4.a. Beds, pods and clumps

For a genetic interpretation of the skeletal concentrations, it is important to distinguish between layered beds with physically reworked material (e.g. due to a storm event) and other concentrations due to biological activity (e.g. burrowing and bioturbation). Yet another category consists of clustered biogenic material introduced to the site of deposition from different intraformational sources. These three categories can be illustrated with examples of shell accumulations within the Sognefjord Formation (Early to Middle Oxfordian) of well 32/4-4, described as (1) beds, (2) pods and (3) clumps (Figure 2).

1. (1) Physically reworked, disarticulated but generally complete bivalve shells were accumulated mostly convex-up together with other minor components in a fining-upward bed (Figure 2a). They are part of a ca. 70 cm thick, coarse-grained, sandy unit deposited above an erosive surface (not illustrated), probably part of a minor channel on the upper shoreface. The sandstone matrix is affected by patchy calcite cementation due to partly dissolved bioclasts, although a close-up view reveals almost intact, only slightly abraded shell surfaces (Figure 2d).

2. (2) Another common feature is pods of skeletal concentrations (Figure 2b). The illustrated tubular structure is a few centimetres in diameter and occurs in firm substrate originating from an erosive surface above (not shown). It is tightly packed with shell and plant fragments and can readably be identified as a large burrow (e.g. shafts and tunnels) in a lower shoreface environment. In addition, undiagnostic mottling can lead to skeletal concentrations with a diffuse outline. Subsequent calcite cementation enhances the visibility of these actively or passively filled burrows.

3. (3) A rounded clump of diverse biogenic material, ca. 10 cm wide and >15 cm long contains bivalve shells, serpulid shells, plant fragments and a belemnite (Figure 2c). Superficially, this type of skeletal concentration resembles category 1 but differs from it by its clumped instead of layered fabric. In addition, the shelly fauna is diverse, including large, small, thick and thin disarticulated and broken articulated shells showing different degrees of alteration. Their surface appears to be irregular, abraded and corroded (i.e. etched, Figure 2e). The fabric within the clumps is tight with shell valves partly stacked into each other. A concentric zonation gives rise to two parts of the clump. The zonation and margin of the clump are defined by tightly packed shells or smearing of smaller bioclasts. Calcite cementation has affected the matrix of the clump. It is assumed that the skeletal material was introduced to the site of deposition (i.e. lower delta front) from different intraformational sources by a biological activity, which will be the objective for the following detailed investigation.

Figure 2.

Examples of a shell layer (physical deposition), burrow (bioturbation) and regurgitalite (biologically introduced), partly calcite-cemented, from the Sognefjord Formation in well 32/4-4 (A-cut). See text for details. Scale bar = 1 cm. (a) Lower part of a shell bed mainly containing disarticulated bivalve shells. Middle Oxfordian, upper shoreface (1374.2 m). (b) Large burrow shaft, passively filled with bioclasts and wood fragments. Early Oxfordian, lower shoreface (1407.3 m). (c) Large, bipartite regurgitalite containing bivalve and serpulid shells, plant fragments and a belemnite (arrow). Holotype of Vomitus concha igen. et isp. nov. (PMO 255.609). Early Oxfordian, lower delta front (1425.4 m). (d) Detail of a, slightly abraded shells. (e) Detail of c, corroded and etched shells.

4.b. Clumps of skeletal remains

Biogenic material clustered together in form of centimetre-sized clumps occurs scattered throughout the successions of the Krossfjord, Fensfjord, Heather and Sognefjord formations. The clumps are not limited to a particular environment but occur across the depositional trend from shelf through delta front to terminal distributary channels. According to their dominating biogenic components, three morphotypes of clumps are distinguished: The benthic components (1) bivalve/brachiopod shells (together with terrestrial plant fragments) and (2) serpulid shells, as well as (3) epibenthic components crinoid ossicles (Figure 3). All components are also recognized as in situ layers in growth position and in reworked sediment layers.

Figure 3.

Examples of in situ and reworked occurrences of bivalve and serpulid shells, which are common components of the clumps. Scale bar = 1 cm. (a) Infaunal bivalves in life position, together with their burrows. Well 31/6-5 (1719.0 m, A-cut), Fensfjord Formation (Middle Callovian), upper delta front. (b) Thin shell bed with reworked, mostly disarticulated bivalve shells in the lower part and plant fragments above. Well 31/3-1 (1563.4 m, B-cut), Fensfjord Formation (Middle Callovian), lower delta front. (c) Infaunal serpulids in life position. Well 31/5-4S (1739.6 m, B-cut, inclined well), Sognefjord Formation (Early Oxfordian), upper delta front. (d) Slightly reworked (parautochthonous) serpulid shells. Well 31/6-2 (1479.9 m, A-cut), Sognefjord Formation (Late Oxfordian), interdistributary bay. (e) Sharp-based (erosive) fining-upward bed (interpreted as tempestite) with reworked bioclasts (mainly bivalve shells) and plant fragments, calcite-cemented. Well 32/7-1 (1490.1 m, A-cut), Sognefjord Formation (Late Callovian), upper delta front.

4.b.1. Bivalve/brachiopod shells and plant fragments

The most common morphotype are biogenic concentrations dominated by bivalve shells together with plant fragments, sometimes also including brachiopod shells (Figure 4). The size of these globular rounded to irregular patchy aggregates varies from 1 to 10 cm (e.g. Figure 2c), although some specimens exceed the size of the core width (ca. 10 cm). The clumps are typically affected by early diagenetic calcite cementation, and their size and shape are partly influenced by subsequent modification, such as reworking (see below). The composition varies from specimen to specimen, with plant fragments constituting 10–30%. Plant fragments are commonly downgraded clasts with a round surface; only in rare cases are larger twigs with rounded edges preserved. The remaining fraction of shells is quite diverse in terms of taxa, completeness and alteration. Aside complete and articulated bivalve shells of different size classes, many of the thick to filamentous shells are broken, abraded or stacked into each other. In addition to bivalve shells and plant fragments, a minor component of other bioclasts (mainly brachiopods, serpulids and belemnites) may occur. The matrix is sandy with fine-grained (i.e. silty to muddy) streak-like portions.

Figure 4.

Regurgitalites mainly containing bivalve shells and plant fragments (Vomitus concha igen. et isp. nov., paratypes), partly calcite-cemented. Scale bar = 1 cm. (a) Poorly constrained clump with complete and fragmented valves together with plant debris. Well 32/4-4 (1465.4 m, A-cut, PMO 255.610), Fensfjord Formation (Late Callovian), muddy shelf. (b) Large clump with diffuse boundary, containing shell fragments of bivalves, articulated brachiopod shell and large plant fragments with partly attached brachiopods (centre). Well 32/4-4 (1568.1 m, A-cut, PMO 255.611), Fensfjord Formation (Early Callovian), prodelta. (c) Same as b, showing twigs. Well 32/4-4 (STR 32/4-4, B-cut, 1568.1 m). (d, e) Erosional abraded clump with small, rounded shell and plant fragments. Well 32/7-1 (1662.85 m, A-cut, PMO 255.612/2), Krossfjord Formation (Late Bathonian), distributary channel. Cf. Fig. 8a. (f, g) Counterpart of d with two abraded clumps with worn bioclasts in sandy matrix, together with a belemnite (arrow). Well 32/7-1 (STR 32/7-1, B-cut, 1662.85 m), Krossfjord Formation (Late Bathonian), distributary channel. Cf. Fig. 8a. (h, i) Clump with defined margin and sandy matrix. Well 32/4-4 (STR 32/4-4, B-cut, 1601.7 m), Krossfjord Formation (Late Bathonian), distributary channel.

