Grzegorz Sadlok

COM PAC TION-RE LATED STYLE OF RUSOPHYCUS

PRES ER VA TION FROM FURONGIAN (UP PER CAM BRIAN)

OF HOLY CROSS MOUN TAINS (PO LAND)

Institute of Paleobiology, Polish Academy of Sciences, ul. Twarda 51/55, PL- 00-818 Warszawa, Poland; address for correspondence: 77 Leadside Road, Aberdeen AB251RX, Aberdeenshire, Scot land, United Kingdom

e-mail: gregsadlok@gmail.com

Sadlok, G., 2013. Com pac tion-re lated style of Rusophycus pres er va tion from Furongian (Up per Cam brian) of Holy Cross Moun tains (Po land). Annales Societatis Geologorum Poloniae, 83: 317–327.

Ab stract: In gen eral, the trace fos sil Rusophycus, pre served as a con cave-up ward struc ture on the top of a bed, is

con sid ered to be a fos sil ized mark ing, made by a trace maker. The struc tures de scribed from the Cam brian (Furon- gian) of cen tral Po land are ge net i cally re lated to Rusophycus. How ever, de spite their oc cur rence on the tops of beds, they are not fos sil ized traces, but com pac tion-re lated fea tures, re sult ing from dif fer en tial sand stone and mudstone com pac tion with pos si ble me di a tion by or ganic-rich, heterolithic sed i ments. The pres er va tion of these struc tures prob a bly was in flu enced by biofilms or biomats.

Key words: Rusophycus, trace fos sil, pres er va tion, com pac tion, Cam brian, Furongian, Po land.

Manu script re ceived 14 November 2013, ac cepted 27 December 2013

IN TRO DUC TION

Rusophycus is a rest ing trace fos sil (cubichnion;

Brom-ley, 1996; Seilacher, 2007). It is most com monly at trib uted to the life ac tiv ity of ar thro pods. Palaeozoic ex am ples oc cur mainly in ma rine strata and have been as cribed to trilobites (Crimes, 1970a, b; Seilacher, 1970). Me so zoic, non-ma rine forms of Rusophycus have been at trib uted to crus ta ceans (Bromley and Asgaard, 1972; Pol lard, 1985; Schlirf et al., 2001). Small ichnospecies of Rusophycus, made by rep re sen ta tives of both of these ar thro pod groups, are mor pho -log i cally in dis tin guish able (Bromley and Asgaard, 1979).

Pre vi ously, con cave, epichnial Rusophycus – a trace fossil on the up per sur face of a sed i men tary bed (Mar tinsson, 1970) – was in ter preted to be ac tu ally a true trace, a de -pres sion in bot tom sed i ment, ex ca vated by a trace maker. There are two hy poth e ses on the mode of for ma tion of the con cave Rusophycus: (1) con cave Rusophycus is a fos sil ized epifaunal trace, pro duced at the sed i mentwa ter in ter -face (Crimes, 1975), and (2) Rusophycus is a fos sil ized infaunal trace, cre ated at the sand-mud in ter face within the sub strate (Seilacher, 1970, 1985).

Both of these sce nar ios as sume that the de pres sions made by trace mak ers were sub se quently filled with sed i -ment, usu ally sand. The tim ing of the ad di tion of the fill is the main dif fer ence be tween these sce nar ios. The epifaunal trace would be filled just af ter the bi o log i cal ac tiv ity or with a de lay, af ter some pe riod of non-de po si tion and pos si ble

ero sion. In the case of infaunal traces, fill ing oc curred im me di ately af ter the bur row ing ac tiv ity and this “in stant cast -ing” mech a nism is con sid ered as lead ing to high-res o lu tion pres er va tion of the shal low traces (Seilacher, 1970, 1985). The sand-filled epifaunal or infaunal true traces are pre served as hypichnia that are nat u ral casts of these traces, oc cur ring on the lower sur faces of a sand bed (Martinsson, 1970). Hypichnia con sti tute the most com monly en coun tered mode of pres er va tion of Rusophycus in the fos sil re cord.

