Earliest known rugosan-stromatoporoid symbiosis from the Llandovery of Estonia (Baltica) Olev Vinn a, ⁎, Mark A. Wilson b , Ursula Toom c , 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 3 March 2015 Received in revised form 8 April 2015 Accepted 21 April 2015 Available online 30 April 2015 Keywords: Symbiosis Bioclaustrations Stromatoporoids Rugosans Baltica Silurian A stromatoporoid, Petridiostroma simplex, from the Llandovery of Estonia was infested by numerous rugosan en- dobiotic symbionts of the species Petrozium losseni (Dybowski, 1874). These rugosans presumably benefitted from the stable growth substrate provided by the stromatoporoid. The effects of the endobiotic rugosans on the stromatoporoid are not known, but it is possible that they reduced its feeding efficiency. The relatively thick skeletons of the rugosans could indicate a short evolutionary history for this symbiotic association. The elevation of the symbionts' apertures above the host stromatoporoid may have been to achieve a feeding advan- tage if the host stromatoporoid and rugosans competed for nutrients. This record and others suggest that com- plex ecological interactions such as symbiosis were common among the macroscopic invertebrates of the Ordovician–Silurian mass extinction recovery fauna. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Endobiotic rugosans often had symbiotic relationships with stromatoporoids, and such interactions are especially common in the Silurian of Baltica (Kershaw, 1987; Vinn and Wilson, 2012; Vinn and Mõtus, 2014a). In addition to rugosans, the tabulate coral Syringopora is also a common endobiotic symbiont in Silurian stromatoporoids of Baltica (Kershaw, 1987; Vinn et al., 2014). The Silurian of Baltica has a relatively rich record of symbiotic interactions between various other macroscopic invertebrates (Kershaw, 1987; Vinn and Wilson, 2010, 2012; Vinn and Mõtus, 2014a,b). Vinn and Wilson (2010) recently de- scribed a symbiotic cornulitid and stromatoporoid association from the late Sheinwoodian of Estonia in which they recorded large numbers of stromatoporoids (77% of the preserved population) as infested by cornulitid endobionts. Syn vivo interactions between different organisms are rather rare in the fossil record. The best studied examples comprise various predatory borings. Similarly important are the endobionts embedded (i.e. bioimmured) by the living tissues of host organisms (see Taylor, 1990 for a review). Microscopic invertebrate symbionts are known from the Cambrian (Bassett et al., 2004). Macroscopic endobiotic invertebrate symbionts appeared later in the Late Ordovician (see Tapanila, 2005 for a summary). Palaeozoic rugosans were sometimes bioimmured by living tissues of stromatoporoids or corals; they differ from bioclaustrations (Palmer and Wilson, 1988) by having their own skeleton. Endobiotic lingulid brachiopod symbionts in stromatoporoids appeared earlier (i.e. Llandovery) than rugosan symbionts (i.e. Wenlock until this work) and are the earliest known skeletal endobiotic symbi- onts of stromatoporoids (Tapanila, 2005). Lingulids often occupied empty borings in the stromatoporoids, and in some cases these borings have been overgrown indicating syn vivo interaction (Stewart et al., 2010). The fauna of stromatoporoids and rugose corals of the Silurian of Estonia is relatively well studied (Nestor, 1964, 1966; Kaljo, 1958). The symbiotic interactions between different animal groups must obviously be after their first evolutionary appearance. In order to better understand the evolution of symbiosis it is important know the times these interactions appeared. The aim of this paper is: 1) to describe the earliest known rugosan symbionts in stromatoporoids of the Silurian of Baltica; and 2) to discuss the palaeoecology of this rugosan-stromatoporoid association. 2. Geological background and localities During the Silurian the Baltica continent was located in equatorial latitudes and drifting northwards (Melchin et al., 2004). The area of modern Estonia was mostly covered by the shallow epicontinental Palaeogeography, Palaeoclimatology, Palaeoecology 431 (2015) 1–5 ⁎ Corresponding author. E-mail addresses: olev.vinn@ut.ee (O. Vinn), mwilson@wooster.edu (M.A. Wilson), ursula.toom@ttu.ee (U. Toom), motus@gi.ee (M.-A. Mõtus). http://dx.doi.org/10.1016/j.palaeo.2015.04.023 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo