py r i ti z a tion of mi c ro fos sils: cr i noid re mains from the mid...

S T U D I A G E O L O G I C A P O L O N I C AVol. 124, Kraków 2005, pp. 37–52.

Methods and Applications in MicropalaeontologyEd ited by J. Tyszka, M. Oliwkiewicz-Miklasiñska,

P. Gedl & M. A. Kaminski

Pa trycja SZCZE PANIK1 & Zbig niew SAW£OWICZ1

Py ri ti za tion of mi cro fos sils: cri noid re mains from theMid dle Ju ras sic of Ogrodzi eniec (Kraków- Czêstochowa

Up land, Po land)

(Figs 1–6)

Ab stract. Py ri tized cri noid skele tal ele ments have been found in the so- called “ore- bearing clays”of the Mid dle Ju ras sic ex posed in Ogrodzi eniec (Kraków- Czêstochowa Up land, Po land). Theiras sem- blage con sists of co lum nals, cir rals and bra chi als; ca lyx plates have not been found. Os si clesoc cur both as unpy ri tized and py ri tized. Three main types of py ri ti za tion have been dis tin guished inin ves ti gated ma te rial: (a) origi nal cal citic skele ton is not re placed by py rite but its void spaces arein filled with py rite; (b) cal citic skele ton is re placed by py rite, with or with out void in fill ing; (c)py ri ti za tion over whelms the pri mary mor phol ogy of the os si cle. The first two types pre domi nate inthe stud ied ma te rial. The types of py ri ti za tion have been ex plained by sev eral sub se quent stages ofthis pro cess. The main stage of cri noid py ri ti za tion hap pened proba bly in the sedi ment dur ing earlydia gene sis and was lim ited to mi cro en vi ron ments of fos sils. Dif fer ent mor pho logi cal forms of py ritereg is tered in the same os si cles (euhe dra, fram boids and mas sive py rite, can be ex plained by dif fer entpo si tion and time of the fos sil py ri ti za tion, the type and lo ca tion of or ganic mat ter, po ros ity, sev eralstages of py ri ti za tion, “open ness” of skele ton, dif fer ent as sem blages of bac te ria, and proba bly manyother fac tors.

Key words: py rite, py ri ti za tion, cri noids, ore- bearing clays, Mid dle Ju ras sic, Batho nian, Po land.

IN TRO DUC TION

Py ri tized fos sils oc cur com monly in dark- coloured sedi men tary rocks (e.g.Hud son, 1982; Fisher, 1986; Brett & Baird, 1986; McNeil, 1990; Briggs et al.,1991, 1996; Raiswell et al., 1993, 1997; B¹k & Saw³owicz, 2000). They are of tenchar ac ter ized by ex cel lent pres er va tion that re flects very fine mor pho logi cal de -tails of the origi nal skele tons. This fea ture of ten al lows for the pre cise re con struc -

1 In sti tute of Geo logi cal Sci ences, Jagiel lo nian Uni ver sity, Ole an dry 2a, 30- 063 Kraków. E- mail:[email protected]; [email protected]

tion of mor phol ogy and func tion of ex tinct or gan isms. Fos sils and the style of theirpy ri ti za tion and mor phol ogy of as so ci ated py rite give also im por tant in for ma tionon sedi men tary en vi ron ments and early dia gene sis.

Py rite re places in or ganic skele ton ma te rial (e.g. McNeil, 1990; Hud son, 1982),or ganic re mains – the so- called soft- bodied fos sils (Al li son, 1988; Briggs et al.,1991, 1996), and fos sil plants (Garcia- Guinera et al., 1998). It en crusts skele tonsand fills in their void spaces (Jensen & Thom sen, 1987; Szcze panik et al., 2004;Briggs et al., 1991). Py rite is also found as in fill ings of di verse trace fos sils (Thom -sen & Vor ren, 1984).

