Upper Ordovician graptolites from the D. complanatus ...

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O?pulcherrimus] \ gre y flag s detrita l limeston e dark grey. - brow n dark gre ...... which it occurs (D. complanatus Zone sensu Ross and Berry 1963) in contrast ...

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Upper Ordovician graptolites from the D. complanatus Zone of the Moffat and Girvan districts and their significance for correlation S. H E N R Y WILLIAMS

Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada


At its type development at Dob's Linn, southern Scotland, the Dicellograptus complanatus Zone is represented by two thin black shale bands some 9 m above the base of the Upper Hartfell Shale Formation. Contrary to previous reports, the fauna is moderately diverse with an assemblage including Dicellograptus complanatus Lapworth, D. minor Toghill, Climacograptus miserabilis Elles and Wood, C. tubuliferus Lapworth, Orthograptus socialis (Lapworth), Orthoretiolites? pulcherrimus (Keble and Harris) and Plegmatograptus? sp. indet. At Girvan the D. complanatus Zone is contained within the Mill Formation of the Upper Whitehouse Group and the lower part of the Shalloch Formation. The basal Mill Formation yields a still more diverse low D. complanatus Zone fauna, with the addition of Dicellograptus alector Carter? (? = D. gravis Keble and Harris), D. cf. minor Toghill, Orthograptus cf calcaratus (Lapworth) and Glyptograptus? cf. occidentalis Ruedemann. The D. complanatus Zone correlates with the D gravis (Ea4) and C. uncinatus (Bol) zones of Australia, and probably with the C. pygmaeus and C. manitoulinensis zones of north-eastern North America.

The upper Ordovician Dicellograptus complanatus Zone was first utilised as a biostratigraphical interval by Lapworth (1880), based on a thin graptolitic band within the U p p e r Hartfell Shale at D o b ' s Linn, some 10 miles north-east of Moffat. T h e zone lies between the Pleurograptus linearis and Dicellograptus anceps zones (Fig. 1); zonal boundaries are within non-graptolitic portions of the U p p e r Hartfell Shale and consequently m a y not be defined accurately. L a p w o r t h (1882) also recognised the presence of the D. complanatus Z o n e at Girvan, where it is n o w k n o w n to span the Mill Formation of the U p p e r Whitehouse G r o u p and the lower half of the Shalloch Formation (Fig. 1). T h e mixed shelly and graptolitic fauna of the Girvan succession permits correlation between the graptolite zonal sequence and shelly-based chronostratigraphy. This suggests that the D. complanatus Z o n e is contained entirely within the lowest Ashgill Pusgillian Stage.


T h e presence of a thin, graptolitic black shale band towards the base of the largely grey and non-graptolitic U p p e r Hartfell Shale Formation was first noted Scott. J. Geol. 23, (1), 65-92, 1987

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BRITISH GRAPTOLITE ZONE 1 P. acuminatus G. persculptus C?



Extraordinarius Band HIGH MAINS SS. P. pacificus Anceps ^ ^ ™ Bands « ^ " D. complexus UR.

P. acuminatus G. persculptus





C? extraordinarius


Rawtheyan |

D. ornatus & C. latus

D. anceps LR.



Complanatus D. complanatus


Cautleyan fauna not known


C. uncinatus


D. gravis


D. hians kirki



o O m





03 13



P. linearis


ir w


0_ Q_

Onnian not to scale





FIG. 1. Upper Ordovician successions at Dob's Linn and Girvan, and their correlation with British and Australian chronostratigraphy and graptolite biostratigraphy. (1) Williams 1982a; (2) Ingham 1979; (3) Ingham and Williams 1974; (4) Ingham 1978; (5) Harper 1984; (6) VandenBerg 1981; (7) VandenBerg et al. 1984.

by Lapworth (1878, p. 316, fig. 28) in his major synthesis of the Moffat Shale Group. Within this publication he identified graptolites from the horizon as "Dicellograptus forchhammeri (Geinitz), Climacograptus scalaris? (Hisinger) and Diplograptus truncatus Lapworth". Subsequently Lapworth (1880) referred the first and last taxa to the new species Dicellograptus complanatus and Diplograptus socialis, which he also recorded from equivalent horizons at Girvan and in Co. Down, Northern Ireland. Elles and Wood (1906) revised the third taxon from Lapworth's list; they erected the new subspecies Climacograptus scalaris miserabilis, based on Lapworth's own specimens. The term " D . complanatus Zone" was first adopted by Lapworth (1879-80, p. 198), who stated that it contained "but few specific forms in addition to the characteristic species". He did, however, recognise the biostratigraphical interval from many localities including the Moffat area, Girvan, Co. Down and Scania, southern Sweden. The lithostatigraphical term "Complanatus Band" was sub-

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sequently adopted, whilst the zone gained international recognition, particularly in North America (see Finney 1982). Elles (1925, p. 348) followed Lapworth (1879-80) in considering the fauna from the Complanatus Band to be of low diversity and somewhat stunted, stating that is was "largely a dwarfed fauna and may be a purely pathological horizon". She considered (op. tit., p. 343; 1937, p. 487) that the zone corresponded to the upper part of the Pleurograptus linearis Zone when found in thicker, more continuously graptolitic strata. Davies (1929, p. 18) additionally recorded Mesograptus spp., Climacograptus supernus Elles and Wood and C. normalis Lapworth from the Complanatus Band at Dob's Linn. These identifications are, however, somewhat doubtful, as C. normalis and C. longispinus supernus are now considered characteristics of the Dicellograptus anceps and later zones (Williams 1982a, also see the systematic description of C. tubuliferus). No further descriptions of the fauna or lithology were made until Toghill (1970) redescribed the three taxa recorded originally and clarified the confusion caused by Elles' (1925) remarks on the zone. Ingham (in Ingham and Williams 1974) proved the existence of a second thin black band 1 cm thick some 0-4 m above the main band. This was demonstrated to lie just over 9 m above the base of the Upper Hartfell Shale at Dob's Linn (Fig. 1), rather than at 1-5-1 8 m as recorded by Toghill (1970) for the Main Cliff section of this locality. The two bands have now become known informally as the lower and upper Complanatus Bands. The lower band is about 4 cm thick and is subdivisible into three distinct black shale units, the basal part being separated from the middle by a narrow interval of pale grey mudstone. The top part is separated from the middle by a thin metabentonite. Two additional graptolite taxa have been found from the lower Complanatus Band during this study, giving a faunal list which includes Dicellograptus complanatus Lapworth, D. minor Toghill, Climacograptus miserabilis Elles and Wood, C. tubuliferus Lapworth and Orthograptus socialis (Lapworth). Unidentifiable Plegmatograptus? fragments also occur rarely. The fauna occurs along a number of discrete laminae, the discovery of each taxon requiring careful splitting of dried samples at precise levels. The upper Complanatus Band has only one graptolitic lamina which yields fairly common D. complanatus, rare Orthoretiolites? pulcherrimus (Keble and Harris) (one specimen) and an inarticulate brachiopod Barbatulella lacunosa Williams and Lockley. The band is overlain directly by about 15 mm of uniformly laminated pale grey mudstone. This is followed by about 13 m of apparently non-graptolitic, locally bioturbated mudstone before the first Anceps Band is reached. The lithological transitions associated with the base of the lower Complanatus Band and top of the upper Complanatus Band have been described in detail by Williams and Rickards (1984) and Williams and Lockley (1983). The total thickness of the Upper Hartfell Shale is now known to be approximately 28 m (Ingham 1979), although Lapworth (1878) originally recorded it as 30 feet. The earlier smaller figure was due probably to measurements taken from the faulted and rotated Main Cliff section.