Ac cord ing to this widely ac cepted in ter pre ta tion, all

Rusophycus-shaped struc tures oc cur ring on bed tops in the

Up per Cam brian Wiœniówka Sand stone For ma tion previou- sly were in ter preted as the true traces of ar thro pods, namely of trilobites (Radwañski and Roniewicz, 1963; ¯yliñska and Radwañski, 2008). The re sults of this study in di cate that at least some of these struc tures, oc cur ring on tops of beds/laminae, ap pear to be com pac tionre lated phe nom ena, re sult -ing from dif fer en tial com pac tion in volv -ing mud and sand (Baldwin, 1971; Nadon and Issler, 1997).

MA TE RIAL AND METH ODS

The ma te rial stud ied co mes from the Wiœniówka Sand-stone For ma tion (Fig. 1). The ob ser va tions and con clu sions pre sented in this pa per are based on nu mer ous field observa-tions and stud ies of trace fos sils from the col lec tion ZPAL Tf. 4. The col lec tion com prises ma te rial col lected by the Au thor

(80%) and by Marcin Machalski (20%) from the In sti tute of Paleobiology of Pol ish Acad emy of Sci ences, War saw.The fol low ing spec i mens from the col lec tion ZPAL Tf. 4 best doc u ment the type of toponomy and pres er va tion de scribed: 192, 513, 518, 600, 791, 1151, 1238, 1468, 1471, 1472, 1473.

The heterolithic in ter vals of the Wiœniówka Sand stone For ma tion were split with a scis sor knife to search lamina-by-lamina for trace fos sils. Both the top and bot tom sur faces of each lamina were in ves ti gated. This mode of bed ex am i na tion al lowed di rect ob ser va tion of the laminae sur round -ing the hypichnial Rusophycus, in clud -ing their re la tion ship with the hypichnial trace fos sil.

LO CA TION AND GEO LOG I CAL

SET TINGS

The ma te rial stud ied was col lected from the Cam brian (Furongian) Wiœniówka Sand stone For ma tion (Or³owski, 1992a, 1992a; ¯yliñska et al., 2006). The siliciclastic depo-sits of this unit com prise quartz-dom i nated sand stones, mudstones and heterolithic in ter vals and are ex posed best at a few quar ries in wes tern most part of the Holy Cross Moun -tains (Wiœniówka Wielka, Wiœniówka Ma³a and Podwiœ-niówka), as well as in the Opatów area (Kowalczewski et

al., 2006 and ref er ences therein). The es ti mates for the thick ness of the Wiœniówka Sand stone For ma tion range from 80–200 m, up to 400–1400 m, de pend ing on the tec tonic model ap plied (Kowalczewski et al. 2006 and ref er -ences therein). The pres ent study was con ducted in the Wiœ-

niówka Wielka Quarry, in the wes tern most part of the nor-thern Holy Cross Moun tains, Po land (Fig. 1).

The Wiœniówka Sand stone For ma tion is poor in in dex body fos sils (¯yliñska et al., 2006). The unit was dated as “Mid dle to Up per Cam brian” by Or³owski (1992a, b), who (Or³owski, 1992b) based his con clu sions on “Cruziana stra -tig ra phy“, an ichnostra-tigraphical scheme ap plied to Lower Palaeozoic non-fossiliferous ma rine siliciclastics (Seila-cher, 1970, 1994, 2007; MacNaughton, 2007). ¯yliñska et

al. (2006), study ing acritarchs and rare tri lo bite body fos

-sils, con cluded that the unit is lower Furongian.

Sed i men tary en vi ron ment and diagenetic his tory The Wiœniówka Sand stone For ma tion is com posed of thick, amal gam ated quartz ite sand stone beds (quartzarenites sensu Nagtegaal, 1978), mudstones and heterolithic in -ter vals char ac -ter ized by flaser, wavy and len tic u lar bed ding (Studencki, 1994; Jaworowski and Sikorska, 2006; ¯yliñska et al., 2006). Studencki (1994) in ter preted the sand -stone in ter vals as de pos ited on the prox i mal in ner shelf, and the heterolithic sed i ments as de pos ited on the dis tal in ner shelf. Jaworowski and Sikorska (2006) in ter preted this unit in a sim i lar way, as, storm-af fected de pos its on a shelf, with the sands as tidal sand ridges or tidal sand waves and the heterolithic in ter vals as tran si tional to shelf muds. How ever, D¿u³yñski and ¯ak (1960) noted in their sedimentological study of the Wiœniówka Sand stone For ma tion that a shift from mud- to sand-dom i nated sed i men ta tion could have taken place in the same depth zone, as a re sult of dy namic changes in the bot tom to pog ra phy and vari a tion in the cur

