Lim i ta tions on the ap pli ca tion of the De vo nian stan dard cono dont zonation
Pi erre BULTYNCK
Bultynck P. (2007) — Lim i ta tions on the ap pli ca tion of the De vo nian stan dard cono dont zonation. Geol. Quart., 51 (4): 339–344. Warszawa.
The most com monly used Lower and Mid dle De vo nian cono dont zonations that some times have been pre sented as stan dard zonations are eval u ated. The au thor ques tions whether the Frasnian stan dard cono dont zonation based on a phylo gen etic suc ces sion of spe cies be long ing to the pe lagic gen era Mesotaxis, Palmatolepis and Siphonodella can be used for world wide cor re la tion. He fa vours the idea of an in ter na - tional cono dont ref er ence scale based on a syn the sis of well es tab lished and doc u mented cono dont suc ces sions (with fig ured spec i mens of first and last oc cur rences of in dex-spe cies) from key ar eas rep re sent ing a va ri ety of fa cies. Graphic cor re la tion is likely to be the most ob jec - tive and pre cise method to pro vide such a syn the sis rep re sented by the com pos ite stan dard. Such stan dards have been al ready elab o rated for the Frasnian and the Mid dle De vo nian. This point of view does not im ply that clas si cal biozonations should be aban doned.
Pi erre Bultynck, De part ment of Pa le on tol ogy, Royal Bel gian In sti tute of Nat u ral Sci ences, Vautierstraat 29, BE-1000 Brussels, Bel - gium; e-mail: pi erre.bultynck@naturalsciences.be (re ceived: Feb ru ary 26, 2007; ac cepted: March 14, 2007).
Key words: De vo nian, stan dard zonations, graphic cor re la tion, com pos ite stan dard.
INTRODUCTION
“… in the case of biostratigraphic units, it must be kept in mind that out of the al most lim it less num ber of over lap ping biozones that could be pro posed, the first to be de scribed is not nec es sar ily the most use ful. This means that work ers must con - tin u ally be free to pro pose new zones or to im prove pre vi ous pro pos als in both scope and no men cla ture...” (Sal va dor, 1994).
The sec ond edi tion of the IUGS In ter na tional Strati graphic Guide (Sal va dor, 1994) does not men tion the term “stan dard zonation”. How ever, a glos sary in cludes the term “stan dard zone” but this is con sid ered as a biostratigraphic term with lim - ited or no ac cep tance. In De vo nian biostratigraphy the terms stan dard zonation/stan dard zone are most com monly used by cono dont work ers (e.g. Ziegler and Sandberg, 1990) and ammonoid work ers (e.g. Becker and House, 2000). In these two pa pers a stan dard zonation im plies, to a greater or lesser de gree, that stan dard zone-de fin ing fos sils oc cur world wide or at least have an interbasinal dis tri bu tion. The meth ods for es tab lish ing a stan dard zonation and the opin ion on the palaeogeographic dis - persal po ten tial of the in dex-spe cies are dif fer ent in the two pa - pers. Strictly speak ing, for the De vo nian there is only a for mally named and de scribed stan dard cono dont zonation for the up per - most Givetian and the Up per De vo nian, from the disparilis to the praesulcata Zone (Ziegler and Sandberg, 1990, text-fig. 1) and
these au thors also stress that it is a zonation for the pe lagic biofacies. Sandberg and Ziegler (1996, text-figs. 1a, b) clearly in di cate that they only rec og nize stan dard zones in the up per - most Givetian and Up per De vo nian and not in the other parts of the Mid dle De vo nian and in the Lower De vo nian. How ever, in the past, com monly used Lower and Mid dle De vo nian cono dont zones have been as sem bled in zonations, oc ca sion ally headed
“stan dard zonation” (e.g. Clausen et al., 1993).
THE DEVONIAN STANDARD CONODONT ZONATION
LOWER DEVONIAN
The Lower De vo nian part of the stan dard zonation in Clausen et al. (1993) should cer tainly not be con sid ered as such. This is clearly dem on strated by sev eral new Lower De vo nian zones that have been in tro duced since 1993 (e.g. Yolkin et al., 1994, Emsian;
Valenzuela-Rios and Murphy, 1997, Lochkovian; Bardashev et al., 2002, Pragian–Emsian; Slavik, 2004, Pragian; Murphy, 2005, Pragian). The two last-men tioned au thors in tro duced new zonations be cause the ap pli ca tion of the 1993 stan dard zonation to the ar eas that they stud ied, the Barrandian and Ne vada, is prob - lem atic. The zon ally de fin ing spe cies of these two new Pragian zonations have a lim ited inter-re gional dis tri bu tion. Ap pli ca tion
on a global scale of the zonations in tro duced by Yolkin et al.
(1994) and Bardashev et al. (2002) is also prob lem atic be cause there is no gen eral agree ment be tween cono dont work ers on the tax on omy of the up per Pragian–Emsian polygnathid in dex-spe - cies they used (e.g. Ma w son, 1997; Murphy, 2005). Con sid er ing a spe cific Lower De vo nian stan dard zone, Valenzuela-Rios (1997) stressed that the pre cise strati graphic level of the base of the up per Pragian pyreneae Zone is un known and that the spe cies is not well char ac ter ized. So it is ob vi ous that there is at pres ent no global Lower De vo nian zonation on which there ex ists a large agree - ment, as would be a nor mal re quire ment for em ploy ing the term
“stan dard”.
MIDDLE DEVONIAN
Sev eral zon ally de fin ing Mid dle De vo nian spe cies (e.g.
Polygnathus partitus, P. costatus, Tortodus aus tra lis, T.
kockelianus, P . ensensis, P. timorensis) have a wide geo graph - ical dis tri bu tion, oc cur in deeper pe lagic and shal lower neritic se quences and were used to es tab lish a Mid dle De vo nian stan - dard zonation (Weddige, 1988). In the same con tri bu tion, how - ever, Weddige wrote that these stan dard zones are use ful for inter-re gional cor re la tions but are too “rough” for de tailed cor - re la tions in smaller ar eas with sim i lar fa cies as for in stance the neritic Ardenno-Eifelian area. So he in tro duced al ter na tive zonations for that area as re fine ments of the stan dard zonation (ibi dem, text-fig. A14-18/11).
Graphic cor re la tion of 9 neritic Mid dle De vo nian suc ces - sions from the Ardenne area (Gouwy and Bultynck, 2003) and 9 suc ces sions from the Mader (pe lagic to neritic fa cies) — Tafilalt (pe lagic-hemipelagic fa cies) re gion in South ern Mo - rocco (Belka et al., 1997; Gouwy and Bultynck, 2002) re sulted in the con struc tion of an Ardenne and a Mader-Tafilalt re gional com pos ite (RC). The graphic cor re la tion of the Ardenne RC with the Mader-Tafilalt RC (Gouwy and Bultynck, 2002) dem - on strates that first oc cur rences of some cono dont spe cies, in - clud ing also zon ally de fin ing spe cies, can be diachronous com - par ing the first oc cur rences in the two re gional com pos ites.
Klapper (1997) had stressed al ready through graphic cor re la - tion of Frasnian se quences in Montagne Noire (MN, France) and West ern Can ada that the en tries of some zon ally de fin ing spe cies of the clas sic-biostratigraphic Frasnian MN zones can be “diachronous to a con sid er able de gree in dif fer ent sec tions of the Frasnian Com pos ite Stan dard”.
Bultynck (1987, fig. 9) in tro duced an al ter na tive zonation for the varcus Zone as de fined and sub di vided by Ziegler et al.
(1976). This al ter na tive zonation is based on a hemipelagic suc ces sion in the north ern Tafilalt (South ern Mo rocco) and in as cend ing or der con sists of the timorensis Zone (= lower part of Lower varcus Subzone), rhenanus/varcus Zone (= up per part of Lower varcus Subzone), ansatus Zone (= Mid dle varcus Subzone mi nus up per most part) and semi-alternans/la - tifossatus Zone (= up per most part of Mid dle varcus Subzone and Up per varcus Subzone) (Fig. 1).