Py ri tized mi cro fos sils, as well as poly fram boi dal py rite ag gre gates, are verycom mon in the so- called “ore- bearing clays” of the Mid dle Ju ras sic in theKraków- Czêstochowa Up land, (Szcze panik, 2003, 2004; Szcze panik in Gedl et al., 2003). Py ri tized sponges, py rite ste ink erns of fo ra mini fers, bi valves, gas tro pods,echi no derms, and as well as en crusta tions on the sur faces of some shells, and py ri -tized trace fos sils have been found. Cri noid os si cles oc cur ring fre quently in thedark- grey clays ex posed in Ogrodzi eniec re veal the whole range of py ri ti za tion2

styles and have been cho sen for pres en ta tion in this pa per. The study of the fos sildis tri bu tion, style of pres er va tion, and py rite mor phol ogy may shed light on the pa -laeoen vi ron ment of the “ore- bearing clay” depo si tion.

GEO LOGI CAL SET TING

Dark- coloured, fine- grained rocks, known as the so- called “ore- bearing clays”,are widely dis trib uted fa cies within the Mid dle Ju ras sic of cen tral Po land. The bi os -tra tigra phy of these sedi ments based on the am mon ite fauna (Zno sko, 1954; Kopik,1974, 1998; Ma tyja & Wierzbowski, 2000, 2003) and mi cro fos sils (fo ra mini fers,os tra cods) (Pazdro, 1967; Bielecka et al., 1980) sug gests a Ba jo cian–Batho nianage. The “ore- bearing clays” are usu ally cov ered by a thick se quence of youngerstrata, ex cept in the Kraków- Czêstochowa Up land where they crop out in sev eralplaces. They are char ac ter ized by the oc cur rence of ho ri zons of sid er itic and cal citic nod ules (Ma jewski, 2000) used as a source of iron ore ex ploited here since the Mid -dle Ages. Sig nifi cant dif fer ences in stra tigraphi cal and fa cial de vel op ment in thisarea were the ba sis for dis tin guish ing two dif fer ent units: a north ern unit (Wieluñ- Czêstochowa- Zawiercie) and a south ern one (Ogrodzieniec- Kraków; Ró¿ycki,1953). The north ern one is char ac ter ized by low fa cies vari abil ity and rela tivelythick de pos its re sult ing from mo noto nous sedi men ta tion. The south ern unit, whichrep re sents a mar ginal part of the epi con ti nen tal sedi men tary ba sin, is char ac ter izedby fre quent fa cies changes, hia tuses and in com plete de vel op ment of ba sal strata ofthe Mid dle Ju ras sic se quence.

The in ves ti gated ex po sure clay pit of the “ore- bearing clays” in the brick- worksin Ogrodzi eniec (Kraków- Czêstochowa Up land) be longs to the south ern sedi men -

38 P. SZCZEPANIK & Z. SAW£OWICZ

2 The term “pyritization” has been used in this article in the sense of pyrite replacement of theoriginal material as well as pyrite infilling of free spaces.

PYRITIZATION OF MICROFOSSILS 39

Fig. 1. A – Geo logi cal map of the Ogrodzi eniec area (sim pli fied af ter Dad lez et al., 2000). B – The section of the so- called ore- bearing clays ex posed in Ogrodzi eniec (Kraków- Czêstochowa Up land,Po land) (origi nally drawn by P. Gedl, pub lished with per mis sion)

tary unit. The ex posed ore- bearing clays (proba bly of Early- Middle Batho nian age;Ró¿ycki, 1953) con sist of dark- coloured, black ish clay stones with sev eral lev els ofsid er ite nod ules and one level of cal citic nod ules (Fig. 1A, B). The thick ness of lay -ers with nod ules does not ex ceed 5–6 cm.