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Fig. 1. Lo ca tion of study area. A. Po si tion of Po land in Eu rope. B. Map of Po land, show ing lo ca tion of Holy Cross Moun tains. C. Holy Cross Moun tains (Palaeozoic core). D. Plan of Wiœniówka Wielka Quarry (stra tig ra phy based on ¯yliñska et al., 2006)

rent paths, typ i cal for shal low sub aque ous en vi ron ments. Nu mer ous and var i ous rip ple types, pre served very well in the heterolithic in ter vals, also were taken as in di cat ing shal -low- to very shal -low-wa ter con di tions (D¿u³yñski and ¯ak, 1960; Radwañski and Roniewicz, 1960).

The Wiœniówka Sand stone For ma tion hosts nu mer ous struc tures that may have orig i nated as a re sult of mi cro bial bind ing of the sed i ment, the so-called microbially in duced sed i men tary struc tures (MISS; Noffke et al. 2001). These oc cur on bed ding planes and ap pear to be ge net i cally di -verse (see the clas si fi ca tion in Eriksson et al., 2007). These struc tures were ob served in the same in ter vals that yielded the trace fos sils stud ied. The most char ac ter is tic of these are wrin kle marks (Fig. 2A–C), cracks de vel oped within the sand stone lithologies (Fig. 2D), in ter fer ence rip ples (Fig. 2E) and “sand chips” (Fig. 2F), that is, oval and flat clasts of sand stone, oc cur ring within sand stone or mudstone litho-logies. This as so ci a tion of struc tures is a strong in di ca tion of the pres ence of biofilms and/or mi cro bial mats within the sed i men tary en vi ron ment (cf. Hagadorn and Bottjer, 1997; Pflüger, 1999; Bottjer and Hagadorn, 2007; Eriksson et al., 2007; Porada and Bouougri, 2007; Porada et al., 2008). All of these MISS re sulted from the pres ence of a microbially pro duced extracellular poly meric sub stance that makes the sandy sub strate atyp i cally co he sive and re sis tant to ero sion (Bottjer and Hagadorn, 2007).

Re cent mi cro bial mats typ i cally oc cur within tidal set -tings, where spe cific pre req ui sites as to sub strate tex ture, high mois ture, low hy dro dy namic en ergy and low metazoan ac tiv ity are met (Porada and Bouougri, 2007). MISS in the fos sil re cord are also most com monly en coun tered in shal -low ma rine con di tions (intertidal to supratidal; e.g. Carmona et al., 2012). How ever, mi cro bial mats are by no

means re stricted to such set tings (Schieber et al., 2007) and in the Pre cam brian and Lower Palaeozoic mi cro bial mats were much more wide spread. Some work ers even pos tu late a Pre cam brian–Cam brian shift from “matground” (mi cro bial mats) to a “mixground” (mi cro bial cov ers around in di vid ual grains) type of sub strate struc ture and link this sig nif -i cant event to the -in creas -ing ac t-iv -ity of b-ioturbators (Se-ila- (Seilacher and Pflüger, 1994). Pflüger (1999) showed a wide en -vi ron men tal range for the MISS that he de scribed from the Si lu rian Tanezzuft Shale and the Acacus Sand stone in Libya. In his ex am ples, the MISS oc curred in strata, char ac ter -ized by ichnofossils typ i cal of the Skolithos and Cruziana ichnofacies (Pflüger, 1999, fig, 7). There fore, the oc cur -rence of MISS in strata, char ac ter ized by the Cruziana ich-nofacies (in clud ing stud ied Rusophycus) as in the case of the Wiœniówka Sand stone For ma tion, is not un usual for the Lower Palaeozoic.

