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, Estonia b Department of Geology, The College of Wooster, Wooster, OH 44691, USA c Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia abstract article info Article history: Received 11 December 2013 Received in revised form 18 March 2014 Accepted 25 March 2014 Available online 4 April 2014 Keywords: Symbiosis Stromatoporoids Endobiont Facies Estonia The distribution of symbiotic endobionts in Silurian stromatoporoids of Estonia is correlated with the diverse 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 offshore deeper 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The earliest macroscopic endobiotic invertebrate symbionts are known from the Late Ordovician of North America and Baltica (Elias, 1986; Tapanila, 2005; Dixon, 2010; Vinn and Mõtus, 2012). These endobionts are among the best examples of symbiotic interactions in the fossil record (Taylor, 1990; Taylor and Wilson, 2002). Symbiotic endobionts are usually completely embedded in the tissues of a host organism, except for an opening on the host surface for feeding. Skeletal endobionts have their own mineral wall separating them from the tissues of the host. Endobionts without mineralized skeletons can leave a living cavity within the hard tissues of the host skeleton. The cavities left by embedment are termed bioclaustrations (Palmer and Wilson, 1988; Tapanila, 2005). Originally Sokolov (1948) interpreted bioclaustrations as the traces of commensal endobionts. The later studies have suggested a parasitic nature (i.e., the organisms have a harmful 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., in press), syringoporids (Nestor, 1966) and cornulitids (Vinn and Wilson, 2010) occur as the endobiotic stromatoporoid symbiont bioclaustrations in the Silurian of Estonia. All these endobiont groups first appeared in the Ordovician (Scrutton, 1997; Tapanila, 2005; Vinn, 2010). Palaeozoic worm bioclaustrations range into the Late Devonian (Zapalski et al., 2008), cornulitids into the Late Carboniferous (Vinn, 2010), and rugosans and syringoporids into the Permian (Scrutton, 1997). Stromatoporoids themselves have a stratigraphic range from the Ordovician through the Devonian (Stock, 2001). Recent symbiotic polychaetes often produce habitation tunnels very similar to the worm bioclaustrations of the Palaeozoic (Tapanila, 2005). Thus, it is likely that at least some of the Palaeozoic worm bioclaustrations may have also been made by polychaete annelids. However, without data on soft body anatomy, the zoological affinities of these ancient worms will remain unresolved. Both syringoporids and rugosans are corals, though they are not direct ancestors of modern corals (Scrutton, 1997). Cornulitid tubeworms have recently been classified as encrusting tentaculitoids (Vinn, 2010). Cornulitids were common encrusters on various biogenic substrates, especially in the middle Paleozoic (Zatoń and Borszcz, 2013). They are presumably ancestors of free-living tentaculitoids (Vinn and Mutvei, 2009; Vinn, 2010). The biological affinities of cornulitids have long been debated. Recently Vinn and Zatoń (2012) showed that they most likely belong to the Lophotrochozoa, and could represent stem-group phoronids (Taylor et al., 2010). There are sclerobiofacies of encrusting and endolithic communities on 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), but these sclerobiofacies are primarily useful within local areas and limited time frames (Brett et al., 2011, 2012). There is no published synthesis of the facies distribution of symbiotic endobionts in the Silurian. However, it is possible that symbiotic endobionts may form various biofacies in the Silurian analogous to the bioeroding organisms and epibionts. The aims of this paper are: 1) to describe the symbiotic endobiont associations of stromatoporoids in the Silurian of Saaremaa (Baltica), Palaeogeography, Palaeoclimatology, Palaeoecology 404 (2014) 24–29 ⁎ Corresponding author. E-mail addresses: olev.vinn@ut.ee (O. Vinn), mwilson@wooster.edu (M.A. Wilson), motus@gi.ee (M.-A. Mõtus). http://dx.doi.org/10.1016/j.palaeo.2014.03.041 0031-0182/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo