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A revised graptolite biostratigraphy for the lower Caradoc (Upper Ordovician) of southern Scotland

A revised graptolite biostratigraphy for the lower Caradoc (Upper Ordovician) of southern Scotland
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  A revised graptolite biostratigraphy for the lower Caradoc(Upper Ordovician) of southern Scotland M ARK  W ILLIAMS 1, *, A DRIAN  W. A. R USHTON 2 , B EN  W OOD 1 , J AMES  D. F LOYD 3 , R ICHARD  S MITH 3 &C HRISTOPHER  W HEATLEY 1 1 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK  2 Department of Palaeontology, Natural History Museum, Cromwell Road, South Kensington,London SW7 5BD, UK  3 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK *Present address: British Antarctic Survey, Geological Sciences Division, High Cross, MadingleyRoad, Cambridge CB3 0ET, UK  Synopsis Biostratigraphical evaluation of graptolites from more than 160 localities in the lowerCaradoc of southern Scotland does not di ff  erentiate a discrete assemblage diagnostic of the Climacograptus bicornis peltifer  Biozone. The lowermost Caradoc  Nemagraptus gracilis  Bio-zone is redefined by the partial-range of   N. gracilis  below the appearance of   Climacograptusbicornis  s.l. The succeeding  C. bicornis  Biozone is divisible into (1) a lower Subzone of  Orthograptus apiculatus  and  Dicranograptus ziczac,  identified also by the appearance of  Amplexograptus leptotheca, Dicranograptus tardiusculus  and several other taxa, and (2) anupper Subzone of   Climacograptus wilsoni  , marked by the first appearance of   C. wilsoni  . Nemagraptus gracilis  and  Didymograptus  (s.l.)  superstes  range into the lower part of the apiculatus-ziczac  Subzone, above which is a poorly characterized interval that is approximatelyequivalent of the ‘  peltifer ’ Biozone of former usage. The  bicornis  Biozone is the approximateequivalent to the  C. bicornis  Biozone of North America, and, at least in part, of the  foliaceus (or  multidens ) Biozone of southern Britain. It is succeeded in Scotland by the  clingani   Biozone. Introduction The  gracilis ,  peltifer , wilsoni  , clingani  and linearis grap-tolite biozones of the Caradoc Series were recognizedin southern Scotland during the period 1878 to 1925,commencing with the classic work of Lapworth (1878)on the ‘Mo ff  at Series’ (see Rushton 2001a) and extend-ing to the synthesis by Elles (1925). More recently, defi-nitions for the mid- to late Caradoc  wilsoni  ,  clingani   andpart of the  linearis  biozones have been refined (Williams1981; 1982, 1994; Zalasiewicz  et al  . 1995), but the rela-tionships of the  gracilis  and  peltifer  biozones remainpoorly constrained (see Rushton 1990, 2001a; Rushton et al  . 1996a, p. 141; Floyd 2001, p. 360). Indeed, theclassical works of Lapworth (1878) and Peach & Horne(1899) made no distinction of this interval in theirgraptolite biostratigraphies of the Southern Uplands.The present work seeks to test the value of the  gracilis and  peltifer  biozones in southern Scotland. The relevantstrata are strongly folded and faulted, and there are fewsections that o ff  er reliable superpositional evidence. Wehave therefore had recourse to a comparison of faunalsuccessions in other parts of the world, aided by astatistical analysis of data from the Southern Uplandsheld in a relational Microsoft Access database. We haveassessed the biostratigraphical distribution of graptolitesfrom more than 160 localities in the lower Caradoc rocksequence of southern Scotland (see Appendix). Most of these have been collected during the British GeologicalSurvey’s (BGS) mapping programme since the latenineteenth century, and they yield a total complement of over 90 taxa. The fauna at these localities is recorded inthe appendices of BGS Memoirs (Stone 1995; Floyd1999; Lintern & Floyd 2000; McMillan 2002) andunpublished British Geological Survey TechnicalReports. Archival data for the faunas is recorded in theBGS’s Palaeosaurus  fossil database.Data on the graptolite assemblages has been compiledin a relational Microsoft Access database that collatesinformation on locality, biostratigraphy, taxa recorded,collector and amount of material collected. The Appen-dix summarizes the biostratigraphical information inthis database. It enables assessment of those graptolitesthat most commonly occur together, and which grapto-lites form the complements of biozones as currentlyidentified. The data provide the basis for a rationaliza-tion of the two lower Caradoc graptolite biozones inScotland, the  gracilis  and  peltifer  biozones, whose dis-tinction has often proved problematical (see Rushton2001a). Caradoc graptolite biozones in Scotland In Scotland, the  gracilis  Biozone (lowermost Caradocaccording to Fortey  et al  . 2000) has been defined by thetotal range of   Nemagraptus gracilis  (see Finney &Bergström 1986).  Didymograptus  (s.l.)  superstes  typicallyco-occurs with  N. gracilis  (see Table 1) and is regarded Scottish Journal of Geology  40 , (2), 97–114, 2004  TABLE 1Graptolites from the Southern Uplands of Scotland whichoccur with: (A) the faunas regarded as lowermost Caradocfrom the Currarie, Craigmalloch and Superstes Mudstoneformations; (B)  Nemagraptus gracilis ; (C)  Climacograptusbicornis  s.l.A.  gracilis  Biozone Cryptograptus  1 Cryptograptus tricornis  1 Dendrograptus  1 Dicellograptus alabamensis  2 Dicellograptus intortus  2 Diplograptus notabilis  1 Dicellograptus patulosus  1 Dicellograptus salopiensis  2dichograptid stipes 1 Dicranograptus brevicaulis  1 Dicranograptus irregularis  1 Dictyonema fluitans  1 Didymograptus superstes  1glyptograptid [narrow form] 1 Glyptograptus euglyphus  2 Hustedograptus teretiusculus  3 Leptograptus validus  1 Nemagraptus gracilis  2 Nemagraptus subtilis  1 Normalograptus  1 Normalograptus brevis  2 Pseudoclimacograptus modestus  3 Pseudoclimacograptus scharenbergi   3 Reteograptus geinitzianus  2 Thamnograptus  1B. With  N. gracilisAmphigraptus  1 Amplexograptus  spp 2 Amplexograptus arctus  1 Amplexograptus leptotheca  2 Amplexograptus perexcavatus  5 Climacograptus  spp 6 Climacograptus antiquus  6 Climacograptus antiquus lineatus  1 Climacograptus bicornis  23 Climacograptus bicornis bicornis  2 Climacograptus bicornis peltifer  5 Climacograptus bicornis tridentatus  2 Corynoides  spp 13 Cryptograptus  4 Cryptograptus tricornis  28 Dicellograptus  spp 4 Dicellograptus alabamensis  1 Dicellograptus divaricatus  4 Dicellograptus exilis  18 Dicellograptus geniculatus  1 Dicellograptus intortus  18 Dicellograptus patulosus  6 Dicellograptus rigidus  8 Dicellograptus salopiensis  18 Dicellograptus sextans  13dichograptid stipes 1 Dicranograptus  spp 3 Dicranograptus brevicaulis  7 Dicranograptus celticus  3 Dicranograptus cyathiformis  1 Dicranograptus furcatus  1 Dicranograptus irregularis  1 Dicranograptus nicholsoni   6 Dicranograptus ramosus  8 Dicranograptus ramosus spinifer  2 Dicranograptus rectus  13 Dicranograptus tardiusculus  7TABLE 1 Continued  Dicranograptus ziczac  1 Dictyonema fluitans  1 Didymograptus serratulus  1 Didymograptus superstes  28diplograptid indet. 1 Diplograptus compactus  1 Diplograptus foliaceus  5 Glossograptus  1 Glossograptus hincksii   8 Glossograptus hincksii fimbriatus  7 Glyptograptus  spp 3 Glyptograptus euglyphus  12 Hallograptus  spp 7 Hallograptus bimucronatus  3 Hallograptus mucronatus  s.l. 15 Hustedograptus teretiusculus  14 Lasiograptus  spp 6 Lasiograptus costatus  8 Lasiograptus harknessi   3leptograptid 2 Leptograptus  1 Leptograptus ascendens  2 Leptograptus validus  2 Leptograptus validus incisus  2 Nemgraptus pertenius  2 Normalograptus  spp 3 Normalograptus brevis  5 Orthograptus  spp 6 Orthograptus apiculatus  2 Orthograptus calcaratus acutus  10 Orthograptus calcaratus  group 13 Orthograptus whitfieldi   3 Pseudoclimacograptus  spp 3 Pseudoclimacograptus modestus  8 Pseudoclimacograptus scharenbergi   17 Pseudoclimacograptus stenostoma  2 Retiograptus geinitzianus  4 Thamnograptus  spp 4 Thamnograptus capillaris  2C. with  C. bicornis  s.l. Amphigraptus  1 Amplexograptus  spp 2 Amplexograptus arctus  1 Amplexograptus leptotheca  7 Amplexograptus perexcavatus  5climacograptid 1 Climacograptus  spp 5 Climacograptus antiquus lineatus  1 Climacograptus antiquus  6 Climacograptus bicornis bicornis  1 Climacograptus bicornis peltifer  5 Climacograptus bicornis tridentatus  2 Corynoides  spp 20 Corynoides calicularis  3 Corynoides serpens  1 Cryptograptus  spp 7 Cryptograptus tricornis  34 Dicellograptus  spp 9 Dicellograptus angulatus  1 Dicellograptus divaricatus  1 Dicellograptus exilis  17 Dicellograptus intortus  17 Dicellograptus patulosus  4 Dicellograptus rigidus  7 Dicellograptus salopiensis  18 Dicellograptus sextans  11dicranograptid stipe 1 Dicranograptus  spp 2 Dicranograptus brevicaulis  4 REVISED GRAPTOLITE BIOSTRATIGRAPHY FOR THE LOWER CARADOC 98  as having a similar range. It has often been used as aproxy for  N. gracilis  in age assignments for the SouthernUplands, though sometimes recorded only from doubt-ful fragments. The  gracilis  Biozone is informally dividedinto two intervals: a lower interval, earlier than the firstappearance of   Climacograptus bicornis  s.l.; and an upperinterval in which the ranges of   N. gracilis ,  C. bicornis  s.l.and several other graptolite taxa overlap. This upperinterval is approximately equivalent to the lower part of the  bicornis  Biozone as recognized in North America(e.g. Berry 1960; Williams 1995) and elsewhere (e.g.Finney  et al  . 1999). Williams (1994) has suggested that a bicornis  Biozone can be distinguished in Scotland.Many lower Caradoc graptolites are long-ranging(e.g. Finney  et al  . 1999, fig. 2). Thus,  Climacograptusbicornis  s.l. extends above the range of   N. gracilis  andinto the  wilsoni   Biozone as recognized by Williams(1994). Most of the graptolite species that occur in thelowermost Caradoc of Scotland (see Rushton  et al  .1996b; Rushton 2001b; Armstrong  et al  . 1998) prior tothe first occurrence of   C. bicornis  s.l. have ranges thatalso overlap with that species, and are also recordedfrom horizons regarded as being of   peltifer  Biozone age,as defined by Elles (1925). Indeed, the  peltifer  Biozoneappears to be, at best, an interregnum without anyage-diagnostic species: graptolites that Elles (1925)identified as diagnostic of the  peltifer  Biozone, namely Dicellograptus patulosus  and  Dicranograptus furcatusminimus , have subsequently been recorded with  N. gracilis  or  Didymograptus  (s.l.)  superstes  (see Floyd1999, appendix 2; herein Appendix). The situation isexacerbated by the questionable subspecific status of  Climacograptus bicornis peltifer  itself (see Riva 1976;Webb  et al  . 1993, p. 8), which overlaps with the range of  N. gracilis  (Table 1). Thus, distinction of the  peltifer Biozone is highly problematical, particularly at spotlocalities and from stratigraphically incomplete sectionsthat do not include zone-specific graptolites, circum-stances that are typical of many graptolite-bearinglocalities in the Southern Uplands (Finney & Bergström1986). Nevertheless, the  peltifer  Biozone has beenwidely, though tentatively, recognized in the Ordovicianof southern Scotland (e.g. Williams 1994; Stone 1995;Floyd 1999; Rushton 2001a), partly for reasons of historical continuity and partly for pragmatic reasonsinvolving the construction of geological maps. Lower Caradoc graptolite-bearing localities in Scotland In order to determine the distinctiveness of the  peltifer Biozone, the faunas from more than 160 lower Caradocgraptolite-bearing localities in southern Scotland havebeen assessed. The largest number of localities occurs inthe New Cumnock–Leadhills district of the SouthernUplands (Fig. 1, Sheet 15), comprising material from 82localities collected during surveying since the nineteenthcentury (e.g. Peach & Horne 1899, Macconochie,and more recent collections including those made byMcMurtry from the Bail Hill area and identified byProf. R.B. Rickards, and those collected by R.A. Smith, TABLE 1 Continued  Dicranograptus celticus  3 Dicranograptus cyathiformis  1 Dicranograptus nicholsoni   11 Dicranograptus ramosus  8 Dicranograptus ramosus spinifer  5 Dicranograptus rectus  13 Dicranograptus tardiusculus  6 Dicranograptus ziczac  4 Didymograptus superstes  20diplograptid indet. 2 Diplograptus  1 Diplograptus compactus  1 Diplograptus foliaceus  4 Glossograptus  spp 5 Glossograptus hincksii   9 Glossograptus hincksii fimbriatus  5 Glyptograptus  spp 2 Glyptograptus euglyphus  12 Hallograptus  spp 4 Hallograptus bimucronatus  3 Hallograptus mucronatus  s.l. 12 Hustedograptus teretiusculus  13 Lasiograptus  spp 8 Lasiograptus costatus  7 Lasiograptus harknessi   4leptograptid 3 Leptograptus  1 Leptograptus ascendens  2 Leptograptus flaccidus  1 Leptograptus grandis  1 Leptograptus validus  2 Leptograptus validus incisus  1 Nemagraptus  spp 9 Nemagraptus gracilis  23 Nemgraptus pertenius  1 Normalograptus  spp 4 Normalograptus brevis  6 Orthograptus  spp 9 Orthograptus apiculatus  7 Orthograptus calcaratus acutus  8 Orthograptus calcaratus basilicus  1 Orthograptus calcaratus  group 12 Orthograptus whitfieldi   3 Pseudoclimacograptus  spp 7 Pseudoclimacograptus modestus  8 Pseudoclimacograptus scharenbergi   17 Pseudoclimacograptus stenostoma  1 Retiograptus geinitzianus  2 Thamnograptus  spp 3 Thamnograptus capillaris  2 Thamnograptus scoticus  1Note that species formerly used to identify the  peltifer  Biozone,such as  A. leptotheca , often occur with  N. gracilis .  C. bicornis peltifer  also occurs in assemblages with  N. gracilis . Somegraptolites co-occurring with  N. gracilis  (e.g.  C. calicularis ) arenot included in the complement of graptolites for the ‘  gracilis ’Biozone given in Table 2. This is because not all horizonsbearing  Nemagraptus  were srcinally identified by the BGSas  gracilis  Biozone. Some species demarcate the lowermostCaradoc, particularly  D. alabamensis  and possibly  D. geniculatus . Many taxa which are recorded from youngerhorizons, and co-occur with  C. bicornis  s.l., are absent from thelowermost Caradoc, particularly  C. bicornis  s.l. , A. leptotheca,O. apiculatus  and many others (see also Fig. 2). The list groupsthose identifications made as ‘a ff  ’, ‘cf’ or ‘?’, so that the numberof occurrences should be considered approximate. For the fullsrcinal identifications of taxa see the Appendix. M. WILLIAMS  ET AL.  99  for which, see Rushton 2003a). The district is also thelocus of the type section for the base of the GlenkilnShale Formation of the Mo ff  at Shale Group (Floyd2001, 2003). Most of the graptolite data from thisarea are taken from unpublished BGS reports and aresummarized in the Appendix.Other key datasets used for this study are from: theCarrick–Loch Doon district (Fig. 1, Sheet 8) where 32localities are identified as  gracilis  or  peltifer  Biozone(Floyd 1999, appendix 2); the Rhins of Galloway(Fig. 1, sheets 1 & 3), where 22 localities are identified asbeing of   gracilis  or  peltifer  Biozone age (Stone 1995,appendix 2; see also the Appendix); Glenkiln Burn, inthe Lochmaben district (Fig. 1, Sheet 10), where there isa rock sequence of lower Caradoc age with severalclosely associated graptolite-bearing horizons (seeWilliams 1994); the New Galloway–Thornhill district(Fig. 1, Sheet 9), where six localities are identified aslower Caradoc (McMillan 2002, appendix 1); theKirkudbright–Dalbeattie district (Fig. 1, Sheet 5), wherefour localities are identified as lower Caradoc (Lintern &Floyd 2000, appendix 1A); and the Kirkcowan– Wigtown district (Fig. 1, Sheet 4), where six lowerCaradoc localities are identified (BGS internal reports).These areas are represented in BGS collections by manythousands of graptolites stored at BGS Edinburgh(Murchison House) and BGS Keyworth, Nottingham(Kingsley Dunham Centre). Some of the historicallyimportant and pertinent graptolite localities from theMo ff  at–Ettrick district have also been reviewed (seeLapworth 1878; Webb  et al  . 1993), though not includedin the database.The database is based on identifications made sincethe 1980 s when BGS biostratigraphical revision of theSouthern Uplands Terrane commenced (particularly thework of Adrian Rushton, Dennis White and SteveTunnicli ff  ). Unless otherwise stated, the 19 th Centurycollections made by Macconochie for Peach & Horne(1899) have been used only after revision of theidentifications. Graptolite names published by Peachand Horne cannot be accepted without checking thesrcinal material (see Rushton 2001a), and even those of Lapworth(1878),whicharegenerallymorereliable,haveto be reviewed in the light of the revision of the wholeBritish graptolite fauna made by Elles & Wood (1918). Analysis of graptolite faunas The lower Caradoc graptolite assemblages from thevarious BGS maps indicated above (see Fig. 1) have atotal complement of over 90 graptolite taxa. Of theselocalities, 80 have been referred to the  gracilis  Biozoneand 19 to the  peltifer  Biozone, some questionably. Theother localities were referred more circumspectly to liewithin the range of the  gracilis  or  peltifer  biozones. Overhalf of the localities studied were of questionable ageassignment, indicating the problems associated withdistinguishing the  gracilis  and  peltifer  Biozones. None of the graptolite assemblages yielded the totalfauna, so that each is a subset of the total graptolitesavailable, controlled by palaeoecological, temporal, preser-vational and collection di ff  erences at each locality. Thoughmost localities yield assemblages of between 3 and 15 taxa,overall assemblages vary from very low- to very high-diversity, ranging between 1 and 25 taxa (see Appendix). In addition to the graptolite-bearing localities fromthe areas indicated in Figure 1, three faunas of the  gracilis  Biozone are considered definitively to lie belowfaunas bearing  C. bicornis  s.l.: they are, (1) in theCurrarie Formation of the Northern Belt of theSouthern Uplands Terrane (see Armstrong  et al  . 1998),(2) the Craigmalloch Formation (Rushton  et al  . 1996b)and (3) the Superstes Mudstone Formation (Rushton2001b). These assemblages are critical for assessing theoverall ranges of the graptolites, as they indicate whichspecies are restricted to the lowermost Caradoc inScotland, and which species occur above that interval(see Table 1).Reconstruction of the distribution of the graptolitetaxa is based on comparison between the complementof graptolites recorded from localities referred to the F  . 1. British Geological Survey map areas (sheet numbers inparentheses) where lower Caradoc graptolite assemblagesin the Southern Uplands of Scotland have been assessedin this paper. The various districts are detailed in thefollowing sources: a, Rhins of Galloway (Stone 1995);b, Kirkcowan-Wigtown (see Tunnicli ff   1990); c,Kirkudbright-Dalbeattie (Lintern & Floyd 2000); d,Carrick-Loch Doon (Floyd 1999); e, New Galloway-Thornhill (McMillan 2002); f, Lochmaben, Glenkiln Burn(see Rushton 1999; also Williams 1994); g, NewCumnock-Leadhills (see Williams  et al  . 2002); h, Mo ff  at-Ettrick (see Webb  et al  . 1993 and references therein). Allof the localities are summarized in the Appendix. REVISED GRAPTOLITE BIOSTRATIGRAPHY FOR THE LOWER CARADOC 100   gracilis  and  peltifer  biozones (Table 2) and on a com-parison of graptolite co-occurrences (e.g. Table 1), aprocess made feasible by the compilation of the data-base. This analysis indicates that distinction of the  peltifer  Biozone, in the sense of Elles (1925), is unten-able. Assemblages referred to the  peltifer  Biozone do notyield a stratigraphically discrete assemblage (Table 2).For example, using the existing diagnoses of graptolitebiozones, the  peltifer  Biozone is identified by the species Amplexograptus leptotheca, Diplograptus foliaceus,Lasiograptus costatus, L. harknessi, Climacograptus antiquus, and  C. bicornis  s.l., including  C. bicornis peltifer , and by the absence of   Nemagraptus  spp. , Leptograptus grandis, D. superstes, D. geniculatus ,  D. alabamensis  and Dicranograptus irregularis  (e.g. see Stone 1995, appendix2; see also Rushton 2001a; Rushton 2003b). Except forthe latter three species (see Table 1A), which are diag-nostic of the lowermost part of the Caradoc (sub- C.bicornis ), all of the graptolites that are typically used toidentify the  peltifer  Biozone have now been recorded ashaving ranges that overlap with those of   N. gracilis  and D. superstes  (Tables 1B, C, 2A, B). Indeed, those grap-tolites that most commonly occur with  N. gracilis  inScotland form an almost identical set with those occur-ring with  C. bicornis  s.l. (Table 1B, C). In addition,comparison of the total complement of graptolitesrecorded from horizons referred to the  peltifer  Biozoneand those from the  gracilis  Biozone is essentially thesame, apart from  N. gracilis . The few exceptions are therarely occurring  Corynoides calicularis, Glossograptusarmatus  and  Leptograptus flaccidus  s.l., all of which haveranges reported to overlap with  N. gracilis  elsewhere(e.g. see Elles & Wood 1901–1918; Bettley  et al  . 2001),including localities in the Southern Uplands. As thereappear to be no species unquestionably arising for thefirst time in assemblages that have been identified as the  peltifer  Biozone, the zone remains without adequatedefinition. It appears to represent either: an interregnumbetween  N. gracilis -bearing and  C. wilsoni  -bearingstrata; or it may have been identified on the basis of assemblages that lack  N. gracilis  or  D. superstes , poss-ibly because of collection failure, or because thosespecies happen to be absent in a particular assemblage.Indeed, many low-diversity assemblages (particularlythose with fewer than 5 identified species) often lack  N. gracilis  and  D. superstes , and were identifiable as  peltifer ,  peltifer  (?), or  gracilis-peltifer , often on the occurrence of species such as  A. leptotheca  (Appendix). In contrast,more precise age-assignments are provided by thoseassemblages with 10 or more species, nearly all of whichindicate a horizon within a single