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Symbiotic endobiont biofacies in the Silurian of Baltica
Olev Vinn a,⁎, Mark A. Wilson b, Mari-Ann Mõtus c
a Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estoniab Department of Geology, The College of Wooster, Wooster, OH 44691, USAc Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia
The distribution of symbiotic endobionts in Silurian stromatoporoids of Estonia is correlatedwith thediverse sed-imentary facies formed in this portion of the Baltica palaeocontinent. These depositional environments are char-acterized by different symbiotic endobiont associations. There are two onshore shallow water and one offshoredeeper water symbiotic endobiont associations. Water depth was not the only controlling factor for their distri-bution: seawater nutrient levels, hydrodynamics (especially substrate stability), sedimentation rates and distri-bution of stromatoporoid hosts may have also played important roles.
The earliest macroscopic endobiotic invertebrate symbionts areknown from the Late Ordovician of North America and Baltica (Elias,1986; Tapanila, 2005; Dixon, 2010; Vinn and Mõtus, 2012). Theseendobionts are among the best examples of symbiotic interactions inthe fossil record (Taylor, 1990; Taylor and Wilson, 2002). Symbioticendobionts are usually completely embedded in the tissues of a hostorganism, except for an opening on the host surface for feeding. Skeletalendobionts have their own mineral wall separating them from thetissues of the host. Endobionts without mineralized skeletons canleave a living cavity within the hard tissues of the host skeleton. Thecavities left by embedment are termed bioclaustrations (Palmer andWilson, 1988; Tapanila, 2005). Originally Sokolov (1948) interpretedbioclaustrations as the traces of commensal endobionts. The laterstudies have suggested a parasitic nature (i.e., the organisms have aharmful effect on the host) for most of these traces (Stel, 1976;Zapalski, 2007, 2009, 2011; Zapalski and Benoit, 2011).
Worms (Vinn et al., in press), rugosans (Nestor, 1966; Vinn et al., inpress), syringoporids (Nestor, 1966) and cornulitids (Vinn and Wilson,2010) occur as the endobiotic stromatoporoid symbiont bioclaustrationsin the Silurian of Estonia. All these endobiont groups first appeared in theOrdovician (Scrutton, 1997; Tapanila, 2005; Vinn, 2010). Palaeozoicworm bioclaustrations range into the Late Devonian (Zapalski et al.,2008), cornulitids into the Late Carboniferous (Vinn, 2010), and rugosans
wooster.edu (M.A. Wilson),
and syringoporids into the Permian (Scrutton, 1997). Stromatoporoidsthemselves have a stratigraphic range from the Ordovician through theDevonian (Stock, 2001).
Recent symbiotic polychaetes often produce habitation tunnels verysimilar to the worm bioclaustrations of the Palaeozoic (Tapanila, 2005).Thus, it is likely that at least some of the Palaeozoicwormbioclaustrationsmay have also beenmade by polychaete annelids. However, without dataon soft body anatomy, the zoological affinities of these ancientwormswillremain unresolved. Both syringoporids and rugosans are corals, thoughthey are not direct ancestors ofmodern corals (Scrutton, 1997). Cornulitidtubeworms have recently been classified as encrusting tentaculitoids(Vinn, 2010). Cornulitids were common encrusters on various biogenicsubstrates, especially in the middle Paleozoic (Zatoń and Borszcz, 2013).They are presumably ancestors of free-living tentaculitoids (Vinn andMutvei, 2009; Vinn, 2010). The biological affinities of cornulitids havelong been debated. Recently Vinn and Zatoń (2012) showed thatthey most likely belong to the Lophotrochozoa, and could representstem-group phoronids (Taylor et al., 2010).
There are sclerobiofacies of encrusting and endolithic communitieson shells in the geological past and in modern seas (Brett et al., 2011,2012). The taxonomic composition of sclerobiont suites has a predict-able variation in marine environments (e.g., based upon depth), butthese sclerobiofacies are primarily useful within local areas and limitedtime frames (Brett et al., 2011, 2012). There is no published synthesis ofthe facies distribution of symbiotic endobionts in the Silurian. However,it is possible that symbiotic endobionts may form various biofacies inthe Silurian analogous to the bioeroding organisms and epibionts.
The aims of this paper are: 1) to describe the symbiotic endobiontassociations of stromatoporoids in the Silurian of Saaremaa (Baltica),
and 2) to discuss whether the distribution of these symbioticendobionts of stromatoporoids depended on sedimentary rocks.
2. Geological background
The Silurian System is represented by various sedimentary rocks inEstonia, but carbonates dominate in shallow water settings (Fig. 1).The exposed strata range from the Rhuddanian to the Pridoli (Fig. 2).In the outcrop area, including Saaremaa and Hiiumaa islands, theSilurian succession is represented by shallow water carbonate rocksrich in shelly faunas (Hints et al., 2008).
During the Silurian, Baltica was located in equatorial latitudes andthereafter drifted northwards (Melchin et al., 2004). The area ofmodernEstonia was variously part of the epicontinental Baltic paleobasin. Thisbasin was characterized by a wide range of tropical environments anddiverse biotas, especially shelly faunas (Hints et al., 2008). Nestor andEinasto (1977) recognized five depositional environments in the BalticSilurian Basin. The three most onshore environments formed a carbon-ate platform containing tidal flat/lagoon, shoal, and open shelf facieszones. Two offshore environments (i.e. basin slope, and a basin depres-sion facies zones) formed a deeper basin with fine-grained siliciclasticdeposits (Raukas and Teedumäe, 1997).
3. Material and methods
Stromatoporoids fromHilliste Quarry (N=40), Panga cliff (N=43),Undva cliff (N = 20), Suuriku cliff (N = 6), Abula cliff (N = 60), Katricliff (N = 18) and Kaugatuma cliff (N = 18) were searched forendobionts by external observation with magnifying lenses andby breaking them with a hammer (Fig. 1). Several stromatoporoidspecimens with endobionts were cut longitudinally and transverselyin the laboratory with a rock saw. Longitudinal and transverse sectionswere then polished and photographed with a Leica IC80 HD digitalcamera. Several thin-sections were made from both transverse andlongitudinal sections. All thin-sections were scanned using an Epson3200 optical scanner. The number of stromatoporoids infested byparticular endobionts was recorded, as were the number of endobiontspecimens per stromatoporoid host. Areal coverage of stromatoporoidsby the endobionts was calculated only for rugosans from Abula cliff.
Fig. 1. Location of outcrops on the Saaremaa and Hiiumaa islands, Estonia. 1
3.1. The lithologic characteristic of the studied beds
Hilliste quarry, Hiiumaa Island (Rhuddanian, Hilliste Formation):Moderately thick horizontal layers of bluish grey packstones with thinmarl interlayes.Massive coral and stromatoporid skeletons are commonin the section, but they are not interconnected.
Panga cliff, Saaremaa Island (lower Sheinwoodian,MustjalaMember):Bluish grey dolomitic marlstone with argillaceous dolostone interbedsand nodules.
Undva cliff, Saaremaa Island (lower Sheinwoodian, MustjalaMember): Blue–green marlstones containing nodules of biomicriticlimestone. The unit contains abundant brachiopods, crinoids,bryozoans, corals and stromatoporoids.
Suuriku cliff, Saaremaa Island (lower Sheinwoodian, MustjalaMember): Blue–green marlstones containing nodules of biomicriticlimestone. The unit contains abundant corals (especially halysitids)and stromatoporoids.
Abula cliff, Saaremaa Island (upper Sheinwoodian, Maasi Beds):Light-grey wavy-bedded pelletal limestones with several discontinuitysurfaces. Stromatoporoids and brachiopods are common.
Katri cliff, Saaremaa Island (Ludfordian, Uduvere Beds):Section contains a biostrome rich in argillaceous material and withlenses and/or irregular lensoidal interbeds of light-beige pelletallimestone. Stromatoporoids, corals and cephalopods are common.
Kaugatuma cliff, Saaremaa Island (lower Pridoli): Section containsgreenish-grey argillaceous limestones and marls. The marl and argilla-ceous limestone layer is extremely rich in fossils and contains abundantin situ buried large crinoid holdfasts (Enallocrinus sp.). It also contains insitu buried tabulate corals and stromatoporoids.
4. Results
Symbiotic endobionts range from theRhuddanian to thePridoli in theSilurian of Estonia (Baltica). They occur in Silurian stromatoporoids ofEstonia throughout several facies zones. Packstones of theHilliste Forma-tion (exposed in the Hilliste quarry, Rhuddanian, of Hiiumaa) (Fig. 1)yielded a single stromatoporoid specimen with 15 Chaetosalpinx? sp.bioclaustrations (Vinn et al., in press) (Fig. 2, Table 1). Marlstones ofthe Mustjala Member (Undva cliff, Panga cliff, Suuriku cliff; all earlySheinwoodian) yielded 19 stromatoporoids (of 69 examined) withcornulitid endobionts Cornulites stromatoporoides (one to twenty per
Fig. 2. Stratigraphic distribution of symbiotic endobionts in the Silurian of Estonia. Localities and lithology of the studied layers: 1 Hilliste (packstones); 2 Panga (dolomitic marlstones); 3Undva (marlstones containing nodules of biomicritic limestone); 4 Suuriku (marlstones containing nodules of biomicritic limestone); 5 Abula (pelletal limestones); 6 Katri(stromatoporoid biostrome, rich in argillaceous material in some parts, and contains lenses of pelletal limestone); 7 Kaugatuma (marl and argillaceous limestone).
26 O. Vinn et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 24–29
host) (Vinn andWilson, 2010) (Figs. 2–3, Table 1). Pelletal limestones ofthe Jaagarahu Formation (Abula cliff, late Sheinwoodian) yielded 48stromatoporoids (of 60 total) with endobionts. The endobionts includedthe abundant C. stromatoporoides (one to three per host) in 75% (N=60)of stromatoporoids and rare rugosans (several hundred per host, ten to20 endobiotic rugosanswere counted per 4 cm2) in 5% of stromatporoids(N = 60) (Vinn and Wilson, 2010, 2012a) (Figs. 2, 4, Table 1). Thebiostromal limestones of the Paadla Formation (Katri cliff, Ludfordian)yielded stromatoporoids with rugosan (in three of 18 stromatoporoids,tens of specimens per host) and syringoporid endobionts (in four of 18stromatoporoids, hundreds of corallites per host) (Figs. 2, 5, Table 1),and abundant worm-like bioclaustrations without mineral tubes (in 14stromatoporoids of 18 with tens of bioclaustrations per host) (i.e.,Chaetosalpinx sp. and Helicosalpinx sp.) (Nestor, 1966; O. Vinn personalobs.) (Figs. 2, 6, Table 1). Packstones of the Kaugatuma Formation(Kaugatuma cliff, early Pridoli) yielded one stromatoporoid (of 18 exam-ined) with a single endobiotic rugosan (Vinn and Wilson, 2012b)(Table 1).
5. Discussion
5.1. Depositional environments
The packstones of the Hilliste Formation (Rhuddanian) areinterpreted as having been deposited in an onshore shallow sea envi-ronment due to the occurrence of normal marine fauna and depositionof calcareous skeletal debris in large accumulations. Marls of theMustjala Member presumably formed offshore in a relativelydeeper sea environment with relatively calm waters (no overturnedstromatoporoids were found). Pelletal limestones of the JaagarahuFormation likely represent an onshore shallow sea environment(“shoal” after Einasto, 1990) with relatively high hydrodynamic activity(overturned stromatoporoids occur, about 10–20%). The biostromallimestones of the Paadla Formation appear to have formed in anonshore shallow sea environment. The packstones and wackestonesof the Kaugatuma Formation formed in an onshore environmentof a relatively shallow sea with high-energy waters (overturned
Table 1Distribution of endobionts in the Silurian stromatoporoids of Estonia. Data on stromatoporoid taxa are from Nestor (1964, 1966) and S. Kershaw (personal comm. 2013).
