Lower Cambrian Fossils from the Hell's Mouth Grits, St Tudwal's Peninsula, North Wales

Home , Agnostus, Calodiscus, Eodiscina, Serrodiscus, Weymouthia (trilobite)

Lower Cambrian fossils from the Hell's Mouth Grits, St Tudwal's Peninsula, North Wales

MICHAEL G. BASSETT, ROBERT M. OWENS & ADRIAN W. A. RUSHTON

SUMMARY The trilobites Hamatolenus (Myopsolenus) dou- of the Issaf6nien Stage of Morocco, and the glasi sp. nov., Kerberodiscussuccinctus gen. et sp. uppermost lower Cambrian trilobite-bearing nov. and Serrodiscus aenoa? Rushton, 1966 are horizon in the Purley Shales of Warwickshire, described from the upper part of the Hell's but it is younger than the Pseudatops viola Mouth Grits, St Tudwal's Peninsula, together horizon in the Llanberis Slates. The top of with hexactinellid sponge spicules, trace the HeWs Mouth Grits is lithologically fossils and a single inarticulate brachiopod. equivalent to the uppermost Rhinog Grits The fauna is of late lower Cambrian age, of the Harlech Dome. within the upper part of the protolenid- Smooth (effaced) eodiscid trilobites are strtmuellid Zone of the Comley Series of shown to be polyphyletic, and to complement British nomenclature. From comparison with the evidence of the HeU's Mouth eodiscids related faunas the horizon is correlated Runcinodiscus index gen. et sp. nov. is described approximately with beds in New York State from the lower Cambrian of Comley, Salop. containing the Admetopus faunule, the top

PRIOR TO 196o , dating and correlation of the lower, westernmost part of the Cambrian sequence of St Tudwal's Peninsula, North Wales (Fig. I), was estimated from consideration of the stratigraphical position of the beds below a well dated middle and upper Cambrian faunal succession occupying the eastern part of the Peninsula, and by lithological comparison with rocks in the Harlech Dome, c. 3 ° km to the east. The full St Tudwal's succession was first described in detail by Nicholas (I915) , who later (Nicholas 1916 ) described trilobite faunas from the middle and upper beds, i.e. upper Caered Mudstones and above. In the absence of fossils below this level a number of authors speculated on the age of the beds, the various views being summarised by Stubblefield (1956 , p. 24-6), who later (Stubblefield 1958 ) included the oldest exposed formation, the HeU's Mouth Grits, together with the overlying Mulfran or Manganese Beds, question- ably within the lower Cambrian; the succeeding Cilan Grits were listed simply as Cambrian, passing up into undifferentiated Caered Mudstones of middle Cam- brian age. These assessments were remarkably well supported by faunal evidence when D. A. Bassett (in Bassett & Walton 196o , p. lO3) reported the discovery of protolenid and eodiscid trilobites in the upper part of the Hell's Mouth Grits, suggesting a late lower Cambrian age; the specimens were assigned provisionally to Myopsolenus and Serrodiscus. Subsequent authors (e.g. Mohr 1964, p. 819; Rushton in Wood 1969, p. 65; Crimes 197oa , Fig. 2, p. 117; Cowie et al. I972 , p. io, 29; Sdzuy 1972, p. 6; Rushton 1974, p. 7 o) have quoted this key fauna in correlation, but until now the species have not been formally described.

Jl geol. Soc. Lond. vol. 132, z976, pp. 623-644, 5 figs. Printed in Northern Ireland.

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D. A. Bassett subsequently recollected from the locality, as did the authors during 1974, and our account is based on all material currently available. The identity of the protolenid is confirmed as Myopsolenus, while the original specimens of ?Serrodiscus are assigned to a new genus; in addition we have discovered a further eodiscid definitely referable to Serrodiscus. The associated fauna consists of numerous sponge spicules (Bassett & Walton I96O, p. 93, IO3, IO9), trace fossils and a single inarticulate brachiopod. We have also re-investigated the succession above the HeU's Mouth Grits, up to and including the lower Caered Mudstones, in the hope of finding new faunas to link with those of the upper Caered Mudstones and in an attempt to locate the position of the lower/middle Cambrian boundary more accurately; so far, however, these horizons have still not yielded macrofossils, but we include some comments on their stratigraphy after the description of the HeU's Mouth fauna. I. Systematic palaeontology Unless noted to the contrary, all specimens described below are from a single locality and horizon in the upper part of the Hell's Mouth Grits, I6. 5 m below the base of the Mulfran Formation, on the southward facing slope of Trwyn Carreg-y-tir, E side of Hell's Mouth (Porth Neigwl), 3o5o m at I92° from Llanengan Church, St Tudwal's Peninsula, Gwynedd [Caernarvonshire], North Wales (SH 2877 24o3). Figured and cited specimens are deposited in the National Museum of Wales (NMW), Institute of Geological Sciences (IGS), Sedgwick Museum (SM), British Museum (Natural History) (BM), and the Geological Survey of Canada (GSC).

TRILOBITA Descriptive terminology of the trilobites is mainly that of Harrington et al. (in Moore x959, P. OI x7-x26), apart from the glabeUar lobe and furrow notation for which we follow Hermingsmoen (I957, p. I2-4, Figs. I, 2). The symbols used in synonymy lists are explained by Matthews (i 973, P. 717-8). The trilobites are preserved as internal and external moulds in a silty and sandy mudstone. Many show strong relief but all show signs ofcompactional deformation and some are slightly distorted tectonically. Order REDLICHIIDA Richter, I933 Suborder REDLICHIINA Harrington, 1959 Superfamily ELLIPSOCEPHALACEA Matthew, t887 Family PROTOLENIDAE Richter & Richter, I948 Genus Hamatolenus Hup6, I953 Type species (original designation): Hamatolenus continuus Hup~, i953 Subgenus Myopsolenus Hup~, 1953 Type species (original designation): Myopsolenus magnus Hup~, I953. The Hell's Mouth protolenid is eharacterised by small eyes and a parafrontal band, a combination of features common to both Myopsolenus and Collyrolenus (type

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species C. staminops Hup6, i953). Each of these genera was erected only on the basis of its type species, both from the late lower Cambrian of Amouslek, western Anti-Atlas Mountains, Morocco, but although Hupe~ (t953, p. 243-4) gave a long diagnosis for both he did not discuss how they might be differentiated. Hennings- moen (in Moore 1959, P. 021 I) gave much more concise diagnoses (although note that those of Myopsolenus and Hamatolenus are transposed), which imply that Myopsolenus is distinguished from Collyrolenus principally by the continuity of the parafrontal band in front of the glabella in the former, and in the stronger taper of the frontal glabellar lobe in the latter. Examination of a plaster cast of the holotype of M. magnus, and of topotypes collected by W. B. R. King (SM A4o8o2-5, 4o8o7), shows that the parafrontal band is not as distinct as reconstructions (Hup6 I953, p. 214, text-Fig. 48:IO; Henningsmoen in Moore I959, p. 02 I% Fig. I5Z:I z) would suggest, although it is better developed in smaller specimens than in larger ones (c£ Hup~ I953, pl. IO, Figs. 14 & z I ; also SM A4o8o5a-b & SM A4o8o7). The type species of CoUyrolenus is apparently based on a single specimen in which the parafrontal band definitely does not run around the frontal

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:O Trwyn 28 29 Cilan 30 31 I .. , I , .I. I Fto. z. Geology of the western half of St Tudwal's Peninsula (mainly after Nicholas i9t5) ; fossil locality marked by an asterisk.

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lobe of the glabella (Fig. 2c). As the distinctness of this band appears to vary within a sample of one species, it seems unwise to use it as a diagnostic character, at least at supraspecific level. Comparison of the cast and the topotypes of Myopsolenus with Hupt's figure of CoUyrolenus shows that the latter can be differentiated from the former in having S I strongly curved backwards proximally, the glabella expanding slightly forwards as far as L2, and only a minute preglabellar field, shorter than the narrow anterior border. We doubt whether these differences are of more than specific rank, but until more is known of Collyrolenus we retain it and Myopsolenus as separate taxa. The HeWs Mouth specimens resemble Collyrolenus in the divergence of the pre- and postocular facial sutures from the palpebral lobe, and in the comparatively greater width of the fixed cheeks; they differ from Collyrolenus, but are like Myopsolenus, because the glabella does not expand forwards between the occipital ring and L2, the preglabellar field is as long (sag.) as the broad anterior border, and S I is not curved strongly backwards proximally (Fig. 2a). We consider that the similarities with Myopsolenus are taxonomically more important than those with Collyrolenus, and therefore assign the HeWs Mouth specimens to the former taxon. Sdzuy (I961) described many new protolenid species from the early middle Cambrian of Spain, and (op. cit. p. 555) considered the standing of genera placed by Hup6 and by Henningsmoen in the Myopsoleninae Hup6, 1953. He noted that the differences between Myopsolenus, Collyrolenus, Hamatolenus and Lotzda Sdzuy, 1958 are small, and suggested that all might be regarded as subgenera of Hamatolautr. Further, he did not consider that the Myopsoleninae, or other protolenid subfamilies, merit recognition within the Protolenidae. We agree with Sdzuy and follow his protolenid classification. Hamatolenus (Myopsolenus) douglasi Bassett, Owens & Rushton sp. nov. Plate I, Figs. I-8; Plate 2, Figs. 1-4; Fig. 2b v. 196o Myopsolenus; Bassett in Bassett & Walton, p. lO3. v. 1969 Myopsolenus; Rushton in discussion of Wood, p. 65. v. z97a Protolenids; Cowie, Rushton & Stubblefield, p. Io, 29. v. I974 Myopsolenus sp.; Rushton p. 7 o.

£1 b ¢ F xo. 2. Outline reconstructions comparing cranidia of: a, Hamatolenus (Myo#solenus) magnus (Htq~, I953) (based on Hup~ I953, pl. xo, Fig. z I and topotype specimens SM A4o8o3-5, A4o8o7); b, H. (M.) douglasi sp. nov.; c, H. (Collyrolenus) staminops (Hup~, x953 ) (based on Hup~ I953, pl. xo, Fig. xo); all c. × z.

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Explanation of Plates 1-3 on following six pages

626

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l ) L A T E I . On fachlg page

Fxc;s. I-8, Hamatolenus (:lIyopsolenus) douglasi Bassett, Owens & Rushton sp. nov. All specimens from Hell's Mouth Grit, Trwvn Carreg-y-tir. (SH 2877 24o3) , I6. 5 m below base of Mulfran Formation. l a,b. External mould and latex cast of almost complete specimen. Holotype, NMW 75.4 G. t. x'2. ,2. Internal mould of fi'ee cheek. Paratype, NMW 74.2IG.5a. x2. 3. Internal mould of cranidium and fi'agmentary thorax, Paratype, NMW 75-5 G.Ia. x2. 4a-c. Internal mould of cranidium, latex cast of counterpart external mould showing detail of marginal terrace lines, and part of right hand side of anterior border region of internal mould showing genal caeca. Paratype, NMW 75.4G.2a,b. a x2, b, c. x6. 5. Internal mould of large, distorted cranidium. Paratype, NMW 74.2IG. 2a. x2. 6-8. Internal moulds of thoracic segments, Paratypes, NMW 75.4 G. 3a, 74.2 IG.I 3, 74.'_,i(;.7a showing deep cxsa~ittal furrow. All x2.

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P L A T E 2. On facing page.

Horizon and locality as for PI. x.

Fi6s. 1-4, Hamatolenus (Myopsolenus) douglasi Bassett, Owens & Rushton sp. nov. 1,2. Internal moulds of incomplete cranidia showing genal caeca and eye ridge. Paratypes, NMW 74.21G. i a, IGS RU6291 showing junction of eye ridge with palpebral lobe. Both x2. 3,4. Internal moulds of thoracic segments. Paratypes, NMW 74.21G.12, 75.4G.4 a. Both x2.

FIc. 5, S~rrodiscus ctenoa? Rushton, 1966. Flattened internal mould of left pleural area of pygidium, N MW 74.2IG.2o. xS.

Fic;s. 6-i4, Kerberodiscus succinctus Bassett, Owens & Rushton gen. et sp. nov. 6a-c. Dorsal, lateral and posterior views of internal mould of pygidium. Holotype, NMW 75.5G.7 x8. 7. Latex cast of external mould of cephalon. Paratype, NMW74.2IG.26b (on same slab as fig. 13). xS. 8. Latex cast of external mould of small, complete specimen. Paratype, NMW 74.21G.21b. xI2. 9 a, b. Internal mould of cephalon and latex cast of counterpart external mould. Paratype, NMW 75.5G. I3 a, b. x8. I oa, b. Internal mould of pygidium and latex cast of counterpart external mould. Paratype, NMW 75.5G.9a, b; the apparent border in this specimen is due to dorso-ventral crushing--there is no border at the middle of the right lateral margin, where the specimen is least crushed, x8. I la-c. Dorsal, lateral and anterior views of internal mould of cephalon. Paratype, NMW 75.5G.16. x8. I2. Internal mould of pygidium. Paratype, IGS RU6296. xS. 13. Anterior view of latex cast of external mould of cephalon. Paratype, NMW 74.2IG.26b. xS. 14. Latex cast of external mould of transitory pygidium. Paratype, NMW 74.2IG.25 b. xI2.

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P L A T E '3" On f(lCillg page.

Horizon and locality as for Pl. 1, except IO which is from 6. 5 m lower in succession.

FIGS. t-4, Kerberodiscus succinctus Bassett, Owens & Rushton gen. et sp. nov. I. Internal mould of ccphalon, Paratype, NMW 75.5G.I5 . x8. 2a-c. Dorsal, lateral and anterior views of internal mould ofcephalon. Paratype, NMW 75.5 G. 14 a. x8. 3a-c. Dorsal, lateral and oblique posterolateral views of internal mould of pygidium (note the enlarged posterior spine in c). Paratype, NMW 75.5G.8, x8. 4- Latex cast of external mould of cephalon. Paratype, NMW 75.5G.x7 . xS.

FI~.. 5, ?Lingulella sp. NMW 74.2iG.~7b, latex cast of external impression, x2o. F I G s. 6- I o., hexactinellid spongespicules. All latex casts of external moulds; 6, NMW 74.2xG.28b; 7, 74 .2~G.28a (counterpart of right hand specimen of Fig. 6); 8, 74.2IG.31 ; 9, IGS RU63oo ; I o, NMW 74.21G. 33 b. All x6.