The use of Lower, Mid dle and Up per varcus subzones is well es tab lished in De vo nian cono dont lit er a ture. But in my view, the al ter na tive zonation pro vides a higher strati graphic res o lu tion than the varcus Zone and its sub di vi sions and re -
flects better the most im por tant changes in the cono dont spe cies of that part of the Givetian.
In the first place, Polygnathus varcus it self is not the crit i cal spe cies for rec og niz ing the base of the Lower varcus Subzone.
Ac cord ing to the def i ni tion of Ziegler et al. (1976) it is de fined by the first oc cur rence of Polygnathus timorensis. From the dis tri bu - tion ta bles in that pa per it ap pears that P. varcus first oc curs well above the base of the zone, in the North Amer i can sec tions stud ied it oc curs only in a few sam ples and there is not any fig ured spec i - men of the spe cies in the pa per. Hud dle (1981) did not rec og nize it in the Givetian of New York. This was the main rea son for re plac - ing the Lower varcus Subzone by the timorensis Zone and the rhenanus/varcus Zone. Ziegler et al. (1976) re garded P.
timorensis as a se nior syn onym of P. rhenanus “ be cause the lat ter seems to have been based on a ju ve nile spec i men of P.
timorensis“. I agree that the holotype of P. rhenanus is not a fully adult spec i men but Bultynck (1987, pl. 7, figs. 13–15) fig ures adult spec i men of P. rhenanus that can be eas ily sep a rated from adult spec i mens of P. timorensis (ibi dem, pl. 7, fig. 9) by the very long blade and the short, clearly asym met ri cal plat form, due to the prom i nent out ward bow ing of the outer an te rior trough mar gin.
John son et al. (1980) and Klapper (1981) in tro duced a late form of P. timorensis, spec i fy ing that it cor re sponds ex actly with P.
rhenanus as es tab lished by Klapper et al. (1970). How ever, the lat ter spe cies is still used by sev eral cono dont work ers and fig ured e.g. Spar ling (1999, in clud ing syn onym list), GarcÍa-LÙpez and Sanz-Lopez (2002) and Kaufmann (1998). So the use of P.
rhenanus is pre ferred herein to the long-winded word ing “ P.
timorensis late form“.
In North Amer ica the late form of Polygnathus timorensis vel P. rhenanus first oc curs in the “Mid dle varcus Subzone”
(John son et al., 1980; Spar ling, 1999).
John son et al. (1980) re corded the “late form of P.
timorensis“ and P. ansatus in the up per most part of the Lower Mem ber of the Denay Lime stone in cen tral Ne vada, as signed to the “Mid dle varcus Subzone”. How ever, there is an im por tant in ter val with out cono dont re cords that may be long ei ther to the
“Mid dle varcus Subzone” or to the “Lower varcus Subzone”.
This in ter val cor re sponds to the “Lower varcus Subzone re gres - sion (up per If)” rec og nized in the up per part of the Lower Mem - ber of the Denay Lime stone in cen tral Ne vada and con sists of dolomites.
In north-cen tral Ohio, Spar ling (1999) re corded P.
rhenanus and P. ansatus from the Prout Do lo mite, as signed to the “Mid dle varcus Subzone”. How ever, in that area there is an im por tant disconformity be tween the Prout Do lo mite and the Plum Brook Shale as signed to the ensensis Zone. The de layed first oc cur rence of P. rhenanus in North Amer ica may be due to the un fa vour able do lo mite rocks in cen tral Ne vada and the disconfomity in north-cen tral Ohio.
Rog ers (1998, fig. 7) rec og nized the “Mid dle varcus Sub - zone” in Iowa by the pres ence of the zon ally de fin ing Polygnathus ansatus and was not able to rec og nize spe cif i cally P. varcus and only men tioned “P. varcus group”, a bucket term that is com - monly used in the Givetian for polygnatids with a small plat form and long blade. In the “Mid dle varcus Subzone” of north-cen tral Ohio Spar ling (1999) rec og nized P. ansatus and P. rhenanus, both fig ured, but no P. varcus. So in my view “ansatus Zone” is a
more ap pro pri ate name for that biostratigraphical in ter val than “Mid dle varcus Subzone”.
Ozarkodina semialternans first oc - curs in the up per most part of the “Mid dle varcus Subzone” and the first oc cur rence of Schmidtognathus latifossatus de fines the base of the “Up per varcus Subzone”.
How ever, in the south ern Mo roc can sec - tions O. semialternans is com mon to abun dant and Schm. latifossatus is rare.
This is also the case in other ar eas (Aboussalam, 2003, p. 13). For that rea - son Bultynck (1987) in tro duced the semialternans/latifossatus Zone, its base be ing only slightly older than the base of the “Up per varcus Subzone”. More over, in the Mid dle De vo nian Com pos ite Stan - dard re sult ing from graphic cor re la tion of sec tions in South ern Mo rocco (Gouwy and Bultynck, 2002), there is lit - tle dif fer ence in com pos ite stan dard units for the en tries and last oc cur rences of O.
semialternans (139, 19–140, 11) and Schm. latifossatus (139, 50–140, 42).
FRASNIAN
Ziegler and Sandberg (1994) em pha - sized the ad van tages of us ing spe cies be - long ing to the pe lagic gen era Mesotaxis, Palmatolepis and Siphonodella for their
“Late De vo nian stan dard cono dont zo - na tion” (1990). (1) These evolved and could be dis trib uted world wide in such a short time that they seemed to ap pear syn chro nously in the geo log i cal re cord;
(2) they are less af fected by physico- chem i cal in flu ences; (3) they evolved more rap idly and pro duce a finer time scale than do spe cies be long ing to shal - lower wa ter spe cies.
The two first-men tioned as ser tions are at least partly con tra dicted by more re cent stud ies on De vo nian ammonoids, a group with a pe lagic life style (Becker and House, 2000). They stressed that
only a few De vo nian ammonoid spe cies are re ally cos mo pol i tan and that gen er ally, the oc cur rence of De vo nian ammo noids shows more fa cies in flu ence than was rec og nized in the past.
They also re ferred to the his tor i cal Ger man De vo nian ammonoid suc ces sion that was used as a stan dard for world wide cor re la tion and that ac tu ally “has hin dered biostra tigraphic prog ress sig nif i - cantly and for long time”. As a re sult of their ob ser va tions Becker and House (2000) rec og nized dif fer ent re gional zonations on the ba sis of which they re con structed an ide al ized scheme of stan dard zones, which may not oc cur in all re gions.
In a com ment on the De vo nian Stan dard Cono dont Scale (Clausen et al., 1993), John son (1993) wrote: “This mode of ex pres sion, em ploy ing the term “Stan dard”, has be come com -
mon place in the ab sence of crit i cal re view, the very thing that could val i date its use”.
Con straints on the Frasnian stan dard cono dont zonation (FrSCZ) for the pe lagic biofacies are dis cussed herein un der four items.
RECOGNITION OF THE STANDARD ZONATION IN PELAGIC FACIES
In pe lagic fa cies the FrSCZ can not be rec og nized world - wide. Cono dont work ers who have tried to ap ply the FrSCZ to pe lagic suc ces sions in dif fer ent parts of the world have been faced with the same prob lems as men tioned by ammonoid
Fig. 1. Chronostratigraphic sub di vi sion of the Givetian, cor re la tion of Givetian cono dont zonations and cor re la tion with the com pos ite stan dard units of a Mid dle De vo nian
Com pos ite Stan dard, ammonoid zones, transgressive-re gres sive cy cles and events 1 — sub di vi sion of the Givetian adopted by the Subcommission on De vo nian Stra tig ra phy (Leicester meet ing, 2006); age of the stage bound aries: left num ber, Gradstein et al. (2004); right num ber, Kaufmann (2006); 2 — zonation based on Clausen et al. (1993) and Weddige and Ziegler (1996); 3 — scale in com pos ite stan dard units based on Belka et al. (1997), Gouwy and Bultynck (2002, 2003); the ref er ence sec tion for the Givetian CS is Bou Tchrafine in the north west ern Tafilalt, South ern Mo rocco (Bultynck 1987; Bultynck and Walliser 2000); 4 — the semialter - nans/latifossatus, ansatus, rhena nus/varcus, timorensis and hemiansatus zones have been in tro - duced by Bultynck (1987); the in dex-spe cies have been il lus trated in Bultynck and Hollard (1980) and Bultynck (1987); 5 — af ter Becker and House (2000); Bou Tchrafine; 6 — trangressive–re gres - sive (T–R) cy cles af ter John son et al. (1985) and John son and Sandberg (1989)
work ers, e.g. an ab sence of marker spe cies of the stan dard zonation, dif fer ent ranges for some marker spe cies, en demic spe cies and so on. They es tab lished al ter native zonations not ex clu sively based on Mesotaxis and Palmatolepis.
