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Palaeontologia Electronica palaeo-electronica.org

The trace Arachnostega in the of (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 (17% of gastropod steinkerns), slightly less abun- dant in the (11% of gastropod steinkerns) and least abundant in the (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 , University of Tartu, 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, University of Technology, Ehitajate tee 5, Tallinn; [email protected]

Keywords: Trace ; 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 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 (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 , 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. ) 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 (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, 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). 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 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, 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- Regional Stage (Middle Sandbian) European Platform was covered by a shallow, epi- (Aluvere quarry, Jänese locality, locality, continental sea with little bathymetric differentia-

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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, lomphala, Pachystrophia, Pararaphistoma, Protur- ditch, Jälgimäe locality, 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 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 originally filled with the statistical differences in Arachnostega infesta- calcite cement that was later dissolved due to tion rates between different substrates and differ- weathering. The studied limestone steinkerns con- ent ages, a non-parametric Mann-Whitney test was tain generally more clay in the Darriwilian and used (result is significant if p < 0.05). All studied Sandbian and are more pure in the Katian. Studied specimens are deposited at the Institute of Geol- steinkerns are similar in size and shape regardless ogy, Tallinn University of Technology (GIT) under of stratigraphic level. Sizes of studied molluscs collection numbers 251, 337, 334, 362, 399, 404, were: gastropods 0,3-16 cm, bivalves: 1-7 cm, 694, 695, 696, and 697. cephalopods: 2-60 cm, hyolithids: 2-11 cm. The host molluscs include: hyolithids (Dorsolinevitus, RESULTS Crispatella, Hyolithes), bivalves (Ambonychia, Arachnostega traces are common in the Mid- Ahtioconcha, Aristerella, Cleionychia, Cypricar- dle and Upper Ordovician of Estonia, occurring in dinia, Cyrtonota, Dystactella, Modiolopsis, Ortho- gastropods, bivalves, cephalopods and hyoliths. nota, Tancrediopsis, Veimarnella), gastropods The Ordovician gastropods of northern Estonia (Brachytomaria, Bucanella, Bucania, Cataschisma, contain Arachnostega traces from the Kunda Cyclonema, Cymbularia, Deaechospira, Ecculio- Regional Stage (early Darriwilian) to the Pirgu

3 VINN ET AL.: ARACHNOSTEGA IN THE ORDOVICIAN

Regional N of steinkerns N of specimens with Stage Arachnostega Series Stage

Porkuni 9 Hirnant.

Pirgu 70 4 (6 %)

Vormsi 142 4 (3%)

Nabala 39

Rakvere Katian 14

Oandu 17 Upper Ordovician

Keila 128 7 (6%)

Haljala 692 68 (10 %) Sandbian Kukruse 73 17 (23 %) Arachnostega gastrochaenae Uhaku 44 7 (16 %)

Lasnamäe 6

Aseri 4 Darriwilian

Middle Ordovician Kunda 39 9 (23 %)

FIGURE 2. Stratigraphic distribution and abundance of Arachnostega gastrochaenae Bertling in the Ordovician gas- tropods of Estonia.

Regional Stage (latest Katian) (Figure 2). The per- Arachnostega traces (Figure 2). Arachnostega centages of gastropods with Arachnostega vary selectively burrowed certain sediment-filled shells. from 3% in the Vormsi Regional Stage (late Katian) In the Aluvere Quarry (Sandbian), the highest to 23% in the Kukruse Regional Stage (early Sand- abundance of Arachnostega is associated with bian). Arachnostega does not show preference for bivalves (26%) and the lowest with gastropods specific areas of the substrate. No signs of re-bur- (12%) (Table 1). A Mann-Whitney test found signifi- rowing were observed. Some burrows are filled cant differences (p<0.01) in Arachnostega abun- with sediment or calcite cement; some are empty. dance between bivalves and gastropods. Gastropods from several stages revealed no Arachnostega was most abundant in the Darriwil-