Locality/endobiont Number of stromatoporids/stromatoporoid diameters
Number of infestedstromatoporoids
Infested stromatoporoid species(number of stromatoporoid specimens)
Stromatoporoid taxa of the locality
Hilliste quarry 40 (15–25 cm) Pachystylostroma ungeri, P. hillistense, P. exile,Clathrodictyon boreale, C. sulevi, C. sarvense,C. lennuki, C. demissum, Ecclimadictyon porkuni,E. microvesiculosum
Worm bioclaustrations 1 (2.5%) –
Panga cliff 43 (15–60 cm) Petridiostroma simplex, Densostroma pexisum,Eostromatopora impexa
stromatoporoids are common) (see Kaljo, 1970, for review of sedimen-tary environments in the Silurian of Estonia).
5.2. Symbiotic endobiont associations
1) Association A is characterized by the co-occurrence of abundantcornulitid (in 75% of stromatoporoids) and rare rugosan endobionts(in 5% of stromatoporoids). This association is established on thestromatoporoid endobiont fauna in the Jaagarahu Formation atAbula cliff (Figs. 1, 2, Table 1). Chaetosalpinx? bioclaustrations in aHilliste Formation stromatoporoid from the Hilliste Quarry onHiiumaa Island may also belong to this association (Figs. 1–2).Association A occurred in an onshore shallow sea with a carbonatesand and mud bottom.
2) Association B is indicated by the co-occurrence of abundantendobiotic rugosans, syringoporids and abundant bioclaustrationsof worm-like organisms (in 77.8% stromatoporoids) withoutmineralized tubes (i.e., Chaetosalpinx sp. and Helicosalpinx sp.). Thisassociation is established on the stromatoporoid endobiont faunain the Paadla Formation exposed at Katri cliff (Figs. 1–2, Table 1).Association B inhabited an onshore shallow sea in biostromes andpossibly also reefs. A similar association of biostrome stromatoporoidswith endobiotic rugosans and syringoporids has been described fromthe Ludlow of Gotland (Kershaw, 1987). Association B differs fromthat of Gotland only by the presence of relatively abundant wormbioclaustrations.
3) Association C is characterized by the occurrence of abundantendobiotic cornulitids (in 16.7–30.2% of stromatoporoids) (Table 1).This association is established on the stromatoporoid endobiontfauna in the Mustjala Member at Panga, Suuriku and Undva cliffs(Fig. 1). Association C inhabited a relatively deep-water offshoreenvironment with a siliciclastic muddy bottom.
5.3. Depth distribution of symbiotic endobionts
The exact water depths of deposition have not been estimated forthe studied sections of the Baltica palaeobasin in the Silurian of Saare-maa, but they were all less than 200 m deep (Kaljo, 1978). Most likelynone were deeper than 90 m (Kaljo, 1978). Endobiotic rugosans andsyringoporids occur only in relatively shallow waters; however, theirfossils are not restricted to onshore environments. The same may betrue for tubeless worm-like bioclaustrations (i.e. Chaetosalpinx?,Helicosalpinx?) in stromatoporoids that have only been found in shallowwater sediments. Thus, it is possible that rugosans and syringoporidsformed symbiotic associations with stromatoporoids only in shallowwaters. In contrast, endobiotic cornulitids occur in both shallowand relatively deep water. The occurrence of symbioses betweenstromatoporoids and endobiotic cornulitids was therefore not entirelycontrolled by water depth. The diversity of symbiotic endobiont groupswas greatest in shallow water (associations A (number of groups two)and B (number of groups three)) and lowest in relatively deep water(association C; number of groups one).