The sig nif i cance of mi cro bial ac tiv ity for the pres er va -tion of trace fos sils is well known (e.g. Carmona et al., 2012). How ever, in the pres ent pa per, a po ten tially new as -pect of the taphonomical sig nif i cance of mi cro bial mats is briefly dis cussed.

The postdepositional his tory of the Wiœniówka Sand stone For ma tion is not known ex actly, es pe cially for postSi -lu rian time (Sikorska, 2000). How ever, it has been pos tu lated that the Wiœniówka Sand stone For ma tion ex pe ri enced deep burial within the Cam brian to Si lu rian time in ter val, reach ing

a burial depth of about 1,700 m in the Si lu rian (Sikorska, 2000). Late diagenetic silification oc curred at that time un der con di tions of high tem per a ture and pres sure, re sult ing in the oblit er a tion of the orig i nal grain boundaries by the ex ten sive for ma tion of quartz overgrowths (see Sikorska, 2000).

RE SULTS

The in di vid ual sand stone and mudstone laminae can be as thin as 1 mm in these in ter vals and are usu ally 1–2 mm thick. Rusophycus is an abun dant trace fos sil in the het erolithic in ter vals stud ied. Hypichnial Rusophycus is em bed -ded in the sur round ing mudstone laminae; mudstone lami-nae run be low and above the Rusophycus, which in cross-sec tion take the form of sand stone lenses (Fig. 3A–E). The tops of these lenses are more or less flat (Fig. 3E). Lo cally, ar thro pod trackways oc cur on the up per sur faces of the len-ses. The sand stone bod ies in the form of lens-like hypich-nial Rusophycus de form the muddy laminae be low and above. The laminae above are slightly con vex-up ward (Fig. 3E) and those be low mimic the mor phol ogy of the lower sur face of the hypichnial Rusophycus (Fig. 3F): the laminae are con caveup ward be low the endopodal lobes and con -vex-up ward be tween the lobes (Fig. 3C, D).

Typ i cally, no in ter rup tion of the lower muddy laminae was ob served. There fore the cross-sec tions of these laminae mostly are not ob served on the lat eral mar gins of the bed-top Rusophycus-shaped struc ture, ex cept for rare, thin cut laminae, as so ci ated with hypichnial Rusophycus (Fig. 5D). The laminae within the Rusophycusshaped fill ing are typ i -cally con tin u ous (Fig. 4A, C–F). Some dis con ti nu ities or “win dows” in the lower muddy laminae were ob served, but these re sult mainly from weath er ing and/or dam age dur ing sam pling (Fig. 4B). Fi nally, bed-top Rusophycus dis plays fewer mor pho log i cal de tails (e.g. sculp tur ing of the lobes), than does the coun ter part, hypichnial Rusophycus (Figs 4E, F, 5A, B). In the sam ple stud ied, the re verse sit u a tion was not ob served, i.e. more de tails were seen on the bed-top

Rusophycus-shaped struc ture.

In ter pre ta tion

Pres er va tion of hypichnial Rusophycus

The sandy lenslike oc cur rences of Rusophycus are typ -i cal hyp-ichn-ia (see Mart-insson, 1965, 1970), w-ith a lat er ally re stricted ex tent of the cast ing me dium (a lenslike ap pear -ance). This mode of pres er va tion may be ex plained as an ex am ple of con cealed bed-junc tion pres er va tion, a spe cial type of trace-fos sil pres er va tion, in which the fill ing mate-rial dif fers in tex ture sig nif i cantly from the host sed i ment and has no con nec tion with sim i lar lay ers above, e.g. a sandy fill -ing of bur rows within a mudstone (Simpson, 1957; Jensen, 1997; Jensen et al., 2005). Hypichnial Rusophycus in the form of sand stone lenses are the re sults of pref er en tial pres er va tion of tracein fill ing sandy ma te rial, de pos ited in a shel -tered de pres sion (the ac tual trace made by the or gan ism).

