Palaeontologia Electronica palaeo-electronica.org The trace fossil Arachnostega in the Ordovician of Estonia (Baltica) Olev Vinn, Mark A. Wilson, Michał Zatoń, and Ursula Toom ABSTRACT In the Ordovician of Baltica, burrows assigned to the ichnogenus Arachnostega Bertling, 1992 occur in steinkerns of bivalves, cephalopods, gastropods and hyolithids. Arachnostega abundance decreased significantly during the Ordovician in Baltica. It was most abundant in the Darriwilian (17% of gastropod steinkerns), slightly less abun- dant in the Sandbian (11% of gastropod steinkerns) and least abundant in the Katian (3% of gastropod steinkerns). This change in Arachnostega abundance correlates well with the regional climatic change (from temperate to tropics) during this interval, along with resulting changes in sedimentary environment, geochemistry and biota. Arachno- stega was substrate selective and preferred bivalves over gastropods. Arachnostega occurs only in the Middle and Upper Ordovician of Estonia and is absent in the Lower Ordovician. 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, Ohio, 44691 USA; [email protected] Michał Zatoń. Department of Palaeontology & Stratigraphy, University of Silesia, Będzińska 60, 41-200 Sosnowiec, Poland; [email protected] Ursula Toom. Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, Tallinn; [email protected] Keywords: Trace fossils; burrows; worms; steinkern; carbonate rocks INTRODUCTION al., 2013). Arachnostega traces constitute irregular, branching burrows in the sediment fill of shells; it is The ichnogenus Arachnostega was originally visible on the surface of steinkerns. Previously, described from the Upper Jurassic of Germany Arachnostega has been reported from Ordovician (Bertling, 1992). The earliest Arachnostega traces of African and South American Gondwana, but it is are known from the Cambrian (Gil Cid and Lebrón especially widespread in European peri-Gond- Moreno, 2010; Fatka et al., 2011; Fatka and Kozák, wana. It has only rarely been recognized in Ordovi- 2014). They became common in the Ordovician, cian sediments of Baltica and Kazakhstania (Fatka achieving a global distribution (e.g., Aceñolaza et PE Article Number: 17.3.40A Copyright: Palaeontological Association November 2014 Submission: 5 April 2014. Acceptance: 24 October 2014 Vinn, Olev, Wilson, Mark A., Zatoń, Michał, and Toom, Ursula 2014. The trace fossil Arachnostega in the Ordovician of Estonia (Baltica). Palaeontologia Electronica 17.3.40A: 1-9. palaeo-electronica.org/content/2014/960-arachnostega-in-the-ordovician VINN ET AL.: ARACHNOSTEGA IN THE ORDOVICIAN et al., 2011). There is only a single record of Arach- tion. This basin was characterized by an extremely nostega from the Ordovician of Baltica (Mikuláš low sedimentation rate (Mõtus and Hints, 2007). and Dronov, 2005). Mikuláš and Dronov (2005) Along the entire extent of the ramp a series of grey briefly described the ichnogenus from the Middle calcareous - argillaceous sediments accumulated, Ordovician of the St. Petersburg region, Russia. represented now by argillaceous limestones and Most post-Ordovician Arachnostega traces appear marls. There was a trend in sedimentation of to occur in clastic sediments (Fatka et al., 2011). increasing clay and decreasing bioclasts in the off- After the Ordovician there are probable peaks of shore direction (Nestor and Einasto, 1997). During Arachnostega abundance in the Devonian, Juras- the Ordovician the Baltica paleocontinent moved sic, Neogene and Quaternary (Fatka et al., 2011). from the temperate climatic zone to the subtropical However, we must consider that, once exploited, realm (Torsvik et al., 1992; Nestor and Einasto, the sheltered niche produced when nutrient-bear- 1997). In the Middle and Late Ordovician the cli- ing sediment fills a shell would likely have matic change resulted in an increase in carbonate remained exploited. Thus, these abundance production and sedimentation rate on the carbon- “peaks” may be due to collection or preservation ate shelf. During the Late Ordovician the first car- bias. bonate buildups appeared, emphasizing a striking Arachnostega was probably a feeding trace change in the overall character of the paleobasin (fodichnion), but the possibility that Arachnostega (Mõtus and Hints, 2007). represents a dwelling burrow (domichnion) cannot be excluded with certainty (Fatka et al., 2011). The MATERIAL AND METHODS exact paleoecology of Arachnostega is not well A large collection of Ordovician (Darriwilian to understood. Hirnantian) gastropods (N=771) from northern During the Ordovician, Baltica migrated from a Estonia (Figure 1) was systematically searched for temperate climate to the tropics (Torsvik