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D. complanatus and O. socialis were recorded from Girvan by Lapworth (1882) during his original work. Major reassessment of upper Ordovician stratigraphy and structure in this area by Ingham (1978; Ingham and Williams 1974) has permitted a more accurate description of the lithologies and faunas than that summarised for the D. complanatus Zone at Girvan by Toghill (1970). The following discussion is restricted to the Upper Whitehouse Group and overlying beds, many of which have been studied recently for their brachiopod fauna by Harper (1984); lithostratigraphical terminology employed here is taken from those publications by Ingham and Harper, and from unpublished data by J. K. Ingham (pers. com.). The lowest formation of the Upper Whitehouse Group at the Shalloch Mill and Whitehouse shore localities (Ingham and Williams 1974, figs. 29, 30) is the Myoch Formation. This consists of largely red and green mudstones and a variety of coarser grained lithologies. It is non-graptolitic but yields a low diversity trilobite fauna including cyclopygaceans. One coarser grained calcarenite horizon high in the Penwhapple Burn section (Harper 1984, text-fig. 2 explanation) has yielded Tretaspis of the ceriodes species group, indicating a late Caradoc age (Ingham 1978, p. 171). Such an age is supported by the associated, fairly diverse brachiopod fauna (Harper 1979, p. 441). The Myoch Formation is overlain by the Mill Formation. It has been divided informally (J. K. Ingham, pers. com.) into the mottled shale, dark shale, calcareous flags, mud clast conglomerate and Black Neuk shale members (Fig. 2). The mottled shale and Black Neuk shale members yield cyclopygacean trilobites, whilst the dark shale member contains rare trilobites with the relatively rich graptolite fauna detailed on Figure 2. This assemblage contains elements suggestive of both the Pleurograptus linearis and D. complanatus zones as recognised at Dob's Linn. It is therefore likely that the dark shale member lies somewhere close to this zonal boundary, as suggested by Ingham (1978, p. 171). If D. complanatus is taken as restricted to its own zone, the dark shale member must be considered as very early D. complanatus Zone. The fauna indicates that the unit must correlate with part of the unfossiliferous Upper Hartfell Shale below the lower Complanatus Band at Dob's Linn. The matrices of the mud clast conglomerate member and basal Black Neuk shale member contain brachiopods and trilobites including representatives of the Tretaspis seticornis (Hisinger) species group, namely T. hadelandica subsp., suggesting that the lowest Ashgill Series (probably Pusgillian Stage) is here represented. Confirmation of a Pusgillian age is provided by the diverse brachiopod fauna (Harper 1979, p. 441). Ingham and Wright (1970) conclude that at least part of the D. complanatus Zone is equivalent to a portion of the Pusgillian Stage and that the early part of the P. linearis Zone is Onnian in age. Further evidence for this

mottled shale member

dark shale member

calcareous flags member

mud clast conglomerate member

Black Neuk shale member



- :




mottled pale green shale

dark shale with calcareous seams

grey flags with calcareous seams

mud clast conglomerate

dark grey shale bright green shale

dork greygreen shale


complanatus"\ 0. cf. minor I D. alector [I O. cfcalcaratus\\ 0. socialts \ O?pulcherrimus] \


0. complanatus D. cf. minor G?cf. occidentalis 0? pulcherrimus



alector socialis



dark shale member (detail)

mottled pale green

pale grey

dark grey

mottled dark grey

mottled grey

dark grey

dark grey- brown

detrital limestone

grey flags

FIG. 2. Stratigraphy of the Mill Formation at Myoch Bay and Whitehouse shore, Girvan (informal, unpublished member names from Ingham, pers. com.), with details of lithologies and graptolite occurrences in the dark shale member.

D. complanatus Zone

Dictyonema Zone

Dionide Beds


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S. H .


correlation is found within the Venstop Formation of Oslo, Norway and the Fjacka Shale of Sweden, where P. linearis Zone graptolites can occur in association with early Ashgill trilobites. Apparently conflicting evidence between the correlation of shelly and graptolitic biostratigraphy at this level has been discussed recently by Williams and Bruton (1983). The Upper Whitehouse Group is terminated abruptly by the widespread deposition of a basal, unfossiliferous grey mudstone member which defines the base of the overlying Shalloch Formation. The first fossiliferous horizon in this formation consists of a thin detrital limestone some 9 m above the base, which yields D. complanatus and Orthograptus cf. amplexicaulis (Hall). Pusgillian or Cautleyan trilobites occur in similar detrital limestones at a somewhat higher stratigraphical level. Dicellograptus anceps (Nicholson) is found at about 180 m above the base of the formation, signifying the presence of the following D. anceps Zone as defined at Dob's Linn. CORRELATION WITH SEQUENCES IN AUSTRALIA AND NORTH AMERICA

