Staurocephalinae Prantl & Přibyl, 1947

Leidi, Manus G., Mccobb, Lucy M. E., Mcdermott, Patrick D. & Owen, Alan W., 2024, A new Late Ordovician bubble-headed trilobite species from South West Wales and its implications for biostratigraphy, Acta Palaeontologica Polonica 69 (1), pp. 57-72 : 60-68

publication ID	https://doi.org/ 10.4202/app.01120.2023
persistent identifier	https://treatment.plazi.org/id/03F187AC-FFDF-FF96-FCE1-F4A5FBCCF825
treatment provided by	Felipe
scientific name	Staurocephalinae Prantl & Přibyl, 1947
status

Treatment

Subfamily Staurocephalinae Prantl & Přibyl, 1947

Remarks.—Although given full family rank by many earlier workers (e.g., Kielan 1957; Evitt and Tripp 1977; Holloway 1980; Owen and Bruton 1980; Tomczykowa 1987), the concept of Staurocephalidae was largely based on the distinctive “bubble headed” morphology of derived members of the group. We concur with Adrain (2013: 307–308) in regarding Staurocephalinae as a subfamily of Encrinuridae , on the basis of shared morphological traits of protaspid and early meraspid stages of less derived staurocephalines such as Libertella , encrinurines, and “advanced” cybelines (see Edgecombe et al. 1988).

Genus Staurocephalus Barrande, 1846 View in CoL

Type species: Staurocephalus murchisoni Barrande, 1846 , from the Liteň Formation (Wenlock) of Bohemia, Czech Republic .

Emended diagnosis.—Glabella with subparallel-sided posterior portion bearing short S1 and S2; S3 extends across glabella; frontal lobe greatly inflated and overhanging anterior margin of cephalon. Anterior part of rostral plate expanded and continuous with frontal glabellar lobe; short fixigenal spine present in most species. Thorax of 10 segments. Pygidium with 3 pairs of spines. Emended from Curtis and Lane (1997: 50)

Staurocephalus oliveae sp. nov.

Figs. 3 View Fig , 4 View Fig , 5 View Fig ,?6A.

non1885 Staurocephalus globiceps Portlock ; Marr and Roberts 1885: 481 [= Staurocephalus cf. clavifrons ].

?1938 Staurocephalus cf. murchisoni Barrande ; Whittington 1938: 451.

?1973 Staurocephalus clavifrons Angelin ; Price 1973a: tables 1–3, non 7 [= Staurocephalus cf. clavifrons ].

?1980 Staurocephalus cf. clavifrons Angelin ; Price 1980b: 866 (Sholeshook Limestone specimens only), pl. 110: 18.

ZooBank LSID: urn:lsid:zoobank.org:act:99371F20-B988-498A-81E0-

55729B248609

Etymology: After Olive Potts (St. Clears, Carmarthenshire, UK), who discovered the first specimen of this species.

Type material: Holotype:almost complete cephalon, NMW 2013.11 View Materials G.1

( Fig. 3A View Fig ). Paratypes: almost complete cephalon with eyes preserved,

NMW 2013.11G.2 ( Fig. 3B View Fig ); one further cephalon, NMW 2013.11G.8

( Fig. 3E View Fig ); three cranidia, NMW 2013.11G.6, 7, 9 ( Fig. 3C, D, F View Fig );

three librigenae, NMW 2013.11G.3, 10, 12 ( Fig. 4B–D View Fig ); three pygidia, NMW 2013.11G.4, 11, 15 ( Fig. 4F, G, H View Fig ); one rostral plate, NMW

2013.11G.5 ( Fig. 4A View Fig ); one thoracic segment, NMW 2013.11G.13

( Fig. 4E View Fig ). All from the type locality and horizon.

Type locality: Talfan farm, Llanddowror, Carmarthenshire, Wales, UK .

Type horizon: Sholeshook Limestone Formation (Cautleyan, upper Katian), Upper Ordovician.

