Palaeogeography, Palaeoclimatology, Palaeoecology 431 (2015) 1–5 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo 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 b c Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411 Tartu, Estonia Department of Geology, The College of Wooster, Wooster, OH 44691, USA Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia a r t i c l e i n f o 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 b s t r a c t A stromatoporoid, Petridiostroma simplex, from the Llandovery of Estonia was infested by numerous rugosan endobiotic 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 advantage if the host stromatoporoid and rugosans competed for nutrients. This record and others suggest that complex 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 described 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 symbionts 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 ⁎ 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. 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 2 O. Vinn et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 431 (2015) 1–5 Fig. 1. The location of Laukna quarry and Pae karstfield in Estonia. Baltic Basin (Fig. 1). The Baltic Basin was characterized by a wide range of tropical environments that included coral-stromatoporoid reefs and lagoons. The biota of this basin was diverse (Hints et al., 2008). Nestor and Einasto (1977) described the paleoenvironments of the basin as composed of following facies belts: tidal flat/lagoonal, shoal, open shelf, transitional (i.e. basin slope), and a basin depression. The first three facies belts formed a shallow carbonate shelf (i.e. carbonate platform). The latter two facies belts formed a deeper pericratonic basin where fine-grained clastic deposits were deposited (Raukas and Teedumäe, 1997). Laukna (58.929152, 24.186950) is an old quarry in an ancient coastal terrace, about 5 km to north from Koluvere Castle in western Estonia (Fig. 1). A flaggy micritic and coral-stromatoporoid limestone is exposed from the middle part of the Raikküla Formation (middle Llandovery) (Mõtus and Hints, 2007). Other fauna includes the tabulates Multisolenia tortuosaeformis, Parastriatopora celebrata, Sinopora operta, and the rugosans Dokophyllum sp. Pae karstfield (58.99619, 24.916086) is located in western Estonia (Fig. 1). Limestones of the Raikküla Regional Stage containing stromatoporoids are exposed there (H. Nestor, personal comm.). In addition to stromatoporoids, solitary rugosans also occur at Pae karstfield. (Dybowski, 1874) (Fig. 4). Both of the infested stromatoporoids had numerous endobiotic rugosans (N20) (Fig. 3A). The growth surface of the infested P. simplex is evenly covered with apertures of symbiotic rugosans at various growth stages (Fig. 3A). The apertures of the rugosans are elevated above the growth surface of the host stromatoporoid. There is no regular distribution of the symbiont apertures relative to the stromatoporoid morphology; symbionts are numerous at the edges of stromatoporoid as well as in the central regions. The rugosan symbionts are located perpendicularly to subperpendicularly relative to the growth surface of the stromatoporoid. There is a notable change in orientation of the host stromatoporoid growth lamellae around each rugosan symbiont. The growth lamellae of the 3. Material and methods Thirty stromatoporoids were collected from the Raikküla Regional Stage at the Laukna outcrop and five stromatoporoids from Raikküla Regional Stage of Pae karstfield (Fig. 2, Table 1). The studied stromatoporoids were sectioned using a stone saw, and then three thin-sections were made. The thin-sections were scanned with an Epson 4490 scanner. All of the studied specimens are deposited in the collections of the Institute of Geology, Tallinn University of Technology (GIT). 4. Results Among the total stromatoporoid population collected at the Laukna outcrop, symbiosis with rugosans was rare; only a single stromatoporoid (Petridiostroma simplex) specimen of the 30 studied was infested by the symbiotic rugosan Petrozium losseni (Dybowski, 1874) (Fig. 3). Among the five stromatoporoids from Pae karstfield, one (Clathrodictyon turritum = P. simplex) was infested with P. losseni Fig. 2. The stratigraphy of the Llandovery of Estonia. Location of earliest rugosan symbionts in stromatoporoids marked with asterisk. O. Vinn et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 431 (2015) 1–5 3 Table 1 Locality information of studied stromatoporoids. Locality Species Laukna (Raikküla Regional Stage) Ecclimadictyon macrotuberculatum Intexodictyon avitum Intexodictyon olevi Clathrodictyon turritum = Petridiostroma simplex Ecclimadictyon sp. Unidentified stromatoporoids Clathrodictyon boreale C. turritum = Petridiostroma simplex Ecclimadictyon microvesiculosum Unidentified stromatoporoids Pae karstfield (Raikküla Regional Stage) Number of specimen 6 4 3 2 2 14 1 1 1 2 host stromatoporoid are turned upwards around the upper half of the rugosan symbiont (Fig. 3B). The symbiotic rugosans are continuously surrounded by their own skeleton, without any openings, throughout their growth. The thickness of the symbiotic rugosans skeleton is not noticeably reduced (0.2 to 0.5 mm) (Fig. 3. B–D). 