Figure 4 Long description

Panel A: A photo showing a poorly constrained clump with complete and fragmented bivalve shells together with plant debris. Panel B: A photo displaying a large clump with a diffuse boundary, containing shell fragments of bivalves, an articulated brachiopod shell, and large plant fragments with partly attached brachiopods at the center. Panel C: A photo similar to Panel B, highlighting twigs. Panel D: A photo of an erosional abraded clump with small, rounded shell and plant fragments. Panel E: A close-up photo of the erosional abraded clump from Panel D. Panel F: A photo showing the counterpart of Panel D with two abraded clumps with worn bioclasts in a sandy matrix, together with a belemnite indicated by an arrow. Panel G: A close-up photo of the counterpart from Panel F. Panel H: A photo of a clump with a defined margin and sandy matrix. Panel I: A close-up photo of the clump from Panel H.

4.b.2. Serpulid shells

This morphotype occurs in the Fensfjord, Heather and Sognefjord formations, the latter where it is most representative, in accordance with the occurrences of in situ and reworked serpulid shells (Figure 5). The irregularly rounded aggregates are 3–10 cm in diameter with a weak concentric zonation, partly phosphatic (Figure 5c and d). Depending on the hosting formation and environment, they are encased in weakly bioturbated sandstone (mouth-bar deposit) or, more commonly, in totally bioturbated very fine sandstone (lower delta-front deposit). The slightly bent calcitic shells are 10–15 mm long, if completely preserved, however, in many cases only fractions of the shells occur. Most tubes have a circular cross-section, assignable to Pyrgopolon? nodulosum, while those with a quadrangular cross-section can be referred to Nogrobs sp. Small plant fragments and belemnites can be included (Figure 5b).

Figure 5.

Regurgitalites dominated by shells of the serpulid polychaetes Pyrgopolon? nodulosum and Nogrobs sp. (Vomitus serpula igen. et isp. nov.). Scale bar = 1 cm. (a) Small clump with poorly defined margin. Well 31/2-M-41 (1508.5 m, B-cut), Sognefjord Formation (Middle Oxfordian), mouth bar. (b) Clump with diffuse margin (due to partly incorporation into bioturbated matrix), mainly consisting of serpulids admixed with filamentous bivalve fragments, plant debris and a belemnite. Well 32/7-1 (1571.75 m, A-cut, paratype, PMO 255.614), Fensfjord Formation (Early to Middle Callovian), lower delta front. (c) Part of a carbonate-cemented clump tightly packed with serpulids and a few bivalve fragments and plant debris. Well 32/7-1 (1520.8 m, A-cut, holotype, PMO 255.613), Heather Formation (Late Callovian), lower delta front. (d) Counterpart of c, marginal part (STR 32/7-1, B-cut, 1520.8 m).

4.b.3. Crinoid ossicles

Clumps with crinoid ossicles were found in the Fensfjord Formation (upper delta front) and Sognefjord Formation (interdistributary bay and tidal channel) (Figure 6). They are 2–10 cm in diameter and poorly constrained within completely bioturbated sandstone beds (Figure 6a–c). A few filamentous bivalves (complete and fractured) and plant debris are admixed. Columnals are typically visible in cross-section and belong to Isocrinida indet. Many facets are abraded or corroded. Clumps with crinoid ossicles do not show significant cementation, except those with admixed shell fragments of bivalves.

Figure 6.

Regurgitalites dominated by crinoids (Vomitus echinoderma igen. et isp. nov.). Scale bar = 1 cm. (a) Disintegrated clump. Well 31/3-1 (1401.9 m, B-cut), Sognefjord Formation (Late Oxfordian), tidal channel. (b, c) Etched crinoids together with bivalve fragments. Well 32/7-1 (STR 32/7-1, B-cut, 1551.1 m, paratype), Fensfjord Formation (Middle Callovian), upper delta front. (d) Constrained, only partly disintegrated, calcite-cemented clump with etched crinoids, bivalve and plant fragments. Well 31/6-6 (1596.3 m, A-cut, holotype, PMO 255.615), Sognefjord Formation (Late Oxfordian), interdistributary bay.

4.c. Mixed biogenic concentrations

After their allochthonous deposition, the three morphotypes of clumps underwent various taphonomic pathways (Figure 7), which are related to three main processes: (1) bioturbation, (2) cementation and (3) reworking.

Figure 7.

Disintegrated, cemented and reworked regurgitalites Vomitus concha igen. et isp. nov. Scale bar = 1 cm. (a) Disintegrated clump (right) incorporated into the bioturbated sandy matrix. Well 32/7-1 (1567.6 m, A-cut), Fensfjord Formation (Early to Middle Callovian), mouth bar. (b) Various shell fragments, probably originated from a regurgitalite, became integrated together with plant fragments into a large burrow. Well 31/6-3 (1643.2 m, B-cut), Sognefjord Formation (Early Oxfordian), upper delta front. (c) Irregular clump, partly intact and partly incorporated into the background sediment by bioturbation. Well 31/6-2 (1593.6 m, A-cut), Sognefjord Formation (Early Oxfordian), lower delta front. (d) Clump remains as the nucleus of a tight calcite concretion. Well 32/7-1 (1556.5 m, A-cut), Fensfjord Formation (Middle Callovian), mouth bar. (e) Reworked and abraded clump, affected by differential calcite cementation. Well 31/6-3 (1672.7 m, A-cut), Fensfjord Formation (Late Callovian), lower shoreface. (f) Reworked and strongly abraded clumps. Well 31/6-5 (1691.2 m, A-cut), Fensfjord Formation (Late Callovian), lower delta front.

6.a. Nature and origin

The described clumps with skeletal concentrations are challenging to interpret with respect to their origin. In the herein adapted morphological classification of fossil concentrations proposed by Kidwell et al. (Reference Kidwell, Fürsich and Aigner1986), ‘… a clump denotes a concentration with poorly defined margins (e.g. clusters of shells in life position, or small concentrations that have been disturbed by sediment bioturbation)’. An accumulation in life position, including spatfall on a soft-bodied organism (e.g. sponge, see Surlyk, Reference Surlyk2025), can be excluded by the variable preservation and fragmentation of sessile and vagile benthic organisms within the clumps. Although reworked clumps occur as mixed biogenic concentrations (see below), the three morphotypes with their specific composition, fabric and size require an interpretation different from a physical or biological in situ reworking.

The clumps are common in the studied formations but are absent in time-equivalent deposits of different environments, such as delta plain (e.g. Bajocian-Bathonian Tarbert Formation) or barrier-island systems and tidally influenced estuaries (e.g. Bathonian-Callovian Hugin Formation). The composition with stenohaline components such as crinoids and serpulids in a brackish-water influenced delta-front environment makes an allochthonous introduction of the fossiliferous clumps to the site likely. This is confirmed by the co-occurrence of benthic (e.g. bivalves, serpulids, crinoids), nektic (e.g. belemnites) and terrestrial components (e.g. plant fragments) within these clumps.

In two-dimensional sections, the clumps mimic very large burrows (e.g. those described by Wanless and Tedesco, Reference Wanless and Tedesco1993), but their three-dimensional appearance confirms round clusters instead of tubular fills. Although most parts of the succession are intensively bioturbated, large tubular burrows are an exception and no shell beds associated with the clumps could explain their exotic fill. Instead, high density and tightly packed shells, the admixture of complete and broken specimens with partly corroded margins, indicate pre-processed biogenic material before its final deposition.

These characteristics of the clumps are best explained as the material sourced from the digestive system of an organism (i.e. bromalites, digestichnia), which would provide three options: intestinal content (i.e. cololite), ejected faecal material (i.e. coprolite) or regurgitated material (i.e. regurgitalite). Given the large amount of undigested food remains in the investigated specimens, their diffuse and irregular shape, as well as their calcitic instead of phosphatic mineralogy, cololites and coprolites can be ruled out in favour of regurgitalites (Hunt and Lucas Reference Hunt and Lucas2025 a, b).