Material.— Type material and 236 specimens (88 cephala/ cranidia, 33 pygidia, 80 librigenae, 33 rostral plates, one thoracic segment; NMW 2013.11G.13, 14, 16–249) from the type locality. One cranidium, ( NMW 2013.11G.250) and one pygidium ( NMW 2013.11G.251) from the Sholeshook Limestone Formation at Mylet Road near Llanddowror, Carmarthenshire. Price (1980b) reported 15 cranidia of S. cf. clavifrons , ranging throughout the Sholeshook Limestone Formation into the basal Slade and Redhill Mudstone Formation, from the Sholeshook and Prendergast areas near Haverfordwest and from the Llanddowror area but not all of his specimens could be located in museum collections. The single cranidium ( GLAHM: A 9751) figured by Price (1980b: pl. 110: 18), from low in the Sholeshook Limestone in Craig-y-deilo Quarry, Llanddowror, was collected by J. Keith Ingham (Glasgow, UK) and is very likely conspecific with our material (see Remarks, below). The specimen register at the Hunterian Museum, Glasgow, UK, notes those collected by J.K. Ingham in the 1960s from the Sholeshook Limestone that were studied by David Price as part of his Ph.D. research (1971), and these include the figured cranidium and six further Staurocephalus specimens (what became referred by Price 1973a: “one from Loc. 18c, four from Loc. 18d, and two from Loc. 25”). Unfortunately, these specimens cannot currently be located, but are presumably among the 15 Staurocephalus cf. clavifrons cranidia listed by Price (1980b). Two cranidia from the historic collections of the Sedgwick Museum, Cambridge, UK, collected from the Sholeshook Limestone Formation in the Sholeshook railway cutting, Haverfordwest area, were labelled as Staurocephalus globiceps ( SM A31591) and Staurocephalus cf. murchisoni ( SM A31592; Fig. 6A View Fig ) but likely also belong in our new species. Three further fragmentary cranidia labelled “ Staurocephalus globiceps ” in the Sedgwick Museum collections ( SM A31587–9) were collected by Marr and Roberts (1885) from a locality near Pelcomb Cross, in strata now assigned to the basal Slade and Redhill Mudstone Formation (see Price 1973a). These specimens are generally too poorly preserved for specific identification, but one possesses a prominent genal spine ( Fig. 6E View Fig ) and is excluded from S. oliveae sp. nov. (see below).

Diagnosis.—Differs from other congeners by possessing the following combination of characters: Staurocephalus species with right-angled genal angle, lacking a genal spine or protrusion, with four lateral denticles anterior to the genal angle on the fixigena, decreasing in size posteriorly. Librigenae with 13–17 marginal denticles; mode and mean = 15. Inflated anterior glabellar lobe making up around two-thirds of preglabellar glabellar length (sag.). Pygidium around one fifth wider (tr.) than long. Inner of three pairs of spines making up just under two-fifths of total pygidial length. Cephalon (including rostral plate and librigenae) with dense covering of fine to medium granules, median occipital tubercle centrally located or on the posterior half of the occipital ring; pygidium with dense fine granules on spines with larger granules on axial rings and pleural ribs.

Description.—No complete articulated exoskeletons known. Cephalon approximately semicircular in outline; cranidium subtriangular ( Figs. 3 View Fig , 5A View Fig ). Cephalon max. length (sag.) mean = 6.86 mm, range = 2.39–10.08 mm (n = 62). Cephalon max. width (tr.) mean = 10.15 mm, range = 3.37–14.01 mm (n = 42). On those specimens (n = 42) that are complete enough to measure both dimensions, maximum width (tr.) (mean = 10.02 mm, range = 3.37–14.01 mm) of cephalon is about one and a half times (mean = 1.50, range = 0.75– 2.18) the maximum length (sag.) (mean = 6.71 mm, range = 2.39–8.45 mm). Glabella strongly convex (tr.), consisting of a greatly inflated, subcircular anterior glabellar lobe in front of a rectangular glabellar stem. In some specimens, glabellar stem tapers gently forwards, however, in less crushed specimens (e.g., NMW 2013.11G.6; Fig. 3C View Fig ), margins of stem are sub-parallel. Occipital ring accounts for about one fifth of transverse posterior width of cephalon and about a tenth of the total cephalic length (sag.) medially, tapering abaxially by about a third; separated from the posterior border by a very shallow furrow. Median tubercle present, centrally located or on the posterior half of the occipital ring ( Fig. 3A View Fig 1, B 1, C). Occipital furrow shallow and follows the transverse convexity of the glabellar stem and occipital ring, ending in deep pits where it meets the axial furrows. Anterior to the occipital furrow is the narrow (tr.), rectangular glabellar stem, indented by the lateral glabellar furrows S1 and S2; uniformly deep, semi-circular pits. S1 located approx. one fifth of length (sag.) of the glabellar stem in front of occipital furrow; L1 small, an elongate suboval abaxially, narrowing and subparallel-sided adaxially, weakly inflated, angled forwards at around 40° to the axis. S2 located approx. 70% along the length of the glabellar stem; L2 subrectangular, shorter, less inflated and less strongly angled forward than L1; L3 a very weakly inflated, transverse area behind S3. Transglabellar (S3) furrow marks the anterior end of the glabellar stem behind the inflated anterior lobe. S3 is consistently shallow, following the transverse convexity of the glabellar stem, and ends at the deep axial furrow where there is a suggestion of a pit in some specimens although this feature is not clearly preserved in any. Inflated anterior glabellar lobe (mean sag. length: 3.75 mm, n = 20) occupies about two thirds (mean = 0.59, range = 0.50–0.77) of total sagittal glabellar length (mean: 6.31 mm, n = 20), excluding occipital ring, while being approximately three times as wide (tr.) as the glabellar stem (excluding occipital ring), with about half of its length (sag.) protruding over the anterior cephalic margin. The axial furrows are consistently deep alongside the glabellar stem, then anterior of the transglabellar furrow they follow the curvature of the inflated anterior glabellar lobe, between the anterior glabellar lobe and inflated anterior fixigena, remaining consistently deep.

