33

GEOLOGICA BALCANICA, 45, Sofia, Dec. 2016, pp. 33–45.

New record of Diplomoceras cylindraceum (Defrance, 1816) from the Maastrichtian of the Western Fore-Balkan (Bulgaria)

Docho Dochev1, Lubomir Metodiev2

1 Faculty of Geology and Geography, Department of Geology, Palaeontology and Fossil Fuels, Sofia University “St. Kliment Ohridski”, 15 Tsar Osvoboditel Blvd, 1504 Sofia, Bulgaria; email: dochev@gea.uni-sofia.bg2 Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 24, 1113 Sofia, Bulgaria; e-mail: lubo@geology.bas.bg(Accepted in revised form: December 2016)

Abstract: New data on the heteromorph ammonite species Diplomoceras cylindraceum (Defrance, 1816) have been obtained from three localities of the Western Fore-Balkan (West Bulgaria). A retrospection of the earlier Bulgarian records is also presented. This characteristic species is relatively common in Bulgaria and, based on both previously and the newly collected data, it is confined to the Maastrichtian. Owing to the dif-ficulty in collecting enough specimens, the amount of variation in ribbing and whorl-section shape, which have been used by other authors for determination of more species within Diplomoceras, is difficult to judge. Nevertheless, the Bulgarian material reveals that rib density varies in broad limits and the rib index does not show a sufficient degree of variance to warrant the distinction of more than one species. Therefore, it is doubt-ful that all specific names that have been proposed in the literature for rib differences represent other species than Diplomoceras cylindraceum.

Dochev, D., Metodiev, L. 2016. New record of Diplomoceras cylindraceum (Defrance, 1816) from the Maastrichtian of the Western Fore-Balkan (Bulgaria). Geologica Balcanica 45, 33–45.

Keywords: Diplomoceras, ammonites, Maastrichtian, Western Fore-Balkan Mts, Bulgaria.

INTRODUCTION

Maastrichtian ammonites in Bulgaria are usually rep-resented by not large numbers of specimens from scattered localities and single horizons. These include examples of the genera Diplomoceras, Pachydiscus, Pseudokossmaticeras, Hauericeras, Hoploscaphites, Discoscaphites, Anapachydiscus, Pseudophyllites, Sag-halinites, Vertebrites, Phylloceras, Glyptoxoceras, Des-mophyllites, Baculites and Phylloptychoceras (e.g., V. Tzankov, 1932, 1982a; Tz. Tzankov, 1964; Jol-kičev, 1984, 1986; M. Ivanov, 1993, 1995; Ivanov and Stoykova, 1994). In addition, other specimens have been recorded but have not been identified with suf-ficient degree of certainty or have been obtained from ill-defined levels. From the available exposures, only a limited number of ammonite fields have contributed with enough specimens that can be organized into re-liable biostratigraphical range-charts. What is known to date represents, more or less, indications of suc-cessions that are better represented outside Bulgaria.

This scarcity is generally thought to be primarily a result of hostile environments, but it may also be due to non-preservation in unfavourable depositional set-tings. Several sections from the Western Fore-Balkan (West Bulgaria) were found to contain stratigraphi-cally traceable Maastrichtian macrofossil successions that comprise abundant inoceramids accompanied by scattered ammonites. The inoceramids allowed an adequate zonation to be made, but it still remains a working concept and only a few details that support the present ammonite record will be given below. The ammonites that take part in these fossil assemblages correspond, in order of decreasing abundance, to the following genera: Pachydiscus, Diplomoceras, Pseu-dokossmaticeras and Hauericeras. The former two taxa are relatively common and compose parallel rang-es running approximately from the base of the lower Maastrichtian to the base of the upper Maastrichtian, whereas the latter two genera refer to single specimens assigned to the lower and to the upper Maastrichtian, respectively. The current account includes a descrip-

34

tion of Diplomoceras and the rest of the ammonite col-lection will be treated in a different paper.

Diplomoceras is a well-known, nearly cosmopoli-tan genus, for a group of ammonites with heteromorph coiling that first appears in the Campanian and persists to the upper Maastrichtian (e.g., Kennedy, 1986b; Oli-vero and Zinsmeister, 1989; Henderson et al., 1992; Kennedy and Henderson, 1992; Klinger and Kennedy, 2003a; Machalski, 2012; Remin et al., 2015). Despite being amongst the longest ranging Upper Cretaceous ammonites, Diplomoceras is a stratigraphically sig-nificant genus, as it has characteristic morphology and almost worldwide occurrence that make it easily recognizable and commonly recordable. These par-ticularly apply to the Bulgarian occurrences known to date. Owing to the difficulty in obtaining any strati-graphically bound specimens from the Campanian in Bulgaria, Diplomoceras is thought, at least for now, to be a guide fossil for the Maastrichtian.

OCCURRENCE OF THE GENUS DIPLOMOCERAS IN BULGARIA

The first record of Diplomoceras in Bulgaria was that of V. Tzankov (1932, p. 66, Pl. IV, Fig.4). This au-thor described and illustrated a highly deformed speci-men of D. cylindraceum, collected at the vicinity of the village of Drashan (Vratsa District, NW Bulgaria), from rocks that presumably belong to the Kunino For-mation and are of Maastrichtian age. Soon after that, Jélev (1934a, b) published a fragment of a straight shaft of an internal mould of D. cylindraceum, pos-sibly from the Rumyantsevo Formation, which was found near Ralyovo Village, Pleven District, central North Bulgaria (Jélev, 1934a, p. 121; Jélev, 1934b, p. 196, Pl. IV, Fig. 8). According to Jélev (1934a), this specimen was collected from “marly limestones with Bostrychoceras polyplocum A. Roem.” That could be an indication for a late Campanian age, but this record cannot be accepted as reliable because of the unlikely association of names given to the fossils that have been found together with D. cylindraceum and that indicate a Maastrichtian age. Tz. Tzankov (1964, p. 152) reported a few specimens of D. cy-lindraceum, probably from the Kladorub Formation, near the village of Kladorub (Vidin District, NW Bul-garia). Amongst them, a fragment of a ribbed straight shaft was figured (Tz. Tzankov, 1964, Pl. IV, Fig. 2). However, D. cylindraceum was found in association with ammonites that indicate both Campanian and Maastrichtian levels, thus making the assumption of Campanian occurrence of D. cylindraceum doubtful. Furthermore, Hancock and Kennedy (1993) revised some of Tz. Tzankov’s (1964) Campanian taxa and assigned them to Maastrichtian species. A detailed description and a few figures of D. cylindraceum were given by V. Tzankov (1982a, pp. 22–23, Pl. 4, Figs 1–3). The material of this monographic study

was referred to the Maastrichtian. It came from eight localities in central North Bulgaria, possibly from the Rumyantsevo and Kunino formations: Darmantsi, Drashan, Kalen, Oslen Krivodol, Rechkite and Lip-nitsa (Vratsa District); Gortalovo (former Karaguy) and Ralyovo (Pleven District). Jolkičev illustrated an incomplete straight shaft of D. cylindraceum, from the lower Maastrichtian near Darmantsi Village, Vratsa District, NW Bulgaria (Jolkičev, 1984, Pl. 1, Fig. 1) and used several records of D. cylindraceum from the Western and Central Fore-Balkan as indicators for the early Maastrichtian age of the Rumyantsevo, Darmantsi and Kunino formations (Jolkičev, 1986). In more recent times, the extensive studies on the Maast-richtian/Danian boundary interval near the town of Byala (Varna District, NE Bulgaria) have led to the placement of the K/Pg boundary (e.g., Stoykova and Ivanov, 1992, 2002, 2004; Preisinger et al., 1993a, b; Ivanov and Stoykova, 1994, 1995; Rögl et al., 1996; Stoykova et al., 2000; Peybernès et al., 2004), below which a succession of D. cylindraceum was found in beds of the Byala Formation and correlated with the upper Maastrichtian Micula murus and Micula prinsii nannofossil zones.

The above stated localities show that the ammo-nites of the genus Diplomoceras come entirely from the Upper Cretaceous sedimentary rocks exposed in North Bulgaria. Regionally, these rocks are consid-ered to be an integral part of the Moesian Platform and the marginal segments of the Balkan orogen in Bulgar-ia, referred to the Central Balkan–Fore-Balkan and the East Balkan zones (sensu Ž. Ivanov, 1988) (Fig. 1A). With a narrower extent, the Upper Cretaceous rocks also occur in the Kula Zone, which is assumed to represent the Carpathian orogen in Bulgaria (ibid.) and covers the outermost north-western region of the country. In terms of their depositional background, these sedimentary rocks have traditionally been clas-sified into three individual facies types: North Euro-pean, Mediterranean and Carpathian (Dabovski et al., 2009, and references therein). The bulk of the Bulgar-ian localities of Diplomoceras, along with those that are reported in thus study, associate with the North European type, which comprises moderately thick open-shelf carbonate deposits (Dabovski et al., 2009). These include the ammonite fields recorded by V. Tzankov (1932, 1982a), Jélev (1934a, b) and Jolkičev (1984, 1986). The ammonites recorded by Tz. Tzan-kov (1964) belong to a locality, which refers to the Carpathian type Upper Cretaceous sedimentary rocks. This locality is a part of a thick succession of hemipe-lagic to upper slope deposits (Stoykova et al., 2006). The Diplomoceras faunas recorded by Ivanov and Stoykova (1994) originate from a field that is related to the Mediterranean type Upper Cretaceous depos-its. Locally, the latter type corresponds to moderately thick hemipelagic marl-limestone alternation. A map of the main localities of Diplomoceras in Bulgaria is shown in Fig. 1A.

35

Fig. 1. Location maps of the main fields of Diplomoceras cylindraceum (Defrance, 1816) in Bulgaria: (A) Tectonic sketch map of Bulgaria, showing the localities that have yielded ammonites of this species within the framework of the Balkan orogen (after Ž. Ivanov, 1998), the outcrop and subcrop occurrences of the Upper Cretaceous rocks (Dabovski et al., 2009); (B) Geological sketch map of the Western Fore-Balkan in the area around the town of Mezdra, containing the sections used for this study (after Tzankov et al., 1994, 1995; simplified).

There is no well-dated Bulgarian record of Diplo-moceras from the Campanian to date. The foremost members of the genus appear approximately near the base of the lower Maastrichtian and have scattered but decipherable occurrences, which are accompanied by common Pachydiscus and less common Pseudokoss-maticeras and Hauericeras (Tz. Tzankov, 1964; V. Tzankov, 1982a; Jolkičev, 1984, 1986). Similarly, Diplomoceras continues ranging upwards into the up-per Maastrichtian, in combination with Hoploscaph-ites, Discoscaphites, Pachydiscus, Anagaudryceras, Pseudophyllites, Phylloceras, Desmophyllites, Glyp-toxoceras, Saghalinites, Vertebrites and Baculites (V. Tzankov, 1982a; M. Ivanov, 1993, 1995; Ivanov and Stoykova, 1994). Other good markers that constrain the stratigraphical range of Diplomoceras from the previ-ous Bulgarian records are the inoceramids. V. Tzankov (1982b) published a nice collection of lower Maastrich-tian inoceramids, which associate with Diplomoceras localities from central North Bulgaria. For instance, the Diplomoceras-bearing strata of locality Darmantsi (Vratsa District) yielded characteristic lower Maast-richtian inoceramids, such as Platyceramus stephenso-ni (Walaszczyk, Cobban & Harries, 2001) [= Inocera-mus (Inoceramus) wegneri Böhm, 1915 (V. Tzankov, 1982b, Pl. 25, Fig. 3); Inoceramus (I.) sagensis Owen, 1852 (V. Tzankov, 1982b, Pl. 25, Fig. 1); Inoceramus (I.) convexus Hall & Meek, 1854 (V. Tzankov, 1982b, Pl. 28, Fig. 1)]; Platyceramus salisburgensis (Fugger

& Kastner, 1885) [= Inoceramus (I.) planus Münster (V. Tzankov, 1982b, Pl. 33, Fig. 2)]; Endocostea sp. [= Inoceramus (I.) inaequibalis Seitz, 1967 (V. Tzan-kov, 1982b, Pl. 38, Figs 4, 5)]; Cataceramus pallisieri (Douglas, 1942) [= Inoceramus (I.) dariensis Dobrov & Pavlova, 1959 (V. Tzankov, 1982b, Pl. 31, Fig. 3; Pl. 32, Fig. 3)]; and Cataceramus barabini (Morton, 1834 sensu Meek, 1876) [= ?Inoceramus (I.) barabini Morton, 1834 (V. Tzankov, 1982b, Pl. 31, Fig. 1)]. The locality near Drashan Village (Vratsa District) gave in-oceramids of Cataceramus subcircularis (Meek, 1876) [= Inoceramus (I.) planus Münster, 1836 (V. Tzan-kov, 1982b, Pl. 31, Fig. 1)] and that at Ralyovo Village (Pleven District) yielded specimens of Cataceramus pallisieri (Douglas, 1942) [= Inoceramus (Catacera-mus) balticus Böhm, 1907 (V. Tzankov, 1982b, Pl. 34, Figs 4, 5; Pl. 35, Fig. 1)]. Some of these records were confirmed in the area of this study, by new inoceramid evidence. Based on both the earlier and the newly ob-tained data, the stratigraphical distribution of the ge-nus Diplomoceras in Bulgaria seems to be confined to the Maastrichtian.

STRATIGRAPHIC BACKGROUND OF THE COLLECTION OF THIS STUDY

The Western Fore-Balkan, in the area between the towns of Mezdra and Cherven Briag, is a broad foot-

36

hill region, which is composed of gently folded Creta-ceous–Eocene sedimentary rocks. The latter were de-posited into an epiplatform basin with shallow-water to medium-depth terrigenous-carbonate sedimenta-tion. The Upper Cretaceous sequence-slice consists of moderately expanded Campanian–Maastrichtian carbonates, into which four formal lithostratigraph-ic units have been recognized in superposition: the Darmantsi, Kunino, Mezdra and Kaylaka formations (Jolkičev, 1986). These rocks transgressively overlay different levels of siliciclastic sediments of the Lower Cretaceous Roman Formation. From above, they are covered, with parallel unconformity or lithological transition, by Paleogene deposits.

Three sections, located along the Iskar River Val-ley to the southwest and the northeast of the town of Mezdra (Vratsa District), comprise the best outcrops of the Darmantsi, Kunino and Mezdra formations in the Western Fore-Balkan. From SSW to NNE, these stratigraphic sections are as follows: section Morav-itsa, section Darmantsi and section Kunino (Fig. 1B). The sediments yielded abundant inoceramids, rela-tively common ammonites and scattered belemnites and echinoids. Based on the fossil evidence, the sec-tions were found to span from the upper Campanian to the upper Maastrichtian (Jolkičev, 1986). Section Darmantsi is a 13-m thick sequence, which includes the Mezdra and Kunino formations, as well as the type section for the Darmantsi Formation (Jolkičev, 1986). Section Kunino consists of a 14.7-m thick succession that comprises the upper part of the Darmantsi For-mation, the type-section for the Kunino Formation (Jolkičev, 1986), as well as the base of the Mezdra Formation. Section Moravitsa represents a more ex-panded sequence that includes the K/Pg boundary (Sinnyovsky, 2013). The studied interval is 12.3 m thick and includes the Darmantsi and Kunino forma-tions, as well as the base of the Mezdra Formation.

The new fossils collected for this study have enabled us to record anew these instructive sections and to ob-tain bed-by-bed faunas, of which only a few species have already been recorded previously. Along with them, the ammonites compose a continuous record of five fossil-bearing levels in section Moravitsa and three levels in sections Darmantsi and Kunino. Most of the ammonites were collected from the rocks of the Kunino Formation and only one specimen was found in the Darmantsi Formation. The earliest record of D. cylindraceum is that of section Kunino. A single specimen was collected along with a few ammonites of Pachydiscus neubergicus (von Hauer) and Pseu-dokossmaticeras galicianum (Favre), as well as with several inoceramid species of the genera Endocostea and Cataceramus that are diagnostic for the lower Maastrichtian Endocostea typica inoceramid Zone (Walaszczyk et al., 2001, 2002a, b). Another speci-men was fixed to a higher level in section Darmantsi. It is amongst the auxiliary elements of the fossil as-sociation of the Darmantsi Formation, which includes

ammonites of Pachydiscus cf. colligatus (Binkhorst) and P. neubergicus, and which is dominated by the inoceramid genera Cataceramus, Platyceramus and Trochoceramus. The inoceramids yielded a suite of characteristic species that are indicative for the lower Maastrichtian Trochoceramus radiosus inoceramid Zone (Walaszczyk et al., 2001, 2002a, b). A coeval association in the lower half of the Kunino Formation at section Moravitsa was defined in the same inocera-mid zone. At that level, D. cylindraceum is common and co-occurs with also common Pachydiscus neu-bergicus and abundant Cataceramus-Platyceramus inoceramids. Both D. cylindraceum and P. neubergi-cus continue ranging upwards through the section, ap-proximately to the top of the Kunino Formation, but almost alone, and only one inoceramid, identified as “Inoceramus” ianjonaensis morgani Sornay, was re-corded. Although represented by a single specimen, the latter example is a clear indication for the upper Maastrichtian “Inoceramus” ianjonaensis inoceramid Zone (Walaszczyk et al., 2010).

SYSTEMATIC PART

The ammonites included in the present study are pre-served as fragmentary internal moulds of phragmo-cones and a single body chamber. The whorl sections are rarely observable, because most of the specimens are compressed due to an advanced diagenetic com-paction. Suture line is observed in one specimen only. Palaeontological plates were made, showing the am-monites without additional treatment, at magnifica-tion between ×0.4 and ×0.7. Inventory numbers of the studied material are abbreviated to letter-number codes, according to their repository at the Museum of Palaeontology and Historical Geology at Sofia Uni-versity “St. Kliment Ohridski”.

Suborder Ancyloceratina Wiedmann, 1966Superfamily Turrilitoidea Gill, 1871Family Diplomoceratidae Spath, 1926Subfamily Diplomoceratinae Spath, 1926Genus Diplomoceras Hyatt, 1900

Type. Baculites cylindracea Defrance, 1816, p. 160, by original designation (Kennedy, 1986b).Diagnosis. Helicoid spires, consisting of several par-allel to subparallel straight shafts, connected by U-curved elements. The early whorls are unknown. The whorl section is circular to oval and may vary from compressed to moderately depressed. Both the straight shafts and the U-curved sections are ornamented by annular, fine and dense, rectiradiate to feebly prorsira-diate ribs. The internal moulds of the phragmocone are smooth, whereas those of the body chamber are ribbed. The suture line is highly incised and ornate.Remarks. The genus Glyptoxoceras Spath, 1925 was treated previously as a subgenus of Diplomoceras

37

(e.g., Wiedmann, 1962; Klinger, 1976), but based on the different features of the adult coiling, Kennedy (1986b) and Olivero and Zinsmeister (1989) stated that Glyptoxoceras is a different genus.

The considerable range in whorl section shape and whorl proportions, as well as in rib density, has led to the proposal of several specific names for Diplomocer-as to date: D. cylindraceum (Defrance, 1816); D. nota-bile Whiteaves, 1903; D. lambi Spath, 1953; D. maxi-mum Olivero and Zinsmeister, 1989; and D. australe Hünicken, 1965. The extensive studies on the genus, conducted by Kennedy and Henderson (1992), Hen-derson et al. (1992) and Klinger and Kennedy (2003a), however, showed that it is better to unite these forms in a single species, rather than make arbitrary divisions in the morphology. Recently, Machalski (2012) argued that Diplomoceras maximum should be regarded as a distinct species for the upper Maastrichtian, due to the characteristic ribbing of growth stages at whorl height greater than 55 mm that sets it apart from all earlier forms. However, this difference seems to be of in-traspecific, rather than of specific, status and the epi-thet “maximum” is better to be treated as a synonym of “cylindraceum”. Thus, Diplomoceras is assumed to be monospecific and D. cylindraceum (Defrance) is thought to cover all occurrences of the genus.

Occurrence. Campanian and Maastrichtian. Exclud-ing the low latitudes of equatorial regions, the genus has a worldwide distribution, which was summarized by Klinger and Kennedy (2003a). Following their paper, the geographical occurrence of Diplomoceras is as follows: Europe (Biscay Region, Atlantic Pyr-enees, Tercis and Cotentin Peninsula (France); north-ern Spain; the Netherlands; Mons Basin (Belgium); Northern Germany; Denmark; Central and Southeast Poland; Austria, Ukraine; Bulgaria; Serbia; European Russia and Caucasus); Arctic Siberia; northwestern Kamchatka; southern Sakhalin; Tunisia; South Af-rica; Madagascar; South India; Pakistan; Western Australia; Antarctica (Seymour Island); Argentina; Chile; Brazil; Mexico; California; British Columbia; Alaska; Hokkaido (Central and South Japan); West Greenland and New Zealand. A sketch-map show-ing the chief localities of Diplomoceras worldwide is shown in Fig. 2.

Diplomoceras cylindraceum (Defrance, 1816)(Figs 3–5)

1816. Baculites cylindracea Defrance, p. 160.? 1825. Hamites cylindrique (Defrance): de Blainville, p. 382,

Pl. 13, Fig. 1.

Fig. 2. Geographical distribution of Diplomoceras cylindraceum (Defrance, 1816) (after Klinger and Kennedy, 2003, with minor additions).

38

1842. Hamites cylindraceus Defrance: d’Orbigny, p. 551, Pl. 136, Figs 1–4.

1869. Hamites cylindraceus Defrance: Favre, p. 26, Pl. 7, Fig. 1.

1871–1872. Hamites cf. cylindraceus Defrance: Schlüter, p. 103, Pl. 29, Figs 8–9; Pl. 31, Figs 10–14.

1873. Hamites cylindraceus Defr.: Redtenbacher, p. 130. 1903. Hamites elatior Forbes: Weller, p. 418, Pl. 2, Fig. 3. 1903. Hamites sp.: Weller, p. 418, Pl. 2, Fig. 4. 1903. Diplomoceras notabile Whiteaves, p. 335, Pl. 44,

Fig. 4. 1907. Hamites cylindraceus Defr.: Wiśniowski, p. 198,

Pl. 17, Fig. 7. V 1932. Hamites cylindraceus Defrance: V. Tzankov, p. 66,

Pl. 4, Fig.4. 1934b. Hamites cylindraceus Defr.: Jélev, p. 196, Pl. 4,

Fig. 8. 1951. Diplomoceras cf. cylindraceum (Defrance):

Mikhailov, p. 41, Pl. 2, Figs 9–10; Text-fig. 10. 1951. Diplomoceras cylindraceum Defr. var. lvovensis

Mikhailov, p. 42, Pl. 2, Figs 7–8, Text-fig. 11a, b. 1953. Diplomoceras lambi Spath, p. 17, Pl. 2, Figs 1–3;

Pl. 3, Fig. 1. 1953. Diplomoceras notabile Whiteaves: Spath, p. 17,

Pl. 2, Fig. 4. 1959. Diplomoceras cylindraceum (Defrance): Naidin

and Shimansky, p. 181, Pl. 3, Fig. 2. 1962. Diplomoceras (Diplomoceras) cf. notabile

Whiteaves: Wiedmann, p. 208. V 1964. Diplomoceras cylindraceum (Defrance, 1822): Tz.

Tzankov, p. 152, Pl. 4, Fig. 2. 1972. Diplomoceras notabile (Whiteaves, 1879): Lahsen

and Carrier, p. 528, Pl. 2, Figs 1–2. 1976. Diplomoceras cylindraceum Defrance: Klinger, p. 81. 1976. Diplomoceras gr. ex. lambi Spath: Klinger, p. 82. 1976. Diplomoceras gr. ex. cylindraceum Defrance:

Klinger, p. 82. 1976. Diplomoceras (Diplomoceras) notabile Whiteaves,

1903: Klinger, p. 82, Pl. 34, Figs 2, 4. 1980. Diplomoceras cylindraceum lvovensis Mikhailov,

1951: Błaszkiewicz, p. 30, Pl. 54, Fig. 4. 1980. Diplomoceras cylindraceum cylindraceum (De-

france, 1916): Błaszkiewicz, p. 30, Pl. 54, Fig. 2; Pl. 55, Figs 6, 7.

V 1982a. Diplomoceras cylindraceum (Defrance, 1822): V. Tzankov, p. 22, Pl. 4, Figs 1–3.

1984. Diplomoceras cylindraceum (Defrance): Jolkičev, p. 457, Pl. 1, Fig. 1.

1986a. Diplomoceras cylindraceum (Defrance, 1816): Kennedy, p. 51, Pl. 4, Figs 1–2; Pl. 9, Figs 8–10; Pl. 10; Text-figs 3i, 3l, 7 g–m.

1986b. Diplomoceras cylindraceum (Defrance, 1816): Kennedy, p. 181, Pl. 17, Fig. 3; Pl. 18, Fig. 5; Pl. 21, Figs 2–3, 5–6; Pl. 22, Fig. 6; Pl. 23, Figs 1–2; Pl. 24, Figs 1–3; Pl. 25, Figs 1–8; Pl. 26, Fig. 18; Pl. 33, Fig. 16; Pl. 36, Fig. 6; Text-figs 9–10.

1986. Diplomoceras cylindraceum (Defrance, 1816): Kennedy and Summesberger, p. 194, Pl. 15, Figs 1, 2, 5; Pl. 16, Figs 14, 15; Text-fig. 6.

1987. Diplomoceras cylindraceum (Defrance, 1816): Kennedy and Summesberger, p. 32, Pl. 3, Figs 1–5, 10–12.

1989. Diplomoceras lambi Spath, 1953: Olivero and Zinsmeister, p. 627, Figs 2.1–2.4.

1989. Diplomoceras maximum Olivero and Zinsmeister, p. 629, Figs 2–5, 4.1–4.4, 5.1–5.4.

1992. Diplomoceras cylindraceum (Defrance, 1816): Henderson et al., p. 140, Figs 5, 6A–E, H–K, 7.

1992. Diplomoceras cylindraceum (Defrance, 1816): Ken-nedy and Henderson, p. 704, Pl. 6, Figs 1–3, Text-figs. 1B, 3.

1993. Diplomoceras cylindraceum (Defrance, 1816): Bir-kelund, p. 51, Pl. 3, Figs. 3–4; Pl. 9, Fig. 1; Pl. 11, Figs 1–5; Pl. 12.

1993. Diplomoceras cylindraceum (Defrance, 1816): Hancock and Kennedy, Pl. 15, Fig. 15; Pl. 17, Figs 1–4.

1993. Diplomoceras cylindraceum (Defrance, 1816): Ward and Kennedy, p. 49, Figs 42, 43.16, 43.17.

1995. Diplomoceras cylindraceum (Defrance, 1816): Jagt, p. 28, Pl. 26, Figs 1, 2, 7, 9.

1999. Diplomoceras cylindraceum (Defrance, 1816): Fatmi and Kennedy, p. 653, Figs 12.1–12.5.

2001. Diplomoceras cylindraceum (Defrance, 1816): Klinger et al., Pl. 9, Figs 8–11.

2001. Diplomoceras cylindraceum (Defrance, 1816): Küchler and Odin, p. 521, Pl. 6, Figs 12–13.

2001. Diplomoceras cylindraceum (Defrance, 1816): Odin et al., p. 567, Fig. 2.

2003. Diplomoceras cylindraceum (Defrance, 1816): Niebuhr, p. 268, Pl. 1, Fig. 8; Pl. 4, Fig. 1.

2003a. Diplomoceras cylindraceum (Defrance, 1816): Klinger and Kennedy, Figs 1–7.

2003b. Diplomoceras cylindraceum (Defrance, 1816): Klinger and Kennedy, p. 303, Figs 53–55.

2004. Diplomoceras cylindraceum (Defrance, 1816): Ifrim et al., p. 1607, Text-figs 13G–H, 14E.

2010. Diplomoceras cylindraceum (Defrance, 1816): Sa-lazar et al., p. 218, Figs 41–42.

2012. Diplomoceras cylindraceum (Defrance, 1816) sen-su Kennedy, 1987: Machalski, p. 103, Pl. 6; Pl. 7, Figs 1–5; Pl. 8, Figs 9, 11.