The best known is the Montagne Noire cono dont suc ces sion (S France) es tab lished by Klapper (1989), and in clud ing the MN zones 1 to 13. The pe lagic char ac ter is tics of the Frasnian de pos - its in the area are sum ma rized in Feist and Klapper (1985) and Morzadec et al. (2000). The Montagne Noire zonation has been ap plied with suc cess to suc ces sions in widely sep a rated ar eas:
the East ern Ca na dian Cor dil lera (Uyeno, 1991); West ern New York (Kirchgasser, 1994; Kralick, 1994); the Timan-Pechora re - gion of the East Eu ro pean Plat form (Klapper et al., 1996; House et al., 2000); the Can ning Bassin, West ern Aus tra lia and Al berta Rock ies (Klapper et al., 1995); West ern Can ada (McLean and Klapper, 1998); the Rhenish Slate Moun tains-Martenberg, the ref er ence sec tion for the FrSCZ from the punctata Zone to the Lower rhenana Zone (Klapper and Becker, 1999); north ern Tafilalt, South ern Mo rocco (Bultynck and Walliser, 2000;
Becker and House, 2000).
Ovnatanova et al. (1999) es tab lished a suc ces sion of 11 cono dont as sem blages for the en tire Frasnian of the south ern Timan-Pechora Prov ince be long ing to the north east ern part of the East Eu ro pean Plat form. The mid dle Frasnian (base cor re - spond ing to the base of their As sem blage IV with Palmatolepis punctata, Palm. gutta, Ancyrodella gigas, Mesotaxis johnsoni, Polygnathus timanicus and P. vjalovi) and up per Frasnian are rep re sented by slope and basinal de pos its with rich palmatolepid and goniatite fau nas. Palmatolepis hassi does not oc cur and Palm. jamieae only oc curs in an in ter val above the last oc cur rence of Palm. semichatovae and be low the first oc - cur rence of Palm. linguiformis, so in a youn ger strati graphic po si tion than it should be ac cord ing to the FrSCZ. In the south - ern Timan-Pechora Prov ince there oc cur also sev eral Palmatolepis spe cies that are un known from the FrSCZ. House et al. (2000) pro posed a cor re la tion with the MN zonation. The cor re la tion with the MN zones 1, 2 and the bound ary be tween MN zones 12 and 13 are prob lem atic.
BIOSTRATIGRAPHIC RESOLUTION OF MESOTAXIS AND PALMATOLEPIS
Spe cies be long ing to the gen era Mesotaxis and Palmatolepis do not al ways pro vide the high est biostratigraphic res o lu tion.
The up per most Givetian–lower Frasnian (falsiovalis Zone/norrisi Zone and MN zones 1 to 4) are char ac ter ized by the evo lu tion ary ra di a tion of the ge nus Ancyrodella (e.g. Bultynck, 1983; Klapper, 1985; GarcÍa-López, 1986; Sandberg et al., 1989; Kralick, 1994). In this part a dozen short-rang ing valid Ancyrodella spe cies can be rec og nized (binodosa, pristina, rotundiloba, crosbiensis, alata, rugosa, recta, triangulata, pramosica, africana, gigas) and in some of the spe cies dif fer ent morphotypes can be dis tin guished. Their suc ces sion pro vides a much higher strati graphic res o lu tion than the suc ces sion within the gen era Mesotaxis and Palmatolepis. Mesotaxis ovalis, falsiovalis and asymmetricus range from the up per most Givetian/low er most Frasnian into the mid dle Frasnian and the ra di a tion of Palmatolepis starts well above the base of the mid - dle Frasnian. An other ad van tage of Ancyrodella taxa is that they oc cur in both pe lagic and neritic fa cies. So for the up per most
Givetian–lower Frasnian a zonation based on and named af ter Ancyrodella taxa pro vides a better in ter na tional biostratigraphic ref er ence scale than the FrSCZ and re flects the most strik ing event of cono dont evo lu tion in that pe riod. Bultynck (1986, p.