graptolite biozone orsubzone (Appendix). In the case of the SouthernUplands (Fig. 1), nearly all of these indicate a horizon ator above the first occurrence of   C. bicornis  s.l. Redefining the lower Caradoc graptolite biozones inScotland The reconstruction of graptolite ranges presented inFigure 2, which is based on the compilation of graptoliteco-occurrences in the database and on previously pub-lished data, indicates that problems of distinguishingthe  gracilis  from the  peltifer  Biozone (sensu Elles 1925)cannot be resolved by using first appearance data.However, the former  gracilis  to  peltifer  interval can besubdivided by redefining the  gracilis  Biozone as apartial-range zone and recognizing a succeeding  Clima-cograptus bicornis  Biozone, with a lower  Orthograptusapiculatus-Dicranograptus ziczac  Subzone and an upper Climacograptus wilsoni   Subzone.The  gracilis  Biozone is represented by graptoliteassemblages such as those from the Superstes Mudstoneand Craigmalloch formations of the Midland Valley (seeRushton 2001b; Rushton  et al  . 1996b) and CurrarieFormation of the northernmost Northern Belt of theSouthern Uplands (see Armstrong  et al  . 1998). Theseassemblages are older than the first appearance of   C.bicornis  s.l. Species that are restricted to this intervalinclude  D. irregularis, D. alabamensis  and probably  D. geniculatus  (see Table 1 and Fig. 2). The presence of  Leptograptus validus  s.l. in one of these faunas, from theCurrarie Formation (see Armstrong  et al  . 1998), mayindicate a high level in the sub- C. bicornis -bearing partof the Caradoc, equivalent to the  L. validus  Subzoneof Bettley  et al  . (2001, fig. 7). These assemblages areequivalent to the  gracilis  Biozone as identified in NorthAmerica and elsewhere (i.e. as a partial-range zone; seeBerry 1960; Finney  et al  . 1999).The  bicornis  Biozone and its lower division, the apiculatus-ziczac  Subzone, is marked by the first appear-ance of the eponyms. Although numerous other taxathat co-occur with  C. bicornis  s.l. (Table 1C) make theirfirst appearance in Scotland at approximately this level(Fig. 2),  O. apiculatus  is selected to name the Subzonebecause it is based on excellent material (e.g. see Bulman1946) and has been widely recognized, and  D. ziczac because it is a distinctive form, often readily recogniz-able in the field. The lower part, at least, of the  bicornis Biozone contains  N. gracilis  and is equivalent to theupper part of the  gracilis  Biozone as formerly recognizedin Scotland, in which the ranges of   D. superstes, N. gracilis ,  C. bicornis  s.l.,  O. apiculatus, D. ziczac  andmany other species overlap (Table 1). The occurrence of  O. apiculatus  in these assemblages might also signal asimilar level to the lower part of the  foliaceus  graptoliteBiozone of southern Britain (see Bettley  et al  . 2001),though in Scotland it is clear that  N. gracilis  and  O.apiculatus  have overlapping ranges (Table 1 and Fig. 2).Graptolites that are frequently associated with  C.bicornis  s.l. and therefore appear to have their firstoccurrences at about the same or higher level as thatspecies are:  C. bicornis peltifer, A. leptotheca , Dicranograptus tardiusculus, D. ziczac  and  O. apiculatus (see Table 1). Less common graptolites which alsoappear in this interval are  Amplexograptus arctus  and Dicranograptus celticus  and more than 20 other species,though some are known from apparently older horizonsin southern Britain (e.g.  C. antiquus, D. foliaceus, D.salopiensis, H. mucronatus ; see Bettley  et al  . 2001),Scandinavia (e.g.  C. antiquus, H. mucronatus, D. exilis, M. WILLIAMS  ET AL.  101
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