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Palaeontologia Electronica http://palaeo-electronica.org MICROCONCHID-DOMINATED HARDGROUND ASSOCIATION FROM THE LATE PRIDOLI (SILURIAN) OF SAAREMAA, ESTONIA Olev Vinn and Mark A. Wilson ABSTRACT A hardground fauna of moderate diversity from normal marine (shoal to open shelf) of the late Pridoli of Saaremaa (Estonia) contains: microconchids (Palaeocon- chus cf. tenuis, and P. sp.), Anticalyptraea calyptrata, sheet-like trepostome bryozoans, discoid crinoid holdfasts, encrusting graptolites, Aulopora sp., Cornulites sp. and Con- chicolites sp. Microconchids are most abundant in the association and form the second most prominent group by encrustation area in the association after trepostome bryozo- ans. There is a taxonomic polarity between sclerobionts on the upper surface of the hardground and the cryptic fauna beneath. This hardground community shows that the high abundance of microconchids characteristic of Devonian hard substrate communi- ties had been achieved by at least the Pridoli. Microconchids preferred hardground upper surfaces and were able to symbiotically grow within bryozoans when overgrown. Olev Vinn. Department of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia [email protected] Mark A. Wilson. Department of Geology, The College of Wooster, Wooster, OH 44691, USA. [email protected] KEY WORDS: hardground, encrustation, bioerosion, paleoecology, Silurian, Baltica INTRODUCTION less abundant than in the Ordovician (Taylor and Wilson 2003). There are only four detailed studies Carbonate hardgrounds are surfaces of of Silurian hardground communities: Halleck synsedimentarily cemented carbonate layers that (1973), Franzén (1977), Cherns (1980) and Sum- have been exposed on the seafloor. Carbonate rall et al. (2009). However, no hardground fauna hardgrounds are common in calcite sea conditions has been described from the latest Silurian (Pri- because of favorable conditions for early cementa- doli). tion of carbonate sediments in the seafloor (Wilson Silurian hardground faunas are in general and Palmer 1992). Hardgrounds form suitable similar to those of Ordovician. They are dominated attachment surfaces for encrusting and bioeroding by bryozoans and echinoderms, particularly cri- organisms. There was a calcite sea in the Silurian, noids. Devonian encrusting communities are better and hardgrounds were common, though probably PE Article Number: 13.2.9A Copyright: Paleontological Society July 2010 Submission: 17 August 2009. Acceptance: 18 April 2010 Vinn, Olev and Wilson, Mark A., 2010. Microconchid-Dominated Hardground Association from the Late Pridoli (Silurian) of Saaremaa, Estonia. Palaeontologia Electronica Vol. 13, Issue 2; 9A: 12p; http://palaeo-electronica.org/2010_2/212/index.html VINN & WILSON: A LATE SILURIAN HARDGROUND 50 km N Tallinn BALTIC SEA ESTONIA 400 km Finland Sweden Saaremaa Estonia Russia Ohesaare Latvia Lithuania Poland FIGURE 1. Location of the Ohesaare cliff, Saaremaa, Estonia. known on shells than other hard substrates (Taylor (Vinn 2006a). Several other groups of problemati- and Wilson 2003). These communities are domi- cal calcareous tubeworms referred to the Tentacul- nated by microconchids, hederelloids and tabulate ita Bouček, 1964, occur in the Paleozoic (Vinn and corals instead of bryozoans and echinoderms, Mutvei 2005, 2009; Vinn 2010; Taylor et al. in although the latter two groups are still common press). They include Tentaculitida Ljaschenko, (Kesling et al. 1980; Sparks et al. 1980; Bonem 1955, Cornulitida Bouček, 1964, and Trypanopor- 1982; Brett and Cottrell 1982; Alvarez and Taylor ida Weedon, 1991, as well as the genera Antica- 1987; Bordeaux and Brett 1990; Brice and Mistiaen lyptraea (Vinn and Isakar 2007) and Tymbochoos 1992; Grimm 1998, Taylor and Wilson 2003). (Vinn 2006b). Endosymbiotic Streptindytes in the Especially interesting is the question of how and Middle Devonian rugose corals and stromatoporo- when typical Ordovician-Silurian sclerobiont com- ids may also be a tentaculitoid (Vinn and Mõtus munities were replaced by typical Devonian ones. 2008). Hederelloids are extinct colonial animals with In this paper, the researchers hope to: 1) calcitic tubular branching exoskeletons, and they describe for the first time a hardground association appear to be most closely related to phoronids. of Pridoli age; 2) compare the hardground fauna Tentaculitoid tubeworms could also be phylogeneti- from the Pridoli of Saaremaa to other Silurian and cally closely related to the hederelloids (Taylor and Devonian analogues; 3) contrast upper surface Wilson 2008). and cryptic communities; 4) discuss the paleoecol- Small, spirally-coiled calcareous worm tubes ogy of tentaculitid tubeworms; and 5) discern how are common on Paleozoic and