Mainbrookia becki, Adrain & Pérez-Peris, 2021

Adrain, Jonathan M. & Pérez-Peris, Francesc, 2021, Middle Ordovician (Darriwilian) cheirurid trilobites from the Table Cove Formation, western Newfoundland, Canada, Zootaxa 5041 (1), pp. 1-73 : 14-18

Mainbrookia becki n. sp.

Plate 5 , Plate 6 , figs 1–9

Material. Holotype, cranidium, GSC 135207 (Pl. 5, figs 1, 2, 4, 6, 9), and assigned specimens GSC 135208–135214 , all from horizon TCM 18.

Etymology. After Brian Beck.

Diagnosis. Glabella relatively narrow; main glabellar lobe only moderately inflated; L1 very small; fixigenal area bounded by axial furrow, posterior facial suture, and posterior border furrow large; librigena with well defined eye socal and relatively wide (tr.) field; pygidium with large, strongly dorsally produced first pair of spines.

Description. Measurements for the cranidium were made on the specimen of Plate 5 , figure 1. Cranidium sub-triangular in outline, broad (tr.), with maximum width across genal angles (excluding genal spines), tapering anteriorly, short (sag.) with sagittal length 60.6% maximum width, highly vaulted medially, fixigena distally downturned abaxially, in lateral view anteriorly strongly sloping downwards from the horizontal (e.g., Pl. 5, fig. 6); anterior border broadly curved anteriorly (e.g., Pl. 5, fig. 2), short (sag.), medial part overlapped dorsally by the frontal part of the glabella, directed downwards from the horizontal, dense sculpture of medium sized tubercles; anterior border furrow anteriorly bowed as anterior border, short (sag.) slightly expanding distally, shallow; glabella sub-ovoid in outline, bulbous, highly vaulted dorsally (e.g., Pl. 5, fig. 4), especially medially, with sagittal length 133.3% maximum glabellar width (tr.), narrow (tr.) posteriorly, widening anteriorly, anterior margin broadly rounded, posterior margin transverse, posterolateral corners pointed, anteriorly strongly sloping downwards from the horizontal, covered by dense tubercles; S1 short (exsag.), shallow, transverse, forming a slightly posteriorly bowed depression (e.g., Pl. 5, fig. 1), proximally almost in contact with SO; S2 and S3 effaced; SO transverse, narrow (sag., exsag.), narrower sagittally, slightly longer exsagittally, deeply incised, similar in width and depth to axial furrow; LO subrectangular in outline with sagittal length 18.8% cranidial sagittal length, slightly longer exsagittally, anterior margin subtly bowed anteriorly, posterior margin straight; LO highly vaulted dorsally, sparse sculpture of finer tubercles than the glabella and fixigena; L1 reduced, very short (exsag.), wedge shaped, expanding distally; frontal lobe large, comprising the major part of the glabella, swollen, subspherical, slightly less inflated anteroventrally, anterior margin broadly curved anteriorly, posterior margin curved posteriorly; axial furrow narrow, wider than SO, slightly wider anteriorly, deeply incised; fixigenal field broad (tr.) posteriorly and very narrow anteriorly across the interocular field, dense sculpture of medium sized tubercles; anterior fixigena subtriangular in outline, very narrow, slightly broader posteriorly, anteriorly strongly sloping downwards from the horizontal (e.g., Pl. 5, fig. 6); posterior fixigenal field subrectangular in outline, wider than long (sag.), posterior margin abaxially slightly curving anteriorly, in anterior view slightly downturned abaxially (e.g., Pl. 5, fig. 4); posterior border furrow short (exsag.), slightly expanding abaxially, slightly shorter at genal angle, deeply incised, transverse, before reaching the genal angle curved slightly anteriorly, in front of the genal angle strongly curved anteriorly and slightly adaxially directed; lateral border furrow narrower (tr.) than posterior border furrow, shallower, directed anteriorly and adaxially; posterior border short (exsag.), longer near the genal angle, slightly anteriorly directed, extended posteriorly at genal angle by a genal spine, sparse sculpture of finer granules, more densely distributed at genal angle; genal spine subconical in shape, circular in section, running posterolaterally, slightly curving adaxially distally, pointed distal tip, dense sculpture of tiny granules; articulating flange in the posterior margin of the posterior border (e.g., Pl. 5, figs 7, 8), longer distally slightly beyond the medial part of the posterior border, separated by a narrow (sag.) and deep furrow, fulcral socket present at the distal tip of the flange; lateral border short (sag.), triangular in outline, wider (tr.) next to the genal angle, tapering forward, directed anteriorly and slightly adaxially, fixigenal spine present anterior to the base of the genal spine (e.g., Pl. 5, figs 3, 7, 10); fixigenal spine small and triangular in shape, dense sculpture of medium size granules; palpebral lobe narrow (tr.), placed opposite posterior part of the frontal lobe, slightly vaulted dorsally; palpebral furrow narrow, deeply incised; doublure beneath the posterior border very short (exsag.) proximally, longer distally, forming an expanded area beneath the genal angle triangular in outline (e.g., Pl. 5, figs 2, 5); beneath the lateral border doublure triangular in outline, wide posteriorly next to genal angle and narrowing anteriorly.