276) ad vo cated the re place ment of the Lower asymmetricus Zone by Ancyrodella zones.
TAXONOMIC PROBLEMS
The po ten tial for re li able cor re la tions of some Frasnian stan - dard cono dont zones can be ques tioned due to in con sis tent tax o - nomic con cepts and the in suf fi cient range-doc u men ta tion of the in dex-spe cies. This is es pe cially true for the hassi and the jamieae Zones. Ziegler and Sandberg (1990) in clude in the range of vari a - tion of Palmatolepis hassi quite dif fer ent Pa el e ment morphotypes (com pare on plate 2 their fig ure 3 with fig ure 2 = holotype).
Klapper and Fos ter (1993) and Bultynck et al. (1998) used a more re stricted vari a tion around the holotype. Palm. hassi s.s. ap pears in the Up per rhenana Zone/up per part or MN Zone 12. How ever, the Lower hassi Zone of the FrSCZ is di rectly above the punctata Zone and Ziegler and Sandberg (1990) only fig ure spec i mens from the Lower and Up per rhenana Zone and one spec i men from the jamieae Zone. So we do not know the mor phol ogy of the ear - lier spec i mens from the Lower and Up per hassi Zone that they as - signed to Palm. hassi, and whether they re ally can be in cluded within Palm. hassi. Ziegler and Sandberg (1990) dis tin guished two morphotypes within the range of vari a tion of Palm. jamieae, the in dex-spe cies of the jamieae Zone. In my view the holotype of the spe cies be longs to the sec ond morphotype in which the outer lobe is poorly de mar cated and the gen eral out line of the plat form tends to be pyriform (e.g. Ziegler and Sandberg, 1990, pl. 6, figs.
9, 10). The holotype and other fig ured spec i mens close to the holotype are from the Lower rhenana Zone. The only fig ured spec i men from the jamieae Zone be longs to the first morphotype with a well de mar cated lobe, an te ri orly and pos te ri orly. Bultynck at al. (1998) pre ferred to ex clude such forms from the range of vari a tion of Palm. jamieae. Klapper and Becker (1999) also dis - cussed Palmatolepis jamieae in com par i son with Palm. sp B Klapper and Fos ter 1986. For them it is un clear to which morphotype the holotype of Palm. jamieae be longs.
CORRELATION WITH SHALLOWER SHELF DEPOSITS
Ac cord ing to Sal va dor (1994) “...in set ting up new biozones or in se lect ing for use biozones that al ready have been pro posed, prac ti ca bil ity in iden ti fi ca tion and cor re la tion should be con sid ered...”.
Prac ti ca bil ity in cor re la tion with the FrSCZ is se ri ously lim - ited by the fact that it has been es tab lished for the pe lagic biofacies. Look ing at a global scale, ar eas with shal lower shelf de pos its largely pre dom i nate in the geo log i cal re cord, as also in the De vo nian (e.g. Ziegler, 1989, fig. 5). Al though Palmatolepis taxa oc cur in shal lower shelf en vi ron ments, they are rather rare and only well rep re sented dur ing pe ri ods of im - por tant eustatic sea-level rises. Con se quently the ear li est oc cur - rence of the in dex-taxa of the FrSCZ can be diachronous to a con sid er able de gree, re sult ing in in ac cu rate and spec u la tive cor re la tions. I agree that there is a need for a kind of ref er ence scale pro vid ing a com mon ba sis for De vo nian cono dont work - ers. How ever, such a ref er ence scale should not be only based
on suc ces sions in pe lagic fa cies but on a syn the sis of well es - tab lished and doc u mented (with fig ured spec i mens of first and last oc cur rence) cono dont suc ces sions from key ar eas rep re - sent ing the most com mon fa cies/cono dont biofacies. Graphic cor re la tion is likely to be the most ob jec tive and pre cise method to pro vide such a syn the sis rep re sented by the Com pos ite Stan - dard (CS) sub di vided into Com pos ite Stan dard Units (CSU).
