Uchman et al.—New vermetid attachment trace from a Miocene rocky coast
evidence of that interpretation was provided. In a later paper, Radwański (1977, pl. 10c–e) illustrated the same trace and referred it to etchings of serpulids, similar to the modern Serpula vermicularis Linnaeus, 1767 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. (2003) characterized bioerosive structures
ted, further comparisons are difficult. Taylor and Vinn (2006, fig. 1) illustrated the spiral polychaete tube ‘Spirorbis’ incised in the substrate. Tubes determined as the serpulid Pomatoceros invoked by Radwański (1969) can be deeply incised in the rock, as exemplified by specimens from the Oligocene of northern Germany illustrated by Polkowsky (2010), 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., Proto- laeospira strophostoma (Boettger, 1907) from the Miocene) that could produce etchings (Radwańska, 1994) are similar in shape to Spirolites n. igen., but they are a magnitude smaller. The morphology of Spirolites n. igen. also resembles
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 documen-
that of some vermetid gastropods (see Bromley and Heinberg, 2006, fig. 13). Hadfield et al. (1972) reported a few species of the recent Dendropoma from the Hawaiian Islands, the spiral shells of which were embedded in the substrate. Bromley (1978) reported Dendropoma sp. from Bermuda, which produced grooves with a roof. Golding et al. (2014) 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, 1859) from the Mediterranean Sea produces shallow attachment scars and deeper traces, and its shell can be easily detached from the substrate (Rosso et al., 2016). Savazzi (1996, 2001) 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, 2001) and gastropods in general (Schilthuizen and Davison, 2005). Some of the aforementioned Dendropoma sp. fromthe 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, 2001). The last whorls of extant Dendropoma and related bioeroding species are distinctly smaller (6–28mm across, see
889
Golding et al. 2014, for taxonomy and characteristics of Dendropoma) than in the majority Spirolites n. igen. Never- theless, 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, 2001). All these features can be observed in Spirolites n. igen.
Vermetid gastropods have been noted in Skotniki by
Małecki (1966) and Radwański (1967, 1969, 1977), but they are small taxa, which either did not leave attachment scars, or, as Vermetus intortus (Lamarck, 1818), produce the etching trace Renichnus arcuatus Mayoral, 1987. 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, pre- sumably 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 (1969). Smaller clasts, mostly cobbles and pebbles, are bored on all sides. The bored clasts were deposited in a cliff-foot ramp. According to Radwański
(1967, 1969) and Radwański and Górka (2008, 2015), 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, 1977). 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 (1969, 1977). The extant Mediterranean L. lithophaga (Linnaeus, 1758) is a chemical borer, which lives in very shallow (mostly from 0–6m deep) and clean waters (Kleemann, 1973, 1974, 1990; Galinou- Mitsoudi and Sinis, 1995, 1997), commonly on steep or overhanging surfaces (Bromley and Asgaard, 1993a). This suggests that Spirolites n. igen. also originated in very shallow, clean waters, in accordance with previous interpretations (Radwański, 1969). The boring assemblage (Gastrochaenolites, Caulostrepsis, and Entobia) represents a mature stage of bioerosion that developed over the years and can be referred to
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