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Derivation of name. After Dr Douglas A. Bassett, who first discovered trilobites in the HeWs Mouth Grits. Holotype. NMW 75.4G. i, external mould of cephalon with xo attached thoracic segments, H. I, Figs. I a, b. Paratypes. NMW 75.5G.I, cranidium with 5 attached thoracic segments; NMW 75.4G.2, 74.2IG.x-4, IGS RU6291-95, cranidia and cranidial fragments; NMW 74.2IG.5-6, free cheeks; NMW 75.4G.3-4, 75.5G.2-6, 74.2IG.7-I9, thoracic segments. Diagnosis. Palpebral lobe ~ to ~ sagittal length of cephalic axis; distance from posterior end of palpebral lobe to posterior cephalic margin equals 2-3 times the lobe length; width of postocular cheek a little over ~o width of occipital ring; distance from ocular suture to line tangential to pre- and postocular facial sutures is H of distance between it and axial furrow; pygidium and hypostoma unknown; genal caeca present. Description. Cranidium with sagittal length and palpebral width approximately equal over known size range. Cephalic axis ranges from about ] as wide as long on smallest specimen to about ½ on largest, defined by axial and preglabellar furrows which shallow anteriorly. Glabella tapers gently forwards, with frontal lobe well rounded on holotype, but more pointed on three larger specimens. Three pairs of lateral glabellar furrows, decreasing in strength forwards, not confluent with axial furrows. S I bigeniculate and oblique backwards and inwards on holotype, but on larger specimens nearly straight and less oblique; abaxial end about glabellar length from occipital furrow. $2 more or less straight, less oblique backwards than SI, or nearly transverse (P1. I, Fig. 4 a) with abaxial end nearly ½ way along glabella. S 3 opposite point where eye-ridge meets glabella, similar to $2, but shorter, separated by a greater distance from axial furrow and about ] of glabellar length from occipital furrow. Occipital furrow sinuous, with lateral sections as deep as S I. Median section shallower and arched weakly forwards. Occipital ring more than ½ length of frontal area on holotype, but approximately equal to it on a large specimen (P1. I, Fig. 4a), and is longest sagittally. Median node situated near the mid-length, its posterior position on the holotype being a result of compression. Preglabellar field, excluding parafrontal band, equal in length (sag.) to anterior border, each between 16 (P1. I, Fig. I) and ~o (P1, I, Fig. 4 a) sagittal length of cephalic axis. Anterior border evenly curved, of even width. Parafrontal band weakly defined on holotype, but better seen on other specimens (P1. I, Figs. 3, 4 a) running into prominent, well-defined eye-ridges in region opposite $3, and directed backwards at 2o°-3 °° from an exsagittal line. Eye-ridge runs into palpebral lobe without a break (P1. 2, Fig. 2). Palpebral lobe crescentic, raised, defined by a shallow palpebral furrow and about H sagittal length of cephalic axis, its centre opposite L2 on larger specimens, but opposite $2 on holotype. It is situated more than twice its own length from posterior margin in large cranidia. Interocular cheeks as wide as glabella at eye line. Eye small, cresentic, not well preserved on available material. Pre- and postocular facial sutures both convex and divergent from palpebral lobe. A line tagential to both passes well outside ocular suture. Postocular fixed

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cheek broad, trapezoidal, about as wide as the as the occipital ring (slightly less on holotype, shghtly more on largest specimen). Posterior border furrow as wide as lateral part of occipital furrow adaxially, but rapidly broadens and shallows abaxially to merge with lateral border furrow at an obtuse angle at base of genal spine. Field of free cheek narrow, no wider than lateral border. Cephalic border broad, defined anteriorly by deep border furrow which appears to broaden and shallow towards base of genal spine (the prominent, narrow ridge running parallel to the cephalic margin in the holotype is the impression of the inner edge of the doublure, showing on dorsal surface as a result of compression). Genal spine narrow, tapering gently backwards and extending further than tenth thoracic segment. Inner spine angle curved, obtuse. Reticulate, anastomosing genal caeca well preserved on fixed cheeks of large specimens (P1. 2, Figs. t, 2), with those on preocular cheeks much coarser than on postocular cheeks (P1. 2, Fig. I). Some caeca close to anterior border furrow bifurcate distinctly on one internal mould (P1. i, Fig. 4c). Inner part of anterior border of one specimen (P1. I, Fig. 4b) is granulose, and all have a series of raised striae close to and parallel with cephalic margin and on genal spine. Holotype has a principal genal vein extending from middle of eye to lateral border furrow, which it meets just anterior to genal angle. Cephalic doublure weakly convex, it and the border forming a tube, with inner margin of doublure more or less corresponding with the cephalic border furrow. Thorax of at least I o segments, with axis tapering gently backwards. Axial furrows deep, bowed weakly around each axial ring, each of which has a distinct axial node, similar in size and position to median occipital node. Articulating furrow comparable in depth and breadth to occipital furrow (cf. P1. I, Figs. I b, 4 a, 7) ; length of articulating half ring about } that of annulus. First pleura has a distinctly curved spine and is wider (tr.) than succeeding ones. Pleurae of second to fifth segments of nearly constant width; those of sixth backwards become progressively narrower (tr.). Each pleura terminates in a short spine and has a broad, obhque pleural furrow which terminates some distance beyond fulcrum; tips of pleurae not in contact beyond this point. On internal moulds exposing the doublure (e.g. P1. I, Fig. 8) the mould of the panderian protuberance can be seen. Thorax apparently smooth; fine exsagittal ridges on the inner parts of the pleurae of one specimen (P1. x, Fig. 3) are probably a preservational feature rather than primary. Measurements (in ram). Apart from the holotype and one other specimen (P1. x, Fig. 4), the material is either too distorted or incomplete to be measured satisfac- torily. NMW 75.4G.t (holotypc) NMW 75.4G.2a Length of cranidium 8.8 x7"7 Length of cephalic axis 6"8 I5"0 (est.) Length of frontal area 2"o 2"7 Width of glabella at eyelinc 3"7 7"t (est.) Width of fixed cheek at eyclinc 3"t 5"8 Width of occipital ring 3"7 Width of postocular fixed cheek 3"4 8.x

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The estimated length of the holotype, if complete, wouldbe about 2o mm-25 mni. That of the largest specimen (P1. i, Fig. 5) is c. IOO ram. Comparison. The general morphology ofHamatolenus (Myopsolenus) douglasi is very similar to that of/-/. (M.) magnus in glabellar proportions, detail of lateral glabellar furrows, length, and proportional size of preglabellar field. However, the two species differ in that: the palpebral lobe is larger in magnus (about ½ glabellar length, compared with about ~ in douglasi) ; the distance between the posterior end of the palpebral lobe and the posterior margin of the cranidium is equal to the length of the palpebral lobe, or only slightly greater, in magnus, but 2-3 times its length in douglasi; on the holotype of magnus (Hup6 1953, ph IO, Fig. i I) the eye-ridge is represented only by a change of slope, but in all other specimens of this species, both smaller and larger than the holotype, it is well developed, as in douglasi; the width of the postocular fixed cheek is a little over ~o the width of the occipital ring in the latter, while in magnus it ranges from aX~-r~; and genal caeca are unknown in magnus. In Myopsolenus palmeri Parnes (I 97 I, p. 202) the palpebral lobe is ½ the glabeUar length and extends back to the level of the occipital furrow. Order AGNOSTIDA Salter, 1864; Suborder EODISCINA Kobayashi, 1939; Family EODISCIDAE Raymond, 1913; Genus Serrodiscus Richter & Richter, 1941. Type species (original designation): Eodiscus (Serrodiscus) serratus Richter & Richter, 194I. Serrodiscus ctenoa ? Rushton, 1966 Plate 2, Fig. 5 v* 1966 Serrodiscus ctenoa Rushton, p. 14, ph I, Figs. 6-19, text-Figs. 2a, 3. Material. One internal mould of a pygidial fragment, NMW 74.2iG.2o; preserved length 8.6 mm. Description. A fragment of the anterior end of the axis shows no definite ring furrows. Articulating half-ring not preserved but articulating furrow is arched backwards medially. Axial furrow distinct. Flank unfurrowed, triangular, maximum width ] of length (as preserved), very narrow posteriorly but extreme posterior end not preserved. Anterolateral border geniculate at ] distance from axial furrow to lateral border, with a facet abaxially; anterolateral border furrow passes into narrow but deep lateral border furrow. Lateral border with seven marginal spines, whose swollen bases make the lateral margin wavy, the width of the border at the crests being twice that at the troughs; spine bases taper abruptly downwards, passing into thin ventral spines; the total height of the spines is almost ¼ the height of the rest of the pygidium. Surface of internal mould smooth. Comparison. This fragment from the Hell's Mouth Grits differs from most forms of Serrodiscus, and comparable genera with ventral marginal spines, in having a wavy lateral margin and swollen spine bases. However, it does share these features with S. ctenoa from the lower Cambrian of Warwickshire, and with S. subdavatus Rasetti (1966, pl. 8, Figs. 15-18 ) and S. spinulosa Rasetti (1966 , pl. 7, Figs. IO, I I ; 1967, pl. 4, Figs. 4-7) from the lower Cambrian of New York, and in addition all have obscure axial ring-furrows and a backwardly arched articulating furrow. In our specimen the amplitude of the waves in the lateral outline is greater than in