5.4. Controlling factors on symbiotic endobiont facies distribution
Symbiotic endobiont associations (A and B) of both shallow seafacies are different, indicating that thewater depth alone did not controlthe distribution of symbiotic endobionts. Seawater nutrient levels andhydrodynamics (such as substrate stability) and sedimentation mayhave also played an important role.
Modern coral reefs thrive in nutrient poor waters (Muscatine andPorter, 1977). If this was also true for Silurian reefs and biostromes(Kershaw, 1993), differences in nutrient levels may explain the taxo-nomic differences between shallow sea and biostrome symbioticendobiont associations. Cornulitids that are absent in the biostromeassociation may have preferred the more nutrient rich waters of other
Fig. 3. Cornulites stromatoporoides from the Jaani Formation (Sheinwoodian) of Panga cliff,Saaremaa. Scale bar 10 mm. TUG-1328-1.
Fig. 5. Syringoporids in a stromatoporoid from the Paadla Formation (Ludfordian) of Katricliff, Saaremaa. Scale bar 10 mm. TUG 1653-1.
28 O. Vinn et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 24–29
onshore shallow seafloors and shoals. Alternatively, the taxonomic dif-ferences may be explained by different stromatoporoid hosts in thesetwo settings if the endobionts were host-specific.
Why are endobiotic rugosans more abundant in biostromes(association B) and syringoporids lacking in the shallow water associa-tion (A)?Modern reef environments are very diverse ecosystems. If thiswas also true for the Silurian reefs and biostromes in Baltica, it maywellexplain the highest diversity of endobionts at group and possiblyspecies level in the biostrome association (B). The higher abundanceof endobiotic rugosans in the biostrome association (B) as comparedto shallow water association (A) could be explained by the specificfavorable environmental conditions of the biostrome environment.These favorable conditions may include a lack of space competition bycornulitid endobionts (in maximum up to 20 individuals per host) anda large number of available stromatoporoids in a small area (up to 15stromatoporoids per 1 m2). The endobionts may have also preferredthe possible lower sedimentation rates in the biostromes (Kershaw,1993) as compared to other areas of shallow sea.
Fig. 4. Palaeophyllum sp. in Ecclimadictyon astrolaxum from the Jaagarahu Formation ofAbula cliff, Saaremaa. Scale bar 5 mm. TUG1627-4.
In addition to environmental conditions, the taxonomy of the hoststromatoporoids may have influenced the taxonomic composition oftheir endobiont fauna. Different stromatoporoid taxa have a differentfacies distribution in the Silurian of Baltica (Nestor, 1999). It is unlikely,but possible, that stromatoporoidsmay have also been selectively toler-ant with regard to their endobionts. Even if the facies distribution ofstromatoporoid species had some effect on the taxonomic compositionof their endobiont associations, it was not strong as there are endobionttaxa that occur both indeeperwater and shallowwater stromatoporoids,such as C. stromatoporoides.
Acknowledgments
We are grateful to Dimitri Kaljo and Heldur Nestor for identificationof rugosan and stromatoporoid from Abula cliff and Stephen Kershawfor identification of stromatoporoids from Katri cliff. O.V. is indebtedto the Sepkoski Grant (Paleontological Society), Estonian Science Foun-dation grant ETF9064, Estonian Research Council grant IUT20-34 andthe target-financed project (from the Estonian Ministry of Educationand Science) SF0180051s08 (Ordovician and Silurian climate changes,as documented from the biotic changes and depositional environmentsin the Baltoscandian Palaeobasin) for financial support. M.W. wassupported by a grant from the National Geographic Society and FacultyDevelopment funds at The College of Wooster. M-A.M. was supportedby the target-financed project (from the EstonianMinistry of Education
Fig. 6. Helicosalpinx sp. in a stromatoporoid from the Paadla Formation (Ludfordian)of Katri cliff, Saaremaa. Scale bar 2 mm. GIT-656-82.
and Science) SF0140020s08. This paper is a contribution to IGCP 591“The Early to Middle Palaeozoic Revolution”.
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