Fixigenae occupy about four fifths of the posterior (tr.) cephalic width. Palpebral lobe semicylindrical, bounded by a shallow furrow; located at the approximate mid width (tr.) of the fixed cheek, slightly posterior to level of S2 furrow. Fixigenal field inflated, sloping down steeply anteriorly, highest parts positioned above the level of the glabellar stem and commonly reaching half the preserved height of the anterior glabellar lobe in posterior view, although in uncrushed specimens the anterior glabellar lobe was probably somewhat higher. The posterior border is weakly convex exsag.), extending transversely (sag.) from the occipital ring to behind the palpebral lobe beyond which it widens (exsag.) markedly towards the genal angle where it joins the posterior end of the lateral cephalic border at a right angle. No genal spine present, however four small denticles present on lateral border, increasing in size anteriorly and continuous with the larger denticles of the librigena. Posterior border furrow shallow, broadening markedly abaxially alongside the broadening of the posterior border.

The posterior branch of the facial suture is directed transversely from the palpebral lobe curving very gently forwards, until it reaches the lateral border where it curves slightly posteriorly. The anterior branch of the facial suture extends parallel to the sagittal line, from the palpebral lobe to opposite the middle (sag.) of the anterior glabellar lobe. Here the suture curves adaxially and is obscured by the anterior glabellar lobe in dorsal view, meeting the rostral and connective sutures on the flank of the anterior cephalic “bubble”.

Librigena triangular, maximum length subparallel to the lateral border approximately two fifths longer than width normal to this; librigenal field also has a maximum length subparallel to lateral border two fifths longer than width normal to the field ( Fig. 4B–D View Fig ). Librigenae are a little less convex in comparison to the fixigenae. Short (exsag.) eye lies at summit of inflated field, which descends relatively steeply abaxially, separated from it by a shallowing of the slope. Visual surface of eye sub-oval, convex, taller than wide, set on top of a socle of around half its height. Visual surface containing over 120 small hexagonal lenses, with over 16 around the base, ( Fig. 4D View Fig ; NMW 2013.11G.12, max. length 5.94 mm, max. width 4.31 mm), arranged in hexagonal packing. Librigenal border furrow uniformly broad and shallow. Librigenal border comprises a very narrow, flat-lying, very weakly swollen ridge, barely distinguished from the border furrow, and a row of denticles, each longer than the width of the ridge. On complete specimens (n = 36), 13–17 denticles are present with a mean of 15 (rounded from 14.6) and mode of 15 ( Fig. 7 View Fig ). The number of denticles is independent of the size of the librigena. Portion of librigena anterior of inflated field comprises a low-lying subtrapezoidal area, the anteriormost part of which is upturned to form a subtriangular flap, the librigenal precranidial lobe ( Figs. 4B, C View Fig , 5A View Fig 2 View Fig ). Anterior edge of precranidial lobe has a very gentle forwards curvature that matches the curve of the anterolateral margin of the rostral plate, the two librigenal precranidial lobes fitting with the rostral plate along the connective sutures to form the underside of the inflated cephalic lobe (“bubble”).

Rostral plate sub-circular, split into two distinct parts ( Fig. 4A View Fig ). Anterior part of plate lies beneath the anterior glabellar lobe along the rostral suture and is inflated, matching the convexity of anterior glabellar lobe and forming the anteroventrally directed part of the anterior cephalic “bubble” (see Fig. 5 View Fig ). Separated by a transverse furrow, the posterior section consists of a set of six denticles. Behind the denticles there is short, flat ventral section, of similar length (sag.) to the denticles and continuing in the same plane as them.

Hypostome not known.

Internal mould of cranidium (including rostral plate) apart from the furrows and borders densely and evenly covered in relatively small granules. Latex casts of external moulds reveal the granules to be larger and more densely packed on external exoskeletal surfaces, although the number of granules remains consistent ( Fig. 3D 3 View Fig , E 2 View Fig ). Additionally, the larger granules shown on latex casts vary in size. Internal moulds and latex casts of external moulds of the librigenal field have similar granulation to those of the cranidium, with occasional larger granules situated on the marginal border, in line with the denticles.

Thorax known only from internal moulds of two isolated thoracic segments from the type locality that probably belong to this species ( Fig. 4E View Fig ) and are consistent with segments from the posterior part of the thorax (e.g., see specimen of S. clavifrons illustrated by Kielan 1957: pl. 3: 2). Axial ring occupies about a quarter of segment width (tr.). Pleura parallel-sided over most of its length (tr.) turning posteriorly through almost a right angle at the fulcrum and extended as a posteriorly directed pleural spine, almost parallel to the sagittal line, that tapers to a sharp point over the distal 30% of its length. The granulation of these spines is denser and finer than that of the cranidium. Prominent large granules present at the fulcrum. A small number of medium granules developed into short spines visible on a small part of the axial ring of one of the specimens, but preservation too poor to determine overall sculpture.

Pygidium subquadrate in outline, being about one fifth wider (tr.) than long, including spines ( Figs. 4F–H View Fig ). Axis occupies approximately 50% of pygidial length (sag., including spines), and approximately 30% of the anterior pygidial width. Axis convex (tr.) anteriorly, becoming progressively less so and tapering (tr.) posteriorly. Axis has three distinct axial rings, the first two being transverse with the third arching gently forward, separated by axial ring furrows, which become relatively shallower posteriorly following the decreasing convexity of the axis. Each furrow is shallow and narrow medially, widening and becoming deeper laterally. Axis ends with a subtriangular, flat terminal piece. Axial furrows very shallow, little more than a break in slope between the convex axis and flat-lying pleural areas. Axial rings confluent with pleural ribs, the first pair of which are directed transversely for a short distance before curving posteriorly through about 90°, the second and third pairs are essentially parallel to the sagittal line. On external surface ( Fig. 4H View Fig 2 View Fig ), the transverse, inner parts of the anterior two pleural ribs bear shallow furrows directed very slightly obliquely to the anterior edge of the ribs at about one third of the distance from that edge to the posterior edge. The pleural ribs each broaden and become robust, posteriorly directed, spines; the outermost being the longest. The inner edges of the outer and middle pair of spines remains parallel to the sagittal line, with their outer edges curving gently inward, tapering to form a point. On some specimens (e.g., Fig. 4F View Fig ), both inner and outer edges of the inner pair of spines are gently curved to form a more symmetrical spine, although in others the inner edge is straighter (e.g., Fig. 4G View Fig ). The spines are directed parallel to the sagittal line and terminate along an essentially transverse line. The innermost pair of spines (mean = 1.41 mm, n = 4) account for around two fifths (mean = 0.42, range = 0.36–0.46, n = 4) of total length (mean = 3.37 mm, n = 4) of the pygidium.

The pygidium has a varied surface sculpture on the internal mould ( Fig. 4F, G View Fig ) and latex casts of external moulds ( Fig. 4H View Fig 2 View Fig ), with the spines having fine and dense granulation, relatively smaller and denser than on the cranidium. A large prominent tubercle is visible in the centre of each axial ring, similar to the occipital ring, with a row of coarse granules either side (e.g., NMW 2013.11G.11, Fig. 4G View Fig ). Larger granules are also present on the pleural field, of some specimens (e.g., NMW 2013.11G.4, Fig. 4F View Fig ).

Remarks.— Staurocephalus oliveae sp. nov. is most similar to Staurocephalus clavifrons Angelin, 1854 , which was first described from the Staurocephalus Beds (now Ulunda Mudstone, Rawtheyan, uppermost Katian) of Ålleberg, Västergötland, Sweden (cranidium figured by Angelin 1854: pl. 34: 8; refigured by Kielan 1957: pl. 4: 1; see also Kielan-Jaworowska et al. 1991: 240). It was subsequently described in detail from an equivalent horizon in the Holy Cross Mountains, Poland, by Kielan (1957) and from the overlying Mucronaspis mucronata Zone (“ Dalmanitina mucronata zone” then) there (probably Hirnantian, Upper Ordovician; but see Discussion section below) ( Kielan 1960). Staurocephalus oliveae sp. nov. differs from S. clavifrons in lacking genal spines, in possessing four rather than three denticles on the lateral border of the fixigena, with these denticles increasing in size anteriorly whereas they are the same size in S. clavifrons (see e.g., Kielan 1957: fig. 2) and, in most specimens, a greater number of denticles (13–17, mean 15, cf. 13–14) on the lateral margin of the free cheeks. Some specimens also differ in having the medial occipital tubercle posteriorly (cf. centrally) placed. The cephalic and pygidial granulation of S. oliveae sp. nov. is denser and finer than that of S. clavifrons both on internal moulds and on the external surface. Staurocephalus oliveae sp. nov. may differ from S. clavifrons in the number of lenses in the eye (120 cf. 60 recorded by Kielan 1957: 165) although the size of the librigena on which this was counted by Kielan is not known.

Staurocephalus clavifrons was reported or described from Ashgill (Cautleyan and Rawtheyan, upper Katian) horizons in Wales ( Whittington 1965; Price 1973a), northern England ( Ingham 1974, McNamara 1979) and the Republic of Ireland ( Dean 1971). However, Ingham (1977) later described some better-preserved specimens from the Rawtheyan of northern England, which showed that although they were morphologically very similar, there were differences in sculpture from the type and Polish specimens. He concluded that none of the occurrences from Britain and Ireland could confidently be placed in S. clavifrons and should instead be assigned to S. cf. clavifrons or S. aff. clavifrons . As a result, Price (1980b: 866, pl. 110: 18) described as S. cf. clavifrons the material that he earlier (1973a: 245) recorded as S. clavifrons from throughout the Sholeshook Limestone Formation and the base of the overlying Slade and Redhill Mudstone Formation. As is noted above, with the exception of the Slade and Redhill Mudstone Formation specimens, this material is here included in S. oliveae sp. nov., although questioningly so given the poor preservation of the available specimens. The posterolateral part of the left side of the cranidium from the Sholeshook Limestone at Craig-y-deilo quarry figured by Price (1980b: pl. 110: 18) appears to sweep rearwards slightly as if representing the innermost proximal part of a genal spine. However, this area is only slightly beyond the level of the palpebral lobe and is where the width (exsag.) of the border begins to expand. Examination of the specimen in the Hunterian Museum showed that the outermost part of the fixigena curves steeply downwards into the matrix and the genal area is not exposed. Price did not mention a genal spine in his description and the cranidium is similarly proportioned to ours and has a similar sculpture and is assumed to be conspecific. An historic cranidium in the Sedgwick Museum labelled Staurocephalus cf. murchisoni from the Sholeshook Limestone in the Sholeshook railway cutting near Haverfordwest, is within the size range of S. oliveae sp. nov. has similar cranidial and glabellar proportions and has similar sculpture ( Fig. 6A View Fig herein). Although the genal corner is not preserved, the posterior border width (exsag.) and posterior edge of the posterior border (which does not turn sharply rearward to indicate a spine) are consistent with S. oliveae sp. nov. suggesting that it may represent that species.

Hammann and Leone (2007: 76) discussed Ingham’s wider concerns (1977: 89) arising from the sculpture of the material of Staurocephalus View in CoL from the highest Rawtheyan of the Cautley area in northern England in the light of their own specimens from the Ashgill of Sardinia and concluded that the differences that Ingham highlighted were insufficient to provide any separation from S. clavifrons . As a result, they included the material from North Wales described by Whittington (1965) and Ireland by Dean (1971) unequivocally in the synonymy of S. clavifrons and that described by Ingham (1974, 1977) from the uppermost Rawtheyan of the Cautley area of northern England questioningly so. They did, however, consider that Ingham’s (1977) S. aff. clavifrons from the lower Rawtheyan Swindale Limestone of the nearby Cross Fell Inlier differs from Angelin’s (1854) species in its dense, large, granulated cranidial tubercles of various sizes.