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TABLE 1. Occurrence of Arachnostega in the Aluvere (23% of gastropods) is highest for the Ordovician Quarry (Sandbian). of Estonia. However, in the following Keila Total number Number of specimens with Regional Stage (latest Sandbian to earliest Katian), Taxa of specimens Arachnostega Arachnostega abundance declines to 6% of gastro- 506 60 (12%) pods. There is a gap in the Arachnostega distribu- Bivalvia 106 28 (26%) tion in the early Katian (Oandu to Nabala regional Cephalopoda 70 13 (19%) stages). This gap probably represents a very low Hyolitha 16 3 (19%) abundance of Arachnostega rather than their real absence in the section because the sample sizes TABLE 2. Distribution of Arachnostega in gastropods are not large enough. The lack of Arachnostega in from the Ordovician (global stages) of Estonia. the Hirnantian is most likely due to the very small Gastropods with sample size (N=9). The different average abun- Stage Total gastropods Arachnostega dances of Arachnostega in the different stages Katian 282 8 (3%) may have some correlative value within the same Sandbian 765 85 (11%) paleoenvironment (Figure 2). Darriwilian 93 16 (17%) The most numerous records of Arachnostega in the Ordovician are from Gondwana and the TABLE 3. Arachnostega diameter in gastropods in the Armorican Terrane Assemblage (Fatka et al., Ordovician of Estonia. 2011). Slightly fewer records are from Kazakhsta- Stage Arachnostega burrows diameter in gastropods nia and Baltica (Fatka et al., 2011). South Ameri- Katian 0.35-1.14 mm (N=7, Mean 0.55, sd 0.28) can and African Gondwana and Armorican terrane Sandbian 0.22-1.00 mm (N=37, Mean 0.57, sd 0.23) assemblages were located in cold to temperate cli- Darriwilian 0.24-1.37 mm (N=15, Mean 0.76, sd 0.34) matic zones during the Ordovician. It is possible that Arachnostega trace-makers preferred a cold climate, or the Arachnostega type of behavior was ian (17% of gastropods), slightly less abundant in more successful in such climate regime. However, the Sandbian (11% of gastropods) and least abun- for steinkerns to be collectible they have to be sep- dant in the Katian (3% of gastropods) (Table 2). A arated from the matrix, and that requires particular Mann-Whitney test found significant differences in rock types. Pure limestones and dolomites, for Arachnostega abundance between the Sandbian example, are unlikely to yield many free steinkerns, and Katian (p<0.01) and between the Darriwilian which biases the paleogeographic distribution and Katian (p<0.01). The diameter of Arachnos- away from certain regions, especially tropical set- tega tubes is relatively similar in all studied sub- tings far from a source of terrigenous sediment. strate shells in the Aluvere Quarry, though slightly larger in cephalopods (see description). The diam- Substrate Shell Selectivity eter of Arachnostega is similar throughout the Arachnostega is known from several types of Ordovician in northern Estonia (Table 3). invertebrate steinkerns, such as those of trilobites (Fatka et al., 2011), bivalves, cephalopods, gastro- DISCUSSION pods and hyolithids. The burrows were originally Stratigraphic Distribution and filled with the sediment or calcite cement, and their Paleobiogeography fillings were often later dissolved during the weath- ering. There are several factors that can influence Arachnostega traces are not known from the the Arachnostega abundance in particular skele- clastic sediments of the Lower Ordovician of Esto- ton, such as accessibility of shell cavity, sediment nia. Arachnostega traces first appear in the Kunda texture, available space in the shell for the Arach- Regional Stage (early Darriwilian) and are associ- nostega network, nutrient content of the sediment ated with the carbonate part of the Ordovician sec- and availability of shells in the Arachnostega bio- tion (i.e., Middle and Upper Ordovician). In the tope. The data from the Aluvere Quarry indicate Darriwilian, Arachnostega traces occur in the that Arachnostega was substrate shell selective Kunda and Uhaku regional stages. The absence of (Table 1). The trace-makers clearly preferred Arachnostega in the Aseri and Lasnamägi regional bivalves (26%) over the gastropods (12%). Both stages could be explained by very small sample gastropods and bivalves were abundant in the Alu- sizes. In the Sandbian, Arachnostega occurs both vere Quarry community. It is possible that the more in the Kukruse and Haljala stages. The Arachnos- spacious bivalve shells contained more food than tega abundance in the Kukruse Regional Stage