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Fig. 2. Microbially in duced sed i men tary struc tures from Wiœniówka Sand stone For ma tion. A. Top sur face of sand stone bed with wrin -kle marks (marked with ar row and let ter K), note that struc ture oc curs at sand stone-sand stone con tact (ar rowed as s-s) with no clay or silt in ter ca la tion be tween sand stone strata (field pho to graph; com pare Bottjer and Hagadorn, 2007). B. Wrin kle marks with clearly vis i ble re -lief (bed top). C. Thin sec tion of wrin kle marks with vis i ble flat crest and trough (ar rowed) typ i cal fea tures, known from sim i lar struc tures (com pare Calner and Eriksson, 2011; pic ture taken in plane-po lar ized light close to bed top). D. Si nu soi dal crack within sand stone with no clay or silt in ter ca la tion at part ing sur face. E. In ter fer ence rip ples (field pho to graph), pres er va tion of older set might be re lated to mi cro -bial sta bi li za tion (bed top). F. “Sand chips”, rounded and flat sand stone intraclasts, pre served on top of an other sand stone bed (com pare Bottjer and Hagadorn, 2007). These dis play lo cally char ac ter is tic prop er ties, as they seem to be glued to gether (cen tral part of the pic ture; field pho to graph)

Fig. 3. Hypichnial Rusophycus in sand stone, em bed ded in mudstone laminae, and bed-top Rusophycus-shaped de pres sion. A. Hypichnial Rusophycus with un der ly ing mudstone laminae mim ick ing Rusophycus’ mor phol ogy (spec i men in verted, laminae top points to -ward bot tom of pic ture), s1 and s2 – sand stone laminae, m1 and m2 – mudstone laminae (ZPAL Tf. 4/1468). B. Same spec i men as in A., in this case with out un der ly ing laminae (laminae top point to ward top of pic ture), s1 and s2 – sand stone laminae, m2 – mudstone lamina (ZPAL Tf. 4/1468). Note: hypichnial Rusophycus is em bed ded in mudstone laminae (com pare A and B), this mode of pres er va tion may be called con cealed bed-junc tion pres er va tion (see Simpson, 1957; Jensen, 1997; Jensen et al., 2005). C. Un der ly ing mudstone laminae of spec i men, fig ured in A. (ZPAL Tf. 4/1468). Note: bed-top Rusophycus-shaped cav ity (d) and con tin u ous, de formed mudstone laminae (box; lamina top points to ward top of pic ture). D. Fo cus on de tail shown in C., laminae top points to ward top of pic ture (ZPAL Tf. 4/1468). Note: bed-top Rusophycus-shaped cav ity (d) and con tin u ous, de formed mudstone laminae (m). E. Sand stone hypichnial Rusophycus (marked with “s”) and its re la tion ship to over ly ing laminae, m1 and m2 – mudstone laminae (laminae top point to ward top of pic ture; con cealed bed-junc tion pres er va tion; ZPAL Tf. 4/1151). F. Side view of small hypichnial Rusophycus (marked as h) de form ing un der ly ing mudstone laminae (marked as d), the laminae con tinue be low Rusophycus (laminae top points to ward top of pic ture). Note: this is not con cealed bed-junc tion pres er va tion. Rusophycus is “fused” with the over ly ing sand stone bed (ZPAL Tf. 4/1473)

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Fig. 4. Bed-top Rusophycus-shaped struc tures and laminae con ti nu ity in side struc ture. A. Bed-top Rusophycus-shaped struc tures in con tin u ous lamina (edges of lamina shown by ar rows; ZPAL Tf. 4/192). B. Hypichnial sand stone Rusophycus (marked with “s”) de form -ing un der ly -ing mudstone lamina (marked with m; ZPAL Tf. 4/1470). C. Bed-top Rusophycus-shaped struc ture de form -ing con tin u ous lamina (the edge of lamina is marked with box; ZPAL Tf. 4/600). D. Close-up of de tail shown in C, edge (marked with l) of con tin u ous lamina de formed by bed-top Rusophycus-shaped struc ture (ZPAL Tf. 4/600). E. Hypichnial Rusophycus (on right) and bed-top Ru-sophycus-shaped struc ture (on left), m – mudstone lamina, s – sand stone lamina (ZPAL Tf. 4/1471). Note: to tal lack of sculp ture on epichnial struc ture. F. Close-up of de tail shown in E, bed-top Rusophycus-shaped struc ture with no sculp ture and with cross-sec tion of mudstone laminae vis i ble out side struc ture (ar rowed; ZPAL Tf. 4/1471)