MA TE RIAL AND METH ODS

Twenty- four clay stone sam ples have been col lected from a 10 m thick sec tion in the brick- works in Ogrodzi eniec (Fig. 1B). They were treated with a stan dardmethod for mi cro fos sil ex trac tion. Each sam ple (ap proxi mately 0.5 kg) was bro keninto smaller pieces (1–2 cm in di ame ter), soaked in wa ter and boiled (re peated forsev eral times un til com plete rock dis in te gra tion) and washed through a 63-µmsieve. Skele tal ele ments of cri noids were picked out manu ally from the resi due un -der a bin ocu lar mi cro scope. One uni ver sal thin sec tion of se lected mi cro fos silsfrom sam ple Ogrc1-1 was pre pared and ana lysed un der op ti cal po lar iz ing mi cro -scope (trans mit ted and re flected light) and scan ning elec tron mi cro scope. Such atech nique is es pe cially use ful when thin sec tions of the host rock with mi cro fos silsare un ob tain able. Cross- sections of in di vid ual speci mens al low also the stud ies ofin ter nal struc ture, un avail able dur ing ob ser va tion of the sur face. Py rite mor phol -ogy and type of cri noid os si cles pres er va tion (both py ri tized and unpy ri tized) havebeen stud ied us ing a scan ning elec tron mi cro scope with field emis sion (Hi tachiS4700) equipped with EDS (No ran Van tage) in the Labo ra tory of Field Emis sionScan ning Elec tron Mi cros copy and Mi cro analy sis at the In sti tute of Geo logi calSci ences, Jagiel lo nian Uni ver sity, Kraków.

RE SULTS

Stud ied sam ples com prise fre quent skele tal ele ments of fos sil sea lil ies. Eachos si cle shows a po rous struc ture (stereom), which dis plays two main mi cro struc -tural types: ir regu lar, laby rin thic lat tice and regu lar gal ler ied (Macurda et al.,1978). Re cent skele tons are formed from high- magnesium cal cite crys tals (We ber,1969) whereas fos sil ele ments are mostly low- Mg cal cite. The stud ied cri noid cal -cites have low mag ne sium con tent of about 1%.

The skele tons of liv ing cri noids con sist of nu mer ous os si cles held to gether bycol la ge nous fi bers that pene trate the stereom (the ex cep tions are some bra chi als


Fig. 2. A – Non- pyritized ar ticu lum of the Bala no cri nus co lum nal; B – Non- pyritized ju ve nileiso crinid bra chial; C – Euhe dra and fram boids fill in the cal cite skele ton of an iso crinid cir ral plate; D– Euhe dra fill in the cal cite skele ton of a ju ve nile iso crinid co lum nal plate; E – Oc ta he dra, of tenslightly modi fied by cube, fill in the cal cite skele ton of a ju ve nile iso crinid co lum nal plate; F –Euhe dra built of smaller subunits or en crustated by minute py rite grains; G, H – The dis tri bu tion ofthe py rite fill ings fa voured the mar ginal part of an iso crinid co lum nal ar ticula; it fills voids oflaby rin thic stereom over the gal ler ied voids

plates held ad di tion ally by mus cles). All os si cles found in Ogrodzi eniec rep re sentmostly ju ve nile forms of iso crinids (Bala no cri nus, Chariocri nus ?). The os si cles inthe in ves ti gated ma te rial oc cur gen er ally in the form of iso lated plates. In somecases py ri tized seg mented stalks com posed of a few os si cles (up to 2 cm in length)were found. The fol low ing cri noid skele tal ele ments were rec og nized: co lum nals,cir rals and bra chi als. Ca lyx plates have not been found so far.