5. Discussion 5.1. Analysis of the association The set of studied specimens is not large enough to determine the exact nature of this association. We are not certain whether the rugosans were obligatory or facultative symbionts in the stromatoporoids. Some solitary rugosans also occur in the studied outcrops, but they do not belong to Petrozium. It is also not clear whether the symbiotic rugosans were species-specific symbionts in Fig. 4. Petrozium losseni (Dybowski, 1874) rugosans in stromatoporoid Petridiostroma simplex, Raikküla Regional Stage, Llandovery of Pae karstfield, western Estonia (GIT 113-37). the stromatoporoids of the Raikküla Regional Stage. However, among the three specimens of P. simplex, two were infested by rugosans, leaving a possibility that rugosans preferred P. simplex over other species of stromatoporoids. The relatively thick skeletons of the symbiotic rugosans (0.2 to 0.5 mm) could indicate a short evolutionary history of this symbiosis, as the formation of a massive skeleton is energetically costly and an unnecessary investment when the host provided skeletal support. Fedorowski and Gorianov (1973) figured nonsymbiotic P. losseni specimens with wall thicknesses of 0.2 to 0.3 mm. Rugosans definitely benefitted from the stable growth substrate provided by the host stromatoporoid. This association was presumably formed for such stability rather than for a higher feeding tier on the Fig. 3. A–D, Petrozium losseni (Dybowski, 1874) rugosans in stromatoporoid Petridiostroma simplex, Raikküla Regional Stage, Llandovery of western Estonia. A, Petrozium losseni (Dybowski, 1874) rugosans in stromatoporoid Petridiostroma simplex, Raikküla Regional Stage, Llandovery of Laukna quarry, western Estonia (GIT 666-5). B, Longitudinal section of Petrozium losseni (Dybowski, 1874) in stromatoporoid Petridiostroma simplex Raikküla Regional Stage, Llandovery of Laukna quarry, western Estonia (GIT 666-5). C, Transverse section of Petrozium losseni (Dybowski, 1874) in stromatoporoid Petridiostroma simplex Raikküla Regional Stage, Llandovery of Laukna quarry, western Estonia (GIT 666-5). D, Transverse section of Petrozium losseni (Dybowski, 1874) in stromatoporoid Petridiostroma simplex Raikküla Regional Stage, Llandovery of Pae karstfield, western Estonia (GIT 113-37). 4 O. Vinn et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 431 (2015) 1–5 sea floor because the symbiont apertures are evenly distributed over the stromatoporoid growth surface without a preference for the topographically higher locations. The elevation of symbiont apertures above the host stromatoporoid surface could have been an antifouling strategy for the symbionts. Alternatively, it may have been related to achieving a feeding advantage if the host stromatoporoid and rugosans competed for nutrients. The antifouling strategy seems less likely because in that case the host stromatoporoid growth lamellae would have been turned upwards during the entire growth of the symbiont. In modern demosponges, antifouling strategies are very common (Krug, 2006; Ribeiro et al., 2013). It is possible that some antifouling strategies may have been present already in early Palaeozoic sponges. Interference and repulsion have been observed in some intergrown skeletons. This is similar to the effects of aggression in modern scleractinian corals (Tapanila, 2008). Zapalski and Hubert (2011) found a decrease in host stromatoporoid growth rates associated with symbiotic Torquaysalpinx sp. The Devonian Torquaysalpinx sp. may have been a parasite gaining living space and possibly also food resources from its host (Zapalski and Hubert, 2011). It is impossible to detect whether the host stromatoporoids were colonized immediately by the rugosans and the younger symbiont generations represent offspring of earlier colonists, or if the stromatoporoid was colonized by symbionts multiple times during its life. In the cut sections we did not find any completely embedded older generations of rugosans. regions (Mistiaen, 1984; Kershaw, 1987; Vinn et al., 2014). A general picture emerges suggesting that Silurian stromatoporoids were easy to colonize by various invertebrates. 