and Vasilevskaya1971) mentioned that fragments of Diplomoceras have been found in every one of these (Fig. 4), but failed to provide illustrations of this material. It co-occurred with inoceramid bivalves, such as the early Maastrichtian index, Sch. shikotanensis, and the late Maastrichtian index, Sch. kusiroensis.
From the interfluve of the Khatyrka and Svetlaya rivers (Fig. 4) comes another fragment of Diplomoceras, originating from the Vysokorechensk Formation (Volobueva & Krasniy, Reference Volobueva and Krasniy1979) and, more precisely, from the lower upper Maastrichtian Sch. kusiroensis inoceramid Zone (Fig. 4). A few kilometres south of Lake Pekulney, in the Rinotanmelgen River Basin (Fig. 2), Diplomoceras sp. has been noted from the Engilminskaya Formation, co-occurring with the late Maastrichtian index, Sch. kuziroensis (Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979).
From the right shore of Lake Pekulney Lake, in the Tesnaya River valley (Figs. 1, 2), we have now collected fragments of shaft and bend of Diplomoceras from the Kokuy Unit, or lower upper Maastrichtian Sch. kuziroensis inoceramid Zone (Fig. 4). This material we here identify as D. cf. cylindraceum (Fig. 3). Previous finds from this site were noted as Diplomoceras sp. (Igumentzev & Pisorenko, Reference Igumentzev and Pisorenko1979). It is worth noting that, apart from ammonites, we have recovered representatives of other molluscan groups such as inoceramid and non-inoceramid bivalves, gastropods and baculitid ammonites. This collection is under study now and will be the subject of a forthcoming paper (Jagt-Yazykova et al., in prep.). For the moment, we wish to indicate that amongst the inoceramid bivalves from this locality, a specimen of Korjakia kociubinskii Pochialaynen, Reference Pochialaynen1980 has been recognised. This species indicates the late Maastrichtian age of our finds. When describing his new genus and species, Pochialaynen (Reference Pochialaynen1980) mentioned that it had been collected from the upper part of the Maastrichtian section, together with Pachydiscus aff. gollevillensis d’Orbigny, Reference Orbigny1850, which probably is a misidentification of Pachydiscus (P.) flexuosus Matsumoto, Reference Matsumoto1979, a species typical of the upper Maastrichtian in Korjaka, Sakhalin and Japan (Yazykova, Reference Yazykova2004; Jagt-Yazykova & Zonova, Reference Jagt-Yazykova and Zonova2012). Although, recently Shigeta & Tsutsumi (Reference Shigeta and Tsutsumi2019) published the results of radiometric age dating of zircons (238U/206Pb ratios, using the LA-ICP-MS method) from a tuff sample obtained from the middle part of the Pachydiscus flexuosus Zone (taxon-range zone) of the Yezo Group in the Nakatonbetsu area, Hokkaido, northern Japan, correlated with Member 4 of the Krasnoyarka Formation in Sakhalin. This yielded a date of 69.8 ± 0.8 Ma, 95% confidence, which suggests late early to early mid-Maastrichtian under the threefold-subdivision scheme for the Maastrichtian. In twofold-subdivisions it would to the latest early Maastrichtian. It is possible that these data will need to be corrected in light of the zonal subdivision of the Pachydiscus flexuosus zone which is either early late Maastrichtian or late early Maastrichtian. In any case, it is definitively Maastrichtian.
Pochialaynen (Reference Pochialaynen and Copanov1984) illustrated a poorly preserved ammonite fragment, described as D. notabile, from Maastrichtian–Danian deposits (Gangut Formation) in the Velkelveem River Basin (Figs. 2, 4). The exact stratigraphical position of this find, better indicated as Diplomoceras sp., within this formation is unknown. In addition, there is a record of D. notabile from the Bistrorechensk Formation in the interfluve of the Opuka and Khatyrka rivers (Figs. 2, 4), where Diplomoceras originated from a level referred to as the Korjakia kociubinskii Zone, as proposed by Pochialaynen & Byalobzheskiy (Reference Pochialaynen and Byalobzheskiy1984), for the uppermost Maastrichtian part of the section.
Alabushev & Wiedmann (Reference Alabushev and Wiedmann1994) published a single specimen of D. notabile from the lower Campanian of southern Sakhalin. Later, the same authors (Alabushev & Wiedmann, Reference Alabushev and Wiedmann1997, p. 14, pl. 4, Fig. 1) recorded D. notabile on the basis of the same specimen which was again illustrated, and also mentioned that there were two other specimens from the lower Campanian of southern Sakhalin and even one from the lowermost Campanian of Kamchatka. Unfortunately, those specimens have never been illustrated or described in detail. However, the localisation of all specimens is so uncertain, that this age assignment appears highly questionable. Those authors compared this individual of D. notabile with two others (Alabushev & Wiedmann, Reference Alabushev and Wiedmann1997, pl. 4, Figs 2, 3) from the upper Maastrichtian of southern Sakhalin and identified as D. cylindraceum, noting (p. 15), that, ‘The species differs from the cosmopolitan D. cylindraceum (Defrance) by its smaller shell with finer sculpture’ . In our view, this specimen of D. notabile (Alabushev & Wiedmann, Reference Alabushev and Wiedmann1997, p. 14, pl. 4, Fig. 1) is the 7th turn of an adult individual of D. cylindraceum. In short, all three specimens illustrated by Alabushev and Wiedmann (Reference Alabushev and Wiedmann1997, pl. 4, Figs 1–3) belong to D. cylinraceum, but in view of poor photographic documentation are best treated as D. cf. cylindraceum. Unfortunately, Alabushev & Wiedmann (Reference Alabushev and Wiedmann1994, Reference Alabushev and Wiedmann1997) failed to mention any lithological units as established in Sakhalin (Poyarkova, Reference Poyarkova1987). Thus, it is utterly impossible to determine from where these illustrated specimens, and all others in those two papers, originate and their age, either upper or lower Campanian or Maastrichtian, cannot be determined. In addition, we consider one specimen of the associated fauna described by Alabushev & Wiedmann (Reference Alabushev and Wiedmann1997, pl. 4, Fig. 9) as Ryugasella ryugasensis Wright & Matsumoto (Reference Wright and Matsumoto1954) is more closely similar to a species of Glyptoxoceras, and might be of Maastrichtian age.
The best-preserved specimen of Diplomoceras from Far East Russia is the one illustrated by Maeda et al. (Reference Maeda, Shigeta, Fernando and Okada2005), as D. cf. notabile, from unit K3 of the Krasnoyarka Formation in the Makarov area (Sakhalin). Those authors claimed that it was of early late Maastrichtian age, but we cannot be certain of this, because they changed the lithological subdivisions of the Krasnoyarka Formation, failing to provide correlations with the previous stratigraphical scheme (Poyarkova, Reference Poyarkova1987). However, Maeda et al. (Reference Maeda, Shigeta, Fernando and Okada2005) did mention finds of Pachydiscus (P.) flexuosus and Glyptoxoceras sp. from the same level; thus, it might represent the upper Maastrichtian. In stark contrast, the same authors noted the inoceramids Schahmaticeramis shachmati (Salnikova & Zonova in Poyarkova, Reference Poyarkova1987), Sch. kusiroensis and Sphenoceramus hetonaianus (Matsumoto, Reference Matsumoto1952) from Unit K2, which corresponds to the upper Campanian. This is highly surprising because the species listed are all considered to be late Maastrichtian indices (Zonova et al., Reference Zonova, Kasintsova and Yazykova1993; Toshimitsu et al., Reference Toshimitsu, Matsumoto, Noda, Nishida and Maiya1995, and references therein). In short, we cannot be certain about the age, but, as preserved, these specimens show features which are definitively the same as D. cylindraceum.
In summary, it may be stated that finds of Diplomoceras from the Russian Pacific, described as D. notabile, have been noted from the upper Campanian and Maastrichtian. There are also finds of D. cylindraceum that have been dated as Maastrichtian. It follows that, if age assessments are correct, it could be hypothesised that D. notabile was the Campanian precursor of D. cylindraceum in the Pacific Realm, meaning that the centre of origination of Diplomoceras might have been in the Pacific area. However, in view of the poor state of preservation, we might equally well conclude that all specimens belong to D. cylindraceum. Finds from Maastrichtian deposits should also be considered to represent D. cylindraceium, as noted by Kennedy (Reference Kennedy1987) for the specimen published by Dundo (Reference Dundo and Vasilevskaya1971) and subsequently by Klinger & Kennedy (Reference Klinger and Kennedy2003b), who included D. notabile of Alabushev & Wiedmann (Reference Alabushev and Wiedmann1997) into the synonymy of D. cylindraceum and recorded this species from upper Campanian and Maastrichtian deposits of South Africa. Thus, the most parsimonious conclusion is that the cosmopolitan species, D. cylindraceum, is the sole representative of the genus and its stratigraphical range is upper Campanian to uppermost Maastrichtian, with the centre of origination either in South Africa or the Pacific Realm.
Finds from Japan are very important in this respect. The first late Campanian specimens of D. notabile were published by Tatsuo Matsumoto (Matsumoto & Morozumi, Reference Matsumoto and Morozumi1980; Matsumoto in Taira & Tashiro, Reference Taira and Tashiro1981), who also presented a detailed review of the genus and of the species D. notabile (Matsumoto, Reference Matsumoto1984; Matsumoto & Miyauchi, Reference Matsumoto and Miyauchi1984). He suggested that the existence of D. notabile was supported by some morphological features, such as whorl shape and character of the finest ribs, setting it apart from D. cylindraceum. Matsumoto (in Matsumoto & Miyauchi, Reference Matsumoto and Miyauchi1984) actually proposed that every find from the Pacific area would belong to D. notabile, including those from the Falkland Islands (Matsumoto & Miyauchi, Reference Matsumoto and Miyauchi1984) and that the stratigraphical range was upper Campanian. We do not agree that such features can be used to prove the occurrence of two different species, but the fact that late Campanian finds have been mostly made in Pacific or Atlantic areas support the notion of an area of origination mentioned above.
Misaki & Maeda (Reference Misaki and Maeda2009) published a single fragment of Diplomoceras sp. from the Toyajo Formation of south-west Japan. Although the entire assemblage of associated fauna is rather Maastrichtian in character, but the heteromorph, Didymoceras awajiense (Yabe, Reference Yabe1901) dated as late Campanian, then the position is a bit confusing. Subsequent finds from the Maastrichtian Senpohshi Formation in eastern Hokkaido (Shigeta et al., Reference Shigeta, Nishimura and Nifuku2015) have been described as Diplomoceras cf. notabile, while Kurichara et al. (Reference Kurichara, Kano, Sawmura and Sato2016) recorded D. cylindraceum from the uppermost Maastrichtian Kawaruppu Formation (Nemuro Group) in south-east Hokkaido, having been recovered from a siltstone 15 m below the Cretaceous-Paleogene (K/Pg) boundary. The stratigraphical level of the fossil has been estimated to be ∼66.8 Ma, based on the time scale for geomagnetic polarity and the K/Pg boundary. Thus, D. cylindraceum survived at least around 800 kyr prior to the K/Pg boundary event in the north Pacific region. Kurichara et al., (Reference Kurichara, Kano, Sawmura and Sato2016, p. 119) also suggested that all finds of D. notabile should be referred to D. cylindraceum.
There are a number of more recent finds of Diplomoceras from Japan. Masukawa & Ando (Reference Masukawa and Ando2018) mentioned Diplomoceras sp. from the upper Campanian of Japan and also noted that in the north-west Pacific region, Diplomoceras ranged from the mid-Campanian to the uppermost Maastrichtian. They established the age of the Nakaminato Group in south-west Japan as late Campanian, because Diplomoceras co-occurred with Didymoceras awajiense and Inoceramus (Endocostea) shikotanensis (= Schachmaticeramus shikotanensis as understood here). In the upper part of the section, Masukawa & Ando (Reference Masukawa and Ando2018) mentioned the ‘early’ Maastrichtian inoceramid, ‘Inoceramus’ kuziroensis (= Schachmaticeramus kuziroensis as understood here), which leads to a lot of confusion. In many previous papers, these two inoceramid species have been considered to be of early and late Maastrichtian age, respectively (e.g. Noda & Matsumoto, Reference Noda and Matsumoto1976; Poyarkova, Reference Poyarkova1987; Zonova et al., Reference Zonova, Kasintsova and Yazykova1993; Toshimitsu et al., Reference Toshimitsu, Matsumoto, Noda, Nishida and Maiya1995; Yazykova, Reference Yazykova2004). Yet, according to Masukawa & Ando (Reference Masukawa and Ando2018, p. xx) all previous finds of Diplomoceras should be dated as late Campanian. The main argument underlying these changes lies in the outcome of zircon dating. However, these data have never been formally published, but just briefly mentioned as follows, ‘U_Pb age of detrital zircons indicated by Nagata and Otoh (pers. comm., May 2016)’. In short, their dating of stratigraphical levels is based on personal communication, rather than on published data. We are not in favour of this interpretation and shall provide more detailed counterarguments in our planned work on associated ammonites, inoceramid and non-inoceramid bivalves and gastropods from locality 325. The most recently published, comparatively large assemblage from Japan was identified as Diplomoceras sp. and originates from even older levels, mid- to upper Campanian levels (Shigeta & Tsutsumi, Reference Shigeta and Tsutsumi2019).
One last note on finds from Alaska (Jones, Reference Jones1963): D. notabile is known from the upper part of Member 3 of the Matanuska Formation, which could be either latest Campanian or early Maastrichtian. Kennedy (Reference Kennedy1987) included this into the synonymy of D. cylindraceum; we concur. The same should hold true for D. notabile from the Nanaimo Group (uppermost Campanian to lower Maastrichtian) of British Columbia, Canada (Haggart, Reference Haggart and Smith1991).
Conclusions
As noted above, Kennedy (Reference Kennedy1987) and Klinger & Kennedy (Reference Klinger and Kennedy2003b) included into the synonymy of D. cylindraceum some Pacific finds of D. notabile, notably two finds from Maastrichtian strata as described by Anderson (Reference Anderson1958) and two from New Zealand (Henderson, Reference Henderson1970), material from Alaska (Jones, Reference Jones1963), Koryak Upland (Dundo, Reference Dundo and Vasilevskaya1971) and Sakhalin (@@Alabushev & Wiedmann, Reference Alabushev and Wiedmann1997), but did not question any finds from Japan. We agree with Kurichara et al. (Reference Kurichara, Kano, Sawmura and Sato2016) that all material of Diplomoceras from Japan, identified as D. notabile, should be considered to belong to D. cylindraceum. The stratigraphical range of that species is mid-Campanian to uppermost Maastrichtian. Kurichara et al. (Reference Kurichara, Kano, Sawmura and Sato2016) even suggested that D. cylindraceum was the youngest Cretaceous ammonite in the Pacific Realm.
There are three ways to interpret all of the above data, as follows:
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1. Diplomoceras notabile and D. cylindraceum are sibling or vicariant species, in which case all finds from the Pacific area, including Australia, should be listed as D. notabile, and those from European and Atlantic (African) regions as D. cylindraceum;
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2. Diplomoceras notabile occurred in the Campanian, D. cylindraceum in the Maastrichtian, in which case the former might be the precursor of the latter and the centre of origination would be in the Pacific area;
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3. Diplomoceras notabile is synonymous with D. cylindraceum and all morphological differences are those expressed by ecophenotypy or polymorphism, depending on environmental conditions. The deep-water facies of the Pacific is assumed to have led to morphological changes in comparison to the shallower-water chalk facies of Europe.
Our view is that all records of Diplomoceras from the North-West Pacific Province pertain to D. cylindraceum and that the stratigraphical range of this species is mid-Campanian to uppermost Maastrichtian. Long-lived taxa are quite common in the evolutionary history of the Ammonoidea. Klinger & Kennedy (Reference Klinger and Kennedy2003b) already discussed the possibility that D. cylindraceum could have been amongst the longest-ranging heteromorph ammonite species of the Cretaceous Period. Material illustrated here, originating from the upper part of the Maastrichtian, provides evidence that Kurichara et al. (Reference Kurichara, Kano, Sawmura and Sato2016) were right in noting that D. cylindraceum could well have been the youngest Cretaceous ammonite in the Pacific Realm.
Acknowledgements
The present work was supported by the Russian Science Foundation, grant 23-77-01087 (https://rscf.ru/en/project/23-77-01087/). We are grateful to the journal reviewers, Martin Košťák (Prague) and Anonymous, for important suggestions.
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