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these named species, but comes nearest to S. ctenoa; from measurements on comparable sized specimens the percentage value for 2 × wave amplitude -- minimum width of border is 20% in S. spinulosa, 35% in S. subclavatus [specimens in the Rasetti collection at the British Museum (Natural History)] and 70% in S. ctenoa; our pygidial fragment, from a specimen over 1½ times longer, gives a value of 85%. S. subclavatus also differs from the Hell's Mouth form in having proportionally wider and shorter flanks, and also possibly in its granulose (exterior) surface. S. spinulosus has a wider border and the articulating furrow appears to be less strongly arched backwards. Although the present specimen most closely resembles S. ctenoa, its fragmentary nature precludes a firm determination. The 'undetermined pygidium No. 2' of Rasetti (I966 , pl. I I, Figs. i4-I6 ) from New York differs in having only four pairs of widely-spaced marginal spines, which are directed laterally, not ventrally. Genus Kerberodiscus Bassett, Owens & Rushton gen. nov. Type species: Kerberodiscus succinctus Bassett, Owens & Rushton sp. nov. Derivation of name. After the mythical monster Kerberos, guardian of the gates (mouth) of Hell. Diagnosis. Cephalon bluntly rounded in front, with elongate unfurrowed glabella reaching to anterior border-furrow; occipital ring wider than glabella, without spine. Axial furrow weak or obsolete at exterior surface. Lateral border very narrow, in dorsal view almost hidden beneath cheeks. Anterior border cusped in front view. Thorax of two segments (as far as is known). Pygidium elongate with long axis composed of eleven or more rings, axial furrows obsolete at exterior surface in the type species. Lateral border tucked under flanks, hidden in dorsal view, with blunt, ventrally directed spines on the doublure, the posterior pair longest and extended downwards and backwards. Posterior border arched up in rear view. Discussion. Studies of the Suborder Agnostina have shown that a trend smoothing out the axial and other furrows on the dorsal exoskeleton was operative in several lineages (e.g. Opik x96I p. 53)- The end-members of these lineages, known as "effaced" agnostids, are superficially homoeomorphic, especially on the external surface of the exoskeleton, but where suitably preserved (such as exfoliated) specimens show traces of the familial characters, the various effaced forms may be distinguished from each other and be referred to separate genera and families. Similar trends have not been described in the Eodiscina, because although many new blind genera and species have been recognized in recent years (e.g. Rasetti I966 ) there are comparatively few effaced forms, and most of these have been referred to the genus Weymouthia Raymond (type species Agnostus nobilis Ford, I872). Unfortunately, however, Weymouthia nobilis is a nomen dubium; the syntypes, from the lower Cambrian of Troy, New York, have long been lost (Ford in Walcott 1886, p. 15 i) and no topotypes are known; and although Ford's description (I872, p. 42I) is detailed there are slight discrepanices with the figures, making interpretation of the species doubtful. The internal features are not known so the relationships with other genera of the Eodiscidae cannot be assessed. The HelPs Mouth fauna contains a distinctive, new, effaced eodiscid species in which the axial furrows are seen on internal moulds, allowing it to be related to

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'full dress' (non-effaced) genera. To indicate these relationships and to avoid reference to the doubtful Weymouthia it is described here as Kerberodiscus gen. nov. In comparing this form with other eodiscids an additional new, effaced genus and species was recognized in the lower Cambrian of Shropshire, and the opportunity is thus taken to describe it below to facilitate comparison with Kerberodiscus and to further clarify relationships among effaced forms. Kerberodiscus is considered to be an effaced relative of Leptochilodiscus Rasetti, I966 (type-species L. punctulatus Rasetti, i966 , from the lower Cambrian of Columbia County, New York). Both have cephala with a bluntly rounded outline anteriorly, a long, unfurrowed glabella with a sharply tapered front, a wide occipital ring, and in both the border is narrow and bicuspate in front view. In both genera the pygidium is more sharply rounded terminally than the cephalon, the axis is composed of several (more than eight) rings, and the border has ventral spines (of which the posterior pair is the longest and directed backwards) and an upward arch in posterior view. Kerberodiscus differs from Leptochilo- discus principally in having cheeks and flanks which overhang the borders and in the external effacement of the axial furrows (especially in the pygidium). Kerberodiscus has a longer glabella, no occipital spine, a narrower, almost obsolete, anterior cephalic border, and somewhat enlarged posterior spines on the pygidial margin. Other differences are mentioned after the specific description. The pygidium ofBolboparia Rasetti, i966 (type-species B. superba Rasetti, I966) resembles that of Kerberodiscus in having many axial rings, marginal spines, and flanks that almost obscure the lateral border in dorsal view. It differs in having strong axial furrows at the external surface, a large erect spine on the second axial ring, a strong spiny or granulose sculpture, and in lacking the elongated posterior pair of marginal spines and the posterior arch of Kerberodiscus. Bolboparia has a shorter glabella with pit-like glabellar furrows, a strong glabellar spine and a broader border making a more pointed outline anteriorly than in Kerberodiscus; the cheeks are very unusual in shape and swell out anterolaterally to overhang the border. All the forms so far referred to Weymouthia nobilis differ from Kerberodiscus because they have distinct cephalic and pygidial borders of the normal widths for eodiscids (Ford I972; Kiaer I9x6 , pl. 3, Fig. I2; Cobbold I93I ; Rasetti I95~ , pl. 52, Fig. I8); their cephalic borders bear several pairs of dorsal tubercles and are not so unusually narrow as in Kerberodiscus. In no form of W. nobilis is the pygidial border tucked under the flanks laterally or arched up in rear view. Kerberodiscus is unusual among blind eodiscids in having a bisegmented thorax, a feature shared with Dawsonia and Chelediscus. Three segments are present in at least some species ofAcidiscus, Calodiscus, Eodiscus, Ladadiscus, Metadiscus, Serrodiscus, Tannudiscus and 'Weymouthia'. In other genera the thorax is unknown. Kerberodiscus succinctus Bassett, Owens & Rushton sp. nov. Plate 2, Figs. 6-i4; Plate 3, Figs. i-4; Fig. 3 v. z96o eodiscid (?); Bassett in Bassett & Walton, p. io 3. v. z96o Serrodiscus?; Stubblefield in Bassett & Walton, p. Io5 v. x972 Serrodiscus?; Cowie, Rushton & Stubblefield, p. Io, 29 v. z974 Serrodiscus? ; Rushton, p. 70

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Derivation of name. Latin, succinctus, tucked in, referring to the borders. Holotype. NMW 75.5G.7, internal mould of pygidium, Ph 2, Figs. 6a-c. Paratypes. Ten cephala, including counterpart internal and external moulds, NMW 74.2IG. 22-3, 26 (2 specimens), 75.5G.I3-8; ten pygidia, including counterpart internal and external moulds, NMW 74.2zG.24-5, 75.4G.5, 75.5G.8-I2, IGS RU6296-7; one small complete dorsal exoskeleton, NMW 7z.2xG.2I. Diagnosis. As for the genus. Description. Cephalon convex, weakly trilobed or evenly vaulted in transverse section, length ~ to ~0 of width, outline bluntly rounded anteriorly, becoming nearly parallel-sided posterolaterally, posterolateral corners about right-angled. Cephalic axis almost as long as cephalon, sides straight and slightly tapered forward for ~ of its length, then tapered strongly to the sharply rounded front. Axial furrows weak or absent on external surface, distinct on internal moulds. No glabeUar furrows seen. Weak median glabellar node seen in some specimens, especially internal moulds, at about posterior ~ of cephalic length. Occipital ring weakly delimited externally, wider than glabella, without a spine or node, occipital furrow concave forwards, broader and shallower on internal moulds. Cheeks, excluding border, each roughly ] greatest width of glabella, of fairly even width posteriorly, narrowing abruptly forwards at anterior ½, just separated in front of glabeUa. Posterior border furrow strong on internal moulds. Posterior border separated from lateral border at the genal angle by an oblique furrow, just outside which is a low knob. A very short point is seen on the genal angle of two small cephala. Posterior part of lateral border wide and thick, becoming very narrow forwards and in dorsal view hidden or nearly so by the cheeks; laterally the border is level but anteriorly it is bent up into two shallow arcs separated by a median, downward- pointing cusp (Fig. 3). Surface sculpture appears variable; in some examples the external surface is finely punctate, especially on the cheeks, but others are smooth. The lateral border is finely striated parallel to the margin posteriorly. Internal moulds of the cheeks may be faintly granulose, indicating pitting of the interior of the exoskeleton. Thorax of two segments, as seen in the articulated smallest specimen which is a holaspid. No axial spines or nodes. Pleurae with strong fulcra, wider than the axis in the small specimen which has a comparatively narrow pygidial axis. The largest pygidium has two thoracic segments associated with it but they are too fragmentary for description. Pygidial length about equal to or ~-~ of width, but smaller specimens are proportionally wider and the largest is narrower; weakly convex longitudinally, posterior end slightly upturned, not trilobed in transverse section. Axis (excluding articulating half-ring) about a~o pygidial length, anterior end about $ pygidial width, tapered to rounded posterior end which is blunter in the largest specimens. Axis defined on internal moulds by shallow but distinct axial furrows, but barely a trace is visible on external surface; composed of I I rings (only zo seen in some specimens) and a terminal piece; weak median nodes seen on most rings on internal moulds of some specimens (e.g. P1. 2, Fig. Ioa) but not seen in others (e.g. P1. 2,

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...... ".'.

a b d Fxo. 3. Reconstruction ofKerberodiseus sucdnaus gen. et sp. nov.: a, dorsal view of exteriors of cephalon and pygidium; b, dorsal view of internal moulds of the same; ¢, anterior view of internal mould ofcephalon; d, lateral and posterior views of internal mould of pygidium; all ¢. × 8.

Fig. I4). Anterior axial ring shorter than second, its anterior edge sinuous. Articulating half-ring depressed below level of axis. Flanks about ~ width of axis, confluent behind. Anterior border furrow distinct externally, anterior border with fulcrum and facet. Lateral border very narrow, tucked under flanks, mostly hidden in dorsal view. On each side the doublure is drawn downwards into 7 or 8 blunt points, the posterior pair being the largest and extended somewhat backwards (P1. 3, Figs. 3b, c). Posteriorly the border is tilted steeply outwards and backwards to form a transverse upward arch (Fig. 3). External surface sparsely or closely punctate or smooth; lateral border and outer parts of flanks finely striated sub-parallel to margin. The small complete specimen, which is clearly a holaspid even though the pygidium is only I.O5 mm long, shows the axial furrows distinctly, even on the external mould, but the axis is not clearly segmented. A larger external mould about x-3 mm long has distinct axial furrows and axial rings; a thoracic segment is attached but is poorly preserved so we cannot tell if it forms part of a transitory pygidium. In still larger pygidia the axial furrows are effaced. Remarks. In addition to the characters mentioned above under the generic discussion, Leptockilodiscus punaulatus Rasetti (I 966, 1967) differs from K. succinctus in having a longitudinal profile which is 'humped', with the occipital ring depressed, the genal angles have long spines, and the anterior and lateral border is

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Measurements (in mm) of selected specimens. width at base CEPHALA length width of glabella height

NMW 75.5G.I 7 -- (6"4) 2"4 1.6 NMW 75.5G. 15 4"5 (4"9) x-8 i .6 NMW 75.5G.x3a 3"4 (4"3) x'5 I "5 NMW 75.5G.x4a 3" 6 (4"2) i .8 I "3 NMW 75.5G.x6 2"7 (3"4) 1"35 x.o NMW 74.2 xG.26a 2.8 (3" O) I "2 I "2 NMW 74.2xG.26e x'7 2"I5 0"9 0"9 NMW 74.2tG.2Ia I':5 x'7 0-6 0. 4

length width of PYGIDIA length of axis width axis height NMW 75.5G.8 6.8 6-4 (6-0) 2.6 2.2 NMW 75.5G.9a 3"7 -- 4"2 1.8 -- NMW 75.5G.7 3"5 3.0 (3"3) I "3 I. I (Holotype) IGS RU6296 2.6 2.4 (2.7) I .o 5 o.8 NMW 74.2xG.25a -- x'25 i-8 0.65 0.55 NMW 74.2 xG.2 xb t -05 o.9 i "4 o.4 0"3 Figures in parentheses indicate doubled half-widths.

broader and has a pair of pits anterolaterally (though these are difficult to see in some specimens). No median glabella tubercle is seen at the exterior surface, nor a knob on the lateral border just in front of the genal angle. The pygidium of L. punctulatus is more convex and broader in proportion, and has fewer axial rings (8 plus terminal piece), of which the anterior one is barely shorter than the second; no axial nodes are seen at the external surface. Most of the marginal spines are longer and sharper than in K. succinctus but the posterior pair is less enlarged. A cephalon from Quebec (GSC 10565) figured by Rasetti (1967, pl. 3, Fig. 28) as L. punctulatus, differs from the typical form in having weaker axial furrows and a longer glabella. The cheeks drop more steeply to the border but do not overhang it. The occipital spine is broken off but the impression of part of its underside suggests that it was slimmer than in L. punctulatus; the genal spines are also broken but their bases appear likewise to have been slimmer. The frontal and lateral border is narrower than in L. punctulatus, and widens towards the genal angles where there is a faint swelling, similar to but weaker than that of Kerberodiscus. The longitudinal profile and depressed occipital ring resemble those of L. punctulatus. Although this cephalon is referable to Leptochilodiscus (perhaps as a new species), in features of the glabella, cheeks and border, and perhaps in the reduction of the cephalic spines, it approaches a 'full dress' form of Kerberodiscus. The "Eodiscidae, cephalon no. 3" of Rasetti (1967, pl. 5, Figs. 7, 8) differs from K. succinctus in having a wider border and there appears to be a trace of an axial furrow indicating a very wide and forward-expanding glabella, as discussed below' The fragmentary "Eodiscidae, pygidium no. 5" of Rasetti (1967, pl. 4, Figs. 22, 23) is not effaced but otherwise resembles Kerberodiscus, being elongate with a weakly convex axis; the flanks overhang the border which has a large posterior

