Tracemaker of Spirolites radwanskii

Spirolites radwanskii n. igen. n. isp. does not contain any remains of calcareous tubes or shells, which would directly point to the trace maker. Morphology of the borings suggests that it was propagated deep into the rock gradually along a spiral path in the proximal part, increased its size, and steeply directed up in the distal part. In some cases, the distal part of the boring truncates the initial (proximal) part. The shape of the trace fossil resembles spiral tubes of some serpulid polychaetes and shells of some vermetid gastropods, which are considered here as potential trace makers.

The serpulid polychaete hypothesis is cited often because similar spiral trace fossils from the Miocene of Skotniki have been described by Radwański (Reference Radwański1969, pl. 8, figs. 3, 4) as etchings of the serpulid polychaete Pomatoceros sp. However, no direct evidence of that interpretation was provided. In a later paper, Radwański (Reference Radwański1977, pl. 10c–e) illustrated the same trace and referred it to etchings of serpulids, similar to the modern Serpula vermicularis Linnaeus, Reference Linnaeus1767 from the Adriatic Sea coast, but also without any direct evidence. However, this trace fossil is much shallow than Spirolites radwanskii n. igen. n. isp., displays three whorls, and no distinct annulation. Therefore, it represents a different ichnotaxon.

Santos et al. (Reference Santos, Mayoral, Muñiz, Vera-Peláez and Lozano Francisco2003) characterized bioerosive structures produced by Serpulidae (Serpula and Spirorbis) on Pliocene mollusk shells and echinoid tests from southern Spain. They are shallow, sinuous or spiral grooves, bounded a narrow zone, and show perpendicular ribbing. Their depth and width are much smaller than in Spirolites n. igen., which does not show any lateral zone. Because they are not photographically documented, further comparisons are difficult. Taylor and Vinn (Reference Taylor and Vinn2006, fig. 1) illustrated the spiral polychaete tube ‘Spirorbis’ incised in the substrate. Tubes determined as the serpulid Pomatoceros invoked by Radwański (Reference Radwański1969) can be deeply incised in the rock, as exemplified by specimens from the Oligocene of northern Germany illustrated by Polkowsky (Reference Polkowsky2010), if the determination is correct. These papers show that serpulid polychaetes are able to produce attachment scars and bioerode the substrate, but their traces are different than Spirolites n. igen. Some spiral tubes of the polychaete families Spionidae and Sabellidae (e.g., Protolaeospira strophostoma (Boettger, Reference Boettger1907) from the Miocene) that could produce etchings (Radwańska, Reference Radwańska1994) are similar in shape to Spirolites n. igen., but they are a magnitude smaller.

The morphology of Spirolites n. igen. also resembles that of some vermetid gastropods (see Bromley and Heinberg, Reference Bromley and Heinberg2006, fig. 13). Hadfield et al. (Reference Hadfield, Kay, Gillette and Lloyd1972) reported a few species of the recent Dendropoma from the Hawaiian Islands, the spiral shells of which were embedded in the substrate. Bromley (Reference Bromley1978) reported Dendropoma sp. from Bermuda, which produced grooves with a roof. Golding et al. (Reference Golding, Bieler, Rawlings and Collins2014) counted several species of Dendropoma that are coiled and deeply ‘entrenched’ in the substrate, similar to species of the related genera Novastoa, Ceraesignum, and Cupolaconcha. Dendropoma cristatum (Biondi Giunti, Reference Biondi Giunti1859) from the Mediterranean Sea produces shallow attachment scars and deeper traces, and its shell can be easily detached from the substrate (Rosso et al., Reference Rosso, Sanfilippo, Di Geronimo and Bonfiglio2016). Savazzi (1996, Reference Savazzi2001) described deeply endobenthic Dendropoma sp. from the Philippines, including those with spiral shells. The coiling direction of the shell looks to be sinistral (as in actual specimens of Spirolites n. igen.), but taking in account that evolutionally the shell developed down into the substrate, not above the substrate, it must be considered as dextral, as in the majority of vermetids (Savazzi, Reference Savazzi2001) and gastropods in general (Schilthuizen and Davison, Reference Schilthuizen and Davison2005). Some of the aforementioned Dendropoma sp. from the Philippines bore below the level of the substrate. Presumably, they bore chemically by marginal accretion of the shell. During initial development, the boring increases its depth and emerges from the substrate in the final phase. Vermetid gastropods in general bore in order to be better attached to the surface, which is weakened by physical and biological degradation, or the bioerosion is a way to obtain calcium carbonate for building of the shell (Savazzi, Reference Savazzi2001).

The last whorls of extant Dendropoma and related bioeroding species are distinctly smaller (6–28 mm across, see Golding et al. Reference Golding, Bieler, Rawlings and Collins2014, for taxonomy and characteristics of Dendropoma) than in the majority Spirolites n. igen. Nevertheless, vermetid gastropods similar to Dendropoma are more reliable candidates for the trace maker, because: (1) they can bore deeply into the substrate; (2) their shells, as for most of gastropods, show consistently dextral coiling (even if they appear as sinistral because of their development down into the substrate, similarly to Spirolites n. igen.); (3) shells of vermetids show second-order annuli, as in Spirolites n. igen., as opposed to tubes of serpulids that display first-order annulation; and (4) the gradual deepening into the rock, increase in size, and steep emerging between the whorls, with possible erosion of the preceding part of the shell, which is typical of vermetids (see Savazzi, Reference Savazzi2001). All these features can be observed in Spirolites n. igen.

Vermetid gastropods have been noted in Skotniki by Małecki (Reference Małecki1966) and Radwański (Reference Radwański1967, Reference Radwański1969, Reference Radwański1977), but they are small taxa, which either did not leave attachment scars, or, as Vermetus intortus (Lamarck, Reference Lamarck1818), produce the etching trace Renichnus arcuatus Mayoral, Reference Mayoral1987. None of them fits morphologically to Spirolites n. igen.

Living environment

The large boulders contain Spirolites n. igen. and the associated borings mainly on one side, presumably the upper surface. This suggests that they were not rotated. The co-occurrence of degraded and well-preserved Spirolites n. igen. suggests multiphase and long colonization by the vermetid trace makers. This style of colonization at Skotniki was proposed by Radwański (Reference Radwański1969). Smaller clasts, mostly cobbles and pebbles, are bored on all sides. The bored clasts were deposited in a cliff-foot ramp. According to Radwański (Reference Radwański1967, Reference Radwański1969) and Radwański and Górka (Reference Radwański and Górka2008, Reference Radwański and Górka2015), the clasts filling the depression were supplied from a cliff shore of a nearby island, but from its different parts, as evidenced by diverse composition and mechanical abrasion of the clasts. The clasts rolled by waves were bored from all sides, and those that were stabilized on the sea floor were bored only from the exposed sides. Those clasts that were quickly buried were not bored at all. Large numbers of bored clasts suggest good life conditions for the bioeroding organisms. Their borings were readily colonized by encrusters and cavity dwellers sheltering themselves from high wave energy (Radwański, Reference Radwański1977).

The limestones, which laterally replace and cover the clast-bearing facies, contain benthic macrofossils, mainly red algae and pectenids. These fossils and lithofacies features prove a lowering energy in a greater distance from the cliff.

The dominant boring associated with Spirolites radwanskii n. igen. n. isp. is Gastrochaenolites torpedo. This boring was referred to Lithophaga in papers by Radwański (Reference Radwański1969, Reference Radwański1977). The extant Mediterranean L. lithophaga (Linnaeus, Reference Linnaeus1758) is a chemical borer, which lives in very shallow (mostly from 0–6 m deep) and clean waters (Kleemann, 1973, 1974, Reference Kleemann1990; Galinou-Mitsoudi and Sinis, 1995, Reference Galinou-Mitsoudi and Sinis1997), commonly on steep or overhanging surfaces (Bromley and Asgaard, Reference Bromley and Asgaard1993a). This suggests that Spirolites n. igen. also originated in very shallow, clean waters, in accordance with previous interpretations (Radwański, Reference Radwański1969). The boring assemblage (Gastrochaenolites, Caulostrepsis, and Entobia) represents a mature stage of bioerosion that developed over the years and can be referred to the Entobia ichnofacies (Bromley and Asgaard, Reference Bromley and Asgaard1993b), which is typical of Mesozoic–Cenozoic rocky shores (Gibert et al., Reference Gibert, de, Martinell and Domènech1998).