5 VINN ET AL.: ARACHNOSTEGA IN THE ORDOVICIAN gastropod shells, or that feeding on their less Ordovician. This change may have played an curved inner surfaces was easier than in gastropod important role in the decrease of Arachnostega shells. In this case, Arachnostega trace-makers abundance. The climatic change caused changes may have been able to choose the shells they in the sedimentation environment such as faster inhabited and exploited. In the case of closed sedimentation rates (based on thicker sections) bivalve shells, the more protected microenviron- and appearance of carbonate buildups and reefs ment may have been more conducive for burrow- (Nestor and Einasto, 1997). It is possible that ing. Another possible explanation is that generally Arachnostega may have preferred colder climates infaunal bivalves may have possibly contained to tropical carbonate accumulation environments. more food (the carcass), which was preserved The different and more abundant benthos in the more or less in situ. In contrast, gastropod shells tropics may have been among the direct causes were more commonly reworked and only rarely behind the decrease of Arachnostega abundance. contained the carcass. However, the possibility that the increased water temperature also affected Arachnostega cannot be Morphometric Variability ruled out. Arachnostega has no obvious morphotypes GOBE. The change of Arachnostega abundance associated with particular shell substrates. This correlates well with the Great Ordovician Biodiver- indicates that substrates had no influence on the sification Event (GOBE). Alternatively, or in addi- trace morphology. The morphology of Arachnos- tion to the regional climatic change, Arachnostega tega traces shows no stratigraphic trends; both abundance may reflect the changes in marine Darriwilian and Katian traces are similar. The same communities that took place due to the GOBE, applies for the diameter of Arachnostega tubes that especially the appearance of new strategies of does not significantly change through the Ordovi- scavenging and predation. The GOBE may have cian (Table 3). However, the burrows with larger affected Arachnostega via increased levels of bio- diameters tend to be slightly more common in the turbation and possibly also the appearance of vari- Darriwilian than in the Sandbian or Katian. Arach- ous new benthic predators. In modern marine nostega traces in different substrates have rela- environments, predation on rapidly reproducing tively similar diameters, although traces in the invertebrates typically does not go so far as to wipe cephalopods (N=9, Mean 0.9 mm, sd=0.36) tend to them out, but it may cause an adaptive change in be larger than those in bivalves (N=11, Mean 0.40 behavior (Barnes and Hughes, 1999). If the mm, sd=0.12). This may indicate that the generally decrease of Arachnostega abundance reflects the larger cephalopod shells were preferred by larger biotic changes caused by GOBE, it should be a Arachnostega trace-makers (i.e., larger ). global phenomenon and recognizable on other Some specimens (Figure 3.5) have primary tunnels paleocontinents. Future studies should show that are wider than their branches. It is possible whether the GOBE had an influence on the global that the burrowing used the primary tunnels abundance of Arachnostega in the Ordovician. mostly for changing locations and created the branches for searching the food. CONCLUSIONS Dynamics of Arachnostega Abundance in the 1. It is possible that Arachnostega trace-makers Ordovician of Baltica preferred a cold climate, or the Arachnostega type of behavior was more successful in such Arachnostega abundance decreased signifi- a climate regime. cantly during the Ordovician in Baltica. It is not pos- sible to determine whether this decrease involves 2. The data from the Aluvere Quarry indicate the-trace makers themselves (a particular taxon or that Arachnostega was substrate shell selec- taxa) or if it reflects a change in the preferred tive and clearly preferred bivalves (26%) over behavior of the trace-maker(s). the gastropods (12%). Climatic change. The change of Arachnostega 3. Arachnostega abundance decreased signifi- abundance in Baltica correlates well with the cantly during the Ordovician in Baltica possi- regional climatic change that took place as the bly due to a climatic change from temperate to paleocontinent moved from the temperate climate tropical. GOBE may have also influenced the zone to the tropics during the Middle and early Late abundance of Arachnostega.

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FIGURE 3.1. Arachnostega gastrochaenae Bertling in a gastropod from Haljala Regional Stage (Sandbian), Aluvere Quarry, northern Estonia. GIT 399-948-1. 2. Arachnostega gastrochaenae Bertling in a gastropod from the Kunda Regional Stage (Darriwilian), Kunda Ojaküla, northern Estonia. GIT 404-355-1. 3. Arachnostega gastrochaenae Bertling in a bivalve from the Haljala Regional Stage (Sandbian), Aluvere Quarry, northern Estonia. GIT 399-1590-1. 4. Arachnostega gastrochaenae Bertling in a bivalve from the Haljala Regional Stage (Sandbian), Aluvere Quarry, northern Estonia. GIT 399-1601-1. 5. Arachnostega gastrochaenae Bertling in a cephalopod from the Uhaku Regional Stage (Darriwilian), Püssi, northern Estonia. GIT 695-12-1.

7 VINN ET AL.: ARACHNOSTEGA IN THE ORDOVICIAN

FIGURE 4. Arachnostega gastrochaenae Bertling in a hyolith from Kukruse Regional Stage (Sandbian), Harku, north- ern Estonia. GIT 696-15-1.