Cri noid ele ments, al though oc cur ring fre quently through out the whole sec tion,show some mi nor quan ti ta tive fluc tua tions. Sam ples Ogrc1- 10, 12, 20, 22, 24 wererela tively less abun dant in cri noid re mains. The stud ied os si cles are much more di -ver si fied quali ta tively. They show dif fer ent styles of py ri ti za tion that vary within apar ticu lar sam ple. There are os si cles that show no py ri ti za tion (nei ther re place mentnor in fill ings) and such that ex hibit vari ous types and de grees of py ri ti za tion. Alltypes of unpy ri tized cri noid ele ments (Fig. 2A, 2B), oc cur ring in all sam ples arevery well pre served. It is worth men tion ing that bra chi als oc cur gen er ally as non- pyritized whereas cir rals and co lum nals show simi lar types of py ri ti za tion. Threemain types of py ri ti za tion were dis tin guished in the in ves ti gated ma te rial: (a) origi -nal cal citic skele ton not re placed by py rite but with void spaces in filled with py rite;(b) cal citic skele ton re placed by py rite, with or with out voids in fill ings; (c) py ri ti za -tion over whelm the pri mary mor phol ogy of the os si cle. The first two types pre -domi nate in the stud ied ma te rial.

In the type (a) the void spaces in side a stereom are typi cally filled in with euhe -dral py rite crys tals (Fig. 2D, 2E). Their size is typi cally uni form (3–5 µm) but canvary be tween 2 and 10 µm. The number of crys tals in one void ranges from one tosev eral. They are rep re sented by oc ta he dra, of ten slightly modi fied by cube (Fig.2E), and oc ca sion ally by py ri to he dra. Euhe dra are some times built of smallersubunits (Fig. 2F) or en crustated by minute py rite grains. Fram boids are very rare,with a di ame ter from 10 to 50 µm (Fig. 2C, 4C). The dis tri bu tion of the py rite filledvoids var ies sig nifi cantly. In some cases the voids of the laby rin thic stereom of ar -ticu lum are fa voured by py rite crys tals over the voids of gal ler ied stereom (Fig. 2G,H). Apart from py rite crys tals in voids, in di vid ual crys tals (or their moulds), simi lar in mor phol ogy and size to those from voids, and/or their ag gre gates are ob servedlo cally em bed ded in the sur face of cal citic skele ton (Fig. 4D, 4D1).

In the case of type (b) only sur faces of os si cles were avail able for ob ser va tions.Mas sive py rite re places cal cite skele tal ele ments and its smooth sur face closely re -flects their origi nal mi cro struc ture (Figs 3A, 3B, 3C, 4B). The ex ten sion of re place -ment var ies, from pores only (Fig. 3A1, 3B1, 3C1), through the large sur faces of the os si cle (Fig. 3C), and to its com plete re place ment (Figs 3D, 3D1, 4A, 4A1, 4B). It


Fig. 3. A, A1– Partly py ri tized iso crinid cir ral os si cle in filled with fram boids; B, B1 – Lat eral sideof a partly py ri tized iso crinid cir ral; mas sive py rite closely re places the cal citic skele ton; C, C1 –Lat eral side of a partly py ri tized iso crinid cir ral; py ri ti za tion starts from the pores; D, D1 – Com pletepy ri tized iso crinid cir ral; the dis tinc tion between replacement of the skeleton and voids infilling ishardly possible

seems that py rite re place ment starts around pores and pro gresses out wards. In thecase of py rite crys tals fill ing in voids, they re sem ble those from the type (a).

Py ri tized os si cles in which pri mary mor phol ogy is partly or to tally ob scured(type “c”) are very rare (Fig. 5A, 5B, 5C). In some cases the dis tinc tion be tween re -place ment of the skele ton and the voids in fill ing is hardly pos si ble (Fig. 3D1). Inother cases the os si cles are al most to tally en crusted by py rite crys tals, which masktheir origi nal mor phol ogy (Fig. 5A, 5A1, 5B1, 5C1). The en crust ing py rite crys talsare found both on the sur face of car bon ate or py ri tized os si cle (Fig. 5B1, 5C, 5C1)and in their dis so lu tion cavi ties (Fig. 4E, 4E1). They are typi cally in ter grown andshow growth de fects (Fig. 5B2, 5B3).

