Cruziana traces from the Late Silurian ( Pridoli ) carbonate shelf of Saaremaa , Estonia

Late Pridoli Cruziana traces have recently been found in carbonate shelf sediments of the Ohesaare Formation on Saaremaa Island, Estonia. Cruziana isp. is interpreted here as a locomotory trace (repichnia) of an arthropod, possibly a trilobite. Cruziana traces previously known from the Silurian of Baltica differ from Cruziana isp., indicating that the diversity of Cruziana traces in the late Silurian of Baltica was higher than previously thought.


INTRODUCTION
Trace fossils of the Silurian of Estonia and the eastern Baltic in general are poorly studied.A few reports are devoted to borings in the hard substrates of the Silurian of Estonia (Vinn & Wilson 2010a, 2010b).Even less is known about the softground substrate trace fossils.Only Skolithos burrows have previously been described from the late Pridoli of Saaremaa (Vinn & Wilson 2013).However, Silurian rocks of Estonia are rich in trace fossils, which are usually mentioned as worm traces and bioturbation in the geological literature (Kaljo 1970).
Ichnospecies of Cruziana are typically associated with shallow-water marine facies (Seilacher 1985).Cruziana ranges from the lower Cambrian to the Triassic (Zonneveld et al. 2002;Metz 2013) and is both of stratigraphic (i.e.different ichnospecies of Cruziana have a different stratigraphic distribution; Magwood & Pemberton 1990) and palaeoenvironmental significance (i.e.different ichnospecies of Cruziana have a different facies distribution; Seilacher 2007).On the basis of their age and co-occurrence with the body fossils of the probable trace-maker, most of Palaeozoic Cruziana traces have been attributed to trilobites (Seilacher 1970), but some Cruziana are considered as produced by aglaspidids (Fischer 1978), brachiopods (Bromley & Asgaard 1979), as well as vertebrates (Shone 1978).In contrast, Cruziana problematica has often been found from non-marine sediments and was produced by other arthropods than trilobites (e.g.Bromley & Asgaard 1972;Trewin 1976;Pollard 1985).
The Cruziana ichnofauna was less diverse in the Silurian than in the Cambrian or Ordovician.Silurian Cruziana traces have mostly been restricted to specific palaeogeographic regions and localities (Seilacher 2007).Cruziana kufraensis has been described from Lower Silurian rocks of Libya and C. acacensis from the Lower Silurian of North Africa and South America (Seilacher 2007).Cruziana pudica has a wide distribution in the Upper Ordovician and Lower Silurian of North America (Garlock & Isaacson 1977;Seilacher 2007).Records of Rhusophycus pudicum and Rhusophycus pudica should presumably also be included in C. pudica.The Upper Silurian sections contain Cruziana quadrata in North Africa, C. pedroana in North Africa and Spain and C. retroplana in Spain (Seilacher 2007).In contrast to the Silurian, in the lower Palaeozoic several Cruziana species had a global distribution (Seilacher 2007).
The record of Cruziana traces in the Silurian of Baltica is poor.No Cruziana traces have previously been reported from the Silurian of Estonia and the eastern Baltic.Rare late Pridoli Cruziana traces have recently been found from the Ohesaare cliff on Saaremaa Island, Estonia, and are described here.
The aims of the paper are to (1) describe Cruziana traces from the Silurian of Estonia and the eastern Baltic for the first time, (2) interpret the ethology of the tracemaker and (3) discuss the palaeobiogeographic distribution of Cruziana traces in the Late Silurian.