For ma tion and pres er va tion of bed-top Rusophycus-shaped struc tures

The bur row ing ac tiv ity by the Rusophycus trace maker should have re moved the de posit from be low and dis placed it (Fig. 5C), and this ac tion should have pro duced a cav ity in the un der ly ing bed (com pare Seilacher, 1970, 1985). There -fore, the laminae in side the cav ity should be dis con tin u ous

and the crosssec tional views of these laminae should be ex posed at the cav ity side mar gins. On the con trary, the sand stone laminae un der neath the sand stone Rusophycus typ i cally have not been in ter rupted by the trace maker and in -stead are con tin u ous. This type of laminae mod i fi ca tion, with out any cut ting of laminae, in di cates that the

Rusophy-cus-shaped bed-top cav i ties are the re sult of de for ma tion

Fig. 5. Bed-top Rusophycus-shaped struc ture, epichnial Rusophycus and mud laminae com pac tion. A. Hypichnial Rusophycus with well de fined sculp ture on its lobes (ZPAL Tf. 4/1238). B. Bed-top Rusophycus-shaped struc ture with con tin u ous laminae in side cav ity, l1-l2 – laminae (ZPAL Tf. 4/1238). C. Epichnial Rusophycus (true trace or undertrace) with clearly vis i ble dis place ment of ma te rial (ZPAL Tf. 4/513). D. Hypichnial sand stone Rusophycus (marked with “s”) and sin gle muddy lamina it cuts through (marked with “m”; (ZPAL Tf. 4/1468). E, F. Top of rip pled sand stone bed bear ing ar thro pod track or undertrack with 1 mm thick set of mudstone laminae on its top (E), and same bed with out over ly ing set of muddy laminae (F) (ZPAL Tf. 4/1472). Note: in E there are ar thro pod tracks (ar rowed) and in F there are cor re spond ing undertracks (ar rowed)

that af fected the laminae. The mode of de for ma tion in di cates that the laminae were not de formed by the trace pro -ducer it self, but rather by pres sure re lated to the pro cess of com pac tion (Fig. 6), caused by an in crease in the load dur -ing later burial.

The hypichnial Rusophycus stud ied dis plays more mor -pho log i cal de tails, such as sculp ture of the lobes, than the coun ter parts on the bed tops do (Fig. 5A, B). This phe nom e -non is dif fi cult to ex plain in a typ i cal epifaunal or infaunal sce nario, in which con cave struc tures are the ac tual epich-nial traces. In such a case, the bed-top struc tures (epichnia) should dis play more de tails of mor phol ogy. The model of a com pac tion-re lated or i gin for the bed-top Rusophycus-shaped struc tures, de scribed in this pa per, would ex plain the dif fer ences in pres er va tion res o lu tion. The loss of mor pho -log i cal de tails could be ex plained by the “copy ef fect”, where each copy loses the def i ni tion of some qual ity. In the com pac tion-re lated model, the con cave bed-top struc tures are de tail-de pleted cop ies of the hypichnial Rusophycus (sandstone lens-like Rusophycus), whereas the hypichnia them selves are cop ies of epifaunal/infaunal traces.

These ob ser va tions lead to the fi nal con clu sion that the bed-top Rusophycus-shaped struc tures de scribed are not

epifaunal or infaunal traces. The two sub strates were orig i -nally sep a rated. Dur ing diagenesis, the hypichnial Rusophy-

cus fill in the up per layer acted as an in denter that shaped

the sur face of the sandy laminae be neath. The ex act match be tween the hypichnial Rusophycus and its im pres sion on the bed top be low in di cates that ver ti cally act ing pres sure was in volved, rather than lat eral bed slides and stress, where more elon gated grooves would be ex pected. This is how the hypichnial Rusophycus made the bed-top “im print mark” on the top of the bed be low. In this sce nario, the orig i nally bur -rowed sub strate, in which a trace orig i nally was pro duced, is not pre served in the fos sil state and there fore it is pos tu -lated that its orig i nal com po si tion was very sen si tive to diagenetic al ter ation. There fore, the or ganic con tent of this sub strate was likely to be high. The pres ence of microbially in duced sed i men tary struc tures in di cates that mi cro bial biofilms and mats played a role in the sed i men tary en vi ron ment of the strata. Thus it is pos si ble that the or ganicen -riched ma te rial, oc cur ring be tween the laminae and tar geted by the trace pro ducer, was a mi cro bial mat or biofilm.