Triassic hard sub- gradually typical Ordovician-Silurian hardground strates (Taylor and Vinn 2006). Such tubeworms communities were replaced by typical Devonian have been traditionally assigned to the polychaete communities. genus Spirorbis. However, pre-Cretaceous exam- Locality and stratigraphy ples have been reinterpreted as microconchids (Class Tentaculita Bouček 1964; Order Microcon- The Ohesaare cliff (58°0’2˝ N, 22°1’10˝ E) is chida Weedon 1991) on the basis of the early located on the western coast of the Sõrve Penin- ontogeny and microstructure of their tubes sula (Saaremaa, Estonia) near Ohesaare village (Weedon 1991, 1994; Dreesen and Jux 1995; Tay- (Figure 1). The cliff is over 600 m long and up to 4 lor and Vinn 2006; Vinn and Taylor 2007). Two gen- m high (Figures 2 and 3). The total thickness of the era of microconchids are known from the Silurian exposed bedrock is 3.5 m, whereas the thick- 2 PALAEO-ELECTRONICA.ORG M 0.20 Hardground 0.15 (studied here) 0.50 0.40 0.70 Hardground 0.50 0.30 0.15 0.20 FIGURE 3. The Ohesaare cliff. Hammer at the level of studied hardground. 0.35 facies belts can be differentiated in the Baltic Silu- Cross-bedded calcareous siltsones (shoal) rian Basin: tidal flat/lagoonal, shoal, open shelf, Skeletal grainstones (shoal) transitional (basin slope), and a basin depression. Cross-bedded pelletal-skeletal grainstones (shoal) The first three facies belts formed a carbonate Skeletal packstones (open shelf) shelf or carbonate platform and the latter two a deeper pericratonic basin with fine-grained clastic Marlstones (open shelf) deposits (Raukas and Teedumäe 1997). The rocks FIGURE 2. Geological section of the Ohesaare cliff of the Ohesaare cliff were formed in shoal and (modified after Nestor 1990). Location of the studied open shelf conditions. hardground in the section. The Ohesaare cliff has a rich and diverse shelly fauna. The most abundant macrofossils are brachiopods, represented by Delthyris magna nesses of individual beds are rather variable Kozlowsky, 1929, D. elevata Dalman, 1928, throughout the outcrop (Hints 2008). The section is Homoeospira baylei (Davidson 1848), Morino- characterized by the intercalation of thin-bedded rhynchus orbignyi (Davidson 1848), Isorthis ovalis limestones and marlstones (Figures 2 and 3). (Paškevičius 1962). Bryozoans are relatively There are two hardgrounds in the section, one in numerous compared to other eastern Baltic Silu- the upper part (studied here) and another in the rian sections; they include: Fistulipora tenuilamel- lower middle part of the cliff (Figure 2). The studied lata (Bassler 1911), F. aculeata Astrova, 1960 and hardground is underlain by 0.5 m of skeletal pack- Eridotrypa parvulipora Ulrich and Bassler 1913. stone and overlain by 0.15 m silty skeletal grain- Bivalves are represented by Grammysia obliqua stone, the upper surface of which bears large ripple (McCoy 1852), Cardiola interrupta Sowerby, 1839, marks (Hints 2008). The rocks of Ohesaare section Palaeopecten danbyi (McCoy 1851), and Modiol- have a high content of terrigenous material. The opsis complanata Sowerby, 1839. Trilobites are intense influx of fine siliciclastic material into the also common. Corals occur at certain levels in the basin possibly took place at the final stage of its middle part of section. The middle part of the sec- development (Mõtus and Hints 2007). The rocks of tion has also yielded the tentaculitids Tentaculites the Ohesaare cliff correspond to the Monograptus scalaris (Schlotheim 1820) and Lowchidium transgrediens biozone, late Pridoli, Ohesaare inaequale Eichwald 1860. The microfossil associa- Stage (Hints 2008). tion is very diverse and rich, especially the ostra- During the Silurian the Baltica continent was codes (Mõtus and Hints 2007). located in equatorial latitudes drifting northwards (Melchin et al. 2004). The pericontinental Baltic MATERIAL AND METHODS paleobasin in modern Estonia was characterized by a wide range of tropical environments and All hardground samples were collected from diverse biotas (Hints 2008). According to the model the Ohesaare cliff (Saaremaa, Estonia) (Figure 1). worked out by Nestor and Einasto (1977), five main All samples originate from the top 40 cm of the sec- 3 VINN & WILSON: A LATE SILURIAN HARDGROUND FIGURE 4. Ohesaare hardground samples, late Pridoli, Silurian, 1. Upper surface, showing patchy distribution of Try- panites borings and very low encrustation density (TUG 1373-1). 2. Upper surface, showing high microconchid (mc) encrustation density with high Trypanites density (TUG 1373-2). 3. Cryptic roof of a hardground ledge showing high encrustation density (TUG 1373-3). 4. Upper surface showing uneven surface relief and very low encrustation and lack of Trypanites (TUG 1373-4). Abbreviations: ant- Anticalyptraea