Non-technical Summary
Tubeworms form an important part of the modern marine fauna. They were also common in the geological past. We discovered a new genus and species of tubeworms from the latest Ordovician of Estonia. These tubeworms grew on the lithified sea floor during the time of the end-Ordovician mass extinction. Our discovery helps better understand and reconstruct the marine life during this extraordinary time interval.
Introduction
Cornulitids are a group of problematic tubicolous lophophorates that have a stratigraphic range from the Darriwilian (Middle Ordovician) to the late Carboniferous (Vinn, Reference Vinn2010). The zoological affinities of cornulitids have been long debated, but they most likely belong to the Lophotrochozoa (Vinn and Zatoń, Reference Vinn and Zatoń2012) and could represent stem-group phoronids (Taylor et al., Reference Taylor, Vinn and Wilson2010). Their fossils often provide us with important paleoecological information because, as hard substrate encrusters, they generally retain their original position on the substrate through fossilization (Taylor and Wilson, Reference Taylor and Wilson2003). Most cornulitids are general hard substrate encrusters (Zatoń and Borszcz, Reference Zatoń and Borszcz2013; Zatoń et al., Reference Zatoń, Borszcz and Rakociński2017), and all were stenohaline, which differs from their close relatives the microconchids, which were euryhaline (Zatoń et al., Reference Zatoń, Vinn and Tomescu2012, Reference Zatoń, Wilson and Vinn2016). Cornulitids are common fossils in shallow marine sediments of the Paleozoic, especially those associated with carbonate platforms (Richards, Reference Richards1974; Zatoń et al., Reference Zatoń, Borszcz and Rakociński2017; Musabelliu and Zatoń, Reference Musabelliu and Zatoń2018). The Ordovician cornulitids from Estonia have been systematically studied by Vinn (Reference Vinn2013) and Vinn et al. (Reference Vinn, Madison, Wilson and Toom2023a, Reference Vinn, Wilson, Madison and Toomb), but further research on the group is needed to fully understand their diversity and ecology, especially on hardgrounds.
Carbonate hardgrounds are synsedimentarily lithified carbonate layers that have been exposed on an ancient seafloor (Wilson and Palmer, Reference Wilson and Palmer1992). Hardgrounds form excellent substrates for encrusting and bioeroding organisms (Palmer, Reference Palmer1982; Taylor and Wilson, Reference Taylor and Wilson2003). These organisms otherwise dwell on carbonate cobbles or shells of various invertebrates. Ordovician hardground faunas are globally well documented, but not much is known about hardgrounds from the important Hirnantian mass-extinction interval.
The aims of this paper are to: (1) compare a new genus and species of cornulitid tubeworm with various previously known cornulitids and tubicolous organisms, (2) discuss the zoological affinities of the fossil, and (3) discuss the ecology and evolution of hardground faunas in the Ordovician of Baltica.
Geological background and locality
A shallow warm epicontinental sea covered what would become modern northern Estonia during the Hirnantian. The Hirnantian sequence of northern and middle Estonia is represented by tropical carbonate rocks belonging to the Ärina Formation (Porkuni Regional Stage) (Hints and Meidla, Reference Hints, Meidla, Raukas and Teedumäe1997; Hints et al., Reference Hints, Oraspõld and Kaljo2000; Kröger, Reference Kröger2007). There was a significant climatic change in the Katian of Baltica, when the paleocontinent drifted from a temperate climatic zone into the tropical realm (Torsvik et al., Reference Torsvik, van der Voo, Preeden, Mac Niocaill and Steinberger2012). Carbonate sedimentation intensified during the warming of the climate early in the Katian (Nestor and Einasto, Reference Nestor, Einasto, Raukas and Teedumäe1997). The tropical fauna, including tabulate corals and stromatoporoids, that appeared in the early Katian was common in the Hirnantian.
The Reinu quarry is located in northern Estonia (latitude 59.08768°N, longitude 24.74044°E) in Rapla County (Hints et al., Reference Hints, Ainsaar, Meidla, Nõlvak, Toom, Hints and Toom2023) (Fig. 1). Carbonate rocks of latest Katian to Rhuddanian age (Pirgu to Juuru regional stages) are exposed here (Hints et al., Reference Hints, Ainsaar, Meidla, Nõlvak, Toom, Hints and Toom2023) (Fig. 2). The Ärina Formation (~2.5 m thick, Hirnantian) consists of various shallow-marine carbonates and contains the Siuge Member with characteristic kerogenous limestones and a hardground at its upper contact with the Koigi Member (uppermost Hirnantian) (Hints et al., Reference Hints, Ainsaar, Meidla, Nõlvak, Toom, Hints and Toom2023).
Locality map. The locality is indicated by the red dot. Dev.- = Devonian; EST. = Estonia.
Stratigraphic section of Reinu quarry with location of the hardground (modified after Hints et al., Reference Hints, Ainsaar, Meidla, Nõlvak, Toom, Hints and Toom2023, p. 50, fig. 8.3).
Materials and methods
A limestone slab with fossils of Porkuniconchus was collected during a visit to the Reinu quarry in 2023. The limestone slab contained a surface with numerous tubeworm fossils. This surface was cleaned and photographed using an apochromatic zoom system Leica Z16 APO.
Repository and institutional abbreviation
Types, figured, and other specimens examined in this study are deposited in the Department of Geology, Tallinn University of Technology (GIT).
Systematic paleontology
Superphylum Lophophorata Emig, Reference Emig1984
Phylum uncertain
Class Tentaculitida Bouček, Reference Bouček1964
Order Cornulitida Bouček, Reference Bouček1964
Family ?Cornulitidae Fisher, Reference Fisher and Moore1962
Porkuniconchus new genus
Type species
Porkuniconchus fragilis n. gen. n. sp.
Diagnosis
Almost straight, curved to slightly meandering tubes with thin calcareous walls and smooth lumen. The tube exterior is densely covered by fine fusiform irregular transverse ornamentation.
Occurrence
Hirnantian of northern Estonia.
Etymology
After the type horizon, Porkuni Regional Stage, and shell (conch).
Remarks
The new genus is based on the combination of unique fine fusiform perpendicular ornamentation, which separates it from Conchicolites Nicholson, Reference Nicholson1872, and lack of processes, which separates it from Kolihaia Prantl, Reference Prantl1946. The new genus is tentatively assigned to Cornulitidae because of its broad, thin-walled conical calcareous tube.
Holotype
Holotype (complete tube GIT 494-49-1), paratypes (complete tubes GIT 494-49-2, GIT 494-49-3 and GIT 494-49-4).
Porkuniconchus fragilis n. gen. n. sp. from Ärina Formation (Hirnantian), Reinu quarry, northern Estonia. (1, 2) Holotype GIT 494-49-1. (3) Paratype GIT 494-49-2 showing fusiform transverse ornamentation. (4) Paratypes GIT 494-49-3 and GIT 494-49-4. (5) Tubeworms on the surface of hardground.
(1, 2) Arrows point to discontinuous transverse ridges. (3) Longitudinal section of the tube wall (arrows) showing structureless sparry calcite. (4) Transverse section of the tube wall (arrows) showing sparry calcite (Sp).
Diagnosis
Almost straight, curved to slightly meandering, moderately large tubes with thin calcareous walls and smooth lumen. The tube exterior is densely covered by fine fusiform and continuous irregular transverse ornamentation.
Occurrence
Reinu quarry, northern Estonia; Ärina Formation (Hirnantian), Siuge Member.
Description
Almost straight, curved to slightly meandering tubes. Tubes are up to 25 mm long and 2.0–3.5 mm wide at the aperture. Shell wall is thin and calcareous. The exterior is covered by fine dense and irregular transverse ridges. The transverse ridges are very low, and the sides of the tube are almost smooth and only slightly zig-zagged in profile (Fig. 3.2, 3.3). The transverse ridges have convex profiles in longitudinal section. The ridges can fuse and appear in the interspaces between two ridges (Fig. 4.1, 4.2). There are 10–11 ridges in 1 mm near the tube aperture. The transverse ridges are variably developed. Some smaller transverse ridges are conjoined to form larger annuli. The tubes are devoid of any longitudinal ornamentation. Tubes are flattened and contain longitudinal fractures resulted from burial compression. The interior of the tube appears to be smooth (Fig. 4.3). The tube wall is extremely thin (about 0.07 mm). The tube grew moderately to rapidly in diameter. The apical angle of the flattened tube is 13–14°, but the actual value may be smaller as all tubes are compressed. The morphology of the tube's apex is not clear on the studied fossils; it seems to be pointed in some specimens and bulbous in one specimen. The original tube structure is not preserved and is replaced with sparry calcite (Fig. 4.3, 4.4).
Etymology
After fragilis (Latin), meaning “fragile, brittle.”
Materials
Fifteen complete compressed tubes cemented to a hardground surface.
Remarks
Porkuniconchus fragilis n. gen. n. sp. is most similar to Conchicolites rossicus Vinn and Madison, Reference Vinn and Madison2017 from the Kõrgessaare Formation (Katian) (Vinn et al., Reference Vinn, Wilson, Madison and Toom2023b, p. 3–5, fig. 5D, E) in its conical shell that is covered by fine perpendicular ridges, but differs by fusiform perpendicular ornamentation, much larger tubes, and lack of attachment structures. P. fragilis also resembles Kolihaia eremita Prantl, Reference Prantl1946 with its fusiform perpendicular ornamentation and similar size of tubes but differs most remarkably by the lack of radiciform processes. In addition, the perpendicular ornamentation of K. eremita is stronger than in P. fragilis. The tube structure of this new species is not preserved, but considering the extremely thin tube wall, there would not have been space for vesicles in the sense of Cornulites. This new species differs from all other cornulitids by its extremely thin tube wall and its tube size.
Discussion
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.
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.
Acknowledgments
This paper is a contribution to the IGCP project 735 Rocks and the Rise of Ordovician Life (Rocks n'ROL). We are grateful to Z. Chen from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, who collected the slab with the study specimens during the ISOS-14 Estonian excursion. We are also grateful to A. Ernst (University of Hamburg) for identification of the bryozoan. Financial support to O.V. was provided by a Sepkoski Grant (The Paleontological Society). U.T. was funded by the Estonian Research Council, grant number PUTJD1106. We are grateful to G. Baranov, Institute of Geology, Tallinn University of Technology for photographing the specimens. We are grateful to H. Van Iten and an anonymous reviewer for the constructive comments on the manuscript.
Declaration of competing interests
The authors declare none.
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