Klapper et al. (1995) elab o rated a Frasnian CS sub di vided into 34.5 CSU. This Frasnian CS is based on data from 64 sec tions in dif fer ent ar eas in clud ing the Montagne Noire (France), the Al berta Rock ies and the Hay-Trout Rivers (Can ada), the Mid-con ti nent and west ern New York (US), the Can ning Ba sin (West ern Aus tra lia) and the Cen tral De vo nian Field and the Timan-Pechora re gion of the East Eu ro pean Plat form. The sec - tions rep re sent dif fer ent biofacies. Klapper (1997) es tab lished more de tailed cor re la tions be tween the Montagne Noire and Al berta Rock ies with a polygnathid biofacies.
Gouwy and Bultynck (2000) de vel oped a Frasnian re gional com pos ite for the Ardenne Area (Bel gium–north east ern France) on the ba sis of 10 sec tions and us ing data mainly from cono donts but also from cor als and brachi o pods. The Ardenne Frasnian re - gional com pos ite has been cor re lated with the Frasnian CS of Klapper (1997). On page 341 and Fig ure 1 of the pres ent con tri - bu tion in for ma tion on a Mid dle De vo nian CS based on a neritic suc ces sion in the Ardenne Area and pe lagic and pe lagic-neritic suc ces sions in South ern Mo rocco is pro vided. The Mid dle De - vo nian CS is sub di vided into 98.6 com pos ite stan dard units.
CONCLUSIONS
Dur ing the last two de cades a Frasnian and Mid dle De vo - nian Com pos ite Stan dards have been elab o rated. The Frasnian CS is in a more ad vanced stage of de vel op ment than the Mid dle De vo nian CS be cause it is based on more sec tions that are spread across North Amer ica, Eu rope and Australia.
The Frasnian, Givetian and Eifelian com pos ite stan dards better meet the re quire ments of what an in ter na tional cono dont ref er ence scale should be. They pro vide a higher res o lu tion for strati graphic cor re la tions than the Frasnian stan dard cono dont zonation and the most fre quently used Eifelian (partitus, costatus, aus tra lis, kockelianus, ensensis) and Givetian cono dont zones (hemiansatus, Lower, Mid dle and Up per varcus, Lower and Up - per hermanni, Lower and Up per disparilis and low est part of falsiovalis). More over, the com pos ite stan dards in te grate data from suc ces sions with dif fer ent biofacies. So their ap pli ca bil ity is more uni ver sal than a clas si cal stan dard biozonation.
How ever, this point of view does not im ply that clas sic biozonations should be aban doned. They should, though, re - ceive a more ap pro pri ate head ing re fer ring to the geo graph ical area and fa cies in which they were es tab lished and the name of the biozones should em pha size the most im por tant spe cies change(s) that took place. One should also re al ize that in clas - sical biozonations the first oc cur rence of the in dex-spe cies of a zone in the ref er ence sec tion is not nec es sar ily isochronous with its first oc cur rence in other sec tions and there are re li able and less re li able spe cies for cor re la tion, even in the ge nus Palmatolepis (Klapper, 1997, p. 120).
A dis ad van tage of a com pos ite stan dard may be its name - less nu mer i cal scale, ham per ing easy com mu ni ca tion in pre - sen ta tions and de bates and fright en ing the un ini ti ated. The name of a taxon is more in for ma tive and can be linked im me di - ately to a spe cific strati graphic level. A good so lu tion can be to in di cate also the base of a clas sical biozone in a spe cific sec tion by its CSU as has been done for the Frasnian cono dont zonation in the Montagne Noire and the Al berta Rock ies and the biostratigraphic in ter vals rec og nized in the Hay and Trout Rivers (e.g. Klapper, 1997, fig. 5).
Ac knowl edge ments. I wish to ex press my thanks to M.
Narkiewicz for his kind and help ful re view of the manu script.
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