If the or i gin of the bedtop Rusophycusshaped struc -tures sig nif i cantly post dates (pos si bly by mil lions of years?) the for ma tion of the hypichnial Rusophycus, then the pro

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Fig. 6. Sche matic draw ing to ex plain or i gins of bed-top com pac tion-me di ated Rusophycus-shaped struc tures. A. The tri lo bite digs in a muddy sub strate. B. The trace is filled with sand. C. The trace and fill ing sandy ma te rial are bur ied. Note that the sand-filled Rusophycus ap pears to be em bed ded in muddy sed i ment; this is con cealed bedjunc tion pres er va tion (see text for ref er ences). D. Be gin ning of com pac -tion. The de for ma tion of the un der ly ing laminae oc curs af ter the ac tiv ity of the trace-maker and this dif fer en ti ates these struc tures from cleav agere lief pres er va tion (“undertrack”; see Frey and Pem ber ton, 1985). Let ters “m” and “s” re fer to muddy and sandy lithologies, re -spec tively, whereas the num bers in di cate rel a tive ages of laminae (start ing from 1 as the old est). Ow ing to com pac tion, the mor phol ogy of hypichnial Rusophycus is im printed on the top of the lower lamina and the up per laminae also are af fected. The de gree of de for ma tion de -creases away from the hypichnial Rusophycus

posed mode of the for ma tion of these struc tures is dif fer ent from that of cleav age-re lief or undertrack pres er va tion (Fig. 5E, F), in which subsurface laminae are de formed dur ing trace-maker ac tiv ity at the sur face (cf. Lessertisseur, 1955; Heyler and Lessertisseur, 1963; Goldring and Seilacher, 1971; Frey and Pem ber ton, 1985). The trace fos sil is a fos sil ized trace, which is a re sult of sub strateor gan ism in ter ac -tion (Bertling et al., 2006). How ever, the tran si -tion from the orig i nal trace into the fos sil re cord (trace fos sil) is com monly as so ci ated with abiotic pro cesses, such as the for ma tion of hypichnial casts, in which no bi o log i cal ac tiv ity oc -curs. The bed-top struc tures de scribed are not fos sil ized epichnial traces, but a new type of pres er va tion of them, in which a diagenetic pro cess (com pac tion) played a ma jor role. The new term “compactional epichnia” is pro posed here to cover this type of trace fos sil toponomy and pres er -va tion. The fol low ing model is pro posed for the or i gin of the struc tures de scribed: 1) an or gan ism bur rows the sub -strate, which prob a bly was cov ered by a mi cro bial mat or mi cro bial film; 2) the trace is filled with sand; 3) the bur rowed sub strate and the sandy fill ing are bur ied; 4) com pac -tion starts and cre ates a compac-tional epichnion (Fig. 6).

DIS CUS SION

Be cause sands and muds (a mix ture of silt and clay) dif -fer in orig i nal po ros ity, the de pos its rep re sented by these lithologies will tend to com pact dif fer en tially. Sand has a lower po ros ity than freshly de pos ited mud and hence, sand tends to com pact less than mud (Baldwin, 1971; Perrier and Quiblier, 1974; Nadon and Issler, 1997). More over, in mud- stones, the grow ing pres sure in creases sil ica dis so lu tion, but at the same time the pre cip i ta tion of sil ica in the same beds is in hib ited (Mullis, 1992). This means that the vol ume of mud de creases, ow ing to a de ple tion in sil ica. On the other hand, in sands sil ica is pre cip i tated freely as inter-gran u lar ce ment (Mullis, 1992). Thus, the pre cip i tated sil ica, trans ported by pore flu ids from the neigh bour ing muds, can in crease the re sis tance of the sand stone to com pac tion (Ste -phenson et al., 1992). Sim i lar pro cesses prob a bly oc curred in the strata stud ied, as sanddom i nated beds dis play per va -sive sili ci fi ca tion, in ad di tion to pres sure so lu tion (see Si-korska, 2000). This is why com pac tion-re lated de for ma tions are so well pre served in the mudstone laminae, sur -round ing Rusophycus (com pare Nadon and Issler, 1997, fig. 4 therein). As in di cated, the dif fer ence in pres er va tion of de tail, ob served in the bed-top and hypichnion struc ture cou plets, can be ex plained by the “copy ef fect”, with two orig i nally sep a rate sub strates com ing into di rect con tact dur ing diagenesis.