2012. Diplomoceras cylindraceum (Defrance, 1816): Stinnesbeck et al., Fig. 2E; Fig. 3B.

2015. Diplomoceras cylindraceum (Defrance, 1816): Remin et. al., Fig. 3.

2015. Diplomoceras cylindraceum (Defrance, 1816): Witts et al., Fig. 5G.

Neotype: Diplomoceras cylindraceum (Defrance, 1816) (Kennedy, 1986b, p. 183, Pl. 24, Figs 1–3). The type specimen is IRSNB 10511 (Institut royal des Scienc-es naturelles de Belgique, Brussels), from the upper Maastrichtian of the Nekum/Meersen Chalk at St. Pi-etersberg, near Maastricht, the Netherlands. Accord-ing to Machalski (2012, p. 104), the inventory number “IRSNB 10511” is incorrect; the correct number of the neotype is IRSNB 10293 and the type specimen comes

39

from the Kunrade Limestones facies (Maastricht For-mation) at the environs of Kunrade, in the eastern part of southern Limburg, the Netherlands.Material. Seven fragments of straight shafts (one of them a very long part of the phragmocone) and curved sections: section Moravitsa, bed 5, Kunino Formation (Inv.-Nrs. U.S, K2 1650, U.S, K2 1651, U.S, K2 1653, U.S, K2 1655 and U.S, K2 1656); section Darmantsi, bed 3, Darmantsi Formation (Inv.-Nr. U.S, K2 1652); section Kunino, bed 6, Kunino Formation (Inv.-Nr. U.S, K2 1654). One almost complete body chamber, comprising the parallel straight shafts connected with U-curved portion, from section Moravitsa, bed 5, Kunino Formation (Inv.-Nr. U.S, K2 1649).

Description. The heteromorph ammonites of D. cy-lindraceum have paper-clip-like coiling comprising straight shafts and U-curved sections. Our material consists of fragments of internal moulds of straight shafts, comprising the phragmocone or the body cham-ber. No specimens representing the earliest whorls were found. The largest preserved shaft fragment has a length of 453 mm, whereas the smallest is 92 mm long. Usually, the original shape of the whorl section is not extant, but in some better-preserved specimens, it is oval and compressed, with whorl breadth/whorl height ratios ranging between 0.6 and 0.72. All speci-mens are sculptured with sharp annular and transverse ribs. There is no tuberculation. Specimen U.S, K2 1654

Fig. 3a–c. Diplomoceras cylindraceum (Defrance, 1816). Side views and section view of an almost complete and entirely ribbed body chamber with attached part of the phragmocone, comprising two parallel straight shafts connected with U-curved portion. Specimen U.S, K2 1649, section Moravitsa, bed 5, Kunino Formation, upper Maastrichtian, “Inoceramus” ianjonaensis Zone, ×0.4. At whorl height of 47 mm, the rib index is 13, whereas at 59 mm the rib index is 16.

40

(Fig. 5a) is a partly preserved large phragmocone, which is ribbed throughout with dense and straight prorsiradiate to slightly rursiradiate ribs. Specimen U.S, K2 1649 (Fig. 3a–c) is a well-preserved inter-

nal mould, comprising the phragmocone and part of the body chamber that consist of two straight shafts connected by U-curved section. It is also sculptured by dense annular ribs, which are crowded on the U-

Fig. 4a–d. Diplomoceras cylindraceum (Defrance, 1816): a) side view of a fragment of U-curved portion, probably of the body chamber (the rib index is 12 at whorl height of 45 mm), specimen U.S, K2 1650, section Moravitsa, bed 5, Kunino Formation, lower Maastrichtian, Trochoceramus radiosus Zone; b) side view of a fragment of a straight shaft, probably of the body chamber (the rib index is 12 at whorl height of 62 mm), specimen U.S, K2 1651, section Moravitsa, bed 5, Kunino Formation, lower Maastrichtian, Trochoceramus radiosus Zone; c) sp. cf., side view of a deformed fragment of U-curved portion, probably of the body chamber, with unusually distant ribbing, specimen U.S, K2 1652, section Darmantsi, bed 6, Kunino Formation, lower Maastrichtian, Trocho-ceramus radiosus Zone; d–e) side and section view of a fragment of straight shaft, probably of the phragmocone (the rib index is 15 at whorl height of 52 mm), specimen U.S, K2 1653, section Moravitsa, bed 5, Kunino Formation, lower Maastrichtian, Trocho-ceramus radiosus Zone. All figures are ×0.7.

41

Fig. 5. Diplomoceras cylindraceum (Defrance, 1816): a–b) side views of wholly septate long straight shaft (at whorl height of 45 mm the rib index is 15, whereas at whorl height of 52 mm the rib index is 16), specimen U.S, K2 1654, section Kunino, bed 6, Kunino For-mation, lower Maastrichtian, Endocostea typica Zone; c) side view of a fragment of a wholly septate straight shaft (the rib index is 13 at whorl height of 54 mm), specimen U.S, K2 1655, section Moravitsa, bed 5, Kunino Formation, lower Maastrichtian, Trochoceramus radiosus Zone; d) small fragment of a wholly septate straight shaft, specimen U.S, K2 1656, section Moravitsa, bed 5, Kunino Forma-tion, lower Maastrichtian, Trochoceramus radiosus Zone. Figures a, b are ×0.4; figures c, d are ×0.7.

curved portion. With less rib density are specimen U.S, K2 1651, which has a whorl height of 62 mm (Fig. 4b), and the highly deformed specimen U.S, K2

1652 (Fig. 4c). In our material, the rib index (see Oli-vero and Zinsmeister, 1989; Kennedy and Henderson, 1992; Machalski, 2012) varies between 12 and 16 at

42

whorl height from 42 mm to 62 mm. In some speci-mens (Fig. 5a, b), the rib index is directly proportional to whorl height, whereas in others (Fig. 4b) it decreases with growth and, at whorl height of 62 mm, the rib index is 12, due to the increase in distance between the ribs.Discussion. The rib index and the whorl breadth/whorl height ratio have been thought to be of taxonomic val-ue for the genus Diplomoceras (e.g., Whiteaves, 1903; Spath, 1953; Olivero and Zinsmeister, 1989). As stat-ed by Kennedy and Henderson (1992), Henderson et al. (1992) and Klinger and Kennedy (2003a), several specific names have received wide currency in the lit-erature to systematize the wide variety of these mor-phological features that occur in nearly every Diplo-moceras assemblage, but these alterations are nothing else but intraspecific variations. Therefore, D. cylin-draceum takes priority over all of these names, and the genus should be interpreted as consisting of a single species. Concerning the rib index, Kennedy (1986a, b) and Kennedy and Summesberger (1986) pointed out that it can range between 11–13 and 15–17 at a whorl height of 55–60 mm, but in bigger specimens this in-dex may reach, and even exceed, a value of 20.

Nevertheless, some authors believe that the varie-ties of the ribbing correspond to forms of particular stratigraphic levels. For instance, Olivero and Zins-meister (1989) used Diplomoceras lambi Spath, 1953 as a separate specific name for the specimens of the upper Campanian–lower Maastrichtian of Seymour Island (Antarctica), relying on the high rib index of these forms, which varies from 13 to 20. These authors also described ammonites, from the uppermost Maast-richtian levels, having low rib index (ranging between 9 and 13) and circular whorl section that have been interpreted as a new species, Diplomoceras maximum. By inference, Diplomoceras was interpreted as form-ing a lineage of decreasing rib density, from the old-est to the youngest members. Considering the evolu-tion of the Maastrichtian cephalopods from Seymour Island, including those of Olivero and Zinsmeister (1989), Witts et al. (2015) argued, however, that such phyllogenetic lineage cannot be really demonstrated and both D. lambi and D. maximum are junior syno-nyms of D. cylindraceum.

Machalski (2012) made a detailed comparison be-tween D. cylindraceum from the Vistula River Section (Poland) and the materials from other valuable locali-ties, including those of Kennedy (1986b), from Kun-rade (the Netherlands), and of Olivero and Zinsmeister (1989), from Seymour Island. He noted that Diplo-moceras lambi does not differ from D. cylindraceum in rib density and, in the Kunrade material, the rib index increases with the growth. The latter change, however, was found to be an artefact, because rib distance does not change throughout ontogeny and the rib index rises due to the increase in whorl height (Machalski, 2012, p. 105). On the other hand, this author considered Diplomoceras maximum as a distinct taxon, based on the characteristic ribbing pattern of the growth stages at

a whorl height equal to, or greater than, 55 mm, where the rib index is between 9 and 13. Machalski (2012) also stated that, in D. maximum, the rib index is ontoge-netically stable due to a compensation of the increasing distance between the ribs by the increase of the whorl height. In addition, he postulated that it is “reasonable to assign all pre-late late European Maastrichtian re-cords of Diplomoceras to D. cylindraceum, even if they are partly based on specimens of smaller whorl height than the critical value of 55 mm.”

Remin et al. (2015) recorded the stratigraphically earliest occurrence of D. cylindraceum, from the mid-dle Campanian (“Inoceramus” tenuilineatus inocera-mid Zone) in the Roztocze Hills (SE Poland), and dem-onstrated the diachronous appearance of this species across Europe. These authors also retained D. maximum as a valid specific name and considered it as a possible direct descendant of D. cylindraceum, restricted to the late Maastrichtian (Remin et al., 2015, p. 846).

It should be noted that, although being small in numbers, our material does not corroborate with the above stated observations. In particular, the ribs in speci-men U.S, K2 1651, from section Moravitsa (Fig. 4b), are distant and produced a rib index 12 at a whorl height of 62 mm. This specimen was collected from the lower Maastrichtian (Trochoceramus radiosus in-oceramid Zone) and, if we had to follow Machalski (2012), it should be identified as D. maximum. Speci-men U.S, K2 1649, from the same locality (Fig. 3a–c), was assigned to the upper Maastrichtian (“Inocera-mus” ianjonaensis inoceramid Zone), and therefore it should also be recognized as D. maximum, but it is densely ribbed and at a whorl height of 47 mm the rib index is 13, whereas at whorl height of 59 mm this in-dex is 16, which is typical for D. cylindraceum. Thus, these specimens reveal that the rib index is not suffi-ciently variable to warrant species distinction.

Taking into account the values of the rib index by Kennedy (1986a, b) and Kennedy and Summesberger (1986), we should also point out that the previously il-lustrated Bulgarian examples of Diplomoceras, particu-larly those of Tz. Tzankov (1964, Pl. IV, Fig. 2) and V. Tzankov (1982a, Pl. 4, Figs 1–3), fall within the range of D. cylindraceum. The former specimen has a rib index of 19 at whorl height of 59 mm, while the lat-ter specimens have a rib index that varies from 15 to 19 at a whorl height ranging between 62 mm and 75 mm.Occurrence in our sections. Lower Maastrichtian (probably middle and upper parts) to upper Maastrich-tian (lower part): Darmantsi Formation (section Dar-mantsi) and Kunino Formation (sections Moravitsa and Kunino), Western Fore-Balkan Mts.

CONCLUDING REMARKS

New data on the occurrence of the heteromorph am-monite species Diplomoceras cylindraceum (De-france, 1816) have been received from three localities

43

of the Western Fore-Balkan (West Bulgaria). This distinctive species is relatively common in the studied fields, and all known data suggest that it is confined to the Maastrichtian. Due to the difficulty of collect-ing enough specimens from Bulgaria, the amount of variation in ribbing and whorl proportions that have been used by other authors for determination of more species within Diplomoceras is difficult to assess. The Bulgarian material reveals, however, that ribbing may vary in broad limits and does not necessarily corre-spond to particular stratigraphic levels. Therefore, it is doubtful that all the specific names that have been pro-

posed for rib differences in the literature to date rep-resent species other than Diplomoceras cylindraceum.

Acknowledgements

This research was partly supported by the PalSIRP–Sepkoski Grant for 2013. We are immensely grateful to Prof. Nikola Jolkičev, who kindly guided us to the sections that were the basis of this study. Dr Georgi Granchovski and Prof. Iskra Lakova are thanked for highly constructive suggestions and comments on an earlier version of the manuscript.

REFERENCES

Birkelund, T. 1993. Ammonites from the Maastrichtian White Chalk of Denmark. Bulletin of the Geological Society of Denmark 40, 33–81.

Błaszkiewicz, A. 1980. Campanian and Maastrichtian ammo-nites of the Middle Vistula River Valley, Poland: a strati-graphic and paleontological study. Prace Instytutu Geolog-iczego 92, 3–63.

de Blainville, H.M.D. 1825–1827. Manuel de malacologie et de conchyliologie. Levrault, Paris, Strasbourg, 664 pp.

Dabovski, H., Kamenov, B., Sinnyovsky, D., Vasilev, E., Dim-itrova, E., Bayraktarov, I. 2009. Upper Cretaceous geology. In: Zagorchev, I., Dabovski, Ch., Nikolov, T. (Eds), Geol-ogy of Bulgaria. Vol. II. Mesozoic geology. “Prof. Marin Drinov” Academic Press, Sofia, 303–589 (in Bulgarian, with English abstract).

Defrance, M.J.L. 1816. Un dictionnaire des Sciences naturelles, dans lequel on traite méthodiquement des différents êtres de la Nature. Levrault, Paris, Strasbourg, 118 pp.

Fatmi, A.N., Kennedy, W.J. 1999. Maastrichtian ammonites from Balochistan, Pakistan. Journal of Paleontology 73 (4), 641–662.

Favre, E. 1869. Descriptions des Mollusques fossiles de la Craie des environs de Lemberg en Galice. H. Georg librai-re-éditeur, Genève, 187 pp.

Hancock, J.M., Kennedy, W.J. 1993. The high Cretaceous ammonite fauna from Tercis, Landes, France. Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sci-ences de la Terre 63, 149–209.

Henderson, R.A., Kennedy, W.J., McNamara, K.J. 1992. Maas-trichtian heteromorph ammonites from the Carnarvon Ba-sin, Western Australia. Alcheringa 16, 133–170.

Ivanov, M. 1993. Uppermost Maastrichtian ammonites from the uninterrupted Upper Cretaceous–Paleogene section at Bjala (East Bulgaria). Geologica Balcanica 23 (4), 54.

Ivanov, M. 1995. Upper Maastrichtian ammonites from the sec-tions around the town of Bjala (Eastern Bulgaria). Review of the Bulgarian Geological Society 56 (3), 57–73 (in Bul-garian, with English abstract).

Ivanov, M., Stoykova. K. 1994. Cretaceous/Tertiary boundary in the area of Bjala, eastern Bulgaria. Geologica Balcanica 24 (6), 3–22.

