GEOLOGICAL JOURNAL, VOL. 16, 201-216 (1981)

Ashgill trilobite faunas from the Llyn Peninsula, North Wales, U.K.

David Price Sedgwick Museum, Downing Street, Cambridge CE2 3EQ. U.K.

The Crugan Mudstone Formation is formalized and the Dwyfor Mudstone Formation proposed for restricted Ashgill outcrops respectively near Llanbedrog and Llanystwmdwy in the L19n Peninsula. These mudstones contain brachiopods of the Foliomena fauna and a total of 26 determined genera of trilobites, one. Pseudosphaeroxochus represented by a new species. P. scabornei sp. nov. Elements of the trilobite fauna are used to demonstrate a Rawtheyan age for the mudstones. The generic assemblages both of brachiopods and of trilobites present in the mudstones indicate relatively deep water environments of deposition, that of the Dwyfor Mudstones probably being the deeper. The trilobites belong to what is here described as the Nankinolizhus-Opsimasaph~ association.

KEY WORDS Ordovician Ashgill Rawtheyan Trilobitse Brachiopods Biostratigraphy Palaeoecology

1. Introduction

In the Llyn Peninsula (Figure 1) of western North Wales (Gwynedd, formerly Caernarvonshire) rocks of Ashgill age are known at outcrop only in two small areas. One outcrop, just over 1 km2 in extent, stretches to the northwest of the farm of Crugan which lies 1 km northeast of Llanbedrog Church. The second covers a rather larger area (c. 3$ km2) around Llanystwmdwy some 15 km away to the east-northeast. Both of these areas of outcrop have received brief general descriptions and some comment on their faunas from previous workers (see below); the aim of this paper is to deal with the faunas, especially the trilobites, and so estimate the age and discuss the palaeoenvironmental setting.

2. Crugan Mudstone Formation

2a. General description Though the Ashgill rocks to the northwest of Crugan had yielded some fossils

to the early Geological Survey workers (listed in Ramsay 1866 p. 271), it was left

0072-1050/8 1/030201-16$0 1.60 @ 1981 by John Wiley & Sons, Ltd.

Received 2 February 1981 Revised I June 198 I

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Figure 1. Locality map.

to Matley (1938) to map them, collect extensively from them and on the basis of their shelly fauna (identified by Stubblefield) to recognize their Ashgill age. Matley’s work showed these Ashgill rocks which he termed the Crugan Mud- stones (1938 p. 577) to overlie graptolitic black shales (the Penarwel Shales) which Elles considered to belong to the highest part of the Dicranograptur clingani Zone (in Matley 1938 p. 591). No contact with younger rocks is visible but Matley estimated the exposed thickness of the Crugan Mudstones at over 300 m. Though the higher horizons of the formation are only poorly exposed and little is seen of their structure, the figure seems reasonable.

It is here proposed that these mudstones be formally designated the Crugan Mudstone Formation. Their type (and only) development is the area of outcrop mapped by Matley (1938 Plate 50) between their contact with Caradoc black shales in Penarwel Dingle west of Crugan and their disappearance beneath glacial drift to the northwest of Bachellyn Farm; there is no upper contact with ‘solid’ rocks as the formation forms the youngest part of the Lower Palaeozoic succes- sion in this region of the L l ~ Peninsula.

At all exposures the Crugan Mudstones are brown-grey (‘light olive gray’ to ‘yellowish gray’-around 5Y 6/2 on the Geological Society of America Rock Color Chart), compact mudstones. In places they show very thin and closely- spaced dark laminae, though such lamination is not extensively developed, and locally they show some Chondrires-mottling. In the well-exposed lowest part of the formation the mudstones contain frequent discoidal concretions up to 10 cm across. They are also abundantly fossiliferous, again particularly in the lower part of the formation. The fauna is dominated by trilobites but brachiopods too are

ASHGILL TRILOBITE FAUNAS, LLPN PENINSULA, N. WALES 203

reasonably abundant; also present are ostracodes, pelmatozoan debris and rarer conulariids and small gastropods. Simple tube-like trace-fossils, circular in section, gently sinuous and sub-parallel to the bedding are also abundant near the base of the formation where they range between about 2 and 12mm in diameter and from 2 to 13cm long. The trilobites are dealt with separately. The brachiopods form a distinctive, taxonomically restricted assemblage of small, thin-shelled forms. Of 112 specimens collected from locality A of Section 2b below and identified for the author by Dr. L. R. M. Cocks, 40 were referred to Leptestiina prantli (Havlicek), 37 to Christiania nilssoni Sheehan, 27 to Foliomenu folium (Barrande), 3 to Dedzetina sp. and 1 each to Anoptambonites? sp., Eos- tropheodonta sp., Leangella sp., Orbiculoidea sp., and Zygospira? sp. The assemb- lage is clearly related to the provisional Foliomena community of Sheehan (1973); its significance is discussed in Section 5 below.

2b. Trilobite fauna The lowest part of the Crugan Mudstone Formation has been intensively

collected, particularly the beds exposed in a small lane-side quarry about 150 m west of Crugan Farm (= locality A; Nat. Grid Ref. SH 3332 3241). Smaller collections from similar low horizons in the formation come from exposures on the northeast side of the stream about 150m north of Crugan (locality B) and from a rather higher horizon-though probably still within the lower third of the formation-from poor lane-side exposures c.155 rn from the south end of the lane to Bachellyn Farm (locality C; SH 3313 3256). The faunas from each of these localities are listed in Table 1. Here the number of specimens of each trilobite

Table 1. Trilobite fauna of the lower part of the Crugan Mudstone Formation. For locality details see text

Locality Taxa A B C

Nankinolithus 6. granulatus (Wahlenberg) 10 1 X X Opsimasaphus radiatus (Salter) 88 X X Dindymene longicaudara Kielan 71 - X Grauicalymene aff. pontilis Price 63 X X Raphiophorus cf. tenellus (Barrande) 62 ? X Pseudosphaerexochus seabornei sp. nov. 29 X Dionide d. richardsoni Reed (see note on p.214) 24 X Tretaspis caritus Price 20 - Lonchodomas d. drummuckensis (Reed) 16 X X Arthrorhachis tardus (Barrande) 14 X X

Duftonia geniculate Ingham 11 X Phillipsinella parabola uquilonia Ingham 7 X X Amphitryon radians (Barrande) 6 X X Panderia d. megalophthalma Linnarsson 6 - - Cyclopyge sp. 5 X X Ecrinuroides sexcostatus (Salter) 3 Limnemis recumus &innanson) 3 X X ?Pfaylichus glenos Whittington 2 X Ceraurinella intermedia (Kielan) 1 Microparia? sp. 1 Stenopareia? sp. - X

Total number of specimens (locality A) 546

- - -

- Cybeloides (Paracybeloides) giruanensis (Reed) 13 - -

- -

- - - - -

-

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species from locality A is based on counts of all identifiable trilobite material. including fragments, in large collections made by Dr. M. V. J. Seaborne and in bulk samples taken by the author.

The higher parts of the formation are accessible only in very restricted, poor exposures where the rock is largely shattered into small fragments and difficult to collect from. Exposures around Berllan Cottage, 1.65 km northwest of Crugan (SH 3248 3367), produced several specimens of Tretuspis hadelandica St~rmer brachystichus Ingham (see Price 1977 p. 781, Plate 102, Figure 5). a form not known from other localities in the formation. These exposures and others in the wooded area to the southwest (Coed-y-wern) also produced specimens of Cybeloides (Paracybeloides) giruanensis (Reed), Dindymene longicaudata Kielan, Limnemis recumus (Linnarsson)?. Opsimusaphus radium (Salter), Punderia cf. megalophthalma Linnarsson, Phillipsinella parabola (Barrande) aquiloniu Ing- ham, Raphiophorus 6. tenellus (Barrande) and Arthiorhachis tatdus (Barrande). N o higher horizons in the formation have yielded trilobites.

3. Dwyfor Mudstone Formation

3a. General description Rocks of Ashgill age in the Llanymawddwy district first received brief mention

by Fearnsides (1912). Though the Ashgill age of the rocks was not at that time realized, characteristic trilobites had been collected from mudstones exposed below Dynana Farm on the north bank of the Afon Dwyfor 1.3 km northeast of Llanystwmdwy and from mudstones on the east bank of the Afon Dwyfach 1.3 km further west. It was not until 1956, however, that these latter exposures constituting the lower part of the Ashgill succession in the Dwyfach valley were more fully described by Harper (1956). He collected and listed the graptolite fauna from the underlying black shales which he considered to be largely of Pleurograptus Iinearis Zone age. He also listed the shelly fauna from the Ashgill mudstones themselves and noted the occurrence at one locality in these of graptolites referred (1956 p. 391) to Orrhograptus truncatus s.1. ( = 0. ex gr. amplexicaulis (Hall) in modern terms). Subsequently the whole area around Llanystwymdwy was more comprehensively treated by Roberts (1967) whose map of the area (Plate 16) showed the Ashgill outcrop to take the form of an envelope around the elongated strip of Llandovery mudstones (the Llanystwmdwy Mud- stone Formation of Baker 1981) forming the core region of the Llyd Mawr Syncline.

It is here proposed that these hitherto un-named AshgiU rocks around Llanystwmdwy be designated the Dwyfor Mudstone Formation. There is no single section which passes through the entire thickness of the formation but if the whole outcrop area is regarded as the type development a composite section can be obtained from exposures in both the Afon Dwyfach where the base and lower parts are exposed (Harper 1956) and the Afon Dwyfor where the higher parts of the formation are exposed (Roberts 1967, p. 387); total thickness is estimated at 275m. General lithological descriptions have been given by Harper (1956) and Roberts (1967). Most of the formation comprises tough, blocky, dark grey mudstones (N3 to N4 on the Geological Society of America Rock Color Chart), distinctly more indurated than those at Crugan; in higher horizons around Dynana Farm these mudstones show well-developed Chondrites-mottling. The fauna is dominated by trilobites. Brachiopods are relatively less abundant than in

ASHGILL TRILOBITE FAUNAS. LLYN PENINSULA. N. WALES 205

the Crugan Mudstones though the three most abundant froms at Crugan, Christ- iania nilssoni, Foliomena folium and Leptestiina prantli, are all present at both of the main trilobite localities discussed below. In addition the exposures around Dynana have yielded a few ostracodes, tentaculitids and small gastropods.

3b. Trilobite fauna The basal 2 or 3 m of the Dwyfor Mudstone Formation

section (locality 9 of Harper 1956; SH46823976) have trilobites :

Arthrorhachis tardus (Barrande) Nankinolithus 6. granulatus (Wahlenberg) Opsimasaphus radiatus (Salter) Grauicalymene aff. pontilis Price Dindymene longicaudafa Kielan Dionide cf. richardsoni Reed Lonchodomas cf. drummuckensis (Reed) Duftonia geniculata Ingham Raphiophorus cf. tenellus (Barrande) Cybeloides. (Paracybeloides) gimanensis (Reed) ?Platylichas glenos Whittington Amphitryon radians (Barrande) Phillipsinella parabola aquilonia Ingham Pseudosphaerexochus seabomei sp. nov. Cyclopyge sp. Liocnemis recumus Linnarsson Paraproetus giruanensis (Nicholson and Etheridge)

in the Afon Dwyfach yielded the following

No. of specimens

24 22 17 16 11 10

8 6 6 5 4 3 3 3 3 2 1

The specimen numbers given are the totals available to the author from all sources (J. C. Harper collection, his own collection, old Sedgwick Museum collections and National Museum of Wales collections). They are hardly likely to consititute an unbiased sample but serve to give an idea, at least, of relative abundances.

The stratigraphically highest fossiliferous exposure in the formation is a small quarry in the farmyard at Dynana (SH 4802 3963). This quarry has yielded:

Cyclopyge sp., Dindymene longicaudata Kielan, Dionide cf. richardsoni Reed, Illaenus (Parillaenus) sp., Lonchodomas cf. drummuckensis (Reed), Microparia sp., Nankinolithus 6. granulatus (Wahlenberg), Opsimasaphw radiatus (Salter), Paraproetus giruanensis (Nicholson and Etheridge), Phillipsinella parabola aquilonia Ingham, Raphiophorus cf. tenellus (Barrande), remopleurid indet. Sphaeragnostus gaspeensis Cooper and Kindle, ?Sphaerocoryphe kingi Ingham, Symphysops cf. subarmata (Reed), Arthrorhachis tardus (Barrande).

Of these trilobites Nankinolithus and Opsimasaphus strongly dominate the fauna with Lonchodomas, Dindymene, Arthrorhachis and Sphaeragnostus also reasona- bly common and most of the other elements rather rare. While Cyclopyge S . S . is represented by several pygidia, Microparia known from a cranidium and Sym- physops known from two poor pygidia, there are many isolated cyclopy@d visual surfaces present in the mudstones here and cyclopygids probably make up a

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rather higher proportion of the fauna than the number of generically identifiable specimens would suggest.

Only those lowest and highest fossiliferous horizons in the formation from which the faunas are listed above have been collected intensively but intermediate sections along the Afon Dwyfach south and southeast of Tyddyn-crythor (SH 472 403) have yielded specimens of Dindymene longicaudata, Duftonia geniculata, Lonchodomas 6. drummuckensis, Opsimasaphus radiatus and Ar- throrhachis tardus.

4. Age and correlation

The trilobite faunas from the Crugan Mudstones and those from the Dwyfor Mudstones of Llanystwmdwy are not identical in generic composition and the significance of some of the differences at this taxonomic level will be discussed in Section 5 below. They do, however, still have a high proportion not only of genera but of species in common; sufficient, in fact, to argue €or a very close similarity in age if not an exact correlation. In this respect both faunas are undoubtedly Rawtheyan. It is true that some of their species such as Arthrorhachis tardus, Cybeloides (Paracybeloides) gimanensis and Phillipsinella parabola are known to have long ranges but most of them, or forms closely related to them, are known elsewhere only from strata of Rawtheyan age: Amphitryon radians, Platylichas glenos, Opsimasaphus radiatus and possibly Duftonia geniculata (see Ingham 1977 p. 117) from the Rhiwlas Limestone of North Wales (Whittington 1962-68), the same species of Amphityron from Zone 5 of the Cautley Mud- stones, of Duftonia from Zones 5-6 and of Opsimasaphus from Zones 6-7 at Cautley (Ingham 1970-77). The Grauicalymene from these Lljm faunas is a form considered by the author to be intermediate between G. pontilis Price known from Zone 5 and G. quadrata (King) which occurs in strata of Zone 7 age and the Lljln species of Dionide and Lonchodomas are compared with species based on type material from the high Rawtheyan Upper Drummuck Group of Girvan. Stricter age limits, however, can be argued if the Crugan and Llanystwmdwy faunas are taken separately.

Of forms restricted to the Crugan Mudstones, the species of Tretaspis are the most useful in this respect (cf. Price 1977, 1981). T. caritus Price is considered to range up from the base of the Rawtheyan Stage, at which level it appears to replace T. cf. radialis Lamont, and T. hadelandica brachystichus is known to be restricted to Zones 5 and 6, being replaced in Zone 7 by T. ‘latilimbus’ distichus Ingham. On this basis the Crugan Mudstones fall within the Zone 5-Zone 6 interval.

At Llanystwmdwy the presence of Paraproetus gimanensis may indicate a slightly younger age for at least part of the Dwyfor Mudstone Formation. Occurrences of this form elsewhere appear to be restricted to high Rawtheyan strata such as the Upper Drummuck Group of Girvan and the topmost Rawth- eyan of the Lake District (Owens 1973 p. 62). Similarly Sphaerocoryphe kingi, to which a form known only from Dynana has been tentatively referred, occurs in the Cautley Mudstones only in the upper part of Zone 6 (where it is rare) and in Zone 7 (Ingham 1974 p. 71). It is possible, therefore, that the higher part of the Dwyfor Mudstones may be of Zone 7 age and younger than the highest sampled mudstones at Crugan. It may be significant in this respect that Amphitryon

ASHGDU. TRILOBITE FAUNAS, L L w PENINSULA, N. WALES 207

radians, Duftonza genicufata, Grauicalymene aff. pontilis and Pseudo- sphaerexochws seabornei are not known from the higher part of the Dwyfor Mudstones.

5. Generic composition and palaeoecological significance of trilobite faunas

In an earlier discussion of the broad palaeoecological relationships of Ashgill trilobite faunas (Price 1980 p. 882) the author made some initial attempts at dividing these faunas into 3 or 4 main generic associations and pointed to the probable relation between such generic associations and various broadly- conceived lithofacies divisions and their inferred palaeoenvironmental setting. Figure 2 is derived, with some refinement, from that discussion, the associations being placed in their relative positions on a palaeoslope between platform edge and basin floor.

In the highest positions on this slope, near the platform edge or in analogous (e.g. tectonically elevated) locations, relatively pure carbonate lithofacies may be developed containing a distinctive illaenid-cheirurid-lichid trilobite association. Another association, here termed the Nankinolithus-Opsimaaphus association ('mudstone association' of Price 1980) appears to characterize broadly mid-slope

Figure 2. Diagram showing probable palaeonenvironmental relations of trilobite generic associations referred to in the text.

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(in terms of trilobite environmental ranges) positions and gives way down-slope to a more specialized deeper-water association comprising what have been referred to previously as various ‘Cyclopyge faunas’ (e.g. Stubblefield 1939 p. 61; Weir 1959 p. 381) and which may here be termed the Nouaspis-cyclopygid association. Between the Nankinolithus-Opsimasaphus association and the illaenid-cheirurid- lichid association occur faunas of an ‘intermediate’ nature (Price 1980) containing a mixture of genera belonging to both the illaenid-cheirurid-lichid and the Nankinolithus-Opsimasaphus associations. Some genera in these ‘intermediate’ faunas also relate closely to those occurring in shallower clastic and impure carbonate facies of ‘shelf‘ environments; there, appear however to be some genera not typically occurring in any of these other associations (e.g. Encrinuroides).

Some of the generic ‘overlap’ which can exist between different associations and is evident in the nature of the ‘intermediate’ faunas mentioned above can also be seen in the Llgn trilobite faunas. As previously noted (Price 1980 p. 883), overlap of this kind is potentially very useful for permitting h e r palaeoenvironmental discrimination within the overall environmental range of a particular ‘association’. The Lljln faunas are both typical faunas of the ‘mudstone association’ in the numerical dominance in them of the genera Nankinolithus and Opsimusaphw, in the abundance also of Dindymene, Dionide, Raphiophorus and Lonchodomas and in the presence of Duftonia, Liocnemis and Amphitryon. There are, however, differences between the Crugan and Llanystwmdwy faunas which probably relate to differences in palaeoenvironmental setting.

The Crugan fauna contains many specimens of the genus Tretuspis (though this is very much subordinate to Nankinolithus). This genus is much more characteris- tic of ‘intermediate’ faunas and though it is rare in the illaenid-cheirurid-lichid association is again common in shallower clastic and mixed clastic/carbonate shelf sequences. Encrinuroides and Ceraurinella which are typical members of ‘inter- mediate’ faunas, are present in the Crugan fauna, though as much rarer elements. All three of these forms are unknown in the Dwyfor Mudstones of Llanystwmdwy. These mudstones differ also from the Crugan Mudstones in containing overall a rather higher proportion of cyclopygids; Symphysops, is present at Dynana and unknown from Crugan as is the agnostid Sphaeragnosrus. Both Sphaeragnosnss and the cyclopygids are forms more characteristic of the Nouaspis-cyclopygid association, though in typical faunas of this association cyclopygids are both more numerically dominant and more generically diverse than in the Dwyfor Mudstones. Novaspis is not present in the Dwyfor Mudstones.

The Crugan Mudstone fauna then seems to represent a position towards the upper end of the environmental range of the Nankinolithus-Opsimusaphus as- sociation with just a few elements present more characteristic of shallower water associations, the Dwyfor Mudstones to represent a rather deeper water position (Figure 2). In broad terms both formations probably represent deposits well down-slope from any platform edge. That such an environment was a relatively low-energy one is attested by the high proportion of articulated trilobite exoskele- tons. At Crugan specimens of Dindymene, Raphiophorus and Grauicalymene are frequently articulated and articulated specimens are also known of Arthrorhachis, Nankinolithus, Tretuspis, Phillipsinella (see Whittington 1966 Plate 25, Figures 5 and 6), Liocnemis and Dionide (Figure 41). In the Dwyfor Mudstones articulated specimens of Amphitryon (Figure 4e) and Dionide are known from the basal Afon Dwyfach section and of Dindymene, Opsimusaphus (Figure 3j), Paraproetlls (d. Owens 1973 Plate 13, Figure l), Nankinolithus and Sphaeragnostlls (Figure 4d) from Dynana. Delicate structures such as the long frontal and genal spines of

ASHGILL TRILOBITE FAUNAS, LLYN PENINSULA, N. WALES 209

raphiophorids are also frequently preserved intact. The preservation of such structures and of articulated specimens also indicates thay many of the Lljrn trilobites are likely to be autochthonous,

Further evidence for a relatively deep-water environment comes from the presence in the Lljin mudstones of brachiopods of Sheehan’s Foliomenu ‘com- munity’ (see Sections 2a and 3a above)-a low diversity association of forms typically occurring in fine-grained lithofacies deemed to accumulate in deep-water settings (Sheehan 1973 p. 60) and in some places associated with graptolitic shales (Sheehan and Lesp6rance 1978 p. 455). The occurrence of a Foliomenu fauna in the Llfn mudstones is of interest in that elsewhere in Britain such faunas have so far been recorded onIy in conjunction with trilobite faunas quite definitely of the Nouaspis-cyclopygid association (Harper 1979, 1980). Its occurrence at Crugan together with a trilobite fauna containing not only typical members of the Nankinolithus-Opsimasaphus association but also Tretuspis and rare En- crinuroides and Ceraurinella suggests that the Foliomena brachiopod association could have a rather wider palaeoenvironmental range than these other British occurrences might indicate. It should be emphasized, however, that Leptestiina, the most common brachiopod in the Crugan fauna and present also at Llanystwmdwy, is not known in either of the Foliomenu faunas discussed by Harper but at Girvan is a member of his Ortharnbonites-leptestiina brachiopod association interpreted as occupying upper slope to shelf environments (Harper 1979 p. 433). It may also be noted that brachiopods appear to form a much higher proportion of the total fauna at Crugan then they do at either of the other British occurrences of the Foliomenu association so far reported (Harper 1979 p. 440; 1980 p. 190).

6. Systematic section

6a. Description of Pseudosphaerexochus seabornei sp. nov.

specimen orientation follows Temple 1975). (Lateral glabellar lobes (L) and furrows (S) are numbered from the rear;

Family CHElRUF2IDA.E Hawle & Corda, 1847 Subfamily ECCOPTOCHILINAE Lane, 1971 Genus PSEUDOSPHAEREXOCHUS Schmidt, 188 1

Pseudosphaerexochws seabornei sp. nov.

Figure 3a-f.

1956 Pseudosphaeroexochus 6. granulatus (Angelin); Harper p. 391.

Holotype. NMW. 78.65GSa, b, internal and external moulds of large, incom- plete cranidium from lower part of Crugan Mudstone Formation, laneside quarry 150m west of Crugan Farm (locality A of Section 2); M. V. J. Seaborne collection .

Horizons and localities. Lower parts only of Crugan Mudstone and Dwyfor Mudstone Formations of Lljm Peninsula; Rawtheyan stage, Ashgill series.

210 D. PRICE Diagnosis. Elliptical glabella bluntly rounded frontally, only moderately convex

longitudinally; 2s and 3s moderately long (trJY lateral lobes progressively short- ening (exsag.) frontally, frontal lobe shorter than 3L; ornament of closely-spaced granules. Pygidium with 4 pairs of sharply pointed spines, first 2 pairs gradually tapering, third pair thorn-like, fourth in form of isosceles triangles; all terminate in transverse line.

Description. Cranidium about If X as wide (tr.) as long (sag.). Glabella moder- ately convex (tr. and sag), elliptical in outline, about laxas long as wide and bluntly rounded frontally; where widest (w.)~ on level of 2L, occupies almost f total cranidial width. Lobes 1L in form of narrow obliquely oriented ovoids angulate antero-laterally and at their mid-lengths each occupying about one- quarter of glabellar width at that level. 1s deep and broad; on internal moulds reaching occipital furrow only as shallow, broad depression not seen on external moulds, moderately curved but still directed antero-laterally where meets axial furrows. 2s and 3s sub-parallel to anterior third of 1s but narrower and shallower; inner ends of all three exsagitally in line and separated mesially by just over one-third of maximum glabellar width (tr.). Lateral lobes become progres- sively shorter anteriorly in ratio 1:0-85:0-75. Frontal glabellar lobe in lateral view shorter than 3L; drops in steep, convex curve anteriorly. Occipital furrow broad and deep; ring broad (sag. and exsag.), bearing posteriorly positioned median node (Figure 3e). Axial furrows broad and deep. Fixed cheeks moderately convex (tr., exsag.) in front of deep posterior border furrows and broad (exsag.) borders; bearing slender genal spines. Anterior portions of fixed cheeks confluent with narrow, rather flat anterior border. Gently curved palpebral lobe separated from rest of cheek by strong furrow; slightly shorter (exsag.) than 3L and situated opposite posterior half of this and anterior part of 2L. Cranidial surface densely covered with generally evenly-sized granules of c. 0-05 mm-slightly coarser on lateral lobes; in addition fixed cheeks bear well-spaced, well-defined pits of up to 0.15 IIM.

Free cheeks, hypostoma and thorax unknown.

Figure 3 (a-f) Reudospocrexochus scabormi sp. nov. (a and b) HOLOTYPE, NMW 78.65G, internal mould of incomplete aanidium from low Gugan Mudstom Formation of small quarry 150m west of C ~ g a n Farm (hality A), 5% dorsal and 5b, right-lateral view, X l . (c) SMA106263. internal mould of pygidium from same horizon and locality. donal view, ~ 2 . 5 . (d) SM A40716, pygidium with exoskele- ton preserved from basal Dwyfor Mudstone Formation, Afon Dwytach section, Uanystymdwy, dorsal Vim, X2. (e) cast &om SM A106343, external mould of partial midiurn, horizon and locality as for (aHc). oblique view, x4. (f) Cast from SM A106344. external mould of partial pygidium from same horizon and locality, dorsal view, x2-5. (g) Nankinolithur d. granulaw (Wahlenberg), cast from SM A106545b. external mould of cephalon, horizon and locality as for (d). antcrior oblique view, X4. (a and' i) Dtndymcnc longicaudclta Kielan. (h) Cast from SMA106560. external mould of small cranidim, horizon and locality as for (d). dorsal view. ~ 1 0 . (3 Cast from SMA106267b, external mould of pygidium and pardal thorax, horizon and locaiity as for (a)-(c). dorsal view, x6. (j) opsimasaphw mdianu (Salter). SM A106600, internal mould of articulated exoskeleton from the Dwyfor Mudstone Formation, Dyana farmyard, Llanystwmdwy, dorsal view, xl. (k) Tretaspis cwifus Price, cast from SMA106295b, external mould of incomplete ccphalon. horizon and locality as for (a)+), anterior view, x4.

originals of (axe), (e), (0, (i) and (k) from M. V. J. Seaborne collection. Originals of (s) and Q from J. c. Harper mllection. Specimen number prefix SM = Sedgwick Museum, Cambridge, NMW = National Museum of Wales. Cardiff.

ASHGILL TRILOBITE FAUNAS, LLYN PENINSULA, N. WALES 21 1

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ASHGILL TRILOBITE FAUNAS, LLPN PENINSULA, N. WALES 213

Pygidium trapezoid in overall form, with greatest width (tr.), between bases of outermost pleural spines, 12 x maximum length. Strongly convex axis anteriorly occupies over one-third this maximum width and tapers back at 20"; comprises 3 rings and rudimentary terminal piece. First two rings bounded laterally by broad axial furrows and expanded (sag. and exsag.) mesially, particularly second ring which has a pronounced posterior convexity. First two ring furrows broad and deep abaxially but mesially shallowing and curving gently back; third furrow developed only as pair of ovoid slots. First and second pairs of ribs flat (tr., exsag.) where cross pleural lobes, separated from each other and from third pair of ribs by broad, shallow furrows; faint central rib-furrows visible on internal moulds. Third pair of ribs not clearly separated from third axial ring nor fourth pair from terminal piece. Four pairs of pleural spines all sharply pointed posteriorly. First two pairs gently curved and only gradually tapering; with distinct postero-mesial swellings at their bases. Third pair more rapidly tapering and fourth in form of isosceles triangles. All spines terminate posteriorly along single transverse line. No surface ornament seen (irregular apparent granulation on original of Figure 3d probably represents infilings of canals in the exoskeleton).

Discussion. In glabellar outline P. seabomei sp. nov. is similar to specimens of P. ocrolobatus (McCoy) (Lane 1971 p. 48, Plate 8) from the Upper Drummuck Group of Girvan. In P. seabornei, however, 2s and 3 s are much longer (tr.) in both dorsal and lateral views, the glabella has a weaker longitudinal convexity and its ornament lacks the scattered tubercles seen among the fine granulation of P. ocrolobarus. Another form with scattered coarse tubercles is P. boops (Salter)

Figure 4 (a) Duftonia geniculara Ingham, cast from SMA106534, external mould of aanidium from basal Dwyfor Mudstone Formation, Afon Dwyfach section. Llanystwmdwy. dorsal view, x4. (b) Liocnenis recwuus (Linnarsson). SMA106391, internal mould of aanidium from low Crugan Mudstone Formation 150m north of Crugan Farm (locality B), dorsal view, x4. (c) Panderia cf. megalopthalma (Linnarsson), SMA106274, internal mould of pygidium from low Crugan Mudstone Formation 150 m west of Crugan Farm (locality A), dorsal view, X4. (d) Sphaeragnostus gaspeensis Cooper and Kindle, SM A106614. articulated enrolled specimen with exoskeleton preserved from Dwyfor Mudstone Formation, farmyard at Dynana, Llanystwmdwy, pygidium in dorsal view, x6. (e) Amphifryon radians (Barrande), SM A106517, internal mould of partial articulated exoskeleton, horizon and locality as for (a), dorsal view, X4. (f and 9) Cyclopyge sp. (f) SM A106540a. internal mould of glabella, horizon and locality as for (a), dorsal view, X8. (g) Cast from SM A106288b. external mould of pygidium, horizon and locality as for (c), dorsal view, ~ 1 0 . (h and i) Loncfiodomas cf. drummuckensis (Reed). @) SM A106510a, internal mould of incomplete cranidium, horizon and locality as for (a), dorsal view, ~ 2 . 5 . (i) Cast from SMA106506, external mould of pygidium, same horizon and locality, dorsal view, x5. (j) Raphiophorus cf. tenellus (Barrande). cast from NMW 76.14G.20b. external mould of articulated exoskeleton, horizon and locality as for (c), dorsal view, x4. (k and I) Dionide cf. richardsoni Reed. (k) SM A106390, internal mould of partial cephalon, horizon and locality as for (b), dorsal view, x5. (l) Cast from SMA106383b, external mould of articulated throax and pygidium, horizon and locality as for (b), dorsal view x3. (m)? Sphoemoryphe kingi Ingham SM A106605, internal mould of pygidium, horizon and locality as for (d), dorsal view, x4. (n)? Platylichas glenos Whittington cast from NMW 72.18G.l91b, external mould of incomplete cranidium, horizon and locality as for a, dorsal view. X4.

Originals of (a), (e), (0, (h) and (i) from J. C. Harper collection. Originals of (b)-(d), (g). (k) and (1) from M. V. J. Seabome collection.

214 D. PRICE

(McNamara 1979 p. 64, Plate 8, Figures 9-14) from the low Cautleyan of the Lake District. This form also differs from P. seabornei in that 3L is much longer (exsag.) than 2L and 3 s is more transversely oriented. The glabellar characters of P. seabomi differ more markedly from those of the Cautely form P. tecrus Ingham (1974 p. 68, Plate 13, Figures 8-13) with its almost circular glabellar outline and strongly curved basal lateral furrows and from the Sholeshook Limestone form P. juuenis (Salter) (Price 1980 pp. 858-9, Plate 110, Figure 15, Plate 111, Figures 8-11) with its narrower (tr.), frontally parabolic outline and stronger longitudinal convexity.

It is in pygidial characters, however, that P. seabornei differs most strongly from P. octolobattcs and the allied P. tecrus where the pleural spines are short and lobately terminated, with their ends lying on a posteriorly convex curve. In other species which have sharply pointed pygidial spines such as P. juuensis, P. octacan- thus (Angelin) (Kielan 1960 Plate 25, Figure 7) and P. granulatus (Angelin) (Warburg 1925 p. 356, Plate 10, Figures 8-15, 17 and 18) the posterior most two pairs of spines are longer and more gradually tapering than those of P. seabornei and distinctly curved. This difference applies too to the third pair of pygidial spines in the Polish form P. wolkae Kielan (1960 p. 134, Plate 24, Figures 5a and b, Plate 25, Figures 1 and 2; text-Figure 37) but here the fourth pair, also apparently slender terminate well in front of the third pair. In P. wolkae the whole exoskeleton is coarsely tuberculated.

6b. Notes on other taxa Most of the more important trilobites mentioned in the faunal lists of Sections

2b and 3b above and in Table 1 are illustrated in Figures 3 and 4. Forms such as Ceraurinella intermedia, Encrinuroides sexostatus and Arthrorhachis tardus, al- ready well known from many Ashgill faunas are not figured again, nor are those few of the Lljrn forms which have already been figured elsewhere: Grauicalymene aff. ponrilis (Pnce 1982 Plate 1, Figures 7, 9 and l l ) , Opsimusaphus radiatus (Whittington 1966 Plate 24, Figures 5 and 6), Paraproem giruanensis (Owens 1973 Plate 13, figures 1 and 2) and Tieraspis hadelandica brachystichus (Price 1977 Plate 102, Figure 5). Material of the two cyclopygid forms Microparia and Symphysops has proved too poor to provide staisfactory illustrations.

As Ingham (1974 p. 64) noted, the Girvan species Dionide richardsoni Reed was based on material from two separate horizons and two distinct forms are represented. The material from Lljm is compared with specimens from the high Rawtheyan Upper Drummuck Group at Girvan and not with material from the Upper Whitehouse Beds of probable PusgiIlian age.

Specimens of Nankinolithus cf. grunulatus from the Lljrn mudstones, (Figure 3g), in contrast to material recently described by the author from South Wales (Price 1980 p. 851, Plate 109, Figures 1-10), show extensive reticulation on both genal and pseudofrontal lobes. In this they agree with specimens from the Abercwmeiddaw Group of Corris and Dinas Mawddwy. The presence or absence of such reticulation appears to be a feature of whole populations and not merely a matter of preservation as considered by Kielan (1960 p. 173).

The genus Sphaeragnosm Howell and Resser has not previously been recorded from the upper Ordovician of Wales. Apart from its presence in the Dwyfor Mudstones of Llanystwmdwy it is known also from the Slade and Redhill Mudstones of Robeston Wathen in South Wales (B.M.(N.H.) colln.), from Ashgill (?) strata around Dolgarog in North Wales (I.G.S. colln.) and possibly from the low Abercwmeiddaw Mudstones of the Dinas Mawddwy area (author’s tentative identification of distorted specimen in P. M. Magor colln.).

ASHGILL TRILOBITE FAUNAS, LLPN PENINSULA, N. WALES 215

The genus Liocnemis Kielan, first recorded in British faunas by the author (Price 1973 Table 7) from the basal Slade and Redhill Mudstones of the Haverfordwest area, South Wales and here recorded from Lljln (Figure 4b) is now also known from the Bodeidda Mudstone of the Conway area (I.G.S. and author's collections) and from the lower part of the Abercwmeiddaw Group in the area around Dinas Mawddwy (P. M. Magor colln.).

Acknowledgements. I am very much indebted to Dr. M. V. J. Seaborne, Principal of Chester College, for initially stimulating my interest in the trilobites of the Crugan Mudstones, for giving me very willing access to his own collections and for his hospitality both at Chester and in the Lljln Peninsula. Thanks are also due to Dr. P. J. Brenchley for arranging access to the J. C. Harper collection, then at Liverpool, to Dr. L. R. M. Cocks for his comments on the Lljln brachiopods and to Dr. A. W. A. Rushton, Mr. S. P. TunniclifTe (I.G.S.) and h4r. S. F. Morris (B.M. (N.H.)) for arranging the loan of specimens in their care. (Material from the M. V. J. Seaborne and J. C. Harper collections is now in the Sedgwich Museum.) Mr. S. D. G. Campbell kindly read an earlier version of this paper and made several suggestions for improvements.

Note added m pmof

Since completing the text of this paper the author has identified specimens of PseudoJphacrcrochus seabomei sp. nov. in a collection made by Dr. J. K. Ingham from the basal Abercwmeiddaw Mudstones (= Motlfryn Mudstones) of Craig-Tjr-nant, c. 15 km south of Bala. Mr. S. D. G. Campbell has also made a further examination of the brachiopods from Crugan and comments that spechens of Foliomenu folium in the Crugan fauna are considerably larger than specimens from most other Foliomena faunas. He also suggests that the specimen listed on p. 203 above as Anoptambonita? sp. is more probably referable to Chonetoidea.

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