Cornulites sp. and Fistulipora przhidolensis formed a symbiotic association in the Pridoli (latest Silurian) of Saaremaa Island, Estonia. This Cornulites sp.–F. przhidolensis association is the youngest example of cornulitid–bryozoan symbiosis. Symbiosis is indicated by intergrowth of both organisms. The cornulitids are completely embedded within the cystoporate bryozoan colony, leaving only their apertures free on the growth surface of bryozoan. In terms of food competition, this association could have been slightly harmful to F. przhidolensis as Cornulites sp. may have been a kleptoparasite. There may have been a small escalation in the evolution of the endobiotic life mode of cornulitids as the number of such associations increased from the Ordovician to Silurian. It is likely that Palaeozoic bryozoan symbiosis reached its maximum in the Late Ordovician. Most of the symbiotic bryozoans in the Palaeozoic are trepostomes, and the diversity of symbiotic associations was also greatest among trepostomes.


  1. Baltica
  2. bryozoans
  3. cornulitids
  4. Silurian
  5. symbiosis
Among the best examples of symbiosis in the fossil record are endobionts embedded (i.e. bioimmured) by the living tissues of host organisms (Taylor 1990). Bioimmured cornulitids differ from ordinary bioclaustrations as defined by Palmer & Wilson (1988) in having their own skeleton. The bioimmuration of soft-bodied and skeletal organisms within the zoaria of bryozoans has recently been given its own term: bryoimmuration, as it is so common in fossil record (Wilson et al.2019). Organisms that live close can grow into one another to form a fused pair of skeletons. This is different from simple encrustation in that one skeleton overlaps another (Tapanila 2008). The full intergrowth of skeletons provides the best evidence of symbiosis between two animals (Tapanila 2008). Symbiosis is here viewed as any type of a long-term and close biological interaction between two different animals, be it parasitic, commensalistic or mutualistic. Modern ecology and biology textbooks use the ‘de Bary’ definition, or an even broader one, in which all interspecific interactions are termed symbiosis; the narrower definition that symbiosis means only mutualism is no longer used (de Bary 1878; Martin & Schwab 2013; Vinn et al.2019).
Cornulitid tubeworms are palaeoecologically important as hard substrate encrusters that generally retain their original position on the substrate after fossilization (Taylor & Wilson 2003; Vinn & Wilson 2013; Musabelliu & Zatoń 2018). Cornulitids are especially common in shallow marine carbonate platform sediments (Zatoń & Borszcz 2013; Vinn & Wilson 2013). Cornulitids had a diverse ecology including free-living life forms and symbiotic endobionts (Vinn 2010; Vinn & Wilson 2013). They form an order of the encrusting tentaculitoid tubeworms and could be ancestors of free-living tentaculitids (Vinn & Mutvei 2009; Vinn 2010). In older geological literature, cornulitids have been affiliated with various groups of invertebrates, but they most likely belong to the Lophotrochozoa (Vinn & Zatoń 2012). Cornulitid tubeworms could represent stem-group phoronids (Taylor et al.2010). The taxonomy of Silurian cornulitids in Estonia is briefly summarized by Vinn & Wilson (2013).
Eastern Baltic Silurian bryozoans are relatively well studied (Kopajevich 1968, 1971, 1975; Astrova 1970; Astrova & Kopajevich 1970; Pushkin et al.1990), but no cases of intergrowth between these bryozoans and other invertebrates have been previously reported. This differs from the Ordovician of Baltica where bryozoans often formed symbiotic associations with the other invertebrates (Vinn et al.2019).
This paper: (1) describes in detail the intergrowth between cornulitids and cystoporate bryozoan hosts from the Pridoli (latest Silurian) of Estonia; (2) discusses the palaeoecology of this cornulitid–cystoporate association; and (3) discusses symbiosis of Palaeozoic cornulitids and bryozoans.

Geological background

During the Late Silurian, the palaeocontinent of Baltica was located in equatorial latitudes (Melchin et al.2004; Torsvik & Cocks 2013). A shallow epicontinental sea covered the western part of modern Estonia and was characterized by a wide range of tropical environments and diverse biotas (Hints 2008). Nestor & Einasto, 1977 established a facies model for the Baltic Silurian basin with five depositional environments: tidal flat/lagoonal, shoal, open shelf, basin slope and a basin depression. The first three environments formed a platform, and the latter two, a deeper basin with fine-grained siliciclastic deposits (Nestor & Einasto 1997). On Saaremaa Island, the Silurian strata contain shallow shelf carbonate rocks rich in shelly faunas. The best Silurian exposures in Estonia are on Saaremaa Island and are mostly represented by small coastal cliffs. The Lõo cliff section (Lõo Beds, Kaugatuma Formation, Pridoli; Fig. 1) consists of coarse-grained crinoidal grainstones with interlayers of marlstone (Einasto 2008). The section can be interpreted as a normal marine shallow carbonate platform environment influenced by regular wave activity.
Fig. 1 Locality map showing the outcrop of early Palaeozoic rocks in Estonia. [Colour figure can be viewed at wileyonlinelibrary.com]

Materials and methods

A large collection of Silurian shelly fossils was searched for intergrowth of different invertebrates. The collections of the Department of Geology, Tallinn University of Technology (GIT), contain about 260 bryozoans from the Silurian of Estonia and 16 bryozoans from the Kaugatuma Formation. A single bryozoan specimen from the Lõo cliff (Kaugatuma Formation, Pridoli) contained endobiotic cornulitids. Sections were not made from this bryozoan zoarium containing embedded cornulitids because of its rarity as a museum specimen. The specimen was photographed with a Canon EOS5DS R digital camera and a Leica Z16 APO zoom microscope system. The dimensions of both cornulitids and the bryozoan were obtained from calibrated photographs.


At least three slender cornulitid tubes are embedded within the bryozoan colony (Fig. 2). They are orientated more or less perpendicular to the growth surface of bryozoan colony. The diameters of the tubes increase slowly, varying from 1.4 to 3.2 mm. The tubes are almost regularly covered with moderately developed annulations in form of perpendicular ridges. The tube surface between the ridges is covered with poorly developed fine perpendicular growth lines. The tube surface between the ridges is flat to slightly concave. The inner surfaces of the tubes are covered with regular annulations. The external surfaces of the tubes are devoid of any longitudinal ornamentation. The presence of external and internal annulation is characteristic of the genus Cornulites Schlotheim, 1820; we assign the tubes to Cornulites sp.
Fig. 2 Cornulites sp. intergrown with Fistulipora przhidolensis from the lower Pridoli (Kaugatuma Formation) of Lõo cliff, Saaremaa, Estonia (GIT 666-38). A, detailed view of bryozoan, B, Cornulites sp. [Corn] in cross section, C, D, apertures of Cornulites sp. [Corn] on the growth surface of Fistulipora. [Colour figure can be viewed at wileyonlinelibrary.com]
The cystoporate bryozoan colony is elongate and irregularly shaped and built by singular encrusting sheets. The colony diameter along the longer axis is about 60 mm, and the diameter along the shorter axis is about 23 mm. Separate sheets are 1.0-1.5 mm thick. The colony is covered with growth surface all around except for one broken end. Typical characters of this species are regularly spaced maculae consisting of vesicles (covered by calcitic skeleton) and autozooecial apertures with distinct lunaria arranged radially around the maculae. It has been identified as Fistulipora przhidolensis Kopajevich, 1990 (in Pushkin et al.1990; Fig. 2A).


Palaeoecology of the association

Cornulites sp. and Fistulipora przhidolensis Kopajevich, 1990 formed this association during their lives as indicated by complete intergrowth of both organisms and the perpendicular orientation of cornulitids to the bryozoan growth surface. Cornulites sp. and F. przhidolensis most likely grew in height at the same rate, so that the bryozoan could not overgrow the cornulitid aperture and kill it. Similar coordinated growth of symbiont and host is characteristic of many Palaeozoic symbiotic associations between worm-like organisms and massive colonial hosts (Oekentorp 1969; Zapalski 2007; Zapalski 2009; Zapalski & Hubert 2011). F. przhidolensis formed a noticeable elevation around a single embedded cornulitid, which could indicate spatial competition between these two organisms. On the other hand, the lower rims around the other cornulitid apertures do not provide any evidence of the bryozoan’s attempt to overgrow an infesting organism. Thus, spatial competition, if any, was relatively weak between Cornulites sp. and F. przhidolensis.
The high domical bryozoan colony provided cornulitids with a higher tier for suspension feeding. The bryozoan also offered a stable substrate for cornulitids in shallow and hydrodynamically active waters where smaller unattached hard substrates would have often been overturned. The bryozoan colony could have also offered the cornulitid additional protection against predators. Cornulitids often suffered predatory attacks in the Silurian of Baltica (Vinn 2009). Predatory pressure was likely the driving force behind the evolution of the endobiotic life mode in cornulitids (Vinn 2010). The small number of Cornulites sp. tubes in the F. przhidolensis colony presumably did not significantly interfere with the feeding of the bryozoan and cannot be considered harmful to the host. However, the soft body of Cornulites sp. was larger than the zooids of F. przhidolensis, and the former likely had a larger feeding apparatus, so that Cornulites sp. could catch its food before the F. przhidolensis zooids reached it. This is similar to the situation in the cornulitid–Mesotrypa excentrica association in the Upper Ordovician of Estonia (Vinn et al.2018a). Both F. przhidolensis and Cornulites sp. were suspension feeders; if they fed on particles of similar size, there may have been competition for the food (Vinn et al.2018a). In the case of food competition, this association could have been slightly harmful to F. przhidolensis and Cornulites sp. may have been a kleptoparasite similar to Cornulites sp. in the cornulitid–Mesotrypa excentrica association from the Upper Ordovician of Estonia (Vinn et al.2018a). Alternatively, there may have been scramble competition between cornulitids and their host, a relationship that has been reported from a brachiopod–auloporid association (Zapalski 2005). The auloporids probably used water currents produced by the brachiopod's lophophore, impoverishing the host's food availability. Their relationship can therefore be described as scramble competition (Zapalski 2005). Parasitic cornulitids became common in the Devonian where they often formed associations with rhynchonellate brachiopods (Richards 1974; Vinn 2010).
Another possibility is that the larger and taller filter-feeding apparatus of Cornulites may have created significant downward currents, forcing more food-bearing waters onto the underlying surface of bryozoan zooids. This would thus be a mutualistic relationship.
The Cornulites sp.–F. przhidolensis association described here may have been an accidental association, even if symbiotic and parasitic. There is no record of solitary Cornulites sp. from the Kaugatuma Formation (see Vinn & Wilson 2013), but it is not certain whether this symbiotic association was obligatory or facultative for the cornulitid. Bryozoan specimens from the Kaugatuma Formation do not usually have cornulitid symbionts, so this association was likely facultative for the bryozoan.

Distribution of cornulitid–bryozoan symbiotic associations

The Cornulites sp.–F. przhidolensis association is the youngest example of cornulitid–bryozoan symbiosis. Previously, cornulitid–bryozoan symbiosis was known only from the Ordovician. There is only one published record of cornulitid–bryozoan symbiosis from the Late Ordovician of Baltica (Vinn et al.2018a). However, similar cornulitid–bryozoan associations occur also in the Cincinnatian of North America (O. Vinn and M. Wilson personal observations). It is likely that the cornulitid–bryozoan association may actually have a much wider distribution and be more common than previously known.