The succession of Ordovician graptolites in Australia was summarised by VandenBerg (1981), whose discussion was based almost entirely on the type sequence of Victoria. The upper part of the Eastonian Stage, incorporating the zones of Dicranograptus hians kirki (Ea3) and Dicellograptus gravis (Ea4) are here considered to correlate with the P. linearis Zone and lower part of the D. complanatus Zone of Britain. The preceding Climacograptus baragwanathi Zone yields an assemblage similar to that of the Dicranograptus clingani Zone of Britain, including several species of Dicranograptus, Climacograptus caudatus Lapworth, Neurograptus margaritatus (Lapworth) and Corynoides. The D. hians kirki Zone is named after the last Dicranograptus species, but the assemblage including Dicellograptus elegans (Carruthers), Leptograptus capillaris (Carruthers) and Climacograptus tubuliferus Lapworth is otherwise similar to the fauna of the P. linearis Zone at Dob's Linn (Williams 1982). VandenBerg (1981, p. 7) considered Dicellograptus gravis Keble and Harris to be a senior synonym of D. alector Carter; this synonomy is here regarded as likely, but the types of D. gravis are insufficient to make a positive conclusion (see systematic section). The association of D. gravis (? = D. alector) and ilReteograptus" pulcherrimus (Keble and Harris) in the D. gravis Zone suggests a correlation with the low D. complanatus Zone fauna of the Myoch Formation at Girvan. Correlation with the lower and middle Bolindian Stage is uncertain, due to insufficient forms in common with the Complanatus Bands of Dob's Linn. The occurrence of "R. "pulcherrimus in the Climacograptus uncinatus Zone, together with Climacograptus longispinus longispinus Hall which is thought to be the predecessor of the D. anceps Zone subspecies C. longispinus supernus, however suggests a correlation with part of the D. complanatus Zone. The late Bolindian Zone of

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Dicellograptus ornatus and Climacograptus latus has a typical D. anceps Zone fauna, including elements of both the Dicellograptus complexus and Paraorthograptus pacijicus subzones as recognised at Dob's Linn (Williams 1982a). An Australian succession including Ordovician-Silurian boundary graptolites was unknown until VandenBerg et al. (1984) documented a faunal sequence spanning the Climacograptus? extraordinarius to Parakidograptus acuminatus zones. This faunal succession at Darraweit Guim, central Victoria, compares remarkably well with that of Dob's Linn (Williams 1983) and the two may be correlated precisely. Problems surrounding Ordovician graptolite zonation in North America were discussed by Finney (1982), who outlined the discrepancies between zonal schemes adopted for the various regions. It appears that his Orthograptus quadrimucronatus Zone correlates with the P. linearis Zone of Britain, while the zones of Climacograptus pygmaeus and C. manitoulinensis defined by Riva (1969) equate largely with the D. complanatus Zone. The late Ordovician interval equivalent to the D. anceps Zone has been referred to the D. complanatus and D. ornatus zones. Reliable correlation with the British sequence is not possible without further clarification and definition of these biostratigraphical divisions.


The specimens were collected mostly by the author and J. K. Ingham, with a few by A. Mcleod (ex Glasgow University); they are stored in the Hunterian Museum, Glasgow University (HM C). Other material is from the Lapworth Collection (Birmingham University, BU), Gray and Toghill Collections (British Museum, Natural History, BM), the Rushton Collection (British Geological Survey, IGS) and collections in the Sedgwick Museum, Cambridge (SM A). Stippling on text-figures indicates presence of periderm. Terminology is based largely on that given by Bulman (1970).

Family DICRANOGRAPTIDAE Lapworth, 1873 Genus DICELLOGRAPTUS Hopkinson, 1871 Type species (subsequently designated by Gurley 1896, p. 70). Didymograpsus elegans Carruthers, 1867, p. 369, pi. 2, fig. 16a. Dicellograptus complanatus Lapworth, 1880 Figs 3A-H; 5A-C; 6G 1880 Dicellograptus complanatus sp. nov.; Lapworth, pp. 160-168, pi. 5, figs 17a-e.

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9fc H

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?1935 Dicellograptus complanatus Lapworth; Decker, p. 702, text-figs l a - e , 2g. 1956 Dicellograptus complanatus Lapworth; Keller, p. 70, text-fig. 4. 1970 Dicellograptus complanatus Lapworth; Toghill, p p . 12-14, pi. 4, figs 1-5, pi. 5, figs 1-5, pi. 6, fig. 1, text-figs 2 g - l , 4b. For a fuller s y n o n y m y see Toghill (1970), also see Williams (1981, pi. 1, figs. 1, 2, text-figs 3, 4), Briggs and Williams (1981, figs 1, 2) and Williams et al. (1982, figs 6,7) Lectotype. B U 1072b. T h e specimen figured by Elles and W o o d (1904, pi. 20, fig. lb) from the lower Complanatus Band, D o b ' s Linn. L a p w o r t h Collection, selected by Toghill (1970, p. 13). Material. N u m e r o u s specimens preserved flattened and in partial relief from the Lapworth Collection, I n g h a m Collection, Gray and Toghill Collections, collections of the Sedgwick M u s e u m and collected b y the author and b y A. Mcleod. Horizons and localities. Lower and upper Complanatus Bands, D o b ' s Linn. Mill Formation, U p p e r Whitehouse G r o u p and Shalloch Formation, M y o c h Bay and Whitehouse shore, Girvan. Several specimens from Scania and Rostanger, Sweden, collected by Tullberg in 1879 and n o w in the L a p w o r t h Collection, horizon and exact locality unrecorded. Diagnosis (revised). Long slender stipes, straight or gently curved, up to 1 m m wide and over 160 m m long, with an axial angle of 20 to 130°. Thecae simple with slight introversion, pronounced geniculae and straight supragenicular walls, numbering 8 - 1 4 in 10 m m . Sicula rarely preserved complete, virgella short but conspicuous, first t w o to six thecae with short mesial spines. Description. The stipes are commonly very long, reaching up to at least 160 mm, and may be straight or with a proximal concave and distal convex curvature. They normally exhibit gentle right or left-hand torsion. The stipes are 0-3-0-6 mm wide at the aperture of thl 1 , increasing to 0-5-0-7 mm at the aperture of th5' and usually reaching the maximum width of about 0-9 mm within 10 mm. Some small specimens preserved in partial relief only widen to a maximum 0-5 mm. Proximally the thecae number 11-14 in 10 mm, decreasing distally to 8-11 in 10 mm. T h l 1 and l 2 diverge horizontally from the sicula; th.21 grows horizontal initially then bends abruptly up, while th2 2 grows upward at a steady angle, resulting in a wide, rounded axil with 1-1-1-7 mm between the apertures of thl 1 and I 2 . The sicula, whose free portion is invariably missing in mature specimens (probably resorbed), bears a virgella up to 0-3 mm long, T h l 1 and l 2 bear mesial spines. Proximal thecae have slightly introverted apertures which almost fill the semicircular excavations. These normally occupy \ the total stipe width. A prominent genicular hood on all thecae occasionally gives the appearance of genicular spines in flattened material. When specimens are preserved in relief, the proximal dorsal wall is seen to possess conspicuous prothecal FIG. 3. A - H . Dicellograptus complanatus Lapworth, 1880. A - E . Lower Complanatus Band, Dob's Linn. A. HM C14484/2-3 X5. B. HM C14484/la. X5. C. HM C14484/26a. X5. D. HM C14485a. Detail of secondarily thickened axil. X 1 5. E. HM Cl4484/lb. Detail of Ppathological "growth" on dorsal wall. X15. F-H. 9 m above base of Shalloch Fm., Whitehouse shore. F. HM C13100. X10. G. HM C764/2. X2.5 H. HM C764/3. X2-5.