DIS CUS SION

Py rite may re place or ganic mat ter as so ci ated with skele tal ma te rial or “truly” re -place a car bon ate skele ton (Can field & Raiswell, 1991). The lat ter type was de -scribed for am mon ites by Hud son (1982). Py ri ti za tion of cri noids is rela tivelyrarely de scribed (Gas pard & Roux, 1974; Briggs et al., 1991). Py ri tized cri noids inthe stud ied sam ples oc cur gen er ally as in di vid ual pieces. This is not sur pris ingsince cri noids usu ally dis ar ticu late within days af ter death, ex cept in the spe cialcases where rapid in situ bur ial fa cili tates pres er va tion of the whole speci mens(e.g., fos sils from De vo nian Huns rück Slate of Ger many, Briggs et al., 1991).

The py rite for ma tion pro cess re quires re duc ing con di tions at site of its for ma -tion (Gold ha ber & Kaplan, 1974; Ber ner, 1980). It is con trolled mainly by theavail abil ity of suit able re ac tive or ganic mat ter (nec es sary for bac te rial sul fate re -duc tion), dis solved sul phate and iron (Ber ner & Raiswell, 1985). Py rite may beformed via at least three path ways, in clud ing the re ac tion of pre cur sor sul fides withpoly sul phides, the pro gres sive solid- state oxi da tion of pre cur sor iron sul fides, andthe oxi da tion of iron sul fides by hy dro gen sul fide, with a dif fer ent rate of for ma tionfor each pro cess and for a greigite in ter me di ary (see the re view in Rickard et al.,1995). In low tem pera tures py rite growth is usu ally pre ceded by the for ma tion ofun sta ble iron mono sul phides (e.g., Sweeney & Kaplan, 1973; Rickard et al., 1995;Scho- onen & Bar nes 1991). Op ti mal con di tions for py rite re place ment of fos silsare of ten de scribed from re stricted, dy soxic en vi ron ments (Cur tis, 1980; Fisher &Hud son, 1985). How ever, the en vi ron ment of depo si tion (bot tom wa ters and porewa ters of the sedi ment) where py ri tized fos sils are found does not have to be an oxicor even dy soxic. The pro cesses of de com po si tion of a bur ied or gan ism cre ate a lo -


Fig. 4. A, A1 – Com plete py ri tized iso crinid ju ve nile co lum nal; B – Py ri tized ar ticu lum of theChariocri nus? co lum nal; C – Py ri tized ar ticu lum of the Bala no cri nus co lum nal in filled with euhe draand fram boids with a di ame ter from 10 to 50 µm; D, D1 – The in di vid ual crys tals (or their moulds),simi lar in mor phol ogy and size to those from voids, and/or their ag gre gates ob served lo callyem bed ded in the sur face of a cal citic skele ton of the lat eral side of an iso crinid cir ral; E, E1 –Dis so lu tion cavi ties re placed and in filled with py rite (Bala no cri nus)

cal re duc ing mi cro en vi ron ment, which pro motes py rite for ma tion (Kaplan et al.,1963; Ber ner, 1969; Hud son, 1982). Dif fer ent de grees of py ri ti za tion of the stud iedcri noids, rang ing from nonpy ri tized os si cles to com pletely py ri tized ones, sug gestthat py ri ti za tion was lim ited to mi cro en vi ron ments, pos si bly mainly to fos sils. Thesur round ing pore wa ters were proba bly iron- rich to force pre cipi ta tion to oc cur inor at the de cay site, as pre dicted by Raiswell (1997).

Dif fer ent mor pho logi cal forms of py rite have been reg is tered in the same os si -cles (euhe dra, fram boids and mas sive py rite). These could be ex plained by dif fer -ent po si tion and time of the fos sil py ri ti za tion, type and lo ca tion of or ganic mat ter,po ros ity, sev eral stages of py ri ti za tion, “open ness” of skele ton, dif fer ent as sem -blages of bac te ria, and many oth ers, but de tailed stud ies have been rare so far.Modi fi ca tion of the pro cesses of iron sul fide for ma tion by or ganic com pounds wasde scribed, for ex am ple, by But ler et al. (1999).