Whittington (1938: 451) reported Staurocephalus cf. murchisoni and Staurocephalus sp. from the Cautleyan lower Tre-wylan Beds (“ Phillipsinella parabola zone”) at two localities in the Llansantffraid-ym-Mechain district of the south-eastern Berwyn Hills, Mid Wales. Two of his cranidia of Staurocephalus cf. murchisoni (Whittington locality 42; both BIRUG 48862, on same slab) were located in the Lapworth Museum, University of Birmingham. Although both specimens lack their genal areas, precluding definitive identification, their glabellar proportions and granulation are consistent with S. oliveae sp. nov. and they are tentatively assigned to the new species ( Fig. 6B View Fig ). Also from the Berwyn Hills, the historic King collection at the Sedgwick Museum includes three incomplete cranidia (SM A39813, 39814.a and 39814.b) and one librigena (SM A39812) from the (locally) basal Ashgill mudstones (lower Rawtheyan, Ashgill Zone 5, see Price 1980 a, 1981, 1982: 61) at Aber Marchnant, originally assigned to Staurocephalus cf. murchisoni by King (1923: 497) and, presumably, are the material listed as S. clavifrons from there by Whittington (1968: table 4). The cranidia are smaller than those described here and their granulation appears coarser and less dense, but they have similar glabellar proportions and posterior margin and fixigenal corner outline ( Fig. 6C, D View Fig 1), although the fixigenal denticles increase in size posteriorly rather than decrease as in S. oliveae sp. nov. ( Fig. 6D View Fig 2 View Fig ). These cranidia are much closer to S. oliveae sp. nov. than to S. clavifrons and are herein reassigned to S. aff. oliveae . The librigena from the same collection possesses 12–13 marginal denticles, which would place it at the lower end of the range for S. oliveae sp. nov.

The absence of genal spines distinguishes S. oliveae sp. nov. from the other material previously described as, compared with or considered to have affinity to S. clavifrons from North Wales ( Whittington 1965) and northern England ( Ingham 1974, 1977). Dean (1971) only found one cranidium of Staurocephalus from the Ashgill Kildare Limestone in eastern Ireland, with no associated librigenae. Staurocephalus oliveae sp. nov. has a similar internal mould sculpture to Dean’s (1971: pl. 19: 1–4) cranidium and is similar in outline, glabellar proportions and overall size, although the Irish specimen is marginally narrower (tr.). However, the absence of a genal spine distinguishes S. oliveae sp. nov. from the Irish cranidium.

While details of the lateral cephalic margin are not known sufficiently in S. cf. clavifrons and S. aff. clavifrons of Ingham (1977) from the uppermost Rawtheyan and the lower Rawtheyan (Ashgill Zone 5) respectively of northern England, the librigena of material described by Whittington (1965: 53–54, pl. 15: 4–19) from the Rawtheyan Rhiwlas Limestone in North Wales as S. clavifrons has 14 denticles (cf. 13–17, mean and mode = 15, in S. oliveae sp. nov.). In addition to lacking a genal spine, S. oliveae sp. nov. also differs from the Rhiwlas Limestones specimens in having smaller, more closely spaced, granules on both the internal mould and the external surface of the cephalon. Staurocephalus cf. clavifrons of Owen and Romano (2011) from probable Cautleyan strata in eastern Ireland is too incomplete to determine whether there is a genal spine or for the numbers of denticles on the fixed or free cheeks to be determined. The specimens have the sparser but coarser external surface granulation of typical S. clavifrons but the librigena ( Owen and Romano 2011: fig. 2m) also shows extremely fine pits between the granules (as in S. cf. clavifrons of Ingham 1977); the eye contains about 70 lenses. Specimens described as S. aff. murchisoni Barrande by Weir (1959) from the lower Ashgill of Slieve Bernagh in western Ireland are too poorly preserved for useful comparison with any named species.

The absence of genal spines and the surface sculpture also differentiate S. oliveae sp. nov. from material assigned to S. clavifrons from Sardinia from the upper part of the Rawtheyan Punta S’Agricola Member of the Domusnovas Formation underlying the Hirnatian Rio San Marco Formation ( Loi et al. 2023) described by Hammann and Leone (2007). These differences also distinguish S. oliveae sp. nov. from material ascribed to S. clavifrons by Zhou and Zhou (1982) from Inner Mongolia from a formation correlated by them with the Rawtheyan on the basis of the trilobite fauna. There is what appears to be a tubercle at mid-length (sag.) on the occipital ring of one of Zhou and Zhou’s (1982: pl. 1: 6) figured cranidia but it is difficult to discern clearly and genal denticles are not visible on either cranidium. Details of the median occipital tubercle and genal denticles are not known from the Sardinian occurrence. Shaw (2000: 385) considered that material from the Králův Dvůr Formation (upper Rawtheyan) of Bohemia previously assigned to S. clavifrons is too poorly preserved for detailed species comparison. Although the genal area on the single incomplete cranidium known to Marek (1964: pl. 1: 3) from here is not fully exposed, it is possible that it does not possess a spine; S. oliveae sp. nov. differs from it in having markedly smaller and less dense granulation. The incomplete S. clavifrons cranidium featured by Štorch and Mergl (1989: pl. 5: 11) from the Králův Dvůr Formation has a surface sculpture like that illustrated by Marek (1964) and the details of the outermost parts of the fixigena are not preserved.