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folds which bend around the thickened interthecal septal nodes. Two specimens (Figs 3D, E) show large, fold-like structures on the proximal dorsal wall. These are thought to be abnormal, pathological features. The distal portion of the interthecal septa slopes at about 20° to the dorsal wall, while the proximal portion bends almost perpendicular to it. The supragenicular walls remain curved until about th8\ when they become straight and subparallel to the dorsal margin. Prothecal folds become less pronounced distally while the apertures lose their introversion, excavations become narrow and interthecal septa have a distal portion subparallel to the dorsal wall, gradually attaining an angle of about 30° proximally. Differences in appearance of internal and external moulds of thecae, and contrasts between those preserved flattened and in relief have been discussed by Briggs and Williams (1981) and Williams et al. (1982). Remarks. The gentle stipe torsion and curvature are remnants of original openly spiralled stipes (Williams 1981). The thecae are less introverted than any other Dicellograptus species, comparable only to those of D. complexus Davis, D. johnstrupi Hadding and D. carruthersi Toghill. The thecae of D. complexus are almost identical to proximal thecae of D. complanatus but the stipes are tightly spiralled, crossing several times, and have a narrower maximum width of 0-5 mm. Both D. johnstrupi and D. carruthersi have thecae with almost straight supragenicular walls and moderately developed geniculae, but the supragenicular walls slope gently and the apertures are slightly more introverted. The axil is also rather narrower and stouter. The thecal form of D. complanatus is very different from that of D. anceps (Nicholson) and the two are not closely related as has been suggested by some authors. Prothecal folds are prominent proximally when specimens are preserved in relief, a "notch" sometimes occurring at the top of the fold. They are, however, less obvious when flattened owing to differential lateral spread (Briggs and Williams 1981). In the Moffat area, D. complanatus has been found at Dob's Linn and Craigmichan Scaurs. At Girvan it is found in the upper part of the Upper Whitehouse Group and in the Shalloch Formation. The earliest occurrence is in association with a fauna suggestive of the previous P. linearis Zone and predates the level of the Complanatus Bands at Dob's Linn. D. complanatus has been recorded from Northern Ireland (Lapworth 1880; Fearnsides, Elles and Smith, 1907) but not figured. Elsewhere in Europe it has been recorded from the jonstorp Formation and the Upper Dicellograptus Shale of Sweden (see Skoglund 1963; Nilsson 1977). This Upper Dicellograptus Shale is now included in the base of the Jerrestad Mudstone (Bergstrom 1982). D. complanatus has been recorded from North America where it is apparently common and widely distributed (Decker 1935; Ruedemann 1947; Berry 1960; Churkin 1963; Riva 1969). Although D. cf. complanatus was recorded from the Bolindian Stage of Australia (Sherrard and Keble 1937; Harris and Thomas 1938; Thomas 1960), where it was used as a zone index fossil, VandenBerg (1981) concluded that the specimens were in fact representatives of D. ornatus Elles and Wood and D. minor Toghill. D. cf. complanatus and D. complanatus var. nov. were recorded from the Wufeng Shale of China by Mu (1954). These were, however, from an apparently rather later

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interval than the D. complanatus Zone of Britian and the specimens are unlikely to be D. complanatus. The only illustrated description of D. complanatus from Russia is by Keller (1956). Koren' et al. (1979) record it from the C. longispinus and P. pacijicus subzones of the Kolyma River; whilst the earlier of these probably is D. complanatus, the later occurrence is more likely to be D. afF. complexus Davies of Williams (1982a), owing to the associated assemblage which is characteristic of the D. anceps Zone (D. complexus Subzone) of Dob's Linn. Dicellograptus minor Toghill, 1970 Figs 4C, D 1904 Dicellograptus complanatus Lapworth Var. ornatus, var. nov.; Elles and Wood {pars), pp. 140-141, pi. 20, fig. 2c (non figs 2a, b text-figs 85a, b = D. ornatus). ?1945 Dicellograptus graciliramosus sp. nov.; Yin and Mu, pp. 214-216, pi. 1, figs 1, la, 2. 1970 Dicellograptus ornatus minor subsp. nov.; Toghill (pars), pp. 16-17, pi. 6, figs 5-7, text-figs 3d-g (non text-fig. 3c = D. complexus Davis, 1919). mml980 Dicellograptus ornatus minor Toghill; Koren' et al., pp. 122-123, pi. 33, figs 1, 2, text-figs 29a-d (=D. complexus). 1982a Dicellograptus minor Toghill; Williams, pp. 37-38, pi. 2, figs 9-15. 1982 Dicellograptus ornatus minor Toghill; Lenz and McCracken, fig. 3a. 1983 Dicellograptus graciliramosus Yin et Mu; Wang et al., p. 138, pi. 2, figs 11,12. Holotype. SM A19333. Figured Elles and Wood (1904, pi. 20, fig. 2c) and Toghill (1970, pi. 6, fig. 5, text-fig. 3f). From the Anceps Bands, Upper Hartfell Shale, Dob's Linn. Material. Three flattened specimens. Horizons and localities. Lower Complanatus Band, Dob's Linn. Discussion. The present material is insufficient to describe fully, but agrees well with the revision by Williams (1982a). D. minor has not been recorded previously from the Complanatus Bands and is most characteristic of the following D. anceps Zone. The Chinese species D. graciliramosus from the Wufeng Shale appears to be synonymous with D. minor, but as recorded by Williams (op. cit.) the types of the former are insufficient to justify replacing Toghill's name. Specimens figured recently by Wang et al. (1983) are, however, better preserved and clearly identical with D. minor. Dicellograptus cf. minor Toghill, 1970 Fig. 4A Material. Six specimens in partial relief.