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pair of marginal spines and a posterior arch. If effacement is considered to be a progressive character, this pygidium may be regarded as an earlier 'full-dress' representative of Kerberodiscus, although without knowledge of the cephalon such reference can only be tentative. The small "Eodiscidae, pygidium no. 6" of Rasetti (I967, pl. 4, Fig. 2I) is less like K. succinc.tus because it is broader and the border is visible in dorsal view. As observed by Rasetti it more nearly resembles pygidia of L.punctulatus of comparable size. An unnamed pygidium figured by Rushton (i966 , p. 28, text-Fig, i I, pl. 3, Fig. I5) has no lateral border at all and differs from K. succitwtus in having a distinct convex axis. Genus Runcinodiscus Rushton gen. nov. Type species: Runcinodiscus index Rushton sp. nov. Derivation of name. Latin, runcinare, to plane off. Diagnosis. Effaced blind eodiscids with semi-elliptical cephalon, long, simple, forward-tapering glabella, cheeks narrowing forward and confluent anteriorly, and a distinct tuberculate border which is level but widens forwards. Pygidium with long conical axis composed of nine or more rings and a terminal piece, border distinct and level, narrow behind, with ventral swellings or short spines. Discussion. Rundnodiscus is proposed for effaced eodiscids which show internally the characters of Serrodiscus Richter & Richter, as summarized by Rushton (i966 , p. I I, I2). It is appreciated that new evidence may eventually demonstrate that Wey- mouthia nobilis has these Serrodiscus-like features, in which case Runcinodiscus will fall in synonymy with Weymouthia, but at present there is no evidence for this synonymy. In any event it is shown below that R. index is specifically distinct from W. nobilis as described by Ford (I872). The internal mould of 'IV. nobilis' figured by Rasetti (i952) does not show any special affinity with Serrodiscus (Rushton 1966, p. 12) and cannot be referred to Runcinodiscus. The lack of features on the internal mould makes this a difficult form to classify. The "undetermined pygidium no. 3" of Rasetti (i966 , p. 39, pl. IO, Fig. 1 i), though effaced, differs from Runcinodiscus and Serrodiscus in its semi-circular shape and entire pygidial border. Rasetti (I967, p. 56, pl. 5, Figs. 7, 8) described an effaced cephalon resembling Ford's W. nobilis, but its outline is more quadrate than in Serrodiscus and Runcino- discus. A groove on the left side of the glabella seems to be the base of the axial furrow; Rasetti suspected that the cephalon was cracked there but examination of the specimen shows no sign of a break in the exoskeleton, and the even curvature of the glabella and cheek down into the groove is quite suggestive of an original axial furrow. If this interpretation is accepted it indicates that the glabella expands forwards, as in Bathydiscus and Oodiscus (Rasetti 1966), and may indicate a further example of the trend to effacement in blind eodiscid genera. The effaced genera DelgadeUa Walcott, 1912 and Pagetiellus Lermontova, 194 ° exemplify a comparable trend to effacement in the eye-bearing eodiscids of the family Pagetiidae.

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Runcinodiscus index Rushton sp. nov. Fig. 4

v. z93I Weymouthianobilis (Ford); Cobbold, p. 466, pl. 38, Figs. x7-2t [Fig. 22 not seen], non Ford x872 Derivation of name. Latin, index, that which informs (on generic origins in this case). Holotype. SM At536oa , b, cephalon in part and counterpart, showing both internal and external features; figured Cobbold 193 I, pl. 38, Figs. t 7, x8; from the Protolenus Limestone (Ac5), lower Cambrian (upper part), Comley Series, protolenid-strenuellid Zone, Cobbold's excavation no. 2, I8o m S of Comley Quarry, near Church Stretton, Salop [Shropshire]. Paratypes. Three cephala, SM AI5363, IGS 839o6 (collected E. S. Cobbold), IGS 594t8 (collected C. J. Stubblefield) ; six pygidia, SM At536I-2 , IGS 59412-3 (counterparts), IGS 83895 (collected C. J. Stubblefield), BM In356t4- 5 (collected K. P. Oakley) ; all from same locality and horizon as the holotype. Remarks. Cobbold's description (I93z , see synonymy above) is mainly accurate but should include the following. Posterior edge of the undifferentiated occipital ring more than 0-4 cephalic width, curves backwards so that cephalic length is more than 0"9 width. Cephalic height o.28 of width. Cephalon geniculate posterolaterally, posterior border furrow distinct externally. On internal moulds border- furrow widens anteriorly, especially in front ofglabella, and shows lateral tubercles; at external surface border is weakly delimited, widens slightly forwards, marginal tubercles absent or barely visible. Cobbold's Fig. x8 is misleading in showing the right-hand margin as the exterior surface; the edge of the exoskeleton should extend from behind the front of the glabella outwards and backwards to the right

..:" • .

a b c Fro. 4. Reconstruction of Runcinodiscus index gen. et sp. nov.: a, lateral view of cephalon and pygidium; b, dorsal view of exterior; c, dorsal view of internal mould; all c. x 7.

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posterolateral part of the border. The marginal tubercles are correctly drawn but are features of the internal mould. Pygidial height just over 0.33 width. Pygidial border furrow distinct at external surface, border narrow, especially at posterior end where it is almost hidden beneath flanks (Cobbold drew too wide a posterior border in his Fig. 19 partly because it was hidden by matrix). Fresh preparations show at least five pairs of blunt ventral spines (possibly only spine-bases, e.g. IGS 59412). The largest specimen is the pygidium figured by Cobbold (1931 , pl. 38, Fig. 19) which is 5"5 mm long. The present form differs from Weymouthia nobilis, as described by Ford (I872), because the cephalic border widens forwards, the pygidial border narrows backwards, the articulating half-ring is slightly wider in proportion to the pygidial width, and the anterolateral margin of the pygidium is angulated by a distinct geniculadon in contrast to the "feebly rounded" contour of Ford's description and figure. Ford's specimens differ because they show no axial structures yet have prominent marginal tubercles. Cobbold (1931 , p. 467) commented on the "fine lines" on the surface. From Ford's measurements W. nobilis is more convex than R. index; the height/width ratio of his cephala is o.33 and o-36 compared with a consistent o-28 for R. index; and the pygidium of W. nobilis gives a ratio of o'39 compared with an average of o.34 for R. index; Ford's specimens, however, were considerably the larger, more than 9 mm long. The articulating half-ring of R. index is slightly wider than that of "W. nobffis" figured by Kiaer (1916).

BRACHIOPODA ? LinguleUa sp. Plate 3, Fig. 5 Description. One minute valve, c. 0.8 mm long and o'6 mm wide, is oval, gently convex, with umbo at posterior margin. External surface with fine growth lines and a few weak radiating ridges suggesting a juvenile Lingulella.

PORIFERA Hexactinellida gen. et sp. indet. Plate 3, Figs. 6-1o Material. Numerous disarticulated spicules all preserved as internal moulds; from the main fossil horizon in the Hell's Mouth Grits on Trwyn Carreg-y-dr, and also from the same locality but 6. 5 m lower in the succession (i.e. 23 m below base of Mulfran Formation). Description. Spicules mainly cruciform with the tips of the rays curved up out of the horizontal plane, especially in smaller specimens; longer rays 2 to 3 mm long and o.o 9 to o.I8 mm thick at the base. Some spicules are tetracts ('stauracts') but most are pentacts, showing the stump of a fifth, polar ray normal to the centre of the cross (P1. 3, Figs. 6, 8) attached on its concave face. No hexact spicules (with two polar rays) were seen. Two pentacts, each with a broken ray o. I mm thick, show the cast of an axial canal o.o 3 mm across. Numerous straight spicules may include fragments of the above forms as well as simple diacts.