The bur rowed sub strate ap pears to be vir tu ally miss ing in the cases stud ied as cut laminae are mostly not ob served (spec i mens show no mac ro scop i cally vis i ble in ter ca la tions be tween the sandy laminae (Fig. 5A, B)). Dur ing dif fer en -tial com pac tion, beds may ex hibit sig nif i cant re duc tion of thick ness, but typ i cally their con ti nu ity is not dis rupted and at least a thin rem nant of the orig i nal lamina is pre served (Wetzel and Reisdorf, 2007). There fore, it is pos tu lated here that the bur rowed sub strate had a high con tent of or ganic

mat ter and a sig nif i cant re duc tion of its thick ness was caused by de com po si tion pro cesses that could have taken place be fore dif fer en tial com pac tion be gan and dur ing the com pac tion the thick ness of the laminae was fur ther re -duced (see Wetzel and Reisdorf, 2007). The sub strate could rep re sent a liv ing or de cay ing biofilm or mi cro bial mat. The pres ence of mi cro bial biofilms or mats in the sed i men tary en vi ron ment of the Wiœniówka Sand stone For ma tion is indicated by microbially in duced sed i men tary struc tures, as documented in this pa per. The mi cro bial com po nent of these interbeds could have been the real tar get of the dig ging ar -thro pods, namely trilobites feed ing on the biomat.

CON CLU SIONS

The Rusophycus-shaped struc tures, oc cur ring on the tops of beds in the Cam brian (Furongian) Wiœniówka Sand stone For ma tion of the Holy Cross Moun tains (Po land) re -sem ble true epifaunal or infaunal trace fos sils. How ever, a de tailed study of the muddy laminae as so ci ated with the sand stones shows that the laminae un der ly ing Rusophycus (hypichnion) were not in ter rupted by the ac tiv i ties of the trace maker, but are con tin u ous. This mode of mod i fi ca tion of laminae in di cates a com pac tionre lated or i gin with pos si -ble me di a tion by mi cro bial biofilms or biomats, occurring between them.

Ac knowl edg ments

This study is a part of a Ph.D. pro ject su per vised by Marcin Machalski at the In sti tute of Paleobiology of Pol ish Acad emy of Sci ences. It was fi nanced by a grant from the Min is try of Sci ence and Higher Ed u ca tion of Po land in years 2010 to 2012 (no. N N307 243539) to the Au thor and, par tially, by the In sti tute of Paleobiology PAS. I thank the of fi cials and mem bers of staff work -ing on the Wiœniówka Wielka quarry for their help, as sis tance and en thu si asm. I am grate ful to work ers and stu dents in the Fac ulty of Earth Sci ences of the Uni ver sity of Silesia for lo gis tic help and grant ing ac cess to petrographic equip ment. Fi nally, I would like to thank An drew K. Rindsberg (Uni ver sity of Al a bama, Living stone, USA) for lin guis tic cor rec tions and crit i cal re marks. I am grate ful to two ASGP re view ers, Andreas Wetzel (Uni ver sity of Basel, Swit zer land) and one anon y mous re viewer, for their con struc tive crit i cism of the manu script. I thank also Al fred Uchman (Jagiel-lonian Uni ver sity, Kraków) for his con struc tive crit i cism and Frank Simpson (Wind sor, Can ada) for help, re marks and lin guis tic cor rects.

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