The interpretation of fossils

The longitudinal cracks in the walls of most of the compacted tubes indicate that the tubes had rigid mineral walls before sediment compression. Thus, Porkuniconchus n. gen. had biomineralized calcareous tubes. The light microscope study suggests that the microstructure of tubes has likely been diagenetically altered. All tubes originally had circular or oval cross sections. The occurrence of tubes on hardground surfaces suggests that they were cemented with their lower side to the hardground. The well-preserved fine ornamentation of the studied specimens indicates that they were buried relatively quickly after their death as there is no sign of abrasion. All specimens are likely in situ as is usually the case with hard-substrate encrusters (Taylor and Wilson, Reference Taylor and Wilson2003).

Zoological affinities and systematic position

To find a proper systematic position for Porkuniconchus n. gen., one should compare it with the morphologically closest Paleozoic tubicolous organisms. Porkuniconchus shares most characters with tentaculitoid tubeworms and somewhat fewer characters with byroniids and Kolihaia.

Tentaculitoid affinities

Encrusting tentaculitoid tubeworms are usually cemented to hard substrates by one side of their tube, making them similar to Porkuniconchus. Encrusting tentaculitoid tubeworms were all suspension feeders and belonged to the lophophorates (Taylor et al., Reference Taylor, Vinn and Wilson2010; Vinn and Zatoń, Reference Vinn and Zatoń2012). Among the encrusting tentaculitoid tubeworms, microconchids (Zatoń and Olempska, Reference Zatoń and Olempska2017) and anticalyptraeids (Zatoń et al., Reference Zatoń, Vinn, Toom and Słowiński2022, Reference Zatoń, Słowiński, Vinn and Jakubowicz2023) have spiral tubes that differ from tubes of Porkuniconchus. However, cornulitids have conical nonspiral tubes that are almost identical to the tubes of Porkuniconchus. Nevertheless, fusiform perpendicular ornamentation of Porkuniconchus is unusual for cornulitids. On the one hand, the ornamentation of some species of Conchicolites slightly resembles Porkuniconchus. On the other hand, the unique ornamentation of Porkuniconchus is more similar to the ornamentation of the lining of the trace fossil Oikobesalon than to any species of Conchicolites. Despite its rather atypical ornamentation for a cornulitid, Porkuniconchus is still best placed within the family Cornulitidae Fisher, Reference Fisher and Moore1962. The only remaining doubt about this placement is related to the unknown morphology of the tube apex in Porkuniconchus. If it turns out to be pointed instead of bulbous, even the placement within tentaculitoid tubeworms could be in jeopardy.

Byroniid affinities

The Byroniida have a somewhat similar conical conch to Porkuniconchus n. gen. They are externally covered by perpendicular ornamentation that somewhat resembles the ornamentation of Porkuniconchus. Byroniids are small tube-shaped fossils that have a stratigraphic range from the Cambrian to Permian (Bischoff, Reference Bischoff1989). Their composition is variable, with both phosphatic and organic tubes included within the group (Bischoff, Reference Bischoff1989). Their tubes were attached to the substrate by a small disk-shaped holdfast (Holmer, Reference Holmer, Webby, Paris, Droser and Percival2004). Byroniids have been considered an extinct order of thecate scyphozoans (Bischoff, Reference Bischoff1989; Zhu et al., Reference Zhu, Van Iten, Cox, Zhao and Erdtmann2000; Van Iten et al., Reference Van Iten, Marques, de Moraes Leme, Forancelli Pacheco and Guimaraes Simões2014). The attachment to the substrate by only a holdfast and their organo-phosphatic biomineralization rules out byroniid affinities for Porkuniconchus.

Kolihaia affinities

Kolihaia is a problematic tubicolous organism with a calcareous tube and similar fusiform perpendicular ornamentation to Porkuniconchus n. gen. Prantl (Reference Prantl1946) originally placed Kolihaia eremita in the phylum Annelida Lamarck, 1809 with serpulid polychaetes. Later, Fischer (Reference Fisher and Moore1966) included the genus within the family Cornulitidae Fischer, Reference Fisher and Moore1962. An alternative view on the zoological affinities of Kolihaia was published by Kříž et al. (Reference Kříž, Frýda and Galle2001). They interpreted Kolihaia as an epiplanktic cnidarian and possible rugose or tabulate coral. The Kolihaia affinities of Porkuniconchus can be ruled out as the former was a free-living organism or was attached to the substrate with radiciform processes (Gnoli, Reference Gnoli1992; Kříž et al., Reference Kříž, Frýda and Galle2001), which is different from the encrusting life mode of Porkuniconchus. Moreover, Porkuniconchus lacks radiciform processes, which are one of the defining characters of Kolihaia.

