Geological setting

The studied brachiopod-bearing chalk succession crops out in an active chalk quarry run by the cement company Cemex Polska (Fig. 1B), situated in the eastern part of the town of Chełm. Currently, the chalk is accessible along four exploitation levels that are referred to, from the top to the bottom, as II, III, IV and V. Level I has now been completely excavated. A portion of the section below level V temporarily exposed in a dewatering trench was referred to as level VI by Dubicka & Peryt (Reference Dubicka and Peryt2011). The total thickness of the chalk currently exposed at Chełm is c. 40 m, with the top of working level II located at 203 m a.s.l. (above sea level), and the bottom of level VI at 164 m a.s.l. (Dubicka & Peryt, Reference Dubicka and Peryt2011; Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021).

The white chalk exposed at Chełm (Fig. 2) is a fine-grained carbonate deposit composed almost exclusively of coccoliths. Flint layers, burrowed omission surfaces, bored and/or mineralised hardgrounds and other marker beds, which are characteristic of many white chalk successions in Europe, are absent. The only indication that the chalk strata at Chełm lie almost horizontally is provided by the orientation of some fossils, for instance, tests of irregular echinoids (Echinocorys sp.) preserved in life position (Dubicka & Peryt, Reference Dubicka and Peryt2011). In view of this, the exploration levels are the only reference for sampling (Dubicka & Peryt, Reference Dubicka and Peryt2011; Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021). See the ‘Supplementary Information’ in Machalski et al. (Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021) for a full description of the section and its fossil assemblages.

Figure 2. Distribution of brachiopod species recognised in the lower upper Maastrichtian section exposed at Chełm along the section log and macrofossil biostratigraphy; vertical bars mark the occurrence of identified brachiopod taxa: thick – common, thin – rare (see text for further details).

Macrofossil assemblages at Chełm are rather poor in diversity and number of specimens, being dominated by small- to medium-sized bivalves, echinoids and ammonites (Dubicka & Peryt, Reference Dubicka and Peryt2011; Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021). In terms of macrofossil biostratigraphy (Fig. 2), the chalk succession at Chełm has been assigned by Machalski (Reference Machalski2005) and Dubicka & Peryt (Reference Dubicka and Peryt2011) to the lower upper Maastrichtian Belemnitella junior Zone and the Spyridoceramus tegulatus–Belemnitella junior Zone in the conventional subdivisions of the Boreal Maastrichtian in Europe (see, e.g., Błaszkiewicz, Reference Błaszkiewicz1980; Schulz & Schmid, Reference Schulz and Schmid1983; Christensen et al., Reference Christensen, Schmid and Schulz2005). According to Walaszczyk et al. (Reference Walaszczyk, Dubicka, Olszewska-Nejbert and Remin2016), the Chełm succession represented the uppermost ‘true’ (non-tegulated) inoceramid ‘Inoceramus’ ianjonaensis Zone also put in the lower upper Maastrichtian, as based on data from the Vijlen Member in northeast Belgium and the Aachen area in western Germany (see Walaszczyk et al., Reference Walaszczyk, Jagt and Keutgen2010). It is worth mentioning in this context that no ‘true’ inoceramids are known to date from Chełm, Spyridoceramus tegulatus (von Hagenow, Reference Hagenow1842) being the sole representative of the ‘tegulated inoceramids’ throughout the section. Belemnites at Chełm are represented by very rare individuals of Belemnitella junior Nowak, Reference Nowak1913 and B. lwowensis Naidin, Reference Naidin1952 (see Dubicka & Peryt, Reference Dubicka and Peryt2011). Both species are present in the lower part of the upper Maastrichtian of the Hemmoor section, northwest Germany and in the Haccourt-Lixhe area of northeast Belgium (Christensen et al., Reference Christensen, Schmid and Schulz2005). In ammonite terms, levels V and IV belong to the lower upper Maastrichtian scaphitid ammonite Hoploscaphites constrictus lvivensis Zone (Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021; Fig. 2 herein).

