A unique occurrence of stalked crinoids in the Eocene Popiele Beds (Outer Carpathians, Poland)

Abstract

The Eocene Popiele Beds of the Skole Nappe (Outer Carpathians, Poland) are exceptional among the predominantly deep-water lithofacies of Outer Carpathians because of their proximal origin and subsequent mass resedimentation into deeper parts of the flysch basin. Consequently, this lithostratigraphic unit contains unusually rich macrofossil assemblages within otherwise generally fossil-poor flysch successions. During a multidisciplinary study of the fossiliferous mudstone matrix from the Koniusza locality, we identified, in addition to abundant foraminifera, molluscs, bryozoans, other marine organisms, and abundant palynological organic matter, rare remains of stalked crinoids, including isocrinids, and bourgueticrinids. These echinoderms are represented by small columnals that show minimal abrasion, and in some cases little disarticulation indicating rapid burial and limited transport. Their rarity and small size may suggest the presence of a relict population inhabiting environments already strongly affected by post-Mesozoic ecological restructuring. At Koniusza, this setting was most likely a nearshore, nutrient-rich marine environment influenced by strong terrestrial input. Sedimentological and palynological evidence indicates a shallow-neritic setting with normal salinity and elevated productivity. The occurrence of stalked crinoids in such a proximal, probably shallow-marine environment provides new insight into the palaeoenvironmental preferences of these echinoderms and their ecological persistence during the Palaeogene.

Introduction

The evolutionary history of stalked crinoids was profoundly shaped by the ecological restructuring of marine ecosystems during the Mesozoic Marine Revolution (MMR). The rise of durophagous predators during the Mesozoic triggered a major reorganization of benthic communities, forcing many sessile suspension feeders, including stalked crinoids, to retreat from shallow continental shelves into deeper marine environments (e.g.^1–3). As a result, modern stalked crinoids are largely restricted to bathyal and abyssal settings, whereas shallow-water environments are dominated by the more mobile comatulids (e.g.^4,5).

The fossil record suggests that this ecological transition was already well advanced by the Late Cretaceous and continued throughout the Cenozoic. However, reconstructing the palaeoecology and distribution of stalked crinoids during the Paleogene remains difficult because their fossil record is sparse. Crinoid skeletons disarticulate rapidly after death, and their skeletal elements are easily transported, fragmented and selectively preserved in sedimentary environments^6–8. As a consequence, most Paleogene occurrences are represented only by isolated ossicles or short pluricolumnals, which greatly complicates palaeoecological interpretation.

Despite these limitations, several studies have shown that Paleogene stalked crinoids were commonly associated with deeper marine environments, including basin slopes and outer ramp settings (e.g.^8–10). Nevertheless, recent discoveries indicate that the post-MMR shift toward deep-water habitats was not entirely uniform. Local shallow-water occurrences of isocrinids have been documented from the Eocene of Romania and from several Gondwanan regions, suggesting that small relict populations persisted in restricted ecological refugia^11,12. These findings raise important questions regarding the spatial variability of crinoid habitats during the Paleogene and the ecological factors that enabled the survival of stalked crinoids in post-MMR marine ecosystems.

To contribute new data to this issue, we report a unique occurrence of stalked crinoids from the Eocene Popiele Beds of the Skole Nappe in the Outer Carpathians of southern Poland. Most Paleogene deposits of the Outer Flysch Carpathians consist of deep-marine turbidites and hemipelagites, whereas the Popiele Beds represent an unusual lithofacies of proximally derived deposits subsequently resedimented into a deeper flysch basin. Palaeontological analysis of the Popiele Beds exposed at Koniusza revealed a diverse marine fauna preserved within a fossiliferous mudstone matrix, including foraminifera, molluscs, bryozoans, echinoderms, and other fossil groups, together with abundant terrestrial and subordinate marine organic matter.

Among echinoderms, rare remains of stalked crinoids represented by minute columnals and short pluricolumnals of isocrinids and bourgueticrinids show only minimal abrasion, indicating rapid burial and limited transport. These remains suggest the presence of a small relict population inhabiting the primary depositional environment of the Popiele Beds. To reconstruct this environment and its ecological conditions, we undertook a multidisciplinary study of the fossil assemblages. Our sedimentological, palaeontological, and palynological data provide new insight into the palaeoenvironmental conditions of the Popiele Beds and indicate that the Koniusza assemblage may record the persistence of small relict populations of stalked crinoids in nearshore settings during the Paleogene.

Geological background

The Popiele belong to the Skole Nappe, the northernmost tectonic unit of the Flysch Carpathians, thrust over the Carpathian Foredeep (Fig. 1). This lithostratigraphic unit is generally interpreted as a submarine slump derived from the northern slope of the Skole Basin, which during the Palaeogene formed the northern margin of the entire Carpathian basin system^13–15. Consequently, the Popiele Beds crop out along the northernmost part of the Skole Nappe as a narrow belt extending from the city of Przemyśl (Poland) and the Prut River valley (Ukraine^13,16,17).