et al., the Arachnostega traces. In addition, a collection of 1992; Cocks and Torsvik, 2005). This climatic and gastropods (N=506), bivalves (N=106), cephalo- corresponding sedimentological change is well pods (N=70) and hyolithids (N=16) from the Alu- recorded in the Middle to Late Ordovician carbon- vere Quarry (Sandbian) was systematically ate rocks of northern Estonia (Nestor and Einasto, searched for these traces as well. Other faunal 1997). The relatively complete Ordovician (all groups, such as trilobites and echinoderms, were stages are represented) section of north Estonia is also examined for Arachnostega traces. All the rich in potential substrates (i.e., shells filled with studied samples derive from limestone with various sediment) for Arachnostega. amounts of clay and bioclastic material. Echino- This paper addresses the following questions: derm and trilobite fossils did not possess Arachno- 1) Is Arachnostega common in the Ordovician of stega traces for unknown reasons. Baltica? 2) Does the abundance of Arachnostega Kunda Regional Stage (lower Darriwilian) change during the Ordovician in Baltica? If it does, (Mäeküla locality, Nõmmeveski locality, Tsitre local- do such changes correlate with climatic change ity, Kunda-Ojaküla locality, Aseri cliff, Aseri quarry): (temperate versus tropical climate)? 3) Was the limestones and sandy limestones with iron ooids Arachnostega trace-maker substrate and shell and layers of marly limestones. selective? 4) Are the dimensions of Arachnostega Uhaku Regional Stage (upper Darriwilian) burrows different in different shells? 5) Do the (Kiviõli ditch, Püssi locality, Lasnamägi locality, dimensions of Arachnostega change through the Uhaku valley, Osmussaar Island): limestones with Ordovician? layers of marly limestones and oil shale (kukersite). Kukruse Regional Stage (lower Sandbian) GEOLOGICAL BACKGROUND AND SETTING (Kohtla quarry, Küttejõu quarry, Ubja quarry, North- The Ordovician limestones of Estonia are ern Kiviõli quarry, Vanaküla quarry, Oktoobri exposed as a wide belt from the Narva River in the quarry, Kohtla-Järve locality, Tallinn (Punane northeast to Hiiumaa Island in the west (Mõtus and Street), Kukruse quarry, Kohtla quarry, Lasnamägi Hints, 2007). The total thickness of the Ordovician locality, Ubja quarry, Alliku quarry, Harku quarry, system in Estonia varies from 70 to 180 m (Mõtus Harku ditch): intercalation of limestone and oil and Hints, 2007). In the Middle Ordovician and shale (kukersite) layers of various thickness. early Late Ordovician, the western part of the East- Haljala Regional Stage (Middle Sandbian) European Platform was covered by a shallow, epi- (Aluvere quarry, Jänese locality, Peetri locality, continental sea with little bathymetric differentia- 2 PALAEO-ELECTRONICA.ORG 50 km BALTIC SEA Tallinn Aluvere N ESTONIA 400 km Hiiumaa Finland Sweden Saaremaa Estonia Russia Latvia Lithuania Poland FIGURE 1. Location of northern Estonia. The area of Arachnostega gastrochaenae Bertling localities in the Ordovi- cian of northern Estonia is marked with red. Madise cliff, Pääsküla locality, Alliku quarry): marly mphalus, Eccyliopterus, Eotomaria, Fusispira, Hol- limestones and marls. opea, Hormotoma, Kokenospira, Lesueurilla, Keila Regional Stage (upper Sandbian-lower Lophospira, Loxoplocus, Maclurites, Mega- Katian) (Keila quarry, Oandu locality, Aaspere lomphala, Pachystrophia, Pararaphistoma, Protur- ditch, Jälgimäe locality, Saue old quarry, Kasepere ritella, Salpingostoma, Sinuites, Subulites, F-321 drill core): marly limestones and bioclastic Straparollus, Temnodiscus, Trochonema, Worthe- limestones. nia) and cephalopods (discosorids, endocerids, Vormsi Regional Stage (middle Katian) lituitids, orthocerids, tarphycerids). (Paope locality, Kõrgessaare outcrop, Saunja All specimens were cleaned with water and quarry): marly limestones with thin layers of marls. brushes. The best preserved were photographed Pirgu Regional Stage (upper Katian) (Paope with a Nikon D7000 digital camera. Only the maxi- locality, Hosholm locality): pure limestones with mal diameters of each selected Arachnostega layers of marls. trace were measured on the photos digitally pro- All steinkerns in this study are limestone and vided with a scale bar. Abundance data from the were naturally weathered out of marly limestones. regional Keila Regional Stage (latest Sandbian to The pre-Katian specimens contain more clay and earliest Katian) were not included in the analyses the Katian specimens are made of pure limestone. of Arachnostega abundance for global stages (i.e., The bivalve specimens are steinkerns of complete Darriwilian, Sandbian, Katian). In order to check shells. The empty tunnels were