Le Mans: generally fewer than 20 borings per tube were ascertained in 15 serpulid tubes examined by micro-CT and vacuum epoxy method. Among them, the specimen MHNLM EMV 2016.3.22 (Pyrgopolon (P.) deforme) was studied in the most detail (Kočová Veselská et al., 2021). At least 31 boreholes are visible in the CT-shot on this individual (Figure 4A). Some other tubes studied with an SEM have up to 100 burrow openings; without the use of CT or epoxy casting, however, the mouth of Trypanites cannot be reliably distinguished from the entrance shafts of Entobia isp. From the Bohemian Cretaceous Basin (Czechia), the specimen O8728 displays a minimum of 30 borings on the epoxy cast, which includes about one-half of the original serpulid tube. Specimen O8727: Minimum 12 discernible borings on the epoxy cast done from an incompletely dissolved tube. Specimen CZ2: Minimum three incompletely preserved borings on the epoxy cast.

Thin, short, smooth shafts usually pass the whole preserved wall of a serpulid tube. The diameter of individual shafts tends to be constant, but there are notable exceptions (Snapshot 23.tif the specimen upper right; diameter ranges from 0.035 to 0.05 mm). The Axis of the shaft is rectangular or steeply inclined toward the surface of the host tube. The usual length/depth of the tubes is 0.1 to 0.3 mm which corresponds to the thickness of the serpulid tube walls.

Kočová Veselská et al. (2021) studied in detail sclerobionts and borings of the locality Le Mans. These authors stated that in 15 serpulid tubes examined by micro-CT and vacuum epoxy method, about 30 complete borings are oriented perpendicular to the substrate. Another 20 tunnels are inclined at angles of up to 45° to the vertical axis. The perpendicular borings are considered to belong to Trypanites, while the oblique tubes were preliminarily included in the ichnogenus Entobia. However, this attitude gives only approximate results in terms of the number of Trypanites and Entobia individuals. A specimen of Entobia usually has several openings connecting the domichnial chambers to the substrate surface; however, the number of these connections is very variable within the ichnogenus Entobia (e.g., Bromley and D'Alessandro, 1983); thereby, the number of oblique borings cannot be easily recalculated to the number of individuals.

For trypanites, Bromley (1972, pp. 95, 96, Figure G) reported diameter approx. 1 mm and length up to 4 cm. Taylor et al. (2013) stated the diameter of borings 0.35–0.4 mm in belemnites from the Early Cretaceous Speeton Clay Formation. The borings studied in our present publication are substantially smaller than those cited in Bromley's revised diagnosis of the ichnospecies, measuring only about 0.05 mm in diameter. Bertling et al. (2006) stated that the size itself is not a valid ichnotaxobase; thereby, the small size of the finds cannot be considered a clue to discard the above-described finds from Trypanites.

Following this practice, numerous authors (e.g., Nielsen et al., 2003) accommodated microscopic drillings into foraminifer shells to the ichnogenus Oichnus Bromley, 1981 which originally had been established for macroscopic drilling of naticid gastropods through shells of various mollusks (namely gastropods, bivalves, and scaphopods). The present article follows this practice and supports the validity of the approach. Also, other widespread borings than Oichnus may have their microscopic varieties.

On the surfaces of certain tubes from Le Mans more than 100 orifices are visible which could represent the mouths of Entobia isp. (Kočová Veselská et al., 2021). The CT or SEM studies of the epoxy vacuum cast showed that a significant portion of these openings belongs to Trypanites instead (see above) and that the individual of Entobia usually consists of one or two chambers and several (3–7) mouth tubes. Thereby, only a dozen specimens of Entobia isp. are well documented from Le Mans. From the BCB, a sole well casted unicamerate specimen has been found on the sample CZ2. A trinity of two-camerate specimens is present on O8728.

Ovoid or bag-like chambers, 0.2 to 0.7 mm in diameter, connected to the surface by narrow tunnels, usually inclined (angle to the serpulid tube's longitudinal axis ~50°, rarely only 30°). As the chambers occur very close to the surface of the serpulid tubes, some connecting tunnels are very short (length ranges from 0.1 to 2 mm) and narrow (diameter varies from 0.01 to 0.4 mm).

The boring found on CZ2, fortunately, preserved near the cut of the serpulid tube for casting, can be compared with several common ichnogenera: Gastrochaenolites Leymerie, 1842 is typical for rockgrounds but also firmgrounds from the Ordovician till the present; pholadid bivalves are its typical tracemakers, often preserved in situ (Mikuláš et al., 2006). However, these are geometrically exactly bored, due to the use of a mechanical rotational move of the shell toward the substrate. Another possibility, Rogerella isp., is excluded due to the relatively broad aperture in contrast with the slit-like or “comma-like” aperture of Rogerella. On the other hand, the bag-like shape is typical for certain camerate entobians (e.g., E. devonica); these do not show a geometrical exactness of the resultant chamber as it grows using a chemical dissolution of the surrounding substrate. The specimen from O8728 shows a couple of small (up to 100 μm) spherical bodies, partially penetrating each other. From the outside of the substrate, the spheres are smooth, resp. their surfaces correspond to the grain size of the mineralized filling (<10 μm). On the sediment-facing side, approximately six “stems,” shorter than 10 μm, protrude from the two spherical bodies, corresponding to exploratory threads of entobians (Bromley and D'Alessandro, 1984). Another two specimens of E. isp. on the sample O8728 show the comparative morphology and dimensions but are in the back of the epoxy cast and are not easily accessible for study.

Shortly after the year 2000, the vast majority of then known findings of the ichnogenus Entobia were relatively homogeneous in size, with the diameter of the chambers resp. galleries approximately a few millimeters, exceptionally over 1 cm: Entobia gigantea up to 50 mm, Entobia magna 20 mm, Entobia isp. B sensu Bromley and D'Alessandro, 1984 chamber size up to 15 mm. Wisshak (2008) summarized the findings of smaller entobians; he described the rich material from the Lower Pleistocene of the island of Rhodes and recent analogies from various aphotic, cold-water environments, mostly on the NE coast of the Atlantic. After studying a very large material, he described in detail two new ichnospecies of the ichnogenus Entobia, both of microscopic dimensions: E. mikra Wisshak, 2008 has the form of grape or lump clusters and reaches 60–330 μm; E. nana Wisshak, 2008 presents mostly solitary cavities/chambers with a size of 100–750 μm.

The holotypes of the two new entobians determined by Wisshak come from the early Pleistocene (Rhodes Formation), from the coral skeletons of the genus Lophelia. This genus with a finely acicular aragonite structure provides an ideal environment for the formation of idiomorphic (i.e., not taking into account the shape of the substrate) borings (Bromley, 2005). In addition to the ichnospecies E. mikra and E. nana, there are much larger residues of entobians in the bioclasts of the Rhodes Formation, identifiable only to the ichnogeneric level. Other ichnotaxa found include very abundant Saccomorpha clava Radtke, 1991, Orthogonum lineare Glaub, 1994 and Semidendrina pulchra Bromley et al., 2007. Rarely have been found Podichnus centrifugalis, Bromley et Surlyk, 1973, Caulostrepsis cretacea Voigt, 1971 Maeandropolydora isp., Palaeosabella isp., Scolecia serrata Radtke, 1991 and Oichnus simplex Bromley, 1981.

Within the ichnogenus Entobia, it is rather exceptional that direct fibers, known as exploratory threads (Bromley and D'Alessandro, 1984) do not protrude from the chambers in all or several prevailing directions. The behavior of entobians living in the tiny space inside the serpulid shells was apparently directed so that communication with the inside of the tube was not desirable because an exploratory thread would not have an opportunity to be useful in any way. There is no usable space for the boring organism inside the lumen of the tube because it is inhabited by the serpulid animal (except if the tube is already empty because either the serpulid animal has grown and moved to a more anterior position inside its tube, or the serpulid has already died and the lumen is empty.). Similar behavior is recorded by the ichnospecies E. solaris Mikuláš, 1992, from the Lower Cretaceous of the Outer Western Carpathians. Additionally, for these ichnospecies, the exploratory threads are limited to the surface of the substrate (Mikuláš, 1992).