The cranidium described by Owen and Bruton (2012) from the uppermost Caradoc of Maine, USA, as S. cf. clavifrons is too incomplete to determine whether it is closest to S. clavifrons or S. oliveae sp. nov., though the granulation of the external surface is much closer to that of Angelin’s (1854) species than the fine, dense coverage of S. oliveae sp. nov.

The other named Ordovician Staurocephalus species comprise: Staurocephalus pilafrons Owen and Bruton, 1980 (uppermost Caradoc, lower Katian of the Oslo Region, Norway), Staurocephalus djarikensis Apollonov, 1974 (Ashgill, Katian of Kazakhstan), and Staurocephalus elongatus von Freyberg, 1923 (redescribed by Knüpfer 1967 from the lower Ashgill, Katian of Thuringia, Germany). Staurocephalus oliveae sp. nov. differs from S. pilafrons in having a relatively broader and shorter anterior glabellar lobe in relation to the glabellar stem, smaller granules and in lacking a large median occipital spine (although it has a prominent tubercle). Staurocephalus pilafrons also has a distinct eye ridge; the details of its genal angle or numbers of genal denticles are not known. The cranidial border (and therefore the genal angle) is not preserved in the material of S. djarikensis described by Apollonov (1974). Staurocephalus oliveae sp. nov. differs from S. djarikensis in its glabellar proportions, having a relatively wider and longer (sag.) anterior glabellar lobe taking up two thirds of the glabellar length versus around half in S. djarikensis , and a narrower glabellar stem than the Kazakh species. Staurocephalus oliveae sp. nov. differs from S. elongatus by having much finer granules on the anterior glabellar lobe and by having a differently proportioned anterior glabellar lobe, which is about one fifth wider (tr.) than long (sag.), compared to that of S. elongatus , which is about one fifth longer (sag.) than wide (tr.). Knüpfer (1967: 15–16, pl. 1: 13, 14, pl. 17: 7, 8) stated that the posterior section of the genal angle is “directed backwards”, and although he considered that it cannot be determined if there is a genal spine present similar to S. clavifrons , the figured holotype ( Knüpfer 1967: pl. 17: 8) shows the left posterior margin extended strongly rearwards to form a narrow point. Staurocephalus oliveae sp. nov. clearly differs from this species in having a posterior margin that lacks any posteriorly directed portion like that seen on Knüpfer’s cranidium. Ferretti and Barnes (1997) described the conodonts from the Kalkbank Limestone from which S. elongatus was recovered, placing the formation at a level somewhere near the base of the A. ordovicicus Zone of the Katian stratigraphy that was reiterated by Kraft et al. (2023) in their recent review of the Ordovician of the Bohemian Massif. In their paper on Katian conodonts from Wales, Ferretti et al. (2014: 812) noted that: “…the Kalkbank fauna is clearly not older than the A. ordovicicus Zone and it seems likely that it is at least broadly coeval with the Sholeshook Limestone”, making it approximately contemporaneous with S. oliveae sp. nov.

In addition to revising S. clavifrons, Kielan (1957) described specimens that she termed “ Staurocephalus sp. b” and “ Staurocephalus sp. c” from the Ashgill of Sweden “ Staurocephalu s sp. a” was assigned to S. pilafrons by Owen and Bruton 1980: 30). Staurocephalus oliveae sp. nov. differs from “ Staurocephalus sp. b” described by Kielan 1957: 169, pl. 3: 1a–c) from the Upper Jonstorp Formation Rawtheyan) in the Siljan region of Sweden, in lacking a genal spine and in commonly having more (13–17 cf. 14) marginal spines on the librigena and four rather than three on the fixigena. The single cranidium of “ Staurocephalus sp. c” described by Kielan (1957: 170, pl. 4: 4) from the Jonstorp Formation in Västergötland, Sweden, is too incomplete for detailed comparison but S. oliveae sp. nov. appears to have a relatively narrower glabellar stem. Kielan 1957: pl. 4: 5) also assigned a single pygidium to the same species; S. oliveae sp. nov. has noticeably shorter spines, with its innermost pair of spines accounting for just under two fifths of maximum pygidial length (sag.), while those of “ Staurocephalus . sp. c” take up just under half of maximum pygidial length (around 10% longer). Staurocephalus sp. of Weber (1948), from the Ashgill of Mamontovo, Kazakhstan see Kielan 1957: 160) is only known from a single, poorly preserved, cranidium. Staurocephalus oliveae sp. nov. has a distinctly larger anterior glabellar lobe, which occupies two thirds of maximum glabellar length (sag.) (excluding occipital ring), while that of Staurocephalus sp. occupies just over a half.