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Horizons and localities. Dark shale m e m b e r and just below the calcareous flags member, Mill Formation, U p p e r Whitehouse G r o u p , M y o c h Bay and Whitehouse shore, Girvan. Discussion. T h e specimens differ from D. minor only in their narrower m a x i m u m width which never exceeds 0-3 m m . Specimens of D. minor from D o b ' s Linn s h o w pronounced increase in w i d t h distally, but these specimens from Girvan remain an almost uniform 0 - 2 5 - 0 3 m m t h r o u g h o u t their length. It remains a possibility that the width increase of D . minor is a function of diagenetic flattening, but such a pronounced change is not s h o w n normally by other Dicellograptus taxa. Specimens apparently identical to D . cf. minor have been found in the lower Bolindian of Australia (VandenBerg, pers. com.). D . cf. minor may, therefore, represent either an earlier form of the species which was characteristic of the early D . complanatus Zone, or a morphological variety restricted to continental margin environments.

Dicellograptus alector Carter, 1972? Figures 4B; 5 D - H ?1925 Dicellograptus gravis sp. nov.; Keble and Harris, p . 516, pi. 70, figs 2a, b . ?1972 Dicellograptus alector n. sp.; Carter, pp. 4 6 - 4 7 , pi. 1, figs 9, 11, 12, 15, text-figs 2a, g. ?1977 Dicellograptus alector Carter; Carter and Churkin, pp. 18-19, pi. 3, figs 8, 12, 13, 17. Material. O n e almost complete specimen in the Rushton Collection, three proximal ends and several distal fragments in the Gray Collection and several distal fragments collected by the author. Horizons and localities. Rushton Collection and author's specimens from the dark shale member, Mill Formation, U p p e r Whitehouse G r o u p of M y o c h Bay. Specimens in the Gray Collection are from Shalloch Mill, labelled "Ardmillan Series", and probably originate from the same horizon. Description. The stipes are robust, straight and lack torsion. Distal fragments up to 40 mm have been observed. Proximally the stipes measure 0-6-0-8 mm wide, increasing rapidly to

FIG. 4. A. Dicellograptus cf. minor Toghill, 1970. HM C13098. 9 m above base of Shalloch Fm., Whitehouse shore. X5. B. Dicellograptus alector Carter, 1972? IGS RU2966. Dark shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. Rushton Coll. X4. C-I. All from the lower Complanatus Band, Dob's Linn. C, D. Dicellograptus minor Toghill, 1970. C. HM C14490a. X5. D. HM C14474. X10. E, F. Climacograptus tubuliferus Lapworth, 1976. X5. E. HM C14489/16a. F. HM C14489/7. G. Climacograptus miserabilis Elles and Wood, 1906. HM C14487/15. Detail of distal thecae preserved in scalariform orientation. X15. H, I. Climacograptus tubuliferus Lapworth, 1876. X5. H. HM C14489/8. I. HM C14488.

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1-0 mm and widening imperceptibly to a maximum 1-2 mm in some specimens. Juvenile specimens have considerably narrower stipes and less robust overall appearance than mature ones, suggesting secondary cortical increase on both dorsal and ventral margins. Thecae number 8-9 in 10 mm proximally, this density remaining almost constant throughout the rhabdosome. N o complete sicula is present in the Girvan material and proximal development is not clear owing to the poor preservation. Both thl 1 and l 2 appear to grow downward throughout their length, whilst th2' and 2 2 grow initially down then bend upwards at the apertures of the first thecal pair. Considerable secondary thickening of the proximal end occurs during astogeny. At least the first nine thecal pairs possess prominent sub-apertural spines, produced at the sharp bend of the supragenicular wall. Proximal thecae have a slightly introverted apertural selvage, but distal apertures are simple. Interthecal septa are straight and inclined at about 20° to the dorsal wall throughout. Prothecal folds have not been observed, but may have existed prior to flattening. Remarks. T h e only t w o species described previously which are similar to those specimens from Girvan are D. alector and D. gravis. N o other Dicellograptus has such a simple, almost dichograptid thecal style. Carter's (1972) specimens of D. alector from the Phi Kappa Formation, Idaho appear identical to the present material, although her types from Nevada seem rather m o r e slender. Those figured b y Carter and C h u r k i n (1977) are less well preserved, but are similar in thecal style. D. alector is recorded by these authors as restricted to an interval equivalent to the D. clingani and P. linearis zones of Britian. T h e Australian species D. gravis has a similar thecal style and overall form, but revision of the types is necessary before it can be regarded as a senior s y n o n y m of D. alector as suggested b y VandenBerg (1981, p . 7). T h e range of D. gravis given by VandenBerg (op. cit.) appears to be slightly later than that of D. alector, equivalent to the P. linearis Z o n e and possibly extending into early D. complanatus Z o n e (Bol). T h e specimens from Girvan m a y therefore be a m o n g the latest representatives of this species, but are of potential use for intercontinental correlation.

Family DIPLOGRAPTIDAE Lap w o r t h , 1873 Genus CLIMACOGRAPTUS J. Hall, 1865 Type species (by original designation). Graptolithus 268, pi. 73, fig. 2 (pars).

bicornis J. Hall, 1847, p .

Climacograptus miserabilis Elles and W o o d , 1906 Figs 4G; 6 A - C ?1895

Diplograptus (Glyptograptus)

euglyphus Lap w o r t h var. angustus mihi; Perner,

p. 27, pi. 8, figs 14a, b. 1906 Climacograptus scalaris (Hisinger) Var. miserabilis var. nov.; Elles and Wood, p. 186, pi. 26, figs 3 a - h ; text-figs 120a-c.

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FIG. 5. A-C. Dicellograptus complanatus Lapworth, 1880. Lower Complanatus Band, Dob's Linn. X2-5. A. HM C14484/7. B. HM C14464/6. C. HM C14464/11. D-H. Dicellograptus alector Carter, 1972? Dark shale mbr., Mill Fm., Upper Whitehouse Gp. X2-5. D. IGS RU2966. Proximal detail only. Complete specimen figured Fig. 4b. Myoch Bay. Rushton Coll. E-H. Whitehouse shore and Shalloch Mill. Gray Coll. E. BM H2022. F. BM H2037. G. BM H2313. H. BM H2298. 1963 Climacograptus angustus (Perner); Skoglund, p. 40, pi. 3, figs 1, 2, 4-6, pi. 4, fig. 7, pi. 5, fig. 6. 1970 Climacograptus scalaris miserabilis Elles and Wood; Toghill, p. 23, pi. 12, figs 1-11. 1982a Climacograptus miserabilis Elles and Wood; Williams, p. 247, figs lOk-n. 1983 Climacograptus miserabilis Elles and Wood, Williams, pp. 615-6, text-figs 3f-i, ?j, 4f-i, 5a,b. (Summary only—see Williams (1983) for more extensive synonymy.) Lectotype. BU 1150, the specimen figured by Elles and Wood (1906, text-fig. 120a), from the lower Complanatus Band, Dob's Linn, is here designated lectotype. Material. The type material in the Lapworth Collection used by Elles and Wood and numerous flattened specimens. Horizons and localities. Lower Complanatus Band, Dob's Linn. Diagnosis (modified slightly after Williams 1983).

Small rhabdosome usually

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less than 20 mm long, with a rapidly attained maximum width of 1-0 mm (undeformed). Thecae may appear typically Climacograptus in style, or possess slightly curved supragenicular walls, numbering 9-11 in 10 mm. Long virgella and nema commonly present. Remarks. A full description of this species was given by Williams (1983), where it was concluded that specimens from the latest Ordovician and early Silurian may have a more Glyptograptus-like thecal style than those from elsewhere in the upper Ordovician, and that the synonymy of C. miserabilis with C. angustus, suggested by Pribyl (1951) may be true only for the stratigraphically higher material. The maximum width of specimens from the lower Complanatus Band is commonly 1-0 mm as opposed to the 0-8 mm recorded by Williams (1983) for examples from the Birkhill Shale Formation at Dob's Linn. When distorted tectonically this measurement may vary from 0-6 to 1-1 mm, although all specimens preserved in full profile view show a similar rapid attainment of the maximum width.

Climacograptus tubuliferus Lapworth, 1876 Figs 4F, H, I; 6G; 70-Q 1876 Climacograptus tubuliferus Lapw.; Lapworth, pi. 2, fig. 49. 1906 Climacograptus tubuliferus Lapworth; Elles and Wood, pp. 203-204, pi. 27, figs 8a-d, text-figs 134a-c. ?1929 Mesograptus cf. modestus; Davies, pp. 1-3, fig. 1. 1982 Climacograptus tubuliferus Lapworth; Williams, pp. 245-6, figs l l a - n . 1983 Climacograptus tubuliferus Lapworth; Williams and Bruton, pp. 170-2, figs 12c-e, 15a-n. (Summary only—see Williams and Bruton (1983) for complete listing.) Type specimen. Not yet designated. Material.

Twelve flattened, mostly fragmentary specimens.

Horizons and localities. Lower Complanatus Band, Dob's Linn. Remarks. This species has not been recorded previously from the Complanatus Bands. Although rare, the specimens agree well with material from the Lower Hartfell Shale (P. linearis Zone), showing both the thickened nema and occasional proximal membranes growing along the virgella (Figs 41; 7P, Q). VandenBerg (1981) recorded C. tubuliferus from the upper Bolindian Stage of Victoria, Australia (equivalent of D. anceps Zone). It has otherwise been recorded as restricted to earlier strata, equivalent to the D. clingani and P. linearis zones, in Australia (T. S. Hall 1902; Harris and Thomas 1955), North America (Carter and Churkin 1977) and Scandinavia (Williams and Bruton 1983).

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Type species (by original designation). p. 144, pi. 13, figs 1-10.


Lap worth, 1873

Graptolithus quadrimucronatus J. Hall, 1865,

Orthograptus cf. calcaratus (Lapworth, 1876) Figs 8 C - E Material. Many fragmentary specimens preserved in partial relief. Horizons and localities. Dark shale member, Mill Formation, Upper Whitehouse Group, Myoch Bay, Girvan. Remarks. The variation shown by this long-ranging group of diplograptids in the Lower Hartfell Shale (D. clingani and P. linearis zones) at Dob's Linn was discussed recently by Williams (1982 p. 249). The specimens from Girvan cannot be assigned to any previously described taxon, although they are presently not considered sufficiently distinct or well preserved to merit erection of a new species or subspecies. The virgella and first two thecal spines are small, while the rhabdosome reaches a maximum width of 3 0 mm and has a thecal count of 8-10 in 10 mm. The prominent median groove is preservational and it is unclear whether a median septum was originally present. Orthograptus cf. amplexicaulis (J. Hall, 1847) Figs 6D, E; 7A, B Material. Several flattened, mostly fragmentary specimens. Horizons and localities. Just below the detrital limestone 9 m above base of the Shalloch Formation, Whitehouse shore, Girvan. Remarks. These specimens from Girvan have a maximum width of about 2-5 mm, thecal count of 13-16 in 10 mm and overall form similar to O. abbreviatus Elles and Wood from the D. anceps Zone of Dob's Linn. O. amplexicaulis sensu stricto is normally considered somewhat wider with a lower thecal count (see Williams 1982, p. 251). Some larger specimens of O. socialis (Lapworth) approach similar dimensions and it is apparent that a great deal of variation exists in the O. amplexicaulis group. Orthograptus socialis (Lapworth, 1880) Figs 7 C - M ; 8F-H 1880 Diplograptus socialis, Lapw., sp. nov.; Lapworth, p. 166, pi. 4, figs 13a-c. 1907 Diplograptus (Orthograptus) truncatus var. socialis Lapworth; Elles and Wood, pp. 237-8, pi. 29, figs 7a-e, text-figs 157a-d.

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S. H . W I L L I A M S

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FIG. 7. A, B. Orthograptus cf. amplexicaulis (Hall, 1847). 9 m above base of Shalloch Fm., Whitehouse shore. X5. A. HM C13076b. B. HM C13075/7. C - M . Orthograptus socialis (Lapworth, 1880). Lower Complanatus Band, Dob's Linn. X5. C. H M C14471/3. D. HM C14471/11. E. HM C14471/10. F. HM C14471/9. G. HM C14471/8. H. HM C14471/13. I. HM C14465/5. J. HM C14465/5. K. HM C14471/5. L. H M C14465/9. M. HM C14471/12. N. Glyptograptus? cf. occidentalis Ruedemann, 1947. HM C13059. Proximal fragment preserved in relief. Dark shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. X25. O - Q . Climacograptus tubuliferus Lapworth, 1876. Lower Complanatus Band, Dob's Linn. X5. O. H M C14467/5. P. H M C14488. Complete specimen figured Fig. 41. Q. HM C14489/1.