6.b. Comparison

Many examples of regurgitalites (i.e. vomit remains, or the older German terms ‘Speiballen’ and ‘Gewölle’, mainly applied for bird vomit) were studied over the last century and their common features defined, which allows for comparison and delineation (e.g. Bishop, Reference Bishop and Frey1975; Hunt, Reference Hunt1992; Myhrvold, Reference Myhrvold2012; Vallon, Reference Vallon, Hunt, Milàn, Lucas and Spielmann2012; Thies and Hauff, Reference Thies and Hauff2013; Klug and Vallon, Reference Klug and Vallon2018; Gordon et al. Reference Gordon, Roach, Parker and Briggs2020; Hoffmann et al. Reference Hoffmann, Stevens, Keupp, Simonsen and Schweigert2020; Hunt and Lucas, Reference Hunt and Lucas2025 a, b). Several biogenic accumulations of similar size, morphology and composition, mostly from shallow-marine Mesozoic strata, were interpreted as regurgitalites and serve for comparison.

Most relevant in this context are decimetre-size, cemented clumps with densely packed fossils from Middle Jurassic (late Bajocian) claystone of northern Germany, which were interpreted as gastric pellets of marine reptiles (Wetzel, Reference Wetzel1952; Horstmann and Maier, Reference Horstmann and Maier1957). Their content is dominated by a diverse mollusc fauna of varying preservation. A quantitative analysis of their composition shows ca. half part nektic organisms (e.g. ammonites, bivalves, crustaceans and reptile teeth), whereas the remaining part mainly consists of benthic molluscs (Wetzel, Reference Wetzel1952). Completely preserved gastropods and bivalves are interpreted as scavenger that died in the clumps. Admixed wood fragments represent driftwood that was colonized by pseudoplanktic bivalves. Similar faunal aggregates composed of molluscs, brachiopods and echinoderms occur in Bajocian-Bathonian clay deposits in Poland (Zatoń et al. Reference Zatoń, Villier and Salamon2007). The round to elliptical, densely packed fossil aggregates are 6–20 cm in diameter and contain a diverse fauna. They were potentially produced by sharks.

The Upper Jurassic (Kimmeridgian-Tithonian) lithographic limestone of Germany contains patchy accumulations of fossils, such as ammonite lamellaptychi (ca. 7 cm in diameter; Barthel and Janicke, Reference Barthel and Janicke1970) and plant material (6–7 cm in diameter; Schweigert, Reference Schweigert2025). Terrestrial vertebrate forager and predatory fish are the assumed producers.

Fossil accumulations from the Middle Triassic (Anisian) of Poland are elongate, elliptical or subcircular in outline, with dimensions varying from 1 to 10 cm (Salamon et al. Reference Salamon, Niedźwiedzki, Gorzelak, Lach and Surmik2012; Niedźwiedzki et al. Reference Niedźwiedzki, Surmik, Chećko and Salamon2021). Their content is dominated by fragmented bivalve shells occurring together with crinoids. Durophagous sharks and other fish, as well as reptiles and nautiloids, are considered as potential producers.

Götzinger and Becker (Reference Götzinger and Becker1932) report centimetre-sized, elongate, rounded aggregates full of small Inoceramus fragments from the Upper Cretaceous of Austria and discuss their origin either as coprolite or regurgitalite consolidated by mucus, the latter view favoured by Häntzschel et al. (Reference Häntzschel, El-Baz and Amstutz1968). ‘A large number of ossicles of ten species of asteroids, forming, almost certainly, a pellet of indigestible parts ejected by a starfish-eating fish or other animal’, is common in Campanian chalk of England (Wright and Wright Reference Wright and Wright1940, p. 233). Speden (Reference Speden1971) described siltstone with patches of Inoceramus fragments, generally sub-circular to sub-elliptical in shape and about 5–30 cm in maximum length from the Albian-Cenomanian of New Zealand. Another group contains densely packed disaggregated prisms from the outer prismatic shell layer of Inoceramus and reaches a maximum diameter of 2–10 cm. Fish and marine reptiles are the most likely predators (Speden, Reference Speden1971).