Generally, vermetid gastropods live in very shallow and warm waters. The bioeroding Dendropoma sp. from Philippines lives at a depth of 0–1 m (Savazzi, Reference Savazzi2001), and one from Bermuda does not inhabit waters deeper than 4 m (Bromley, Reference Bromley1978). Most species of this genus and related genera live in intertidal to shallow subtidal waters (Schiaparelli et al., Reference Schiaparelli, Albertelli and Cattaneo-Vietti2006). Nevertheless, the vermetid trace Renichnus arcuatus, at a depth of >60 m, also has been noted in the Azores (Wisshak et al., 2011, Reference Wisshak, Berning, Jakobsen and Freiwald2015). In addition, Dendropoma mejillonensis Pacheco and Laudien, Reference Pacheco and Laudien2008 from Chile, which leaves attachment scars, was reported from a submarine wall at a depth of 17 m (Pacheco and Laudien, Reference Pacheco and Laudien2008). Irrespective of the depth, vermetid gastropods require water turbulence for production of the mucus nets for feeding or filter feeding (Schiaparelli and Cattaneo-Vietti, Reference Schiaparelli and Cattaneo-Vietti1999; Schiaparelli et al., Reference Schiaparelli, Albertelli and Cattaneo-Vietti2006). The data on associated borings and bathymetric range of vermetid gastropods suggest that Spirolites n. igen. was produced in very shallow, turbulent waters.

Recent vermetids live in tropical and temperate marine waters between 44°N and 44°S, where they can form small reefs (Safriel, Reference Safriel1975; Schiaparelli and Cattaneo-Vietti, Reference Schiaparelli and Cattaneo-Vietti1999; Silenzi et al., Reference Silenzi, Antonioli and Chemelloc2004; Schiaparelli et al., Reference Schiaparelli, Albertelli and Cattaneo-Vietti2006). Generally, the period between 17 Ma and 15 Ma (Pińczów Formation, which can be related to the late phase of this because the overlying Miocene series is dated to NN5–NN6; see Radwański and Górka, Reference Radwański and Górka2008) is referred to the Middle Miocene Climatic Optimum, which is characterized by global warmth, high content of CO₂ in the atmosphere, and a significant decrease in the Antarctic icecap (Foster et al., Reference Foster, Lear and Rae2012). As evidenced by data on echinoids, the temperature rose up to tropical levels (sea surface temperatures 16–17°C during winter and up 28°C during summer) from the Karpatian (late Burdigalian) stage to the early Badenian optimum for the Central Paratethys, but the Carpathian Foredeep remained in the temperate climate zone (Kroh, Reference Kroh2007). However, data on facies from the eastern part of the Carpathian Foredeep (Stebnyk Formation, Ukraine) suggest that the increase in temperature also touched this region (Oszczypko et al., Reference Oszczypko, Uchman and Bubniak2016). Only local, colder waters related to an upwelling are interpreted for the Paratethys sea at that time (Key et al., Reference Key, Zágoršek and Patterson2013). According to Bałuk and Radwański (Reference Bałuk and Radwański1977), the slightly younger biota of the nearby Korytnica Clay (NN6) suggests tropical or subtropical environment. However, common occurrences of echinoids and large foraminifera in the lower Badenian deposits that purportedly might suggest thermal conditions typical of subtropical or tropical climate are questioned by some authors. Studencki (Reference Studencki1999) and Randazzo et al. (Reference Randazzo, Müller, Lelkes, Juhasz and Hamor1999) suggested that paleo-temperature in the Paratethys basin during that time was rather typical of moderate climatic zones, on the border with the subtropical zone. This is suggested by poorly developed coral reefs, scarcity of green algae, and inorganic calcareous grains, especially ooids. According to Studencki (Reference Studencki1999) and Randazzo et al. (Reference Randazzo, Müller, Lelkes, Juhasz and Hamor1999), red algae, bryozoans, and mollusks that are very common in the lower Badenian deposits are typical of the rhodalgal biolithofacies, which occurs in Recent moderate climatic zones (e.g., near the coast of Scotland). Studencki (Reference Studencki1999) postulated that the strong expansion of fauna known from the Korytnica Clay and the Heterostegina Sand, including large mollusks and large foraminifera, was a local effect of peculiar topographic conditions during the climatic optimum. The latitude of Skotniki (recently N50°25') during sedimentation of the gravelstone might be lower by 10° or more (e.g., Vrielynck et al., Reference Vrielynck, Odin and Dercourt1997). This, in coincidence with the Middle Miocene Climatic Optimum, suggests that the temperature of waters in the Paratethys Sea on the northern margin of the Carpathian Foredeep was higher at that time than the Atlantic waters in the same latitudes at present. Such waters were inhabited by vermetid gastropods, the shells of which are preserved and have small traces of Renichnus and the larger trace Spirolites n. igen.