The mac ro en vi ron ment of depo si tion of the pyrite- bearing rocks seems to haveno in flu ence on the py rite crys tal mor phol ogy, in con trast to the mi cro en vi ron mentof or ganic re mains (Dill et al., 1997). The scar city of fram boids in the stud ied os si -cles is sur pris ing as they are very typi cal for many other py ri tized fos sils. Most ofthe euhe dral crys tals are oc ta he dral, some times slightly modi fied by cube. Mor -phol ogy of crys tals in the cu bic crys tal lo graphic sys tem de pends on the growth rateof crys tal faces (Franke, 1987). Ac cord ing to Dill et al. (1997) oc ta he dral py ri tesrep re sent an early stage of py ri ti za tion in the im me di ate mi cro en vi ron ment of or -ganic re mains. Or ganic re mains could sup port a higher sulphate- reducing bac te riapopu la tion than sedi ment with out the or ganic ma te rial and con se quently lead to ahigher H2S con cen tra tion and higher su per satu ra tion (Dill et al., 1997). Dif fer ences be tween mor phol ogy and forms of ag gre ga tion of the euhe dral crys tals in fill ingvoid spaces in side stereom and the py rite crys tals en crust ing skele tons might be ex -plained by vari ous con tents of or ganic mat ter and the time of crys tal li za tion. Theformer are very regu lar, well de vel oped, and not in ter grown and were formedproba bly ear lier, pos si bly in or ganic mat ter ma trix which in hib ited their growth.Con trast ingly, the lat ter are in ter grown and show vari ous mor pho logi cal de fects,proba bly be cause they grew later in the ab sence of or ganic mat ter and de fects mayre sult from sec on dary dis solv ing pro cesses.

Dif fer ent types of py rite in cri noid os si cles might be ex plained by sev eral sub se -quent stages of py ri ti za tion (Fig. 6):

A. Euhe dra, built of smaller subunits, be gin to form first in voids where the ge -lati nous or ganic mat ter (meso der mic tis sue) is still pres ent, and per haps grow (re -crys tal lize) dur ing later stages. Higher amounts of py rite crys tals in the voids of


Fig. 5. A, A1 – Py ri tized iso crinid co lum nal plate; its pri mary mor phol ogy is partly or to tallyob scured; B, B1, B2, B3 – Py ri tized co lum nal of Chariocri nus (B), py rite en crusta tion masks itspri mary mor phol ogy (B1, B2), in ter grown py rite crys tals show ing growth de fects (B3); C, C1 –Py rite en crusta tion on the lat eral side of ju ve nile co lum nal (?Chariocri nus)

laby rin thic stereom of ar ticu lum in com pari son to the gal ler ied one in some os si cles could be ex plained by the lack of fi bres in the laby rinth which re sulted in morespace avail able for crys tals for ma tion.

B. Re place ment of the car bon ate skele ton can over lap the former one or be thefirst in case of ex haus tion of or ganic mat ter be fore py rite euhe dra for ma tion invoids. It be gins when ex ter nal or ganic cover is de stroyed and pore wa ters are si mul -ta ne ously un der satu rated with re spect to car bon ates, and satu rated to over satu ratedwith re spect to iron sul phides. Py ri ti za tion can be fa cili tated by the trans for ma tionfrom the high- Mg less sta ble cal cite to low- Mg cal cite dur ing early dia gene sis; apro cess that in volves dis so lu tion (Petr et al., 1997) and can be driven by the oxi da -tion of or ganic mat ter. The re place ment seems to be gin from voids out wards sug -gest ing sul phide pro duc tion by bac te rial sul phate re duc tion al lowed by de com po si -tion of more re sis tant or ganic mat ter, e.g. col la ge nous fi bers.