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Remarks. The arrangement of spicules in the body of the sponge is not known, and the HelPs Mouth forms are difficult to assign to any known hexactinellid genus. Typical Protospongia (e.g.P. fenestrata Salter, P. hicksi Hinde) have no pentacts and the stauracts reach a large size (Rushton & Phillips i972 ). Pleodioria tomads Opik (I96i , p. 50), from the middle Cambrian of Australia, has tetracts and stauracts similar in size and shape to those described above but is charac- terized especially by pentacts with split tips to the rays, a feature not seen in the Hell's Mouth material. Caldhexactinafranconica Sdzuy (I969, p. I4O), from the early middle Cambrian of Germany, has pentact spicules but they are less numerous than the hexacts and differ from our material in having rays whose lengths vary widely on a single spicule. In the light of comments by Stubblefield (in Bassett & Walton I96O , p. io5-6), we have re-examined the spicules (IGS ZsI418 ) described by Greenly (i919, P. 394) from the Careg-onen Beds of Anglesey in order to compare them with our St Tudwal's specimens. They include cruciform spicules with rays 0"7 mm to o-26 mm in thickness in which the axial canals are generally less than ½ of the outer diameter (Greenly I919, text-Fig. I93 ). There is evidence that tetracts or pentacts are present, but hexacts are not proved. In size and shape the Careg-onen specimens are comparable with those from the HeWs Mouth Grits. Greenly (i 919, pp. 395-8) argued that the Careg-onen Beds were younger than the Mona Complex but pre-Ordovician, but left the age undecided between Cambrian and upper Precambrian, and he later (Greenly i946 , p. 238) considered them to be probably Precambrian (pre-Arvonian). On balance the sponge spicules are more indicative of a Cambrian age, but such spicules are long ranging stratigraphically and of uncertain value in correlation.

TRACE FOSSILS From some arenaceous units of the Hell's Mouth Grits, Bassett & Walton (i96o , p. 92) reported irregular and often bifurcating casts of worm tracks, I-2 cm in diameter. Similar structures occur as burrows in the silty mudstones on Trwyn Carreg-y-fir, in association with the trilobites, but are generally only i- 3 mm in diameter. All are preserved as featureless limonitic moulds of fairly constant diameter, and are short and straight, or gently sinuous, or bifurcating; these structures may include both types of burrows reported by Crimes (I97ob , p. I io, Fig. 4), from Trwyn y Ffosle (Fig. I) at the northern end of the Hell's Mouth outcrop, under the separate names of Planolites and 'bifurcating burrows' (see also Crimes 197oa, Fig. 7, P. 125-6).

2. Succession above the Hell's Mouth Grits Mulfran Formation [Beds] and Gilan Grits. Rushton (1974, P- 7o-I ) has reviewed the stratigraphy of these horizons, based on the original work of Nicholas (i915). Siltstones similar to those of the fossiliferous level in the HelPs Mouth Grits are present in both units, but have not as yet yielded any faunas. From the upper part of the Cilan Grits south of Trwyn Llech-y-doll (Fig. x) Crimes (I97ob , p. I IO, Fig. 4) reported five different kinds of trace fossils: Planolites, bifurcating burrows,

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Sinusites, intertwining burrows, and burrow network (cf. Paleodictyon) (see also Crimes 197oa, p. 13 I). Lower Caered Mudstones. Nicholas (I 915, 1916) reported no fossils from these beds, but subsequently a complete trilobite identified as Agnostus was collected by Dr R. B. McConnell from about 3o m (ioo ft) above the top of the Cilan Grits south ofTrwyn Llech-y-doll (Matley etal. I939, p. 85). We have contacted a number of people who were present at the time of this discovery, including Dr McConnell, but unfortunately the subsequent history of the specimen is unknown, and we have not located it in any major museum collection. Prolonged fieldwork at Trwyn Llech-y-doll has failed to reveal additional specimens, but at its face value the record of Agnostus suggests a link with the abundant middle Cambrian agnostid fauna of the Upper Caered Mudstones. A few metres below the unconformable basal Arenig sandstone, at the base of the path leading down to the beach (SH 3ooo 234 o) south ofTrwyn Llech-y-doll, the dark black mudstones in the Lower Caered contain branching burrow structures; the most common are slender and threadlike, about o.2 mm in diameter, but there are slightly coarser examples ranging from o. 5 to o. 9 ram, and also relatively thick, unbranched forms from 2 to 3 mm. All three types are intimately associated and probably include the mat-like associations recorded by Crimes (i97oa , p. I34 ) from the same horizon, although some of his burrows are up to 3 cm across. Crimes referred to larger, non-branching burrows as Planolites, and recorded the presence of other undescribed forms. All our specimens are preserved as rusty weathered moulds, but all appear to have been originally circular in cross section. A short length of one horizontal burrow, about I mm in diameter, shows definite annulations spaced at about 5 ram, while similar, but less regular, annulations were observed on one larger specimen (2.25 mm diameter). 3-Correlation There is a satisfactory lithological correlation of the lower, manganiferous part of the Mulfran Formation with the lower, Ore-bed Shales division of the Hafotty (Manganese) Formation in the Harlech Dome (see Cowie et al. i972 , p. 29 and Rushton I974, p. 7o, 71 for summaries). This is supported by geochemical data (e.g. Mohr I959, I964, Mohr & Allen i965). The top of the Hell's Mouth Grits is accordingly correlated with the top of the Rhinog Grits of the Harlech Dome succession. A comparable manganese ore-bed occurs at the base of the Chamber- lain's Brook Formation on the Avalon Peninsula in south-east Newfoundland (Fig. 5), lying possibly at a non-sequence, between fossiliferous lower Cambrian (Protolenus Zone) and middle Cambrian (Paradoxides bennetti Zone) beds (Hutchinson I962 ). The geochemical composition is again similar to that of the Mull'ran and Hafotty formations 'strongly suggesting that the manganiferous horizon... [in North Wales]... is strictly equivalent to that of SE Newfoundland (Mohr I964, p. 8x9), and that all these beds 'were deposited in the same geosyn- dine' (Mohr & Allen I965, p. 336). The trilobites from the Hell's Mouth Grits compare most closely with species which occur in Morocco, New York and Nuneaton (Warwickshire) near the top

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of the respective lower Cambrian sequences. Myopsolenus was originally recorded (Hup6 1953) from the penultimate lower Cambrian horizon in the top lower Cambrian zone (Zone VIII) in Morocco. Subsequently Hup6 (I 960) extended its range through the whole of the Aguilizian Substage (the upper Substage of the Issaf6nien Stage in the Anti-Arias), and also recorded M. magnus in the basal bed of the overlying middle Cambrian (with Acadoparadoxides). Rasetti (I 966, 1967) has recognized three faunal divisions in the lower Cambrian of New York, the Elliptocephala fauna (below), an Acimetopus faunule and a Pagetides faunule (above), the latter passing up into middle Cambrian beds. Leptochilodiscus punctulatus, the form closest to Kerberodiscus, was first recorded (Rasetti 1966) in an Adraetopus faunule at Griswold farm, Columbia County, New York, associated with Olenellus. Subsequently it was found in the same assemblage near Malden Bridge, New York, and with the slightly later Pagetides faunule near Griswold farm (Rasetti 1967). In Quebec the cephalon assigned by Rasetti (1967, pl. 3, Fig. 28) to L. punctulatus but compared here (p. 634 ) with K. succinctus, was associated with Bolboparia, a genus from the Acimetopus assemblage, together with a Calodisaus species known from Malden Bridge. The EUiptocephala asaphoides fauna, which includes Pseudatops reticulatus and Serrodiscus spedosus, may be correlated approximately with the upper divisions of the Comley Limestones (Aca-A%) of Salop, and with the lower part of the Brigus Formation of Newfoundland, both of which yield S. bellimarginatus and a form close to P. reticulatus (Cobbold 1921, 1936, Hutchinson 1962). These horizons lie near the base of the protolenid-strenuellid Zone of Cowie et al. (1972 , p. IO). The Admetopus assemblage correlates approximately with the fauna from Rushton's (i 966) locality 2A in the Purley Shales of Warwickshire, which clearly overlies his locality I (yielding S. bellimarginatus) and underlies middle Cambrian Paradoxid, s- bearing beds (P. pinus Zone). The locality 2A fauna, from the highest lower Cambrian trilobite-bearing beds in the Pufley Shales, includes S. ctenoa, with which the HeU's Mouth Serrodisms compares most closely, suggesting that the HelPs Mouth assemblage may also be correlated with the Acimetopus faunule. The horizon yielding Pseudatops viola and Serrodiscus? sp. in the Llanberis Slates of Caernarvonshire (Howell & Stubblefield 195 o) appears to be older than the HelPs Mouth Grits, approximately equivalent to beds elsewhere containing S. beUimar- ginatus and Pseudatops (Fig. 5) (Rushton 1974, p. 80). In summary, although the lower-middle Cambrian boundary is not necessarily drawn at precisely the same level in Morocco, New York and Warwickshire, the relationships of the Hell's Mouth Grits fauna are consistently with other faunas assigned to the late lower Cambrian rather than to the middle Cambrian. In British terminology (Cowie et al. 1972) this indicates a correlation within the upper part of the protolenid-strenuellid Zone of the Comley Series (Fig. 5). The overlying manganese-bearing beds may therefore be approximately at the level of the lower-middle Cambrian boundary. Further support for this correlation is provided by the evidence ofpalynological data from the St Tudwal's sequence. From six horizons in the HelPs Mouth Grits Dr T. L. Potter (pers. comm.) has extracted acritarchs indicative of a level close to the lower-middle Cambrian boundary. A similar flora occurs in the base of the