SYSTEMATIC ICHNOLOGY burrow systems that form irregular, polygonal meshes. The tunnels have rounded cross-sections. Ichnogenus ARACHNOSTEGA Bertling, 1992 The diameters of burrows in each system vary on Type ichnospecies. Arachnostega gastrochaenae molds depending on how they are exposed on the Bertling, 1992. steinkern. The largest systems can occupy the Arachnostega gastrochaenae Bertling, 1992 whole interior of the substrate shell. Arachnostega Figures 3-4 traces can occur in gastropods throughout their shell interiors. 2002. Arachnostega gastrochaenae Bertling; Dimensions (mm). Tunnel diameter in bivalves: Aceñolaza and Aceñolaza, p. 182, fig. 6D. 0.22-0.59 (N=11, Mean 0.40, sd=0.12), diameter in 2011 Arachnostega gastrochaenae Bertling; Fatka gastropods: 0.22-1.20 (N=37, Mean 0.57, et al., p. 372-375, text-figs. 4, 5. sd=0.23), diameter in cephalopods: 0.43-1.36 2013 Arachnostega gastrochaenae Bertling; (N=9, Mean 0.9, sd=0.36) Aceñolaza et al., p. 315-322. (see for synonymy). Description. Straight, curved or angular tunnels that ACKNOWLEDGEMENTS form burrows or burrow systems on the surface or at partial contact with the surface of steinkerns of We thank two anonymous reviewers for sug- skeletal fossils (bivalves, gastropods, cephalo- gesting improvements to our submitted manu- pods, hyolithids). Most common simple forms are script. O.V. is indebted to a Sepkoski Grant usually branching at an angle of 45–90°, but the (Paleontological Society), the Palaeontological branching patterns are variable. Less common are Association Research Grant program, Estonian

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Science Foundation grant ETF9064 and Estonian Fatka, O., Mikuláš, R., Szabad, M., Micka, V., and Research Council grant IUT20-34 for financial sup- Valent, M. 2011. Arachnostega Bertling, 1992 in the port. M.W. thanks the Faculty Development Fund Drumian (Cambrian) sediments of the Teplá-Barran- at The College of Wooster. This paper is a contri- dian region (Czech Republic). Acta Geologica Polon- bution to IGCP 591 “The Early to Middle Paleozoic ica, 61:367-381. Fatka, O. and Kozák, V. 2014. A new type of entomb- Revolution.” ment of Peronopsis (Agnostida) in a hyolithid conch. Carnets de Géologie, 14:191-198. REFERENCES Gil Cid, M.D. and Lebrón Moreno, J.A. 2010. Taphonomy Aceñolaza, G. and Aceñolaza, F.G. 2002. Ordovician of trilobites and associated invertebrates from SW trace fossils of Argentina. In aspects of the Ordovi- Spain and NW Portugal. Neues Jahrbuch für Geolo- cian System in Argentina. In Aceñolaza, F.G. (ed.), gie und Paläontologie, Abhandlungen 257:169-179. INSUGEO, Serie Correlación Geológica, 16:177- Mikuláš, R. and Dronov, A.V. 2005. Trace Fossils, p. 33- 194. 38. In Dronov, A.V., Tolmacheva, T., Rayevskaya, E., Aceñolaza, G., Gutiérrez-Marco, J.C., and Peralta, S. and Nestell, M. (eds.), Cambrian and Ordovician of 2013. Arachnostega gastrochaenae Bertling (traza St Petersburg Region. St Petersburg State University fósil) en las secuencias volcaniclásticas de la For- and A.P. Karpinsky All-Russian Research Geological mación Suri, Sistema de Famatina, Argentina. Institute, St. Petersburg. Ameghiniana, 40:315-322. Mõtus, M.A. and Hints, O. 2007. Excursion Guidebook. Barnes, R.S.K. and Hughes, R.N. 1999. An Introduction In 10th International Symposium on Fossil Cnidaria to Marine Ecology, 3rd Edition. Wiley-Blackwell. and Porifera. Excursion B2: Lower Paleozoic geology Bertling, M. 1992. Arachnostega n. ichnog. – burrowing and corals of Estonia. August 18-22, 2007. Institute traces in internal moulds of boring bivalves (late of Geology at Tallinn University of Technology, Tal- Jurassic, Northern Germany). Paläontologische linn. Zeitschrift, 66:177-185. Nestor, H. and Einasto, R.1997. Ordovician and Cocks, L.R.M. and Torsvik, T.H. 2005. Baltica from the carbonate sedimentation basin, p. 192–204. In Rau- late to mid-Palaeozoic times: the gain kas, A. and Teedumäe, A. (eds.), Geology and Min- and loss of a terranes identity. Earth-Science eral Resources of Estonia. Estonian Academy Reviews, 72:39-66. Publishers, Tallinn. Torsvik, T.H., Smethurst, M.A., van der Voo, R., Trench, A., Abrahamsen, N., and Halvorsen, E.1992. Baltica. A synopsis of Vendian– palaeomagnetic data and their palaeotectonic implications. Earth Sci- ence Reviews, 33:133-152.

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