Fig. 1.

Geological map of the study area (A; after¹⁸), locality map (B; after¹⁹), and lithostratigraphic scheme of the Popiele Beds within the Skole Nappe succession (C; after²⁰). The dashed rectangle in (C) marks the location of Popiele Beds exposures along the Koniuszanka (Zalesie) Stream (see¹⁹ for details).

Most currently known outcrops of the Popiele Beds are situated in Ukraine (e.g.²¹), whereas in Poland they occur mainly south of Przemyśl^22,23. Among the Polish localities, the exposures near the village of Koniusza, from which the material studied herein was collected, are among the best known (see e.g.¹⁹). The term Popiele Beds was introduced by Kropaczek²⁴ on the basis of the stratotype section near the village of Popiele close to Borysław, a locality that was then within Poland and is now situated in Ukraine.

The tectonic origin of the Popiele Beds remains controversial^25,26 interpreted them as tectonic slides and reported their occurrence from two boreholes in the Stebnik Unit, i.e., the belt of folded Miocene strata developed in front of the northward-migrating Carpathian nappes²⁷, see also²⁸.

Alternative interpretations include olistostromes within the Stebnik Unit²⁹, cf.²¹, the flysch substratum of the Stebnik Unit¹⁷, and the most widely accepted interpretation, which regards this unit as a submarine slump (e.g.¹³).

The Popiele Beds consist of dark brown to black, unstratified silty mudstones forming a matrix that contains variable amounts of clasts, ranging from small fragments to blocks several metres in diameter. The clast assemblage includes intraclasts derived from flysch succession (e.g., Hieroglyphic Beds) and much rarer extraclasts (exotics), represented by Jurassic limestones and probably Carboniferous coals^13,30,31, cf.^21,32. Precise estimation of the thickness of the Popiele Beds is difficult because of tectonic deformation and the absence of complete, undisturbed exposures showing contacts with the underlying Hieroglyphic Beds and the overlying Menilite Beds.

For more than a century, the Popiele Beds have been known for their rich macrofossil content^23,33, which makes them exceptional within Flysch Carpathian successions that are otherwise typically poor in large macrofossil. Molluscs (bivalves, gastropods and scaphopods) were studied by^{22,23,33–35}, and³⁴. Microfossil investigations have documented foraminifera^33,36–38, calcareous nannoplankton³⁶, and dinoflagellate cysts^19,39.

Based on these fossil groups, the age of the Popiele Beds is generally considered to range from the Bartonian–Priabonian to the Priabonian–early Rupelian intervals^{19,34,36,37,39,40},see also Age interpretation below).

Palaeogeography

The upper Eocene strata of the Popiele Beds represent the northeasternmost record of marine transgression within the Carpathian Basin. Whether this basin was connected to the epicontinental basin that extended farther north across the Polish Lowlands remains uncertain. The closest site with coeval marine strata was described from a borehole near the village of Łukowa in south-eastern Poland⁴¹, some 60 km from the recent front of Carpathian overthrust, and some 100 km from the presumed Carpathian sea shore in late Eocene. But the dinoflagellate cyst data from Łukowa, despite some similarities with Popiele Beds flora, does not solve that problem unanimously. Alternatively, Gedl⁴¹ suggested a land barrier separating these two marine areas.

Materials and methods

Approximately 20 kg of dark grey to nearly black mudstone were collected from a natural exposure of the Popiele Beds along the Koniuszanka (Zalesie) Stream near the village of Koniusza (Fig. 1B; 49°40′55.6"N, 22°42′03.4"E). The locality is publicly accessible and situated outside protected land; therefore, no permits or special permissions were required for sampling.

Part of the material disintegrated during initial processing, whereas approximately half of the sample proved relatively resistant to slaking. Consequently, a sequential treatment involving drying, soaking in water, and repeated freezing–thawing cycles was applied. Thawing was conducted both at room temperature and by pouring boiling water over the sample. This procedure was repeated several times. After each cycle, the material was rinsed with clean water and wet-sieved through two meshes: an upper sieve with a mesh size of 2.00 mm and a lower sieve with a mesh size of 0.35 mm, from which the finest fraction was recovered. The coarser fraction was returned for further disintegration following the procedure described above. As a result, the remaining sediment was reduced to particles of approximately 2 mm.

Complete slaking was not achieved. Because of the increased risk of damaging delicate microfaunal remains, sodium sulfate (Glauber’s salt), commonly used for clay-rich carbonate sediments, was deliberately avoided.