Ivanov, M., Stoykova, K. 1995. The Cretaceous/Tertiary boundary and the “Mass extinction” problem as could be seen in the sections around Bjala (east Bulgaria). Comptes rendus de l’Académie bulgare des Sciences 48 (2), 77–79.

Ivanov, Ž. 1998. Tectonics of Bulgaria. Professorship thesis, Sofia University “St. Kliment Ohridski”, 579 pp. (in Bul-garian, unpublished).

Ifrim, C., Stinnesbeck, W., Lopes-Oliva, J.G. 2004. Maastrich-tian cephalopods from Cerralvo, north-eastern Mexico. Palaeontology 47 (6), 1575–1627.

Jagt, J.W.M. 1995. A Late Maastrichtian ammonite faunule in flint preservation from northeastern Belgium. Mededelin-gen-Rijks Geologische Dienst 53, 21–47.

Jélev, S.T. 1934a. Géologie des environs de Pléven, I. Stratigra-phie, paléogéographie et tectonique. Review of the Bulgar-ian Geological Society 6 (2), 110–142 (in Bulgarian, with French abstract).

Jélev, S.T. 1934b. Géologie des environs de Pléven, II. Paléon-tologie. Review of the Bulgarian Geological Society 6 (3), 166–204 (in Bulgarian, with French abstract).

Jolkičev, N. 1984. Stratigraphy of the North European type Up-per Cretaceous in the area of the Balkanides and the Moe-sian Platform to the east of the Ogosta River. Part II. PhD thesis, Sofia University “St. Kliment Ohridski”, 536 pp. (in Bulgarian, unpublished).

Jolkičev, N.A. 1986. Lithostratigraphic units related to the Up-per Cretaceous in the West and Central Fore-Balkan. Re-view of the Bulgarian Geological Society 47 (3), 49–61 (in Bulgarian, with English abstract).

Kennedy, W.J. 1986a. The ammonite fauna of the Calcaire à Baculites (Upper Maastrichtian) of the Cotentin Peninsula (Manche, France). Palaeontology 29, 25–83.

Kennedy, W.J. 1986b. The ammonite fauna of the type Maas-trichtian with a revision of Ammonites colligatus Binkhorst, 1861. Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre 56, 151–267.

Kennedy, W.J., Henderson, R.A. 1992. Heteromorph ammo-nites from the Upper Maastrichtian of Pondicherry, south India. Palaeontology 35, 693–731.

Kennedy, W.J., Summesberger, H. 1986. Lower Maastrichtian ammonites from Neuberg, Steiermark, Austria. Beiträge zur Paläontologie von Österreich 11, 149–206.

Kennedy, W.J., Summesberger, H. 1987. Lower Maastrichtian Ammonites from Nagoryany (Ukrainian SSR). Beiträge zur Paläontologie von Österreich 13, 25–78.

Klinger, H.C. 1976. Cretaceous heteromorph ammonites from Zululand. Memoirs of the Geological Survey of the Repub-lic of South Africa 69, 1–142.

Klinger, H.C., Kennedy, W.J. 2003a. Observations on the systematic, geographic and stratigraphic distribution and

44

origin of Diplomoceras cylindraceum (Defrance, 1816) (Cephalopoda: Ammonoidea). Annals of the South African Museum 110 (4), 171–196.

Klinger, H.C., Kennedy, W.J. 2003b. Cretaceous faunas from Zululand and Natal, South Africa. The ammonite families Nostoceratidae Hyatt, 1894 and Diplomoceratidae Spath, 1926. Annals of the South African Museum 110 (6), 219–334.

Klinger, H.C., Kennedy, W.J., Lees, J.A., Kitto, S. 2001. Upper Maastrichtian ammonites and nannofossils and a Paleocene nautiloid from Richards Bay, Kwa Zulu, South Africa. Acta Geologica Polonica 51 (3), 273–291.

Küchler, T., Odin, G.S. 2001. Upper Campanian–Maastrichtian ammonites (Nostoceratidae, Diplomoceratidae) from Ter-cis les Bains (Landes, France). In: Odin, G.S. (Ed.), The Campanian–Maastrichtian stage boundary. Characteriza-tion at Tercis les Bains (France) and correlation with Eu-rope and other continents. Developments in Paleontology and Stratigraphy 19, 500–528.

Lahsen, A., Charrier, R. 1972. Late Cretaceous ammonites form Seno Skyring-Strait of Magellan area, Magallanes Province, Chile. Journal of Paleontology 46, 520–532.

Machalski, M. 2012. Stratigraphically important ammonites from the Campanian–Maastrichtian boundary interval of the Middle Vistula River section, central Poland. Acta Geo-logica Polonica 62 (1), 91–116.

Mikhailov, N.P. 1951. Upper Cretaceous ammonites from the southern part of European Russia and their importance for zonal stratigraphy (Campanian–Maastrichtian). Trudy In-stituta Geologicheskich Nauk Akademii Nauk SSSR 129 (50), 1–142 (in Russian).

Naidin, D.P., Shimansky, V.N. 1959. Cephalopoda. In: Moskvina, M.M. (Ed.), Atlas of the Upper Cretaceous fau-na of the northern Caucasus and Crimea. Gostoptekhizdat, Moscow, 166–220 (in Russian).

Niebuhr, B. 2003. Late Campanian and Early Maastrichtian ammonites from the white chalk of Kronsmoor (northern Germany) – taxonomy and stratigraphy. Acta Geologica Polonica 53 (4), 257–281.

Odin, G.S., Courville, P., Machalski, M., Cobban, W.A. 2001. The Campanian–Maastrichtian Ammonite from Tercis (Landes, France); a synthetic view. In: Odin, G.S. (Ed.), The Campanian-Maastrichtian stage boundary. Charac-terization at Tercis les Bains (France) and correlation with Europe and other continents. Developments in Paleontol-ogy and Stratigraphy 19, 550–567.

Olivero, E.B., Zinsmeister, W.J. 1989. Large heteromorph am-monites from the Upper Cretaceous of Seymour Island, Antarctica. Journal of Paleontology 63 (5), 626–636.

d’Orbigny, A. 1840–1842. Paléontologie française: Terrains Crétacés 1, Céphalopodes. Masson, Paris, 662 pp.

Peybernès, B., Fondecave-Wallez, M.-J., Stoykova, K., Ciszak, R., Ivanov, M., Nikolov, T. 2004. Détermination de la limite Crétacé-Tertiaire en Bulgarie par les foraminifères planctoniques. Geobios 37 (6), 755–769.

Preisinger, A., Aslanian, S., Stoykova, K., Grass, F., Mauritsch, H.J., Scholger, R. 1993a. Cretaceous/Tertiary boundary sections on the coast of the Black Sea near Bjala (Bulgaria). Palaeogeography, Palaeoclimatology, Palaeoecology 104 (1–4), 219–228.

Preisinger, A., Aslanian, S., Stoykova, K., Grass, F., Mauritsch, H.J., Scholger, R. 1993b. Cretaceous/Tertiary boundary sections in the East Balkan area, Bulgaria. Geologica Bal-canica 23 (5), 3–13.

Redtenbacher, A. 1873. Die Cephalopodenfauna der Gosau-schichten in den nordöstlichen Alpen. Abhandlungen der Kaiserlich-Königlichen Geologischen Reichsanstalt 5, 91–140.