Palaeozoic examples were given by Zangerl and Richardson (Reference Zangerl and Richardson1963, p. 135, pl. 44, fig. c), who describe a ca. 10 × 8 cm large ‘… gastric residue pellet containing minced Mytilus (therein Myalina) shells’ from Pennsylvanian black shale of Indiana, USA. Similar pellets also contain palaeoniscoid fish remains. The predator is not recorded, but large sharks are common in the succession and could account for it.

6.c. Potential producers

Assuming a correct interpretation of the clumps as regurgitated remains, the search for potential producers must fulfil some requirements for being qualified. First, the body size of the producer must be large enough for allowing to vomiting clump sizes up to 10 cm or more. Secondly, the regurgitated material indicates durophagous predation. Thirdly, the compacted and consolidated vomit requires some processing and maturation before it became expelled and preserved as clumps. At this stage, and with contrasting content in the clumps, it cannot be excluded that different producers were involved in their origin.

Most invertebrates can be ruled out because of their limited size and different feeding behaviours. Only cephalopods merit consideration due to their abundance and diversity in the Jurassic, size and durophagy. Ammonoids developed into Middle and Upper Jurassic megafauna (Klug et al. Reference Klug, Fuchs, Pohle, Korn, De Baets, Hoffmann, Ware and Ward2025) and could be potential tracemakers of the clumps. Hoffmann et al. (Reference Hoffmann, Stevens, Keupp, Simonsen and Schweigert2020) regarded coleoids as predators of ammonites, which produced aptychi-bearing regurgitalites. Nevertheless, these supposed cephalopod-produced regurgitalites differ in composition and size from the described clumps.

In contrast, a wide range of vertebrates flourished in the Jurassic seas, many of which are known for their durophagous behaviour. Fish can regurgitate, but the size of vomit produced by Osteichthyes (such as Teleostei) would be rather small, whereas Chondrichthyes (such as Neoselachii) potentially could produce larger clumps. Hybodont sharks flourished in shallow-to marginal-marine and freshwater environments during the Middle Jurassic and a great ecological diversity is documented from England (Rees and Underwood, Reference Rees and Underwood2008). Cutting, clutching and piercing are inferred from dental morphologies, with a widespread crushing feeding strategy among numerous taxa. An unnamed Hybodus species was equipped with teeth suitable ‘… for grasping prey such as molluscs attached to the bottom or living within the substrate’ (Rees and Underwood, Reference Rees and Underwood2008, p. 144). Extreme crushing dentition also occurs in Asteracanthus magnus and A. ornatissimus, for which a similar feeding strategy is assumed.

A diverse marine reptile fauna is documented from Middle to Upper Jurassic outcrop analogues, foremost the Oxford Clay Formation (Callovian) and the Corallian Group (Oxfordian) in the UK (e.g. Martill and Hudson, Reference Martill and Hudson1991; Benton and Spencer, Reference Benton and Spencer1995). Based on fossil teeth, Foffa et al. (Reference Foffa, Young, Stubbs, Dexter and Brusatte2018) analysed the dietary guilds of marine reptiles in the Jurassic Sub-Boreal Seaway, to which the herein studied regurgitalites belong to. Out of their five categories, the crunch guild with strongly ornamented teeth is inferred as hard-object feeders (durophages). This applies to the croco-dylomorphs Geosaurinae and Teleosauridae but would also partly include the generalist guild of Pliosauridae and Plesiosauroidea. An ichthyosaur vertebra was found together with clumps containing bivalve shells and plant fragments (Figure 8), although no genetic relationship is inferred because ichthyosaur primarily consumed nektic soft-bodied marine organisms (such as coleoid cephalopods) and fish (e.g. Pollard, Reference Pollard1968; Fischer et al. Reference Fischer, Weis, Delsate, Della Giustina, Wintgens, Fuchs and Thuy2025).