C. En crusta tion is the lat est stage of py ri ti za tion pro cess and over lap the formerones dur ing dia gene sis. Py rite crys tals can grow and weld on the sur face of both,car bon ate or al ready py ri tized os si cle. In the case of ear lier dis so lu tion of both types of skele ton py rite crys tals can also fill cavi ties.

The role of bac te ria in the pro cesses of cri noid py ri ti za tion is proba bly sig nifi -cant but dif fi cult to evalu ate. Py rite can re place dead sym bi otic bac te ria liv ing onfos sils. It is pos si ble that bac te ria served as nu clea tion sites (Fer ris et al., 1987) forthe growth of in di vid ual crys tals and their ag gre gates are ob served lo cally, em bed -ded in the sur face of cal citic skele tons of cri noid os si cles. Dis crete, or gan ized sul -phide par ti cles oc cur also in spheri cal mul ti cel lu lar bac te ria of 3–8 µm in size(Mann et al., 1990; Fa rina et al., 1990).


Fig. 6. The model of pyritization of crinoid remains from the “ore-bearing” clays in Ogrodzieniec

It is not clear why the de gree of py ri ti za tion var ies so much be tween in di vid ualcri noid os si cles, rang ing from non- pyritized os si cles to com pletely py ri tized ones,in the very ho mo ge ne ous clay stone. Sev eral pos si ble ex pla na tions can be pro -posed, e.g., parts of stems bur ied in sedi ment were py ri tized pref er en tially (such aphe nome non was ob served in cri noids from the Mid dle Ju ras sic clays fromGnaszyn by A. Boc za rowski, pers. com mun., 2004) or that mi cro bio logi cal de com -po si tion of or ganic mat ter of some os si cles hap pened at dif fer ent times, e.g. be forepo ten tial py ri ti za tion. Dif fer ences be tween non- pyritized bra chi als and py ri tizedcir rals and co lum nals can re sult from the dif fer ent pri mary con tent of Mg, mak ingthem more or less sus cep ti ble to dis so lu tion and sub se quent py ri ti za tion. We ber(1969) has shown that the con tent of Mg2+ var ies within dif fer ent skele tal ele ments.A 3-D diffusion- with- precipitation model by Raiswell et al. (1993) dem on stratedthat the rate of de com po si tion of or ganic mat ter de ter mines whether or ganic mat terpy ri ti za tion can oc cur. The main stage of cri noid py ri ti za tion hap pened proba bly inthe sedi ment dur ing early dia gene sis be cause a fast rate of or ganic mat ter de com po -si tion is sug gested for cri noids.

CON CLU SIONS

1. Three dif fer ent types of py ri ti za tion have been dis tin guished in the stud iedcri noids and ex plained by sev eral sub se quent stages of this pro cess.

2. Py ri ti za tion was lim ited to mi cro en vi ron ments, proba bly mainly to fos sils.3. The main stage of cri noid py ri ti za tion took place proba bly in sedi ment dur ing

early dia gene sis.4. Bra chi als oc cur gen er ally as non- pyritized whereas cir rals and co lum nals

show simi lar types of py ri ti za tion.5. Dif fer ent mor pho logi cal forms of py rite, reg is tered in the same os si cles can

be ex plained by dif fer ent po si tions and times of the fos sil py ri ti za tion, the type andlo ca tion of or ganic mat ter, po ros ity, sev eral stages of py ri ti za tion, the “open ness”of the skele ton, dif fer ent as sem blages of bac te ria, and many other fac tors.

Ac know ledge ments

The authors wish to thank P. Gedl for the en cour age ment to un der take this study and the help withsam ple col lec tion; and A. Kaim and P. Le on owicz for field as sis tance.

We thank E. G³uchowski for help in taxo nomic in ter pre ta tions and for re view ing the manu script.Com ments by J. Tyszka are kindly ac knowl edged.

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