First hardground fauna from the Hirnantian of Baltica

The Hirnantian is an important interval of the end-Ordovician mass extinction. There are few data from Baltica on the hard substrate faunas of the Hirnantian interval, but these faunas are important for understanding the influence of mass extinctions on hard substrate encrustation and the fate of encrusting organisms. The observable hardground area is about 100 cm², and about 12% of the area is covered by encrusters. Thus the hardground surface is rather densely encrusted for the Ordovician hardgrounds of Baltica (i.e., usually less than 1.3% of the hardground area is encrusted; Vinn, Reference Vinn2015; Vinn and Toom, Reference Vinn and Toom2015) and does not show any adverse effects of mass extinction on the process of encrustation. The encrustation area of studied hardground is similar to the Middle Ordovician Kanosh Shale hardground, where 10.5% of the area is covered by encrusters (Wilson et al., Reference Wilson, Palmer, Guensburg, Finton and Kaufman1992). It has been suggested that low encrustation of the other Estonian hardground could be due to low productivity (Vinn and Toom, Reference Vinn and Toom2015). Thus it is possible that the studied hardground represents a high-productivity site (Lescinsky et al., Reference Lescinsky, Edinger and Risk2002) in the Hirnantian of the Baltic Basin. This is also supported by the high organic content of the host rock (Hints et al., Reference Hints, Ainsaar, Meidla, Nõlvak, Toom, Hints and Toom2023). It is also possible, however, that the amount of encrustation on any marine hard substrate is a function of exposure time on the seafloor (Taylor and Wilson, Reference Taylor and Wilson2003; Zuschin and Baal, Reference Zuschin and Baal2007). Substrates exposed longer may accumulate more skeletal encrusters regardless of productivity rates.

The hardground fauna is numerically (N = 15) and by encrustation area dominated by cornulitids. The other groups are represented by only a single specimen of the sheet-like cystoporate bryozoan Ceramopora. The cornulitid specimens represent different growth stages (i.e., sizes), which suggests that the hardground was continuously colonized by new cornulitid larvae. The lack of orientation of the cornulitids indicates that there were no unidirectional currents near the seafloor as it would have forced cornulitid tubeworms to orient their apertures toward the current for most efficient suspension feeding. The lack of signs of abrasion indicates quiet conditions or moderate water movements. Encrusting unilaminar growth habits of bryozoans may indicate an environmental setting near the shore (Tolokonnikova and Ernst, Reference Tolokonnikova and Ernst2017).

One specimen of P. fragilis n. gen. n. sp. is encrusting a sheet-like bryozoan (Fig. 5). Some cornulitids have been overgrown by the other cornulitids (Figs. 3.4, 4). The sheet-like bryozoans are good competitors for space, but their compactness means that they do not disperse their zooids very widely (Taylor and Ernst, Reference Taylor and Ernst2008). However, no certain signs of spatial competition occur on the studied hardground surface.

Figure 5. Tubeworm P. fragilis n. gen. n. sp. (Corn.) and encrusting sheet-like cystoporate bryozoan Ceramopora sp. (Br) on the surface of the hardground (GIT 494-49-5).

In Baltica, the temporally closest lower Katian hardground is from the Vasalemma Formation, where cornulitids are similarly accompanied by bryozoans but with the occurrence of some Trypanites borings (Vinn and Toom, Reference Vinn and Toom2015). The most striking characteristic of this hardground fragment is the lack of bioerosion, but the studied area is too small to claim that there was no bioerosion at all. The lack of borings could be caused by some local environmental conditions such as location in a shallow topographical depression, where Trypanites borings were less common. Nield (Reference Nield1984) suggested that, in addition to selecting favorable water currents, larvae of the Trypanites producers concentrated on topographic highs. With gradual sedimentation, topographic highs would be exposed longest, allowing more time for bioerosion (Nield, Reference Nield1984; Knaust et al., Reference Knaust, Dronov and Toom2023). There are reefs in the Ärina Formation in Reinu quarry, and bioerosion is not common in the reef facies in the Ordovician Basin of Estonia (Toom, Reference Toom2019). However, bioerosion is not common in the Porkuni Regional Stage and was long considered absent there (Toom et al. Reference Toom, Vinn and Hints2019, Reference Toom, Kuva and Knaust2023), which could point to the influence of the end-Ordovician mass extinction event in the Baltic region.