The recovered residue was examined under a PZO MST-131 stereomicroscope at × 10 magnification. Scanning electron microscope images were obtained using a Hitachi S-4700 instrument at the Institute of Geological Sciences, Jagiellonian University, Kraków, Poland.

Thin sections were prepared at AGH University of Kraków, Poland, and used for facies and microfacies analyses. Photomicrographs were taken using an Olympus SZX 7 microscope equipped with an Olympus SC 30 camera and Olympus Stream 1.7 software.

A small portion of the same sample was processed for palynological analysis to constrain the age of the studied material. Fifty grams of cleaned fresh sediments were treated successively 38% hydrochloric acid (HCl), 40% hydrofluoric acid (HF), and heavy-liquid separation (ZnCl₂ + HCl; density 2.0 g·cm^-3), followed by ultrasonic treatment for 10–15 s and sieving through a 10 µm nylon mesh. No nitric acid (HNO₃) treatment was applied. Microscopic slides were prepared using glycerine jelly as mounting medium. Two slides were analysed for the palynological spectrum, and all dinoflagellate cysts together with 1,000 pollen grains and spores were counted in total. Unprocessed rock material, palynological residues, and slides are housed in the collection of the Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Kraków.

All figures in this article were prepared using CorelDraw 2021 (version 23.5.0.506), licensed to the University of Silesia in Katowice, Poland.

Results

The fossil assemblage comprises foraminifera (> 400 specimens), gastropods (167 specimens), bivalves (33 specimens), scaphopods (66 specimens), bryozoans (48 specimens), corals (17 specimens), barnacles (11 specimens), serpulids (12 specimens), ostracods (2 specimens), and fish remains (25 specimens, including teeth and otoliths). Echinoderms are represented by a single irregular echinoid specimen, isolated spines, and fragments of test plates. Asteroids are recorded by four lateral arm plates. Crinoids include stalked isocrinids (order Isocrinida), represented by 33 columnals (including pluricolumnals) and several cirrals, as well as stalked comatulid-related forms assigned to bourgueticrinids (16 columnals).

Crinoid description

Isocrinids, Isocrinida indet. (Fig. 2C-F): columnals are rather small, with diameters ranging from 1.2 mm to 2.1 mm. Their heights range from 0.8 mm to 1.3 mm. Internodal columnals are circular, (sub)pentagonal, rarely stellate. Pentagonal and stellate columnals have acute interradial angles. Articular surfaces of distal (circular) and median (sub-, pentagonal) columnals are flat. They can be covered by small, drop-like petal floors. They are separated by belts of radial crenulae consisting of two parallel sets of minute tubercles numbering from 2 to 6. Marginal crenulae, with a maximum number of 28, are of similar size. Facets between internodals and the proximal facet of nodals have narrow petals and strong crenulae. Fossulae are restricted to a thin and cryptosymplectial facet. Nodal columnals are as wide as high and stellate in outline. They are only slightly larger than internodals. Nodal columnals have short, uniform marginal crenulae fading towards a smooth margin. Fossulae are poorly developed. Cirrus sockets are of different size, but small, oval ones predominate, directed outward and lacking cupules. Cirral scar width is 50–55% of nodal diameter. Lumen is circular and small. Latera can be smooth, straight, concave, and/or convex. Cirrals are smooth. They are elliptical or circular in outline. Proximal cirrals increase in length distally with an irregular adoral crest.

Fig. 2.

Crinoid remains recovered from the Popiele Beds sample at Koniusza. Scale bar = 1 mm. Images A–D were obtained using a Hitachi S-4700 scanning electron microscope; images E–F were taken with a PZO MST-131 stereomicroscope. (A) Bourgueticrinid proximale, oblique view; (B) bourgueticrinid columnal, articular facet; (C) isocrinid cirri, lateral view; (D, E) isocrinid columnals, articular facets; (F) isocrinid pluricolumnal, lateral view.

Comatulids (bourgueticrinids, Bourgueticrinina; Fig. 2A, B): columnals are small, with diameters ranging from 1.0 mm to 1.8 mm. Their heights range from 0.9 mm to 1.5 mm. They are cylindrical or barrel-shaped in outline. Most columnals display poorly visible facets due to sediment/cement coating, and only some reveal poorly visible synarthrial facet with small and circular lumen. Latera is smooth and being either planar, concave or convex. The proximale is elongate and cylindrical, with a subcircular outline and gently rounded margins; the lateral surface is smooth. Its height is 2.98 mm, and its diameter ranges from 1.65 mm at the narrowest point to 2.11 mm at the widest point.