GEOLOGICAL BACKGROUND AND LOCALITY
An epicontinental basin covered most of the area of Estonia in the Silurian.During the Late Silurian the Baltica palaeocontinent was located in equatorial latitudes and was drifting northwards (Cocks &Torsvik 2005).A wide range of tropical environments and diverse biotas characterized the Silurian Baltic palaeobasin (Hints et al. 2008).Five main facies belts have been recognized in that basin: tidal flat/lagoonal, shoal, open shelf, basin slope and a basin depression (Nestor & Einasto 1977).The first three facies belts comprise a carbonate platform succession (Raukas & Teedumäe 1997).The rocks exposed at the Ohesaare cliff mostly represent the open shelf facies belt, but the upper part of the section, which formed during a considerable shallowing event (Hints et al. 2008;Vinn & Wilson 2013), represents the shoal facies belt.
The Ohesaare Cliff (58°00′02″N, 22°01′10″E) is located on the western coast of the Sõrve Peninsula of southwestern Saaremaa Island (Estonia) near Ohesaare village (Fig. 1).The cliff is over 600 m long and up to 4 m high (Fig. 2).On average, about 3.5 m of bedrock is exposed.The thicknesses of individual beds (lenticular and slightly wavy) vary throughout the outcrop (Nestor 1990;Hints et al. 2008;Vinn & Wilson 2013).The Ohesaare section is characterized by the intercalation of thin-bedded limestones and marlstones of normal marine shelf origin (Fig. 2 The figured specimens are deposited at the Geological Museum, Museum of Natural History, University of Tartu (TUG).Their collection number is 1670.
Notes.The longest, bilobate trace is up to 23 cm long.
It is characterized by well-developed relatively deep linear scratch marks on both sides of the wide median groove.The median groove is about 1/3 as wide as the trace.Scratch marks form a V-shaped pattern.The angle between scratch marks and the median groove is 42-44 degrees.Six to seven scratch marks per 10 mm occur on both sides of the trace.The scratch marks are 0.5-0.7 mm deep.The trace has variable width and can be divided into sectors with increasing diameter (in one specimen).The minimum width of the trace is 6.0 mm and the maximum width is 11 mm.The scratch marks are less developed in the widest parts of the trace.
Remarks.The specimens described here resemble Cruziana pudica because of their somewhat similar V-shaped scratch marks pattern.However, Cruziana isp.has a more V-shaped scratch marks pattern than C. pudica.It also has a wider median furrow and lower relief than C. pudica.It is possible that these Cruziana isp.traces may belong to a new ichnospecies.

DISCUSSION
Cruziana isp. of the Ohesaare Formation is interpreted here as a locomotory feeding trace (repichnia), presumably formed by an arthropod which excavated sediments by scarping its endopodites towards its midline.During this movement the trace-maker may have filtered food from the disrupted sediment (Seilacher 1985(Seilacher , 1990(Seilacher , 1994)).
Ohesaare Cruziana may have formed as a surface furrow, since it occurs on a bedding plane.This is an unusual example as it was found on the sole of a limestone bed, while mostly Cruziana occur on the soles of sandstones.This is because sand has cast the bottom of the furrow during a sedimentation event such as a storm.It is known that sedimentation, particularly in shelf settings, appears to enhance the preservation of shallow-tier and interface structures such as Cruziana (Crimes 1975).Three sections of the trace with an increasing diameter are interpreted here as episodes of faster movement of the trace-maker separated by two episodes of rest or slower movement.During the initial phase or episode of movement the trace-maker probably moved higher above the bottom and was less in contact with the sediment (the narrowest part of the trace).At the end of the moving episode the trace-maker dug deeper into the sediment.The widest part of the trace at the end of the movement was presumably left by the head shield of the trace-maker, because the shape of that part of the trace resembles the shape of the trilobite head shield.
Trilobites were the most likely Cruziana trace-makers in the Palaeozoic (Seilacher 2007).They also are the most common fossilized arthropods in the Ohesaare Formation.Several trilobites of the Ohesaare Formation, such as Calymene, had a suitable body size for the Cruziana isp.traces.The trilobite fauna of the Ohesaare  most common trilobites at the Ohesaare cliff locality are Calymene conspicula, C. soervensis and Acaste dayiana (Hints et al. 2008).
The occurrences of Cruziana drop off after the Ordovician, due to changes in global diversity (Seilacher 2007).The highest diversity of Silurian Cruziana traces is associated with North Africa and Spain (Seilacher 2007).Most of the Silurian Cruziana traces have been found in Gondwana (Seilacher 2007).Cruziana pudica had a slightly wider distribution in the Early Silurian (Garlock & Isaacson 1977;Seilacher 2007).The diversity and stratigraphic distribution of Cruziana traces are poorly understood in the Silurian of Baltica.There are  records of two types (A and B) of Cruziana traces from the Late Silurian of Norway (Davies et al. 2006).Both of these traces differ from Cruziana isp. of the Ohesaare Formation, indicating that the diversity of Cruziana traces in the Late Silurian of Baltica was higher than previously known.
; Vinn & Wilson 2013).Only one Cruziana specimen was collected from the section.The other two specimens were obtained from loose material.The beds with rare Cruziana traces are situated in the upper part of the cliff below the hardground (Figs 2, 3; for description see Vinn & Wilson 2010b).The Cruziana traces were found in the beds of open shelf origin where they occurred as hyporelief along bed junctions.The bed surface with Cruziana has a wavy relief.