Modern carnivore crocodiles dispose indigestible material by oral ejection of solid gastric pellets (Andrews et al. Reference Andrews, Axelsson, Franklin and Holmgren2000; Myhrvold, Reference Myhrvold2012), which could also be the case for Mesozoic marine croco-dyliforms (Thalattosuchia) with an omnivore and herbivore diet (Melstrom and Irmis, Reference Melstrom and Irmis2019). Crocodylomorpha had a high taxonomic and morphological diversity in the Mesozoic and were adapted to catching fast-moving prey such as fish and cephalopod (Young et al. Reference Young, Brusatte, de Andrade, Desojo, Beatty, Steel, Fernández, Sakamoto, Ruiz-Omeñaca and Schoch2012; Ballell et al. Reference Ballell, Moon, Porro, Benton and Rayfield2019). This feeding strategy is quite different from bottom feeding as inferred by the described regurgitalites and makes Crocodylomorpha less likely their producers. In turn, a Teleosauroidea (Thalattosuchia) is recorded from an Upper Jurassic regurgitalite of Italy as the product of a pelagic predator or scavenger (Serafini et al. Reference Serafini, Gordon, Foffa, Cobianchi and Giusberti2022). Several large crocodilian coprolites occur together with the regurgitalites, which indicate a vertebrate diet rather than invertebrate prey.

The last group of marine reptiles to consider are Plesiosauria, including the ‘plesiosauromorphs’ with long necks and small heads, and the ‘pliosauromorph’ with a short neck and a large head. A change of composition of marine reptile communities happened across the Early-Middle Jurassic transition, where plesiosauroids diminished and were replaced by pliosaurids as apex predators from the Middle Jurassic to the Late Cretaceous (Fischer et al. Reference Fischer, Weis and Thuy2021). Plesiosauria occupy the ecological guilds pierce, cut and general, with a wide range of feeding behaviour (Massare, Reference Massare1987). Stomach content shows that plesiosaurs ate coleoids (Sato and Tanabe, Reference Sato and Tanabe1998; see Hoffmann et al. Reference Hoffmann, Stevens, Keupp, Simonsen and Schweigert2020), but wood fragments are also reported (Janensch, Reference Janensch1929). A pliosaurid-attacked nautiloid from the Middle Jurassic of Poland also indicates cephalopods as preferred prey (Jain et al. Reference Jain, Salamon, Klug, Lomax, Płachno and Duda2025).

Cryptoclididae is a family of long-necked plesiosaurs that existed from the Middle Jurassic to the Early Cretaceous. They fed on small soft-bodied prey such as small fish and crustaceans (Zammit et al. Reference Zammit, Daniels and Kear2008; Fischer et al. Reference Fischer, Weis and Thuy2021). One member of this family, an elasmosaurid plesiosaur from the Early Cretaceous of Australia, is preserved with its stomach content, which contains ‘… the remains of many benthic invertebrates, predominantly bivalve shell fragments but also the remains of gastropods and pieces of crinoid’, implying a bottom-feeding behaviour (McHenry et al. Reference McHenry, Cook and Wroe2005). From the ichnological record, gutter-like furrows, up to 60 cm wide and 9 m long, are recorded from a Lower Callovian shallow-marine bedding plane and attributed to the bottom-feeding activity of pliosaurs and ichthyosaurs (Geister, Reference Geister1998). Such trace fossils are assigned to the ichnogenus Megaplanolites, originally described from the Upper Oxfordian of Spain (Calvo et al. Reference Calvo, Gil and Meléndez1987).

In conclusion, two groups of potential tracemakers of the regurgitalites come into closer consideration, hybodont sharks and plesiosaurs. As a rule in ichnology, it is possible that the same type of regurgitalite was produced by different taxa, which would explain the wide stratigraphic occurrence of similar regurgitalites from the Palaeozoic to the Cretaceous. However, the limited stratigraphical and environmental distribution of the described specific regurgitalites in the study area coincides with the dominance of cryptoclidid plesiosaurs (known from the Bajocian onward; Fischer et al. Reference Fischer, Weis and Thuy2021; Otero et al. Reference Otero, Acuña, Vargas, Rojas, Ortiz and Aguirrezabala2026), which are assumed as the likely producers. This is consistent with the composition of stomach content of a specimen of the family Cryptoclididae (McHenry et al. Reference McHenry, Cook and Wroe2005). For example, the genus Cryptoclidus is a moderately sized long-necked plesiosaur with adults about four to eight metres long (Brown, Reference Brown1981; Martill et al. Reference Martill, Taylor, Duff, Riding and Bown1994). It is known from many individual specimens from the Oxford Clay Formation (Callovian). Cryptoclidus probably used its numerous sharp, thin and evenly spaced teeth to sift bottom sediments for benthic animals such as crustaceans, molluscs, annelids, brachiopods and echinoderms, in addition to catch squid and fish (Martill et al. Reference Martill, Taylor, Duff, Riding and Bown1994).