Palynology

The sample yielded abundant palynological organic matter composed predominantly of terrestrially derived plant debris, including cuticles, dark brown to black phytoclasts, and subordinate sporomorphs. Aquatic palynomorphs constitute only a few per cent of the total palynofacies, and are represented by numerous organic linings of foraminifera, dinoflagellate cysts, and the prasinophyte alga Tasmanites. All aquatic palynomorphs are very well preserved, being pale coloured and retaining intact wall structures with no evidence of thermal alteration. Some specimens, however, show mechanical damage in the form of tearing or breakage.

The dinoflagellate cyst assemblage consists of 35 taxa. The most abundant forms are Deflandrea spp. (Fig. 3E), Homotryblium spp. (Fig. 3B), and Spiniferites spp. (Fig. 3C). Other taxa include chorate gonyaulacoids such as Operculodinium, Areosphaeridium (Fig. 3D, G), Enneadocysta, and Hystrichokolpoma, together with less common peridinioids including Charlesdowniea (Fig. 3I), Rhombodinium (Fig. 3F), and Selenopemphix. A single specimen of Impagidinium was recorded.

Fig. 3.

Dinoflagellate cysts, organic lining of foraminifera, and palynofacies from the Popiele Beds sample at Koniusza (photographs by P. Gedl). The scale bar in A represents 15 μm and applies to all photomicrographs except J and K, where it represents 500 μm. (A) Foraminifera organic lining; (B) Homotryblium plectilum; (C) Spiniferites ramosus; (D) Areosphaeridium michoudii; (E) Deflandrea phosphoritica; (F) Rhombodinium perforatum; (G) Areosphaeridium diktyoplokum; (H) Samlandia chlamydophora; (I) Charlesdowniea sp.; (J, K) palynofacies.

Sporomorphs are very abundant but of low diversity and generally poorly preserved, although most specimens remain identifiable (Fig. 4). In total, 25 fossil-species were recognised, including eight species of spores, ten species of gymnosperm pollen, and seven species of angiosperm pollen. In addition, several small tricolporate/tricolpate and triporate pollen grains were encountered, but their poor preservation precludes reliable taxonomic assignment beyond broad morphological categories (Table 1).

Fig. 4.

Spores (A–G) and pollen grains (H–T) recorded in the Popiele Beds sample at Koniusza (photographs by E. Worobiec). The scale bar in A represents 10 μm and applies to all photomicrographs. (A, B) Retitriletes sp., same specimen, different focal planes; (C, D) Neogenisporis crassicus, same specimen, different focal planes; (E) Neogenisporis cf. neogenicus; (F) Leiotriletes sp.; (G) Verrucatosporites cf. favus; (H) Inaperturopollenites concedipites; (I) Sciadopityspollenites sp., (J) Sciadopityspollenites verticillatiformis; (K) Pinuspollenites labdacus; (L) Sciadopityspollenites miniverrucatus; (M) Zonalapollenites sp.; (N) Pinuspollenites sp.; (O, P) Tricolporopollenites cf. staresedloensis, same specimen, different focal planes; (Q) Tricolporopollenites fallax; (R) tricolporate/tricolpate pollen grain; (S, T) cf. Tricolporopollenites deliberatus, same specimen, different focal planes.

Table 1.

Spores and pollen grains recorded in the sample from Koniusza.

Fossil taxa	Botanical affinity	Element	Number of specimens
Spores of plants
Leiotriletes sp.	Lygodiaceae and other ferns	P	2
Neogenisporis crassicus Krutzsch + Neogenisporis cf. neogenicus Krutzsch + Neogenisporis sp.	Gleicheniaceae, Cyatheaceae	P1	5
Retitriletes sp.	Lycopodiaceae: Lycopodium	A	1
Stereisporites sp.	Sphagnaceae: Sphagnum	P/A	1
Triplanosporites sp.	unknown	P	1
Verrucatosporites cf. favus (Potonié) Thomson and Pflug	Davalliaceae, Polypodiaceae, and other ferns	P/A	2
Pollen of gymnosperms
Cathayapollis sp.	Pinaceae: Cathaya	A1	13
Inaperturopollenites concedipites (Wodehouse) Krutzsch	Cupressaceae: Taxodium, Glyptostrobus	P2/A1	2
Piceapollis sp.	Pinaceae: Picea	A	2
Pinuspollenites labdacus (Potonié) Raatz + Pinuspollenites sp.	Pinaceae: Pinus	A	891
Sciadopityspollenites miniverrucatus Kohlman-Adamska + S. serratus (Potonié and Venitz) Raatz + S. verticillatiformis (Zauer) Krutzsch + Sciadopityspollenites sp.	Sciadopityaceae: Sciadopitys	A1	65
Zonalapollenites sp.	Pinaceae: Tsuga	A1	1
Pollen of angiosperms
Arecipites sp.	Arecaceae, Araceae, Butomaceae	P/A	1
Ericipites sp.	Ericaceae	P/A	2
Tricolporopollenites fallax (Potonié) Krutzsch	Fabaceae	P/A	1
Tricolporopollenites cf. staresedloensis Krutzsch and Pacltová	Hamamelidaceae	P2	2
Tricolporopollenites sp.	Hamamelidaceae	P2/A1	3
Trivestibulopollenites betuloides Pflug	Betulaceae: Betula	A	1
cf. Tricolporopollenites deliberatus Durska	Salicaceae?, Zygophyllaceae?	unknown	1
small tricolporate/tricolpate pollen grains	Fagaceae?	unknown	2
small triporate pollen grains	Juglandaceae?	unknown	1
SUM			1000

Taxonomic assignments and botanical affinities follow^42–45). The following palaeofloristical elements have been distinguished: palaeotropical (P), including tropical (P1) and subtropical (P2), and arctotertiary (A), including warm-temperate (A1) and temperate (A2), and cosmopolitan (P/A).

The spore-pollen spectrum is strongly dominated by bisaccate pollen grains derived from Pinaceae, chiefly Pinus (Fig. 4K, N), which accounts for 89.1% of the assemblage. Cathaya (1.3%) and Picea occur only in minor amounts. Non-bisaccate conifer pollen grains are the second most numerous, among which Sciadopitys (Fig. 4I, J, L) accounts for 6.5%. In addition, few pollen grains from Cupressaceae (Fig. 4H) and Tsuga (Fig. 4M) were found. Similar to those mentioned above, angiosperm pollen grains occur in single specimens and represent the families Hamamelidaceae (0.5% of the total; Fig. 4O, P), Ericaceae, Fabaceae (Fig. 4Q), Betulaceae, and probably also Fagaceae, Juglandaceae, and palms. Fern spores are rare, with Neogenisporis which accounts for 0.5% (Fig. 4C–E) is being the most common. There are also a few specimens from the fossil-genera Leiotriletes (Fig. 4F), Triplanosporites and Verrucatosporites (Fig. 4G). Only single spore of Sphagnum and a lycopod (Fig. 4A, B) were observed. No fungal spores or other recognizable fungal remains were encountered.

Facies description

The studied deposits consist of dark-gray, sandy, glauconite-bearing, slightly calcareous bioclastic mudstone forming the fossiliferous matrix of the Popiele Beds (e.g.^13,19). This matrix contains numerous well-preserved fossil remains and encloses variable amounts of clasts (intraclasts, extraclasts), ranging from small fragments to blocks several metres in size^13,30,31. Subordinate intercalations of clayey bioclastic sandstone were also observed. Microscopic observations reveal abundant bioclasts, plant-derived organic particles, and inorganic grains dispersed within a silty matrix (Fig. 5A-C). In addition to calcareous skeletal debris, the matrix itself contains an admixture of carbonates, as indicated by a moderate reaction to HCl (Fig. 5E).

Fig. 5.

Photomicrographs of the matrix of the Popiele Beds. (A) Sandy mudstone containing abundant angular to subangular quartz grains and glauconite (arrowed); sample Ko2a; (B-C) Sandy mudstone with bioclasts of thin-shelled bivalves, foraminifera, and plant detritus; sample Ko1b, c; (D) Bioclastic sandy mudstone containing echinoids fragments and bivalves; sample Ko3b; (E) Sandy mudstone with a slighty calcareous micritic matrix; an isolated sandstone clast is visible in the centre of the photomicrograph; sample Ko3c; (F) Bioclastic sandy mudstone with echinoids fragments, other bioclasts, and numerous sandstone aggregates; sample Ko2 b; red arrows – bioclasts; green arrows – glauconite; blue arrows – aggregate grains; yellow arrows – plant detritus.

The bioclastic fraction is dominated by thin- and thick-shelled bivalves, calcareous benthic foraminifera, gastropods, bryozoans, and echinoids, and numerous unidentified skeletal fragments (Fig. 5B, D, F). Elongated dark-brown to black organic particles, reaching up to 3 mm in length, occur commonly and are chaotically distributed throughout the sediment (Fig. 5B, C). The siliciclastic component includes abundant angular, less commonly subangular quartz grains up to 1 mm in diameter, although most are smaller than 0.3 mm. Rare grains attain 1.5 mm (Fig. 5A). Subangular glauconite grains measuring 0.3–0.4 mm are also common (Fig. 5A, E). In addition, irregular aggregate grains (lumps) composed of several densely packed angular quartz grains and silt were observed (Fig. 5F). Within the mud-supported matrix, distinctly larger ellipsoidal grains reaching approximately 0.6 cm in thin section were identified (Fig. 5E). These aggregates consist of quartz sandstone composed of tightly packed fine angular quartz grains.