The determination of this ichnofossil at the ichnospecific level is complicated. It is obvious that it is a camerate ichnospecies (typical examples are E. cretacea and E. ovula; Bromley and D'Alessandro, 1984). Non-camerate examples, i.e., cylindrical or conical ichnospecies would certainly create a gallery at first and not spherical hollows. It is also clear that the microscopic ichnospecies described by Wisshak (2008), i.e., E. mikra and E. nana, are morphologically different from E. isp. from both Le Mans and the BCB. Le Mans findings compared to the BCB collections may represent a single ichnospecies, but the comparative material is still relatively small. For the same reason, we also abandon the determination of new ichnospecies based on the material described herein.

From a couple of E. mikra and E. nana, none of them can be considered a typical product of chamber growth on exploratory threads. Wisshak (2008; p. 221, Figure 4) showed epoxy castings of E. mikra as grape-like forms with individual chambers measuring 3–6 μm in diameter. The chambers intersect the neighboring ones; therefore, they cannot be connected by galleries or threads. In the case of the specimen shown by Wisshak (2008, Figures 4A,B), 2–3 threads are clearly visible in an area limited by approximately 10 chambers. The specimen in Figures 4E,F resembles an elongated, partially branched grape composed of approximately one hundred balls. Tubular or fibrous forms are not recognizable here at all. Closer to the “classic” form of Entobia isp. with one or more chambers of “potato” shape and thin but distinct network of exploratory threads are Entobia nana Wisshak, 2008. A detailed numerical analysis of the ichnospecies E. mikra and E. nana was given by Wisshak in Tables 2 and 4. Interestingly, Wisshak (2008; p. 219) stated that the chambers of most entobians, if well preserved, have a cuspate microsculpture that may be lacking in gerontic individuals. In terms of these data, Entobia isp. studied herein, with a relatively smooth surface (the grain size of the surface is related to the grain size of the surrounding substrate) and the lack of exploratory threads represent an adult, if not a gerontic specimen. In addition, in the BCB findings, none of the studied substrates found “transient” forms between small entobians in serpulid shells and large, mostly as yet unstudied E. cf. cretacea from more massive substrates (mostly oysters; Žítt et al., 1997).

From Le Mans site, two tubes probably corresponding to Maeandropolydora were found in a CT projection of two specimens of Pyrgopolon (Kočová Veselská et al., 2021; Figures 7E,F). From the Bohemian Cretaceous Basin, a huge specimen is developed inside the O8728 sample.

Specimens from Le Mans appear as rather short, winding cylindrical galleries, constant in diameter (0.2–0.3 mm), branched, and thereby have 2–4 openings. The galleries tend to follow the surface of tubes (Kočová Veselská et al., 2021). The specimen O8728 from the Kaňk locality is much larger. Long, cylindrical galleries have several apertures, running through the substrate in irregular, not too sharp turns. Some of the views of the epoxy cast show that the galleries are duplicated, partially touching each other; in the remaining part, they are somewhat less densely spaced but remain more or less parallel. The length of the individual boring is 5.8 mm, which corresponds to the overall length of the studied serpulid. The diameter of the galleries is somewhat variable but basically circular; it ranges from 0.3 to 0.5 mm. The surface of the galleries is smooth.

Contrary to the diagnosis of Maeandropolydora proposed by Bromley and D'Alessandro (1983), no enlarged sections (i.e., vanes and pouches) were observed. The serpulid tube O8728 was apparently filled with calcium carbonate and was redeposited under a high physical energy environment of the rocky shore. Then this cylindrical object was available for drilling. The resulting configuration of borings is therefore quite different from the other studied specimens, which have the character of relatively thin-walled tubes.

Three possible fragments of ?Iramena isp. in the serpulid specimen no. CZ2.

Three short cylindrical or somewhat conical fillings of borings through the entire wall of the serpulid specimen CZ2. The epoxy casts show the stolons of the main borings, which are characterized by a significantly smaller, variable diameter compared to the main boring. The axes of the main bored cylinders/cones form an angle of 70–85° with the surfaces of the tubes, which is close to the values found for Trypanites. The axes of the stolons make an angle of 30–60° with the axis of the main borings.

Ichnotaxonomic determination of the borings described above is problematic. However, the presence of tunnels of different diameters and their branching resembles the ichnogenus Iramena, Boekschoten (1970), which is typically diagnosed as “borings consisting of long tunnels in an irregular network, with a round to reniform apertures situated in alternating position laterally to and close by the tunnels” (Boekschoten, 1970). The stenomorphic form of Iramena in a thin tube could look just like an irregular letter “V.” However, it is necessary to make this determination with caution. In the specimen described, the ichnogenus Conchotrema Teichert, 1945 has a similar potential for making “V”-shaped borings (for details on Conchotrema see, e.g., Bromley and D'Alessandro, 1987).

From Le Mans, 209 Rogerella specimens were ascertained on 55 serpulid specimens selected for detailed statistics (Kočová Veselská et al., 2021); they occur in 68.75% of the non-moldic serpulid specimens (55 specimens out of 80).

Elongated, almond-shaped, unornamented pits. Dimensions of borehole openings vary from 0.2 to 2.7 mm in length and 0.1–0.9 mm in width. The depth ranging from 0.4 to 0.6 mm was measured using micro-CT and polymer resin casts. For further details concerning morphology, taphonomy, and interactions, see Kočová Veselská et al. (2021).

Reproduction of clionaid sponges (i.e., the causative agents of most representatives of the ichnogenus Entobia). The Porifera group (sponges) belongs to the relatively diverse and biologically important, but still relatively little studied representatives of benthic fauna, especially (but not only) in shallow tropical and subtropical seas (Schönberg, 2021). A separate ecological group of boring sponges was soon recognized, initially often called “parasitic” due to their destructive activity on the shells of living bivalves of various genera (Nasonov, 1883), on coral skeletons, and also in the limestone pebbles of the Northumberland coast (Hancock, 1849). These sponges were classified by the mentioned authors into the genera Cliona Grant, 1826 and Vioam Nardo, 1839. Nasonov (1883) approached the question of the technique of boring clionaid sponges into limestone substrates through detailed observations in aquariums; his observations were specified and supplemented by Topsent (1900). Nasonov returned to the subject in 1924. The process of reproduction, colonization of the substrate, and its drilling is described in detail by the mentioned authors, while it is evident that within the family Clionaidae there is a certain diversity. For example, Nasonov (1883) observed that clionaid sponges in aquariums produced eggs, while Topsent (1900) observed larval hatching. The same authors described the process of nesting the larva into the substrate.

This process can be observed by placing thin slices of a prismatic layer of bivalve shells in an aquarium containing the larvae of the sponges. The larvae attach to the surface of these slices and the part of their body facing the substrate emits short, thin protoplasmic expansions, which sink into the substrate and, when viewed from above, the sponge has a rosette structural pattern at this stage. Through these expansions, the larva separates small particles that the larva draws into the body. The particles then pass through the soft tissue and then leave the body of the sponge. (This creates a large amount of micritic fraction, which can be transported and accumulated in other parts of the sedimentary basin; cf. Flügel, 2004). We can conclude that the work of piercing the chambers and galleries of the sponge into bivalve substrates, whose shell construction also contains substances other than CaCO₃, is carried out first by a chemical process, i.e., by the excretion of acid and an enzyme that promotes the dissolution of conchiolin. In some sponges, the authors observed the ability to dissolve resistant substances, such as chitin. In the boring sponges, the mechanical process is manifested by the ejection of cut particles out of the body. According to Nasonov (1924) and other authors cited above, the minimum dimensions of the structure attributable to the ichnogenus Entobia are microscopic: young sponges begin drilling at a diameter of about 0.06 mm. The measured values of the diameter of the chambers of the entobians on the Pyrgopolon tubes slightly exceed 0.1 mm; within the above-mentioned morphological and physiological elasticity of this group, it is possible to imagine the formation of an egg in a chamber of this diameter, from which a larva equipped with a flagellum emerging very quickly.

