RUGOSAN EPIBIONTS ON VERTICAL STEMS FROM THE LUDLOW AND PRIDOLI OF SAAREMAA, ESTONIA (BALTICA) OLEV VINN 1 AND URSULA TOOM 2 1 Department of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia 2 Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia e-mail: olev.vinn@ut.ee ABSTRACT : The earliest known rugosans attached syn vivo to vertical stems occur in the late Silurian of Saaremaa, Estonia. These rugosans display vertical to subvertical attachment scars and are more common in the Ludfordian than in the Pridoli. The unknown hosts provided a higher tier for the feeding, making the association beneficial for the rugosans. Several rugosans were themselves syn vivo encrusted by bryozoans and unknown endobiotic tubicolous organisms, possibly cornulitids. Estonian rugosans appear to have been host size selective and preferred substrates of certain size. Silurian symbiotic rugosans are more often endobionts in stromatoporoids than epibionts on the vertical stems. INTRODUCTION Syn vivo interactions between different extinct organisms are seldom un‐ equivocally identifiable in fossil material. The best studied examples of syn vivo interactions comprise various predatory borings (Huntley and Kowalewski 2007) and endobionts bioimmured (i.e., embedded) by the living tissues of host organisms (see Taylor 1990 for a review). Although the earliest microscopic invertebrate symbionts are known from the Cambrian (Bassett et al. 2004), macroscopic invertebrate symbionts are not common in the fossil record until the Late Ordovician, mostly as bioclaustrations (Palmer and Wilson 1988; Tapanila 2005). Rugosans have often been found bioimmured by living tissues of stromatoporoids or corals (Vinn and Wilson 2012; Vinn and Mõtus 2014; Vinn et al. 2015), but although the fauna of rugose corals from the Silurian of Estonia is relatively well studied (Kaljo 1958), there is no published record of syn vivo rugosan epibionts from the Silurian of Estonia and relatively little is known about symbioses involving Silurian rugosans in general (Kershaw 1987; Vinn and Wilson 2012; Vinn and Mõtus 2014; Vinn et al. 2015). The aims of this paper are to describe the earliest known rugosan symbionts on vertical stems of unknown organisms from the Silurian of Baltica and to dis- cuss the paleoecology of this rugosan-host association. GEOLOGICAL BACKGROUND During the Silurian, the area of present-day Estonia (Fig. 1) was part of the Baltica paleocontinent, which was located in equatorial latitudes (Melchin et al. 2004). The middle and western part of modern Estonia, including Saaremaa Island, was covered by the epicontinental Baltic paleobasin. This basin was characterized by diverse tropical faunas and extensive shelf environments (Hints 2008; Raukas and Teedumäe 1997). The sedimentary facies of the Silu‐ rian of Estonia are divided laterally into five facies belts along a reconstructed bathymetric profile (Nestor and Einasto 1977). The first three environments (i.e., tidal flat/lagoonal, shoal, and open shelf) comprised a carbonate platform, whereas the latter two (i.e., basin slope and basin depression) formed at greater water depths characterized by fine siliciclastic deposits. The tidal flat/lagoonal, shoal, open shelf, and basin slope sediments are represented in outcrops on Saaremaa Island, but rugosans occur only in rocks of shoal, open shelf, and basin slope origin. The Ludfordian fauna of studied localities includes rugosans, tabulates, stromatoporoids, trilobites, ostracodes, fish, conodonts, nautiloids and microconchids. The rugosans are most abundant among the Ludfordian fauna. The Pridoli fauna of studied localities includes rugosans, tabulates, stromatoporoids, brachiopods, crinoids, bryozoans, trilobites, ostra- codes, conodonts, and microconchids. MATERIAL AND METHODS A collection of rugosans (N559) from the late Silurian of Saaremaa (Figs. 1, 2) contains specimens with circular attachment scars. Fifty six (56) rugosans originate from four Ludfordian (Kuressaare Regional Stage) localities: Ilpla (N513), Kuressaare (N510), Muratsi (N530) and Kärsa (N53). The remain- ing three rugosans come from the lower Pridoli (Kaugatuma Regional Stage) localities: Väike Rootsi (N51) and Kaugatuma cliff (N52). Both Ludfordian and Pridoli rugosans were from open shelf argillaceous limestones and marls. The rugosans with attachment scars were photographed using a Nikon D7000 digital camera. The diameter and length of specimens as well as the diameter of attachment scars were measured from calibrated photographs. Thir- teen rugosans were thin sectioned and identified to genus level. All epibionts cemented to the rugosans were counted and identified to the finest taxonomic level possible. The studied specimens are deposited at the Institute of Geology, Tallinn University of Technology (GIT). RESULTS Fifty nine (59) rugosans of the total examined (391) have a single lateral scar with vertical to subvertical orientation (Figs. 3A–3F, 4A, 4B, 5A–5D); in one case, two merged rugosans have a common scar. Scars are sometimes complete cylindrical shafts through the lateral part of the corrallum, but more commonly they are semicylindrical furrows (Neumann 1968). Cylindrical shafts and semi- cylindrical furrows are not morphologically variable, have a smooth surface, and do not vary much in size (Fig. 6); they are more common in smaller than larger specimens (Figs. 7, 8). Scars occur in four genera: Entelophyllum Wede- kind 1927, Hedstroemophyllum Wedekind 1927, Kyphophyllum Wedekind 1927, and Pilophyllum Wedekind 1927, but are most numerous in Entelophyllum (seven of 13 identified) (Figs. 3D, 4B). Cylindrical shafts and semicylindrical furrows are more common in the Ludfordian (56 of total 202 rugosans) than PALAIOS, 2016, v. 31, 35–40 Research Article DOI: http://dx.doi.org/10.2110/palo.2015.051 Published Online: February 2016 Copyright E 2016, SEPM (Society for Sedimentary Geology) 0883-1351/16/031-35/$03.00