Interpretation

Age

The co-occurrence of Rhombodinium perforatum (Fig. 3F), Areosphaeridium diktyoplokum (Fig. 3A), and A. michoudii (Fig. 3D) indicates an early Priabonian age of the dinoflagellate cyst assemblage recovered from the studied sample^46–48.

This age assignment is broadly consistent with previous palaeontological studies of the Popiele Beds, although precise correlation remains difficult because the unit comprises several lithologies, and not all author clearly refer to this aspect. However, most studies based on fossils derived from the fossiliferous dark-coloured mudstone matrix have likewise indicated a predominantly Priabonian age^19,34,37,40. Comparable ages were also suggested for greenish shales interpreted as clasts by¹⁷ (uppermost Priabonian–lower Rupelian) and³⁹ (Priabonian). The youngest age estimate was proposed by³⁶, based on early Oligocene calcareous nannofossils, although no information was provided regarding the lithology of the source material. Moreover, the spore-pollen assemblage from Koniusza is generally similar to the upper Eocene (Priabonian) palynoflora from Łukowa⁴⁹, in which the most common taxa were Pinus, Sciadopitys, Cathaya, Taxodium/Glyptostrobus, and Hamamelidaceae.

Sedimentary setting reconstruction

The studied interval of the Popiele Beds is represented by a sandy, glauconite-bearing, bioclastic-rich mixed siliciclastic-carbonate mudstone facies. This facies closely resembles other early Paleogene siliciclastic and mixed siliciclastic–carbonate mudrock of the Carpathian Tethys realm (e.g.^{14,19,21,30,50} and references therein, cf.⁵¹). The commonly well-preserved fauna forms an assemblage typical of inner- to mid-shelf environments. Sediments comprised of mixed siliciclastic and carbonate material are most commonly interpreted as deposits of near-coastal inner-shelf settings (e.g.⁵²). In the study area³⁴, see also³⁷ interpreted this faunal assemblage as neritic, corresponding to water depths of several tens of metres.

The abundance of angular terrestrially derived quartz grains, extrabasinal glauconite grains, and plant-derived organic particles indicates deposition in a proximal nearshore marine setting. At the same time, microscopic observations reveal chaotic orientations of shells and elongate plant debris, whereas the fauna remains generally well-preserved, suggesting only limited sediment transport. Evidence for limited redeposition is also provided by the observed ellipsoidal aggregate sandstone clasts. These forms resemble similar structures described from the Eocene silts of the Carpathian Flysch within the overlying so-called Menilite Beds (e.g.⁵³), which were classified as sandstone whirl-balls. According to⁵³, these structures represent isolated sandstone balls produced by vortices developed in dense turbidity currents or mud flows (for details see⁵³).

Furthermore, the occurrence of variable amounts of larger clasts and blocks within the silty mudstone matrix in the Koniusza area also indicates sediment transport^13,19 however rather limited in comparison to others depositional setting of the Popiele Beds (e.g.²¹). In summary, the above-mentioned sediment features indicate a near-coastal shelf/upper slope setting for the Popiele Beds in the Koniusza area.

Climatic reconstruction

Because the spore-pollen assemblage is strongly dominated by bisaccate pollen grains, a feature typical of marine sediments, conclusions regarding palaeovegetation and palaeoclimate remain necessarily limited. Pine pollen is produced in large quantities and may have been transported over considerable distances from habitats differing in humidity and trophic conditions, thereby reducing its local environmental significance.

In contrast, Sciadopitys pollen grains are much less prone to long-distance transport, so their presence and relatively good preservation and diversity indicate the significant role of these trees in the coastal vegetation of that time. Cupressaceae pollen (Taxodium and/or Glyptostrobus) may originate from swamp forests growing in permanently flooded areas. More mesophilous vegetation included representatives of the Hamamelidaceae, Betulaceae, Ericaceae, Fabaceae, Fagaceae and Juglandaceae families, as well as some conifers. These plant communities also included some thermophilic taxa, such as ferns from the Lygodiaceae, Gleicheniaceae and Cyatheaceae families, and possibly also palms. The occurrence of such “palaeotropical” elements as Leiotriletes sp., Neogenisporis spp., Triplanosporites sp. and Tricolporopollenites cf. staresedloensis (Table 1) indicates that the climate was warm.