Due to the need to study benthic communities based on mere pictorial documentation which becomes an often-pronounced requirement, Schönberg (2021) introduced a classification system of Porifera based on a strictly functional interpretation of traditional spongal morphologies. The goal is to deliver a standardized approach that can be optionally display-based and can be applied anywhere. A sophisticated system was introduced which works with 21 morphologies in its end categories; 4 morphologies are considered basic: 1—true crusts, endolithic bio-eroding, and creeping sponges, 2—simple massive, globular massive, composite massive, and tubular sponges, 3—bowls and barrels, 4—one-dimensional, two-dimensional and three-dimensionally erected forms; stalked sponges. From this point of view, based on a study of the extensive literature on recent sponges from various environments, our presented endolithic microforms represent a specialized sub-group within basic morphology no. 1. Their difference/specialization lies mainly in the following aspects: (1) Typical endolithic sponges usually inhabit the environment with the highest hydrodynamics; (2) endolithic sponges have to protect themselves from being buried by rapid sedimentation, and therefore erect forms are common in these environments. Both of these facts do not apply to sponge domichnia in serpulid tubes. The location of Le Mans ring road is characterized by a rather low moderate energy environment (Kočová Veselská et al., 2021) and the absolute predominance of small bioclasts.

Maeandropolydora is produced by polychaetes, mostly of the family Spionidae (Bromley and D'Alessandro, 1983, 1987; Farinati and Zavala, 2002) which ranges from intertidal to the depth of 50 m (Wisshak et al., 2005). Spionids are very small polychaete worms (lengths from a few millimeters up to 2–3cm) living in a wide variety of substrates and various habitats (e.g., Blake and Evans, 1973; Blake et al., 2019; Rouse et al., 2022).

Spionids may feed by gathering particles that deposit on the surface of the sea floor using a pair of long, grooved feeding palps with cilia that transport the particles along the palps to the mouth. Spionid tubeworms are selective deposit feeders (Fauchald, 1977) with two grooved palps for caption and prey. Palps may also be used to gather plankton and particles suspended in the water. Their glandular pouches produce mucus that cements sand grains and detritus for building their tubes. These worms are capable of chemical and mechanical boring into different calcareous substrates, including corals, mollusk shells, coralline algae, barnacles, and brachiopods (e.g., Abe et al., 2019; Blake et al., 2019; Abe and Sato-Okoshi, 2020; Malan et al., 2020; Demircan et al., 2021; Villas et al., 2021; Rouse et al., 2022). Shell boring ability is considered species-specific (Sato-Okoshi, 1999; Sato-Okoshi and Okoshi, 2000), albeit some exceptions are mentioned in Radashevsky and Pankova (2013). Some spionid worms may live freely. Spinoid traces described as Caulostrepsis Clarke, 1908 and Maeandropolydora Voigt, 1965 are known from the fossil record as well as from recent environments. The genus Caulostrepsis is characterized by U-shaped boring of various shapes, whereas the genus Maeandropolydora is characterized by more complex, multiple U-shaped, occasionally branching boring systems with often welldeveloped cylindrical galleries (Wisshak and Neumann, 2006). Both ichnogenera are found in many different calcareous and non-calcareous hard substrates. Their fossil record reaches back to the Palaeozoic (Bromley, 2004). The central portion of U-shaped boreholes is filled with detritus and particles of the dissolved substratum. The two openings communicate with the outside of the substratum, and the mud tube protrudes from each opening (Sato-Okoshi and Okoshi, 1997, 2000). Blake and Evans (1973) summarized all known records of polydorid borings in calcareous substrates, boring patterns, specificity of the attack, and larval development. These authors mentioned three main types of polydorid borings described in bivalve shells: surface fouling, U-shaped borings, and mud-blisters. The morphological variations of the traces depend on the type of substrate, abrasion, and density of occupation, and are determined by the changing trajectories of the boring tunnels and channels. Circular to elliptical boreholes in our material show maximum diameter varying between 0.3 and 0.5 mm and length up to 5.8 mm. Compared to dimensions of various Maeandropolydora in fossil and recent substrates (Zottoli and Carriker, 1974; Diez et al., 2011; Demircan, 2012), Maeandropolydora tunnels in our studied serpulids are very tiny; therefore, the tracemaker polydorid worms must have been juveniles or dwarfed adults (Kočová Veselská et al., 2021).