6.d. Palaeobiology

Given that an interpretation as regurgitalites of a long-necked plesiosaur (e.g. Cryptoclidus) is correct, the described clumps provide important insights into trophic interactions and food chains of these marine reptiles during the Middle and Upper Jurassic. This data complements the rare findings of stomach contents, which provide direct evidence for diet (Pollard, Reference Pollard, Briggs and Crowther1990). In contrast, regurgitalites contain undigested food particles that verify the diet of the producer, thus offering information about its behaviour.

Skeletons and bones of the snake-like plesiosaurid Cryptoclididae (e.g. Cryptoclidus) are relatively common in the Oxford Clay Formation (Callovian) of England. Cryptoclidus had a long neck and small head, with adults reaching a length of ca. 4 m. Cryptoclidus had numerous, tightly packed, small, sharp teeth with reduced ornamentation (Brown, Reference Brown1981), which allowed it to capture squid and fish (‘trap guild’, Chatterjee and Small, Reference Chatterjee, Small and Crame1989). Another peculiarity is the limited mobility of the Cryptoclidus neck, especially sideways, as inferred by qualitative anatomical observations (Wintrich et al. Reference Wintrich, Jonas, Wilke, Schmitz and Sander2019). Instead, a greater mobility of the neck of Cryptoclidus existed in dorsoventral direction compared to the lateral direction. Both features would support bottom-feeding on prey, like the evidence of equivalent stomach content in early Cretaceous Cryptoclididae (McHenry et al. Reference McHenry, Cook and Wroe2005).

The regurgitalites contain a blend of fossil remains out of their original habitat context, which belong to bivalves and polychaetes (both infaunal), stenohaline echinoderms (epifaunal) and plant fragments (introduced from land). Bivalves included different taxa from the shelf to the nearshore (including delta front) but cannot be taxonomically assigned due to their fragmentary preservation. The serpulids Pyrgopolon? nodulosum and Nogrobs sp. are common constituents in normal-marine sediments and seep deposits (Vinn et al. Reference Vinn, Hryniewicz, Little and Nakrem2014). Similar forms were described as Ditrupa nodulosa from Middle Jurassic storm-generated shell beds (offshore shoals) of Greenland (Fürsich and Heinberg, Reference Fürsich and Heinberg1983). The crinoids Isocrinida are stenohaline organisms, which prefer fully marine salinity. In the Middle and Late Jurassic, they occur in shallow-marine environment, mostly on the shelf (Salamon and Feldman-Olszewska, Reference Salamon and Feldman-Olszewska2018). Finally, some regurgitalites (particularly those with bivalve shell fragments) contain a higher amount of land-derived plant fragments that probably were ingested from the sea floor together with benthic food. In addition, driftwood with sessile organisms (such as brachiopods) was also collected by the predator.

This variety of composition provides good evidence that the plesiosaur preferred bottom-feeding in shelf environments but came nearshore into deltaic systems to regurgitate. Modern analogues for such a behaviour are fur seals (e.g. Clarke and Trillmich Reference Clarke and Trillmich1980) and dolphins (e.g. Silva-Jr. et al. Reference Pandolfo and Sazima2004). The term ‘emetolite’ was proposed by Myhrvold (Reference Myhrvold2012) to specify regurgitalites comprising fossilized gastric pellets produced by routine emesis. In contrast to many other sporadically produced regurgitalites with a fluidized origin, the content of these gastric pellets was compacted and probably stiffened by a gastric adhesive agent (e.g. mucus).