Discussion

Reconstruction of the sedimentary setting

The studied sample of the Popiele Beds matrix yielded a palynofacies composition indicating deposition in a marine setting strongly influenced by terrestrial input. The relatively diverse dinoflagellate cyst assemblage, together with abundant organic linings of foraminifera, indicates normal-marine salinity conditions. There is no clear evidence for salinity reduction depsite the strong continental influx, which was most likely caused by river activities; the latter, however, may result in slightly increased nutrient availability in surface waters benefited by peridinioid Deflandrea (e.g.^54,55) frequently occurring in material studied. Palynological data presented give no direct evidences related to basin depth and the distance from the shore. The dinoflagellate cyst assemblage is typical of shelf setting, whereas the frequent occurrence of Homotryblium may indicate inner-shelf to marginal-marine environments, as this genus is commonly associated with restricted lagoonal settings (see⁵⁵). The near absence of offshore oligotrophic indicators (only a single specimen of Impagidinium was found) supports this interpretation. Generally, composition of aquatic palynomorphs in the sample studied indicates shelf eutrophic waters as the environment of its origin. It cannot be, however, excluded that the whole rock body that originally accumulated in shelf area was subsequently resedimented en masse into deeper parts of the flysch basin.

The overwhelming dominance of terrestrial plant remains (cuticles, cortex fragments, phytoclasts) indicates intense continental influx, likely related to to proximity to river mouths. Such settings are commonly characterized by increased nutrient availability and elevated turbidity, the latter reducing light access but simultaneously enhancing food supply for suspension feeders.

Comparable environmental interpretations were proposed by previous authors studying the dark mudstone matrix of the Popiele Beds. Krach³⁴ suggested a neritic marine setting with water depths not exceeding 100 m. Liszka and Geroch³⁷ regarded the foraminiferal assemblages as indicative for a neritic conditions. Gedl¹⁹ interpreted the palynological record as reflecting intense terrestrial influx and slightly eutrophic waters, manifested by increased frequencies of Congruentidioideae, a group considered to favour nutrient-enriched environments (e.g.^56,57).

In summary, the fossiliferous matrix of the Popiele Beds accumulated in a neritic shelf environment characterised by normal salinity and utrient-rich waters. Despite high organic-matter input bottom-water oxygenation remained sufficient to sustain a diverse benthic fauna.

Palaeoecological and taphonomic context of stalked crinoids

Stalked crinoids constitute one of the rarest components of Cenozoic marine faunas, and their Paleogene fossil record is notably fragmentary (e.g.^8,12,58–66). This scarcity reflects the ecological legacy of the Mesozoic Marine Revolution (MMR) but can be also related to taphonomic bias. Increased predation pressure from durophagous predators led to the progressive retreat of sessile suspension feeders, including stalked crinoids, from shallow shelves into deeper and more stable refugia, while their delicate skeletons were highly susceptible to post-mortem disarticulation and destruction^1–4,8,67. Although such taphonomic biases are inherent to the fossil record, their relative impact likely increases in the Paleogene, as the decline in abundance and the shift of stalked crinoids into deeper, less favorable preservational settings significantly reduce their likelihood of fossilization.

By the Eocene, this bathymetric trend was already largely established. Roux and Plaziat⁸, analysing crinoid assemblages from the Pyrenees, the Basque Country, and southern France, demonstrated that Conocrinus, Bourgueticrinus, and Isocrinus were mainly associated with epibathyal and bathyal facies, usually below approximately 150–200 m water depth, along basin slopes and carbonate ramps. These assemblages were commonly parautochthonous or allochthonous and dominated by isolated ossicles and short stem fragments, indicating rapid post-mortem disarticulation and a short interval between death and burial.

A similar palaeoecological and taphonomic signal is observed in well-documented isocrinid assemblages from the Lutetian (middle Eocene) of northern Italy. In the Cambrigar section (Monte Baldo), remains of Isselicrinus diaboli and I. baldoensis occur in slope deposits containing redeposited shallow-water fauna within transitional facies between basin and platform (glauconitic calcarenites and calcirudites), indicating an upper bathyal to epibathyal setting¹⁰. Some stems are preserved in near-life position, with fragments oriented perpendicular to bedding and anchored within soft slope sediments. However, most of the material was rapidly transported and buried by gravity flows. The dominance of long but broken pluricolumnals displaying irregular oblique fractures unrelated to articulation surfaces indicates mechanical destruction during transport and early diagenesis rather than calm in situ disarticulation.

At the same time, data from the Transylvanian Basin demonstrate that this deep-water restriction of stalked crinoids during the Paleogene was not complete¹². documented well-preserved pluricolumnals of Isselicrinus from shallow-water Eocene carbonate ramp deposits at Turnu Roșu, deposited in warm open-marine inner- to middle-ramp environments dominated by coralline red algae, benthic foraminifera, echinoids and bryozoans, with associated nearshore ichthyofauna indicating water depths of several tens of metres. These grain-supported calcarenites contain long, only weakly abraded stem fragments, suggesting short transport and rapid burial under relatively high-energy shoal conditions. This constitutes the first unequivocal record of shallow-water Eocene isocrinids in the Northern Hemisphere. However, these occurrences do not contradict the general post-MMR pattern, rather, they indicates that small, geographically restricted relict populations could locally persist in shallow-water environments long after the principal shift of stalked crinoids into deep-water refugia (e.g.^12,66,68).