FIG. 6. A - C . Climacograptus miserabilis Elles and Wood, 1906. Lower Complanatus Band, Dob's Linn. X10. A. H M C14472/1. B. H M C14477/la. C. H M C14466/7. D, E. Orthograptus cf. amplexicaulis (Hall, 1847). 9 m above base of Shalloch Fm., Whitehouse shore. X10. D. HM C13086. E. HM C13093/1. F. Orthoretiolites? pulcherrimus (Keble and Harris, 1934). HM C130971. Dark Shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. X10. G. Dicellograptus complanatus Lapworth, 1880 with Climacograptus tubuliferus Lapworth, 1876. HM C14464/12, 14467/9. Lower Complanatus Band, Dob's Linn. X5. H. Glyptograptus? cf. occidentalis Ruedemann, 1947. H M C13058. Dark shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. X10.

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1963. Orthograptus truncatus var. socialis (Lapworth); Ross and Berry, pp. 151-2, pi. 11, fig. 21. 1970. Orthograptus truncatus socialis (Lapworth); Toghill, p. 24, pi. 13, figs 7-9, pi. 16, fig. 7. Type specimen. Not yet designated. Material.

Many flattened specimens.

Horizons and localities. Lower Complanatus Band, Dob's Linn. Dark shale member, Mill Formation, Upper Whitehouse Group, Myoch Bay, Girvan. Diagnosis (revised). Small amplexicaulis group diplograptid up to 2-0 mm wide. Thecae everted, numbering about 13 in 10 mm. Sicula and first two thecae with small spines. Description. The rhabdosome is small, the largest specimen seen being 15 mm long. It is 0-8-1-0 mm wide proximally, increasing to an average 1-4 mm in 5 mm. Most specimens are under 8 mm long with a maximum width of 1-5 mm, although larger specimens increase slowly to a maximum of about 2-0 mm in 10 mm and narrow somewhat distally. The sicula is about 2 mm long and bears a small but conspicuous virgella. The first two thecae also bear conspicuous mesial spines up to 0-5 mm long. Remaining thecae are straight with gently inclined supragenicular walls and everted apertures which occasionally have a slight "lip". Growth fusellae are commonly visible. A short nema occasionally projects distally. Remarks. O. socialis is very similar to other amplexicaulis group diplograptids but differs from all others by its consistently small size. The rate of width increase is similar to O. pauperatus Elles and Wood, but rather less than O. abbreviatus Elles and Wood. The thecal count is rather higher than O. pauperatus, but similar to that of O. abbreviatus and it is evidently closely related to both these species. O. socialis occurs crowded on several bedding laminae of the lower Complanatus Band at Dob's Linn, and occurs in a similarly gregarious manner at Girvan. Records from outside Britain are restricted to North America (Ross and Berry 1963) and Australia (Thomas 1960). Genus


Type species (by original designation). 526, pi. 19, figs 10-13.

Lapworth, 1873.

Diplograpsus tamariscus Nicholson, 1868, p.

Glyptograptus? cf. occidentalis Ruedemann, 1947 Figs 6H; 7N Material. Five specimens in varying states of compression from flattened to almost full relief.

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Horizons and localities. Dark shale member, Mill Formation, Upper Whitehouse Group, Myoch Bay, Girvan. Description. The rhabdosome is small, measuring about 15 mm long. It widens from 0-6 mm proximally to 1-3 mm in 5 mm and reaches the maximum 1-5 mm within 8 mm. Thecae number about 12 in 10 mm proximally, reducing distally to about 10 in 10 mm. The sicula is exposed for 0-5 mm of its length in obverse view, but its total length is unknown. Thl 1 grows downwards initially until it reaches some 0-2 mm below the sicula aperture before bending upwards. Thl 2 appears to grow upwards throughout its length. The first two thecae possess mesial spines, and the sicula has a rather robust virgella. Thecae are alternate throughout the rhabdosome with gently curved interthecal septa and strongly curved, convex supragenicular walls which lack geniculae. The apertures are slightly introverted, opening into excavations which occupy \-j the total rhabdosome width. Prominent transverse grooves occur at the level of each aperture; these are probably artefacts of preservation.

Remarks. Insufficient material has been found to allow precise identification but the overall form agrees well with G? occidentalis (Ruedemann 1947, p. 409, pi. 69, fig 61). Both Ruedemann (1947) and Ross and Berry (1963) recorded the holotype of this species to have attained a maximum width of 2 mm within 7 mm. Measurements from their illustrations indicate, however, a maximum width of about 1-5 mm. Neither the holotype nor the remaining specimen figured by Ross and Berry possess complete proximal ends showing spines. Ruedemann's "G. teretiusculus occidentalis" is here raised to full specific status owing to the horizon at which it occurs (D. complanatus Zone sensu Ross and Berry 1963) in contrast to the much earlier stratigraphic range of G. teretiusculus teretiusculus (Hisinger). The proximal development of the specimens described here differs from the type species of Glyptograptus, G. tamariscus (Nicholson) in having a much squarer proximal end and spinose first thecal pair. It is therefore concluded that the Girvan material may only be assigned questionably to this genus. The only similar species from an approximately equivalent stratigraphical level to the Girvan specimens is G. tenuissimus Ross and Berry. This has a similar overall form, widening gradually from 0-6 mm to 1-6 mm, and similar thecae numbering 10-12 in 10 mm. It appears to be very much like the holotype of G? occidentalis and may be a junior synonym, although Ross and Berry (1963, p. 142) recorded it as restricted to the somewhat earlier O. quadrimucronatus Zone of North America. The prominent transverse grooves on the specimens described herein are similar to those shown by specimens of G. asiaticus Keller (Keller 1956, pi. 3, fig. 9), although this Russian species has a much more robust rhabdosome. They are probably artefacts of compression induced by diagenetic flattening of a rhabdosome with alternate thecae, although their exact mode of formation is presently unclear.

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Family RETIOLITIDAE Lap worth, 1873 Subfamily ARCHIRETIOLITINAE Bulman, 1955 Genus ORTHORETIOLITES Whittington, 1954 Type species (by original designation). 614, figs 1-13.