The absence of a genal spine distinguishes S. oliveae sp. nov. from most of the Silurian species of Staurocephalus , such as the type species S. murchisoni Barrande, 1846 (Beroun, Czech Republic, Wenlock), S. susanae Thomas, 1981 Dudley and Malvern, UK, Wenlock), S. clowni Tomczykowa, 1987 ( Poland, Sheinwoodian, Wenlock), S. oarion Holloway, 1980 (Arkansas, USA, Sheinwoodian, Wenlock), S. aff. oarion Holloway, 1980 (Arkansas, USA, Wenlock), S. lagena Holloway, 1980 (Arkansas, USA, Whitwellian, Wenlock), S. aff. lagena Holloway, 1980 ( USA, same locality as S. lagena ), S. obsoletus ( Weller, 1907) (Illinois, USA, Ludlow), S. struszi Chatterton and Campbell, 1980 (Canberra, Australia, Whitwellian, Wenlock) and S. mitchelli Chatterton and Campbell, 1980 (New South Wales, Australia, Ludlow). Both S. struszi and S. mitchelli have pygidia with proportionally smaller and flatter spines than those of S. oliveae sp. nov., which also has more lateral denticles on its free cheeks, 13–17, compared to “11 or 12” for S. struszi , and 10 for S. mitchelli . Staurocephalus mitchelli is one of the youngest species of Staurocephalus known, occurring in the Yarwood Siltstone Member of the Black Bog Shale, which Bicknell and Smith (2021) placed within the Ludfordian. Staurocephalus obsoletus is potentially of similar age, but the stratigraphical horizon (Niagaran, from Chicago Drainage Canal, near Lemont, Illinois) provided by Weller (1907) for his single specimen is too imprecise to be certain.

Another Ludfordian species, S. azuella Šnajdr, 1980 , was described from the Cromus beaumonti horizon in the Kopanina Formation (see Vaněk and Valíček 2002: 52; dated as Ludlow, Ludfordian, see Lehnert et al. 2007) in the Czech Republic. Although Šnajdr (1980) listed his single specimen as an “incomplete pygidium with corroded exoskeleton”, his description and illustration focus only on the pygidium, which lacks the tips of the spines. Excluding the spines, S. oliveae sp. nov. differs in having a relatively longer pygidium (just over four fifths as wide [tr.] as long [sag.]) than that of S. azuella (twice as wide [tr.] as long [sag.]) The shape and proportions of the pygidial spines are also different, those of S. oliveae sp. nov. being slightly wider in the middle than the base and rapidly tapering to a point, with the abaxial edge of the first two pairs of spines curving gently abaxially, while the adaxial side of the spines remains straight giving these spines a conical, slightly outwardly inflated appearance. The preserved portions of the spines of S. azuella are distinctly narrower (tr.) and of more uniform width, a gently tapering rectangle in outline and appear to be directed straight rearwards.

Staurocephalus sp. of Whiteley et al. (2002) from Rochester Shale, New York, USA (Aeronian, Llandovery) is known only from a single librigena from which that of S. oliveae sp. nov. can be differentiated by having more lateral denticles, there being 12 in the American species. Like S. oliveae sp. nov., S. trichochin Kobayashi and Hamada, 1985 ( Japan, Whitwellian, Wenlock) lacks genal spines but S. oliveae sp. nov. has a relatively smaller and less inflated anterior glabellar lobe as well as relatively larger fixigenae. Staurocephalus sp. of Curtis and Lane (1997, 1998) from the Aeronian, Llandovery, of northern England is too incompletely known for extensive comparisons to be made, although S. oliveae sp. nov. can be distinguished from it by having fewer denticles on the rostral plate.

Stratigraphic and geographic range.—Sholeshook Limestone Formation, upper Katian, Upper Ordovician. Llanddowror, Carmarthenshire and Haverfordwest area, Pembrokeshire, Wales, UK.