Such shallow-water refugia are particularly well documented in the Gondwanan record. In the Leticia Formation of Tierra del Fuego, Malumián and Olivero¹¹ described nearly autochthonous aggregations of Isselicrinus in which stems are locally preserved close to life orientation. This preservation style indicates very rapid burial in a calm yet productive shallow-marine environment, permitting dense suspension feeder populations. Comparable assemblages are also known from Antarctica, where metacrinids and isocrinids persisted in shallow waters until the late Eocene⁶⁹ and (literature cited therein).

In the Northern Hemisphere, however, the pattern appears more restrictive, as particularly illustrated by data from northern Italy⁵⁸, in a revision of the Michelotti collection, showed that the Eocene and Miocene crinoid material from Veneto and Piedmont is dominated by cups of non-isocrinid bourgueticrinids and holopodids (Conocrinus, Cyathidium, and Holopus), which are characteristic of deep-water settings, whereas columnals of both isocrinids and bourgueticrinids are rare and usually isolated. This discrepancy likely reflects differences in taphonomic resistance between massive cups and much more delicate columnal elements rather than the true structure of living communities.

Important insight into this environmental mosaic comes from studies of the Lessini Mountains carbonate ramp. In the ruditic tuffites of the Calcari Nummulitici Formation at Cava Boschetto, columnals of isocrinids and rhizocrinids co-occur with cups of cyrtocrinids, conocrinids and rhizocrinids, together with rare holopodids associated with hard substrates, indicating the presence of diverse microhabitats across the ramp⁶⁰. Later studies documented additional holopodid and cyathidiid taxa, confirming the high diversity of this fauna and suggesting biogeographic links with other western European carbonate ramps^10,59.

Morphological analysis of bourgueticrinid cups from Chiampo (Chiampocrinus, Conocrinus, Fabianicrinus, Lessinicrinus, Pseudoconocrinus) indicates compact and robust skeletons characterised by high basals and weakly expressed sutures⁵⁹, corresponding to taphonomic type III of⁷⁰. Such skeletons could survive gravitational redeposition and downslope transport as relatively complete units, whereas delicate isocrinids tended to disintegrate almost entirely into smaller ossicles. Consequently, assemblages from Chiampo appear dominated by non-isocrinids, although isocrinids may originally have formed a more significant component of the living community but remain underrepresented in the fossil record.

In this context, the Koniusza locality provides an important additional example. The assemblage contains only a few small columnals and short pluricolumnals of isocrinids, together with isolated bourgueticrinid columnals. These ossicles show only minor abrasion or evidence of prolonged transport, and articulated stem fragments indicate relatively rapid burial. Their small size is therefore unlikely to reflect simple hydrodynamic sorting; instead, it more plausibly records genuinely small-bodied relict populations inhabiting environments already strongly modified by post-MMR ecological restructuring.

Taken together, the Koniusza assemblage supports an interpretation in which the Paleogene history of stalked crinoids was not a uniform continuation of Mesozoic ecological patterns. Rather, it involved a mosaic of geographically restricted refugial populations controlled by predation pressure, environmental heterogeneity, and taphonomic filtering of the fossil record.

Conclusions

A rare occurrence of stalked crinoids in a nearshore Paleogene setting is documented, indicating that shallow-water refugia for stalked crinoids persisted locally after the ecological restructuring associated with the Mesozoic Marine Revolution.

The good preservation and minimal abrasion of skeletal elements indicate rapid burial and limited transport, supporting a largely parautochthonous to near-autochthonous origin of the assemblage.

The Popiele Beds record a transient depositional window in which proximal shelf-derived ecosystems were transferred into a deep-marine setting, preserving palaeoecological signals that are commonly lost from the fossil record.

These findings challenge the concept of a uniform post-MMR deep-water distribution of stalked crinoids and instead suggest survival in spatially restricted, environmentally controlled refugia.

Author contributions

MAS, PG designed the research and were responsible for the overall writing of the manuscript. MB conducted field works and acquired fossil specimens for research. MAS, PG, MK, EW, BJP contributed to the execution and interpretation of analytical data. MAS supervised the project and contributed to the conceptual development. All authors participated in the research and contributed to the writing of the paper.

Funding

The spore-pollen research (EW) was supported by statutory funding from the W. Szafer Institute of Botany PAS. MK was supported by a subsidy granted to AGH University of Krakow by the Ministry of Science and Higher Education. The research has been partially supported by the Ministry of Science and Higher Education of Poland, within the statutory activities of Institute of Botany, Faculty of Biology, Jagiellonian University (N18/DBS/000002) to BJP.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.