Orthoretiolites hami Whittington 1954, p.

Remarks. Strachan (1976) discussed the shortcomings of the Archiretiolitinae. He considered the subfamily to include two unrelated lineages; Reteograptus and Orthoretiolites were probably evolved from the diplograptids, while Plegmatograptus was evidently derived from the lasiograptids. Other members of the group, namely the late Ordovician Chinese genera Orthoretiograptus Wang et al. and Pararetiograptus Mu are known only from flattened examples (see Wang et al. 1983, p. 143 for discussion). Orthoretiolites7. pulcherrimus (Keble and Harris, 1934) Figs 6F; 8A, B 1934 Retiograptus pulcherrimus sp. nov.; Keble and Harris, pp. 178-9, pi. 22, fig. 1, text-fig. 6. 1960 Retiograptus pulcherrimus Keble and Harris; Berry, p. 96, pi. 9, figs 9, 10b. 1963 Retiograptus sp.; Ross and Berry, pi. 13, figs 5, 10, ?12 ?1963 Retiograptus pulcherrimus Keble and Harris; Ross and Berry, pp. 158-9, pi. 13, figs 13, 13a, 14. ?1964 Retiograptus pulcherrimus Keble et Harris; Obut and Sobolevskaya, p. 79, pi. 16, figs 8-11. 1966 Retiograptus pulcherrimus Keble and Harris; Berry, pp. 445-6, pi. 50, fig. 8. nonYTIl Reteograptus pulcherrimus Keble and Harris; Lenz, pp. 1950-2, pi. 1, figs 3-7 (=Orthoretiograptus denticulatus Wang et al.). non 1982 Reteograptus pulcherrimus Keble and Harris; Lenz and McCracken, figs. 3i, j ( = 0 . denticulatus). Holotype. National Museum of Victoria No. P14385. The specimen figured by Keble and Harris (1934) and by Berry (1966), from the lower Bolindian near Matlock, Victoria, Australia. Material. Many rather poor, fragmentary specimens.

FIG. 8. A, B. Orthoretiolitesl pulcherrimus (Keble and Harris, 1934). Dark shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. X5. A. HM C13030. B. HM C13044/2. C-E. Orthograptus cf. calcaratus (Lapworth, 1876). Dark shale mbr., Mill Fm., Upper Whitehouse Gp., Myoch Bay. X5. C. HM C13002. D. HM C13036. E. HM C13003/la. F-H. Orthograptus socialis (Lapworth, 1880). Lower Complanatus Band, Dob's Linn. X5. F. HM C14486/9. G. HM C14486/10. H. HM C14471/3.

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Horizons and localities. Dark shale member, Mill Formation, Upper Whitehouse Group, Myoch Bay, Girvan. Description. The rhabdosome of the Girvan specimens reaches a little over 50 mm long, although Keble and Harris originally recorded a maximum length of 120 mm. It widens from about 1 mm proximally to be maximum width in about 20 mm. The maximum width in oblique orientation is 3 mm, but this is reduced to about 2 mm when preserved in normal profile view. Proximally the thecae number about 10 in 10 mm, reducing distally to 7-8 in 10 mm. Proximal development has not been observed. All thecae possess single apertural spines up to 0-8 mm long. Thecae are simple, having everted apertures with thickened lists and straight, sloping supragenicular walls. Interthecal septa are straight and thickened; they are attached to a zig-zag virgula, related to the alternating thecae. Proximally the rhabdosome is moderately well sclerotised, but distally sclerotisation is weak or lacking. Remarks. The original description of O? pulcherrimus was a largely qualitative, generic one with no diagnostic, specific information. This was rectified by Berry (1966) who recorded the types to vary from 2-8-3-0 mm wide and to have a distal thecal count of 7-10 in 10 mm. The types possess short apertural spines up to 0-7 mm long, the internal structure is clearly similar to that of the material described here, and they originate from an equivalent horizon. The specimen figured as R. pulcherrimus by Berry (1960) appears similar to those specimens described here, as does the form figured (but not described) as Retiograptus sp. by Ross and Berry (1963). Those described as "R. pulcherrimus" however have a heavily sclerotised periderm and lack spines, well thickened apertural lists or interthecal septa. Obut and Sobolevskaya (1964) recorded the species to range from the C. peltifer to O. quadrimucronatus zones in Taimyr, Russia, with maximum widths of 3-5-3-7 mm. It is unclear how many of their specimens may be assigned to O? pulcherrimus owing to their greater widths and longer stratigraphical range. The specimens described by Lenz (1977) from the D. ornatus Zone of the Yukon, northern Canada were referred to Orthoretiograptus denticulatus by Williams (1982a), as should those figured by Lenz and McCracken (1982). The correct generic position of O? pulcherrimus is unclear. The type species of Reteograptus (R. geinitzianus) lacks both a lacina and apertural spines, although it does possess a similar framework construction of the reticulum (see Bulman 1970, V130). Both the type material of O.? pulcherrimus and the Girvan specimens appear to show single apertural spines. The similar "boxed" construction to Orthograptus quadrimucronatus, which also has similar overall dimensions, however leads I. Strachan (pers. com.) to suggest that O? pulcherrimus may have had a similar arrangement of paired apertural spines and affinities with the Diplograptidae. The type species of Orthoretiolites possesses single apertural spines and was described using three-dimensional, isolated material; if O? pulcherrimus does indeed have paired apertural spines it could not be assigned to this genus. It is unlikely that the problem of generic identity will be solved without similarly preserved material; until such a discovery is made, the species described by Keble and Harris

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(1934) as "Reteograptus pulcherrimus" Orthoretiolites.


is here transferred tentatively to the genus


T h e w o r k was undertaken largely during the tenure of a N . E . R . C . studentship at Glasgow University, supervised by D r s . J. K. Ingham and R. B . Rickards. I a m especially indebted to J. K. I n g h a m for guidance in the field, for making his collections available for study, for unpublished stratigraphical detail and for reading an earlier version of the manuscript. I also thank D r s . R. A. Fortey, R. B . Rickards, A. W. A. Rushton and I. Strachan for readily making available collections under their control, M r . A. Mcleod for donation of material collected during a student excursion, and M r . A. H . M . VandenBerg for invaluable discussion and unpublished data on graptolites from the Victorian sequence of Australia. T h e manuscript has benefited greatly from critical c o m m e n t s by D r s . D . A. T. Harper and I. Strachan.


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