Remin, Z., Machalski, M., Jagt, J.W.M. 2015. The stratigraphi-cally earliest record of Diplomoceras cylindraceum (het-eromorph ammonite) – implications for Campanian/Maas-trichtian boundary definition. Geological Quarterly 59 (4), 843–848.

Rögl, F., Salis, K. von, Preisinger, A., Aslanian, S., Summes-berger, H. 1996. Stratigraphy across the Cretaceous/Paleogene boundary near Bjala, Bulgaria. In: Jardiné, S., Klasz, J., Delenay, J.-P. (Eds), Géologie de l’Afrique et de l’Atlantique sud: Actes Colloques Angers, 1994. Elf Aquit-aine Edition, Angers, France, 673–683.

Salazar, C., Stinnesbeck, W., Quinzio-Sinn, L.A. 2010. Ammo-nites from the Maastrichtian (Upper Cretaceous) Quiriqui-na Formation in central Chile. Neues Jahrbuch für Geolo-gie und Paläontologie, Abhandlungen 257, 181–236.

Schlüter, C. 1871–1872. Cephalopoden der oberen deutschen Kreide. Palaeontographica 21 (1–5), 1–120.

Sinnyovsky, D.S. 2013. Upper Cretaceous calcareous nan-noplankton biostratigraphy in Bulgaria. “St. Ivan Rilski” Publishing House, Sofia, 80 pp.

Spath, L.F. 1953. The Upper Cretaceous cephalopod fauna of Graham Land. Falkland Islands Dependencies Survey Sci-entific Reports 3, 60 pp.

Stinnesbeck, W., Ifrim, C., Salazar, C. 2012. The last Creta-ceous ammonites in Latin America. Acta Palaeontologica Polonica 57 (4), 717–728.

Stoykova, K., Ivanov, M. 1992. An interrupted section across the Cretaceous/Tertiary boundary at the town of Bjala, Black Sea coast (Bulgaria). Comptes rendus de l’Académie bulgare des Sciences 45 (7), 61–64.

Stoykova, K., Ivanov, M. 2002. Event and sequence stratigra-phy of the Maastrichtian and Danian in Bulgaria. Geologica Balcanica 32, 55–61.

Stoykova, K., Ivanov, I. 2004. Calcareous nannofossils and sequence stratigraphy of the Cretaceous/Tertiary transition in Bulgaria. Journal of Nannoplankton Research 26 (1), 47–61.

Stoykova, K., Ivanov, M., Belivanova, V., Kostov, R., Tzan-karska, R., Ilieva, T. 2000. Integrated stratigraphical, sedi-mentological and mineralogical-geochemical study of the Cretaceous/Tertiary boundary in Bulgaria. Review of the Bulgarian Geological Society 61 (1–3), 61–75.

Stoykova, K., Dinarès-Turell, J., Ivanov, M. 2006. Calcareous nannofossil record along the Paleocene–Eocene transition at Kladorub and Riben sections, Bulgaria. Climate and Bio-ta of the Early Paleocene, 2006, Abstracts, 133.

Tzankov, Tz. 1964. Ammonites from the Maastrichtian near Kladorub village, Belogradchik region (north-western Bul-garia). Travaux sur la Géologie de Bulgarie, Série Paléon-tologie 6, 143–168 (in Bulgarian, with Russian and English abstracts).

Tzankov, Tz., Nedjalkova, L., Aladžova-Khrischeva K., Khris-chev, Kh, Angelova, D. 1994. Geological map of the Re-public of Bulgaria 1:100 000, Cherven brjag map sheet. Committee on Geology, Company for Geophysical surveys and Geological mapping, Sofia.

Tzankov, Tz., Nedjalkova, L., Angelov, V., Aladžova-Khris-cheva, K., Yanev, S., Haidutov, I., Sapunov, I., Tchou-matchenko, P. 1995. Geological map of the Republic of Bulgaria 1:100 000, Vraca map sheet. Committee on Ge-ology, Company for Geophysical surveys and Geological mapping, Sofia.

Tzankov, V. 1932. Mollusques fossiles de la craie supérieure dans la Bulgarie de nord. Review of the Bulgarian Geologi-cal Society 4 (1), 46–84.

Tzankov, V. 1982a. Les fossiles de Bulgarie. Va. Crétacé su-périeur. Cephalopoda (Nautiloidea, Ammonoidea) et Echi-

45

nodermata (Echinoidea). Académie bulgare des Sciences, Sofia, 125 pp. (in Bulgarian, with French abstract).

Tzankov, V. 1982b. Les fossiles de Bulgarie. Bivalvia I. In: Tzankov, V. (Ed.). Les fossiles de Bulgarie. V. Crétacé supérieur. Grandes foraminifères, anthozoaires, gastéro-podes, bivalvia. Académie bulgare des Sciences, Sofia, 73–151 (in Bulgarian, with French abstract).

Walaszczyk, I., Cobban, W.A., Harries, P.J. 2001. Inocera-mids and inoceramid biostratigraphy of the Campanian and Maastrichtian of the United States Western Interior Basin. Revue de Paléobiologie, Genève 20 (1), 117–234.

Walaszczyk, I., Odin, G.S., Dhond, A.V. 2002a. Inoceramids from the Upper Campanian and Lower Maastrichtian of the Tercis section (SW France), the Global Stratotype Section and Point for the Campanian – Maastrichtian boundary; taxonomy, biostratigraphy and correlation potential. Acta Geologica Polonica 52 (3), 269–305.

Walaszczyk, I., Cobban W.A., Odin, G.S. 2002b. The inoceramid succession across the Campanian – Maastrichtian boundary. Bulletin of the Geological Society of Denmark 49, 53–60.

Walaszczyk, I., Jagt, J.W.M., Keutgen, N. 2010. The youngest Maastrichtian ‘true’ inoceramids from the Vijlen Member (Gulpen Formation) in northeast Belgium and the Aachen

area (Germany). Netherlands Journal of Geosciences 89 (2), 147–167.

Ward, P.D., Kennedy, W.J. 1993. Maastrichtian ammonites from the Biscay region (France, Spain). The Paleontologi-cal Society Memoir 34, 1–58.

Weller, S. 1903. The Stokes collection of Antarctic fossils. Journal of Geology 11, 413–419.

Whiteaves, J.F. 1903. On some additional fossils from the Van-couver Cretaceous, with a revised list of the species there-from. Geological Survey of Canada, Mesozoic Fossils 5, 309–416.

Wiedmann, J. 1962. Ammoniten aus der Vascogotischen Krei-de (Nordspanien), 1. Phylloceratina, Lytoceratina. Palae-ontographica 118A, 119–237.

Wiśniowski, T. 1907. Über die Obersenone Flyschfauna von Leszcyny. Beiträge zur Paläontologie und Geologie Öster-reich-Ungarns und des Orients 20, 191–205.

Witts, J.D., Bowman, V.C., Wignall, P.B., Crame, A.J., Fran-cis, J.E., Newton, R.J. 2015. Evolution and extinction of Maastrichtian (Late Cretaceous) cephalopods from the López de Bertodano Formation, Seymour Island, Antarc-tica. Palaeogeography, Palaeoclimatology, Palaeoecology 418, 193–212.