Symbiotic endobionts among Silurian cornulitids

In the Silurian of Baltica, cornulitids formed symbiotic associations with large stromatoporoids. The cornulitid–stromatoporoid associations are common in the Sheinwoodian of Saaremaa, Estonia (Vinn & Wilson 2010). A cornulitid–tabulate association has been described from the earliest Silurian of North America (Dixon 2010). Partially embedded cornulitids occur on crinoid stems in the Silurian of Gotland (Franzén 1974). Thus, Silurian cornulitids were prone to symbiosis with other invertebrates. There may have been a slight escalation in the evolution of the endobiotic life mode in cornulitids as the number of such associations increases from the Ordovician (Dixon 2010; Vinn & Mõtus 2012; Vinn et al.2018a) to the Silurian (Franzén 1974; Dixon 2010; Vinn & Wilson 2010; Vinn & Wilson 2016). The latter fact supports predator-driven evolution of endobiotic life modes among cornulitids (Vinn 2009, 2010). Predation pressure increased over this time interval (Signor & Brett 1984; Brett & Walker 2002; Huntley & Kowalewski 2007) and caused defensive behaviours and morphologies to become more common among prey organisms (Vermeij 1977, 1987; Vermeij et al.1981).

Symbiosis in Palaeozoic bryozoans

The earliest known bryozoan endobionts are worm-like Anoigmaichnus bioclaustrations that occur in Mesotrypa bystrowi from the Middle Ordovician of Estonia (Vinn et al.2014). Anoigmaichnus bioclaustrations (i.e. A. odinsholmensis, A. bretti and A. zapalskii) are common in the Upper Ordovician of Estonia (Vinn et al.2018b). Bioclaustrations made by ascidian tunicates or colonial hydroids occur in the bryozoans of the Upper Ordovician of North America (Palmer & Wilson 1988). Skeletal endobionts, such as rugosans, conulariids and cornulitids, are also common in Upper Ordovician bryozoans (Vinn et al.2019). In contrast, symbiosis in Silurian bryozoans has not received attention. It is most likely a study bias and does not indicate real decline in bryozoan symbiosis after the Ordovician. More data are available about symbiotic bryozoans in the Devonian when bryozoans formed symbiotic associations with Chaetosalpinx (a bioclaustration) (Ernst et al.2014), rugosans (Sendino et al.2019; Plusquellec & Bigey 2019) and tabulate corals (McKinney et al.1990). However, the data for the Devonian are still limited as compared to the Upper Ordovician. Late Palaeozoic bryozoan symbiosis is again an unstudied topic. Regardless of obvious study bias, it is possible that Palaeozoic bryozoan symbiosis could have reached its maximum in the Late Ordovician, though there are many palaeontological sites to be discovered that could provide more cases in the late Palaeozoic.
Most symbiotic bryozoans in the Palaeozoic are trepostomes (18 associations, with 72 genera according to Paleobiology Database 2020), while cystoporates were less symbiotic (seven associations, with 47 genera according to Paleobiology Database 2020) (Palmer & Wilson 1988; McKinney et al.1990; Ernst et al.2014; Vinn et al.2014; Vinn et al.2018b; Sendino et al.2019; Plusquellec & Bigey 2019; Vinn et al.2019). The other bryozoans are not known to have formed symbiotic associations in the Palaeozoic. The diversity of symbiotic associations was also greatest among trepostomes, which lived together with worm-like organisms (various bioclaustrations), conulariids, rugosans, tabulates and cornulitids. Cystoporates formed symbiotic associations only with rugosans, worm-like organisms (i.e. Chaetosalpinx bioclaustrations) and cornulitids. The reasons why trepostomes were the most symbiotic group among the Palaeozoic bryozoans are unknown, but may have something to do with their colony architecture.


Financial support to O.V. and U.T. was provided by Estonian Research Council grants (IUT20-34 and PRG836). We are also grateful to G. Baranov, Department of Geology, Tallinn University of Technology, for digital photographing of the specimen. This paper is a contribution to project IGCP 653 ‘The onset of the Great Ordovician Biodiversification Event’.


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Information & Authors


Published In

Go to Lethaia
Volume 54Number 11 January 2021
Pages: 9095


Received: 8 January 2020
Accepted: 3 April 2020
Published online: 11 May 2020
Issue date: 1 January 2021



Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A 50411, Tartu, Estonia;
Institut für Geologie, Universität Hamburg, Bundesstr. 55 20146, Hamburg, Germany;
Department of Earth Sciences, The College of Wooster, Wooster, OH 44691, USA;
Ursula Toom [email protected]
Department of Geology, Tallinn University of Technology, Ehitajate tee 5 19086, Tallinn, Estonia;

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