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Muffran Formation, but at a higher horizon in these beds the flora proved to be less diverse. One sample from the Cilan Grits and four from the Lower Caered Mudstones all yielded acritarchs interpreted as being of middle Cambrian age. The floras, which include a number of new forms, are being described by Dr Potter elsewhere.

ACmNOWLEDO~.~rCTS. We thank Dr D. A. Bassett for agreeing to us working on his original material and him, Sir, James Stubblefield and Mr T. C. Nicholas for valuable help in discussion; to the British Museum (Natural History), Sedgwick Museum, Cambridge, Geological Survey of Canada and United States National Museum for loans of comparative material. A. W. A. Rushton's contribution is by permission of the Director, Institute of Geological Sciences.

4. References

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MATTHEWS, S. C. x973. Notes on open nomenclature and on synonymy lists. Palaeontology x6, 713--9. MOHR, P. A. x959. A geochemical study of the shales of the Lower Cambrian Shale Group of the Harleeh Dome, North Wales. Geochim. cosmochim. Acta. x7, I86-2oo. 1964 . Genesis of the Cambrian manganese carbonate rocks of North Wales. J. sedim. Petrol. 34, 819-29. & ALTON, R. I965 , Further considerations on the deposition of the Middle Cambrian manganese carbonate beds of Wales and Newfoundland. Geol. Mag. to2, 328-35. MooRE, R. C. (ed.). x959. Treatise on invertebratepaleontology: Part 0 Arthropoda 1. Geol. Soe. Am. & Univ. Kansas Press. NICHOLAS, T. C. 19I 5. The geology of the St. Tudwal's Peninsula (Caernarvonshire). Q. dl geol. Soc. Lond. 7 x, 83-I43. J I9t6. Notes on the trilobite fauna of the Middle Cambrian of the St. Tudwal's Peninsula (Caernarvonshire). Q. dl geol. Soc. Lond. 7x, 451-72. OPIK, A. A. I96I. The geology and palaeontology of the headwaters of the Burke River, Queens- land. Bull. Bur. Miner. Resour. Geol. Geophys. Aust. 53, 1-249. PAm~-ES, A. x97 I. Late lower Cambrian trilobites from the Timna area and Har 'Amram (southern Negev, Israel). Israel J. Earth Sd. 20, 179-2o 5. R~s~a'~, F. t952. Revision of the North American trilobites of the family Eodiscidae. J. Paleont. 26, 434-5 I. 1966. New Lower Cambrian faunule from the Taconie sequence of New York. Smithson. misc. Collns, x48 (9), x-52. ~I967. Lower and Middle Cambrian trilobite faunas from the Taconie sequence of New York. Smithson. misc. Collns, x52 (4), I-I I t. RAYMOND, P. E. xgI 3. Trilobita. In EAS~, C. R. (ed.). Text-book off'Paleontology, adalJtedfrom German ofK. A. yon Zittel. 2nd edn., p. 692-729 Macmillan, London. RXCH'rER, R. x933. Crnstacea (Pal~iontologie). In Handw~rterbuch der Naturwissenschaflen, 2, 8~j:~64~ Jena. & Rzcm'zR, E. I94 Z. Studien im Pal~iozolkum der Mittelmeer-Lgnder. 6. Die Fauna des Unter-Kambriums yon Cala in Andalusien. Ser~kenbrrg. natu~f. Gesell. Abtmndl. 455, I-9O. & ~ 1948. Studien im Pal~iozoikum der Mittelmeer-I.~nder. 8. Zur Frage des Unter- Kambriums in Nordost-Spanien. Senckenbergiana 29, 23-9. RUSHTON, A. W. A. x966. The Cambrian trilobites from the Purley Shales of Warwickshire. Palaeontogr. So¢. [Monogr. ], x-55. i974. The Cambrian rocks of Wales and England. In Holland, C. H. (ed.). LowerPalaeozoi¢ rocks ofthe world. 2 ~Cambrian of the British Is/es, Norden, and Spitsbergen. p. 43-12 L Wiley, London, New York, Sydney, Toronto. & Prm.r.Tps, W. E. A. I973. A Protospongia from the Dalradian of Glare Island, Co. Mayo, Ireland. Palaeontology x6, 23I-7 . SaLa~R, J. W. x864. A monograph of the British trilobites from the Cambrian, Silurian and Devonian formations. Palaeontogr. Soc. [Monogr. ], x-8o. SDztrY, K. x958. Neue Trilobiten am dem Mittelkambrium yon Spanien. Senckenberg. leth. 39, 235-53. .... x961. Das Kambrium Spaniens, Tell 2: Trilobiten; (I) Abh. math..naturw. Kl. Akad. Wiss. Mainz, Jg. x96x , 7, 499-594 (2x7-3x2). "' x969. Unter- mad mittelkambrische Porifera (Chancelloriida mad Hexactinellida). Paliiont. Z. ~, 115-47. I972. Das Kambrium der aeadobaltischen Faunenprovinz. Zbl. PalO.ont. x972 2, I-9x. Sa-tmBu~t,mta~, C. J. x956. Cambrian palaeogeography in Britain. In El Sistema Gdmbrieo, su paleogeografla y d problema de su base: symposium: Part I : Europa, Africa, Asia. p. 1-43.2oth Int geol. Congr., Mexico. x958. In WHrrT~'~D, W. F. & StepsoN, S. (ed.). Lexique stratigraphique international: Europe Fascicule 3a England, Wales & Scotland: Part Sa III Cambrian. t-95, Centre National de la Recherche Scientifique, Paris. WAa.eOa'T, C. D. i886. Second contribution to the studies on the Cambrian faunas of North America. Bull. U.S. geol. Surv. 3o, 727-to95.

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Received 24 December x975; revised typescript received 27 March I976.

MICHAEL G~v'YN BASSE~rr & ROBERT 1VIAURIC~E O~NS, Department of Geology, National Museum of Wales, Cardiff CFI 3NP. ADRL~ Wu.~AM AMSLBR RusIrroN, Department of Palaeontology, Institute of Geological Sciences, London SW7 2DE.

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