5.2. Comparison with other rugosan stromatoporoid symbiosis 5.6. O/S extinction and symbiotic interactions Similar rugosan-stromatoporoid associations have been described from the late Sheinwoodian of Estonia (Vinn and Mõtus, 2014a). A symbiotic rugosan has been recently described from a stromatoporoid of Pridoli of Saaremaa, Estonia (Vinn and Wilson, 2012), but in this association only a couple of rugosans occur in a relatively large stromatoporoid. In contrast, the late Sheinwoodian stromatoporoidrugosan association from Saaremaa (Vinn and Mõtus, 2014a) is generally similar to the association described here. The rugosans in this described association have a larger size than their Sheinwoodian equivalents, but the occupied area of the feeding surface of the host is similar. Numerous rugosans occur in the stromatoporoids of the Silurian of Gotland (Mori, 1969, 1970; Kershaw, 1987) and in the Devonian of Spain (Soto and Méndez Bedia, 1985). Records of Late Ordovician and Silurian macroscopic symbionts in stromatoporoids and tabulate corals are numerous. Vinn et al. (2013) found that numerous macroscopic symbionts occurred among the stromatoporoids of the Ordovician/Silurian recovery fauna in Baltica. It seems that the record of macroscopic symbionts from the mass extinction recovery interval is still growing. This record also indicates that complex ecological interactions (such as symbiosis) were common among the macroscopic invertebrates of the Ordovician–Silurian mass extinction recovery fauna. Thus, it is possible that the mass extinction did not have a severe effect on the symbiotic interactions, or these interactions were very shortly re-established after the extinction event. 5.3. Comparison with other stromatoporoid symbiosis Financial support to O.V. was provided by a Palaeontological Association Research Grant and Estonian Research Council projects ETF9064 and IUT20-34. M.W. thanks the Luce Fund at The College of Wooster. We thank Stephen Kershaw for identifying the stromatoporoid, and Dimitri Kaljo for identification of the rugosan. This paper is a contribution to IGCP 591 “The Early to Middle Palaeozoic Revolution”. We are grateful to G. Baranov, Institute of Geology, Tallinn University of Technology for photographing the specimens. We are grateful to journal reviewers for their constructive comments. Silurian stromatoporoids host many symbiotic endobionts (Tapanila, 2005). Some of stromatoporoid endobionts are preserved as bioclaustrations since they lack their own skeleton (Tapanila, 2005; Vinn and Mõtus, 2014b). Bioclaustrations in the Silurian stromatoporoids are also known from Estonia. In the Ludlow of Estonia, spiral traces belonging to Helicosalpinx occur in the biostromal stromatoporoids (Vinn and Mõtus, 2014b). These traces are more numerous per stromatoporoid specimen (N 100 in a larger stromatoporoid specimen) than the rugosans in the association described here. However, regarding the large size (i.e. removed space from host's feeding surface) of the rugosans, their impact on the host's physiology may have not been smaller. Similar bioclaustrations in tabulates have recently been interpreted as traces of parasites (Zapalski, 2007, 2011). Parasitic tubeworm-like Torquaysalpinx was described from the Devonian stromatoporoids by Zapalski and Hubert (2011). Silurian stromatoporoids in the Sheinwoodian of Estonia also hosted bioimmured cornulitid tubeworms (Vinn and Wilson, 2010). Similarly to bioclaustrations and rugosan symbionts, stromatoporoids can host large numbers of cornulitids (Vinn and Wilson, 2010). There are also records of symbiotic brachiopods living within the Silurian stromatoporoids (Tapanila, 2005). In addition, syringoporids occur in large numbers in the Silurian stromatoporoids of Baltica and other 5.4. Comparison with other rugosan symbiosis In addition to stromatoporoids, rugosan symbionts also occur in tabulate corals and crinoids. A symbiotic Streptelasma sp. was an endobiont within the tabulate coral Paleofavosites prolificus in the Llandovery of Ohio (Sorauf and Kissling, 2012). Living crinoid stems were infested by rugose corals in the Devonian of Morocco and Germany (Berkowski and Klug, 2011; Bohatý et al., 2012). However, the cases of symbiosis between rugosans and stromatoporoids (Kershaw, 1987; Vinn and Mõtus, 2014a) seem to be more common than the other associations in the Palaeozoic. 5.5. Other cases of symbiosis by bioimmuration in the early Palaeozoic Early Palaezoic tabulates hosted various symbiotic endobionts similarly to the recent corals (Nishi and Nishihira, 1996, 1999). 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