Dubicka & Peryt (Reference Dubicka and Peryt2012) assigned the Chełm section to their local foraminiferal zones VIII (lowest part), IX (bulk of section) and X (top of section). Based on planktic foraminiferal spectra (Dubicka & Peryt, Reference Dubicka and Peryt2011, Reference Dubicka and Peryt2012) and the shape of the carbon isotope curve, Machalski et al. (Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021; Supplementary Information) concluded that the Chełm section corresponded to the interval between c. 180 and 115 m in the Stevns-1 reference borehole in Denmark, as described by Thibault et al. (Reference Thibault, Harlou, Schovsbo, Schiøler, Minoletti, Galbrun, Lauridsen, Sheldon, Stemmerik and Surlyk2012) and Surlyk et al. (Reference Surlyk, Rasmussen, Boussaha, Schiøler, Schovsbo, Sheldon, Stemmerik and Thibault2013). This interval represents the Hvidskud Member of the Møns Klint Formation, which comprises the topmost part of the UC19, entire UC20a and lower part of the UC20b-c nannofossil zones, and three magnetostratigraphic zones, namely 31n, 30r and 30n. This corresponds to the topmost part of the lower Maastrichtian (level VI not studied for brachiopods for the present paper) and the lower part of the upper Maastrichtian (remaining levels, see Supplementary Information figure 2 in Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021).

The Chełm chalk was deposited in an epicontinental setting, in the eastern part of the Boreal Chalk Sea of Europe (e.g., Surlyk et al., Reference Surlyk, Dons, Clausen and Higham2003; Engelke et al., Reference Engelke, Esser, Linnert, Mutterlose and Wilmsen2016, Reference Engelke, Linnert, Mutterlose and Wilmsen2017). The Chełm chalk matches the characteristics of the ‘benthos-poor’ variety of white chalk in the Boreal Chalk Sea facies model, representing the deepest epicontinental chalk facies, deposited below the photic zone and storm wave base (Surlyk et al., Reference Surlyk, Dons, Clausen and Higham2003, Fig. 13.7; see also Surlyk & Birkelund, Reference Surlyk, Birkelund, Kauffman and Hazel1977 and Hansen & Surlyk, Reference Hansen and Surlyk2014). Similar to other examples of this white chalk subfacies, the Chełm chalk is monotonous and lacks hardgrounds, omission surfaces, scour horizons, tabular fossil concentrations and flint nodules. Its macrofossil content is typified by a rather poor macrobenthic assemblage, a predominance of Zoophycos burrows (at least in the lower levels), a relative abundance of ammonites and an extreme rarity of belemnites represented solely by adult individuals (Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021). A shallowing-upwards trend is inferred from planktic foraminiferal data for the succession (Dubicka & Peryt, Reference Dubicka and Peryt2011, Reference Dubicka and Peryt2012), with depth estimates ranging from >100 m for the lower levels to several dozen metres in the uppermost part of the section (Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021). Correlation with the planktic foraminiferal spectra recorded from equivalent Maastrichtian levels in the Middle Vistula River section (central Poland) suggests that the record of the maximum mid-Maastrichtian sea level rise should be expected just beneath the bottom of the section exposed at Chełm (Dubicka & Peryt, Reference Dubicka and Peryt2011, Reference Dubicka and Peryt2012).

Taxonomy

In total, 15 brachiopod taxa have been recovered from the chalk succession at Chełm (Table 1). These are Lingula cretacea Lundgren, Reference Lundgren1885, Isocrania costata (Sowerby, Reference Sowerby1823) (Fig. 3G–I), Cryptoporella antiqua Bitner & Pisera, Reference Bitner and Pisera1979 (Fig. 4A–H), Cretirhynchia sp., Neoliothyrina sp., Carneithyris sp., Terebratulina chrysalis (Schlotheim, Reference Schlotheim1813) (Fig. 3C), T. faujasi (Roemer, Reference Roemer1841), T. longicollis Steinich, Reference Steinich1965 (Fig. 3A, B), Terebratulina spp., Gisilina sp., Bronnothyris bronni (Roemer, Reference Roemer1841) (Fig. 3E, F), Magas chitoniformis (Schlotheim, Reference Schlotheim1813) (Fig. 3D), Leptothyrellopsis polonicus Bitner & Pisera, Reference Bitner and Pisera1979 and ?Aemula sp. (Fig. 2; Table 1). Only Cryptoporella antiqua and some species of Terebratulina are common in the material studied, while others are rare (Fig. 2; Table 1). Six taxa were identified at all levels sampled for brachiopods (Fig. 2). Of these, the presence of Lingula cretacea is worth noting; this is represented by poorly preserved fragments throughout the section. Cretaceous to Recent representatives of Lingula have been characterised by a very low taphonomic potential in view of thin fragile shell, composed of chitinous and phosphatic layers (Emig, Reference Emig1990; Kowalewski & Flessa, Reference Kowalewski and Flessa1996).

Figure 3. Selected brachiopods from the lower upper Maastrichtian white chalk exposed at Chełm. A, B. Terebratulina longicollis; A – dorsal view of complete specimen, level IV, ZPAL Bp.50/8; B – inner view of dorsal valve to show brachidium, level V, ZPAL Bp.50/9; C – Terebratulina chrysalis, dorsal view of complete young specimen, level III, ZPAL Bp.50/7; D – Magas chitoniformis, inner view of dorsal valve, level III, ZPAL Bp.50/10; E, F – Bronnothyris bronni, ventral and dorsal views of complete specimen, level II, ZPAL Bp.50/11; G–I – Isocrania costata; G – inner view of ventral valve, level II, ZPAL Bp.50/1; H, I – outer and inner views of dorsal valve, level II, ZPAL Bp.50/2. All SEM photomicrographs.

Figure 4. Cryptoporella antiqua from the lower upper Maastrichtian white chalk exposed at Chełm, ZPAL Bp. 50/3-6. A, B – Dorsal view of complete specimen and enlargement of umbonal part to show details of beak, respectively. C–E – Inner view of ventral valve, enlargement of umbonal part and oblique view to show dentral plates, respectively. F – Inner view of ventral valve. G, H – Inner view of dorsal valve and enlargement of posterior part to show details of cardinalia, respectively. All SEM photomicrographs.

Table 1. Ecological groups of brachiopods identified from the lower upper Maastrichtian chalk exposed at Chełm and their relative abundance in the section (generalised, based on all levels). In parentheses, numbers of the groups after Schrøder & Surlyk (Reference Schrøder and Surlyk2020) are shown.

There is some variation in brachiopod diversity throughout the section, ranging from a minimum number of 9 taxa recorded from level IV to a maximum of 13 taxa from level II (Fig. 2). The number of specimens also varies between samples, ranging from a minimum number of 87 specimens in a sample from level IV to a maximum of 267 from level III (Fig. 2).

Most of the taxa identified at Chełm are common in the white chalk facies of northwest Europe, which are characterised by long stratigraphical ranges and a wide geographical distribution (compare Fig. 2 and Table 1 herein with Surlyk, Reference Surlyk1984, Fig. 2; see also Surlyk, Reference Surlyk1972). A notable exception is Cryptoporella antiqua (Fig. 4A–H), known so far only from the lower to upper Campanian of Mielnik (Bitner & Pisera, Reference Bitner and Pisera1979; stratigraphy based on Bojanowski et al., Reference Bojanowski, Dubicka, Minoletti, Olszewska-Nejbert and Surowski2017). Contrary to the Mielnik assemblage, this species dominates in the material from Chełm, constituting more than 40% of the material. Another species erected based on Mielnik material by Bitner & Pisera (Reference Bitner and Pisera1979) is Leptothyrellopsis polonicus. This was recovered by these authors from the chalk interval corresponding to the lower Campanian in currently accepted stratigraphy of the Mielnik section (Bojanowski et al., Reference Bojanowski, Dubicka, Minoletti, Olszewska-Nejbert and Surowski2017). This species has subsequently been recognised in the chalk facies of late Campanian and early Maastrichtian age in northern Europe (Surlyk, Reference Surlyk1982; Johansen & Surlyk, Reference Johansen and Surlyk1990; MacKinnon et al., Reference MacKinnon, Simon and Bitner1998; Simon & Mottequin, Reference Simon and Mottequin2018).

With a total of 15 identified taxa, the Chełm assemblage is rather poor in comparison with the most prolific white chalk brachiopod assemblages, described from the lower lower and upper upper Maastrichtian of Denmark (Surlyk, Reference Surlyk1972, Fig. 3; Surlyk, Reference Surlyk1984, Fig. 1). For example, the assemblage from the Argyrotheca stevensis–Magas chitoniformis Zone at the top of the Danish Maastrichtian comprises 23 species, and those from the Rugia spinosa–Terebratulina subtilis, T. subtilis–Trigonosemus pulchellus and T. pulchellus–T. pulchellus zones from the middle portion of the lower Maastrichtian yield 22, 23 and 24 species, respectively. In contrast, the assemblage from the Rugia tenuicostata–Meonia semiglobularis Zone of Surlyk (Reference Surlyk1984) comprises 16 species, which is the minimum number for the Danish Maastrichtian, matching well the Chełm assemblage as far as taxonomic diversity is concerned.

In terms of number of individuals per standard 10 kg sample, the Danish chalk assemblages yield from over 3,000 specimens (Argyrotheca stevensis–Magas chitoniformis and Terebratulina subtilis–Trigonosemus pulchellus zones) to several hundred specimens in the Rugia tenuicostata–Meonia semiglobularis Zone (Surlyk, Reference Surlyk1972, Fig. 3). The specimen abundance at Chełm, ranging from 87 to 267 specimens per sample, is much lower than in any of the samples studied by Surlyk, including those from the Rugia tenuicostata–Meonia semiglobularis Zone, which are the poorest in specimens within the entire Danish Maastrichtian.

Stratigraphical position

As pointed out above, the Chełm assemblage is of relatively low diversity and abundance and is dominated by stratigraphically long-ranging forms which are known from the upper Campanian and Maastrichtian. In terms of the micromorphic brachiopod zonation of the Danish chalk (Surlyk, Reference Surlyk1972, Reference Surlyk1984), the assemblage studied herein is best comparable to the brachiopod assemblage from the Rugia tenuicostata–Meonia semiglobularis Zone (= tenuicostata-semiglobularis Zone in Surlyk, Reference Surlyk1984, abbreviated t-s, referred also to as Zone 7 in Surlyk, Reference Surlyk1970). The base of this zone was defined by Surlyk (Reference Surlyk1984) by the last occurrence of Rugia tenuicostata Steinich, Reference Steinich1963, and the top by the appearance of Meonia semiglobularis (Posselt, Reference Posselt1894). Both these morphologically distinctive species are absent at Chełm (Fig. 2; Table 1). The stratotype of the Rugia tenuicostata–Meonia semiglobularis Zone is sample 82 from the Hemmoor section, northern Germany, corresponding to the upper lower Maastrichtian part of the Hemmoor succession (Surlyk, Reference Surlyk1984).

Schulz & Schmid (Reference Schulz and Schmid1983, Fig. 3) correlated Zone 7 of Surlyk (Reference Surlyk1970), that is, the Rugia tenuicostata–Meonia semiglobularis Zone of Surlyk (Reference Surlyk1984), with the upper lower Maastrichtian belemnite Belemnella cimbrica and B. fastigata zones, making the appearance of Meonia semiglobularis, a marker for the higher Zone 8 of Surlyk, just beneath marl layer M-900. This bed has been selected as the conventional boundary between the lower and upper Maastrichtian at Hemmoor (e.g., Schulz & Schmid, Reference Schulz and Schmid1983; Christensen et al., Reference Christensen, Schmid and Schulz2005). The same upper lower Maastrichtian position of Zone 7 is presented by Reich & Frenzel (Reference Reich and Frenzel2002, Fig. 7) for the Maastrichtian chalk succession of the Isle of Rügen, northeast Germany. In contrast, the Chełm assemblage is recorded from the lower upper Maastrichtian Spyridoceramus tegulatus–Belemnitella junior Zone sensu Schulz & Schmid (Reference Schulz and Schmid1983), so comes from the Chełm eqivalents of Zone 8 at Hemmoor. On the other hand, the lower upper Maastrichtian position of the Rugia tenuicostata–Meonia semiglobularis Zone is suggested on the log of Stevns 1 borehole by Surlyk et al., (Reference Surlyk, Rasmussen, Boussaha, Schiøler, Schovsbo, Sheldon, Stemmerik and Thibault2013, appendix 2). The problem of apparent diachroneity of various biostratigraphical markers in the Boreal Maastrichtian of Europe is beyond the scope of the present work.

Palaeoecology

Except for burrowing Lingula, all brachiopods from the European white chalk in general, and from the Chełm succession in particular, represent a guild of fixo- and libero-sessile epifaunal suspension feeders as distinguished by Engelke et al., (Reference Engelke, Esser, Linnert, Mutterlose and Wilmsen2016, Reference Engelke, Linnert, Mutterlose and Wilmsen2017) in their study of early Maastrichtian macrobenthos at Kroonsmoor, northwest Germany. In view of the ecological requirements of this group, the development of the Late Cretaceous brachiopod communities in the Boreal Chalk Sea of Europe must have been controlled largely by two interrelated environmental factors: 1) input of food and nutrients, and 2) availability of minute skeletal substrates which would have been suitable for brachiopod settlement (Surlyk, Reference Surlyk1972; Hansen & Surlyk, Reference Hansen and Surlyk2014; Engelke et al., Reference Engelke, Esser, Linnert, Mutterlose and Wilmsen2016, Reference Engelke, Linnert, Mutterlose and Wilmsen2017). Both factors were, in turn, indirectly influenced by the depth of the Boreal Chalk Sea and its variations (e.g., Hansen & Surlyk, Reference Hansen and Surlyk2014). The substrate–brachiopod relationship is easiest to grasp based on simple geological and palaeontological evidence and is considered in more detail below.

According to Schrøder & Surlyk (Reference Schrøder and Surlyk2020), the white chalk brachiopods may be subdivided into seven groups in terms of their adaptations and strategies to live on the sea bottom. Based on that paper, we characterise these groups as follows: 1) micromorphic species attached with a pedicle and able to use very small, hard substrates; 2) medium- to large-sized species attached with a pedicle to large, hard substrates; 3) species attached directly to the soft sediment with a rootlet pedicle; 4) secondarily free-living pedunculate species, attached to the substrate by pedicle at the juvenile stages; 5) species attached by cementation by the whole surface of the ventral valve; 6) species attached by cementation in the juvenile stage, but secondarily free-living in the adult stage, therefore possessing a small attachment surface on the ventral valve; and 7) species burrowed into the soft-bottom substrate. Table 1 presents the assignment of the brachopod taxa from Chełm to these categories along their relative abundance. Except for group 5, all the groups distinguished by Schrøder & Surlyk (Reference Schrøder and Surlyk2020) are present in the Chełm material, with a clear predominance of group 1 (Table 1).

As demonstrated by Surlyk (Reference Surlyk1972, Fig. 3), there is a striking parallelism between the washing residue curve (approximately corresponding to the bryozoan curve) and the curves illustrating the brachiopod species and specimen-abundance throughout the Danish chalk succession (see also Surlyk & Birkelund, Reference Surlyk, Birkelund, Kauffman and Hazel1977). According to these authors, the low brachiopod diversity and abundance in the upper lower and lower upper Maastrichtian assemblages, including that from the Rugia tenuicostata–Meonia semiglobularis Zone, resulted from low availability of substrates suitable for colonisation by brachiopods on the sea floor, reflected in low numbers of skeletal hash, mainly bryozoans, in the washed residues. In contrast, the lower lower and uppermost Maastrichtian maxima in brachiopod diversity and abundance reflect a superabundance of predominantly bryozoan hash on the sea bottom, discernible as relevant peaks in washing residues (Surlyk, Reference Surlyk1972, Fig. 3). The three curves seem, in turn, to be indirectly related to bathymetry, with depauperate brachiopod communities linked to the deeper-water portions of the chalk succession (Surlyk, Reference Surlyk1972; see also Hansen & Surlyk, Reference Hansen and Surlyk2014).

The brachiopod assemblage from the Chełm succession matches the above observations, as the Chełm chalk represents a relatively deep-water, ‘benthos-poor’ subfacies of the white chalk (Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021), characterised by a paucity of minute skeletal substrates on the sea floor. Minor variations in brachiopod diversity and abundance at Chełm (Fig. 2) are not easy to explain with the evidence at hand. The differences in diversity may simply result from sampling bias, but the highest abundance at level III (Fig. 2) may reflect a temporal improvement of environmental conditions on the sea floor. Notably, an increase in abundance of epifaunal suspension-feeding bivalves has been recorded from that level by Machalski et al., (Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021, Supplementary Information). Minor temperature variations inferred from oxygen stable isotopic analyses of benthic foraminiferal tests from Chełm (Machalski et al., Reference Machalski, Owocki, Dubicka, Malchyk and Wierny2021, Fig. 2) are not correlatable in any way with the changes in brachiopod abundance reported herein.