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

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Lower Cambrian Fossils from the Hell's Mouth Grits, St Tudwal's Peninsula, North Wales 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/6/623/4885552/gsjgs.132.6.0623.pdf by guest on 29 September 2021 624 M. G. Bassett, R. M. Owens & A. W. A. Rushton 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 num- erous 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/6/623/4885552/gsjgs.132.6.0623.pdf by guest on 29 September 2021 Lower Cambrianfossils from the Hell's Mouth Grits 625 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 reconstruc- tions (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 ~)Ig -,.- /.::':::::::::::"!!i?i!!i!!!i30!i!/:;::::;:::::I;;;: :::::.'! ~~NO~ Porth Neigwl or Hell's Mouth ALES ? .... :::::::::::::: ::::::::: ............. :::::,%.:::::.::.: ~ :-:::!!~ .... ~,:::..:.... ::.::............ ..... :.~ ~.... .............. :..:. ::.:::.~,..::. %~._ Trwyny Ffosle :::::::::::::::::::::::::::::::::: ::! ! i ! i~: i i ! :: :: i~'.~!,~ i ! ! i i ii i i ! ! i ~.' " map area !--_-'~::::::::::::::::::::.%::::::::: ..,.,, :::::::::::::::::::::::: .................... °~ Alluvium :::::::::::::::::::::: :::.-::::.: : ~:. 0 ~ ARENIG R D ...... "....'~...:.::.'..: Trwyn ~. : ..... .... .~. ..~ ..... "" .... : ......... "~ ~" U, ~ CaeredMudstonesI .~ Cilan Grits M Trwyn y ~!ii!iii!iiiiiii!ii: B MulfranBeds R Hell~Mouth Grits N 400 800metres " Ll~ch'-y':doll --" "- --" Fault ! I 88J0yards :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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/6/623/4885552/gsjgs.132.6.0623.pdf by guest on 29 September 2021 626 M. G. Bassett, R. M. Owens & A. W. A. Rushton 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 differen- tiated 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 compara- tively 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.
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