Librigenal measurements were made on the specimen of Plate 5 , figure 17. Librigena sub-triangular in outline; eye large, bulbous, highly vaulted, subspherical in shape, set in the middle part of the librigena in a dorsal elevated position compared with the rest of the librigena; furrow separating the eye from the librigenal field narrow and shallow, shallower anteriorly; librigenal field subtrapezoidal in outline, short and narrow, width of field 47.4% length along the lateral border furrow, length of the field 41.3% length of the border; in lateral view field sloping downwards distally (e.g., Pl. 5, fig. 16, 18), dense sculpture of medium sized tubercles and sparse small pits over entire surface; border furrow narrow, deeply incised, shallowing posteriorly before reaching the posterior branch of the facial suture, separating the librigenal field from the border, slightly bowed laterally; lateral border relatively broad, with width 88.9% that of the field, outer margin gently curved laterally; border slightly more vaulted anteriorly than posteriorly, posterior projection longer than anterior projection, tapering distally, slightly curved medially in dorsal view; anterior projection slightly shorter and curved laterally, in lateral view directed downwards from the horizontal (e.g., Pl. 5, fig. 18), with dense sculpture of medium sized tubercles; anterior facial suture short, distally bowed adaxially, changing slope opposite border furrow, curving posteriorly across the field (e.g., Pl. 5, fig. 17); posterior facial suture long, describing a laterally directed arch, distally turning medially; lateral border doublure narrow posteriorly, expanding anteriorly, anteriormost part visible in dorsal view, smooth and lacking sculpture.

Rostral plate unknown.

Hypostome unknown.

Thorax unknown.

Pygidial measurements based on specimen of Plate 5 , figure 11, Plate 6 , figure 1. Pygidium subtriangular in outline (excluding the pleural spines), with sagittal length (excluding articulating half-ring and pygidial pleural spines) 39.2% (33.3–45%) maximum width; fulcrum set close to the axis; articulating flange short (exsag.), set along the anterior margin of the first pleural segment, ending at fulcrum, separated from the pleural segment by shallow and short (exsag.) furrow, fulcral process visible in dorsal view (e.g., Pl. 5, fig. 11, Pl. 6, fig. 2); articulating half-ring with sagittal length 12.7% (11.1–14.3%) sagittal length of pygidium, slightly shorter exsagittally, highly vaulted dorsally, anterior margin slightly bowed anteriorly with medial part almost transverse, posterior margin transverse; half-ring with sculpture of tiny granules; articulating furrow deeply incised, short (sag.; exsag.), transverse; axis composed of three axial rings and a terminal piece (e.g., Pl. 5, fig. 11, Pl. 6, fig. 1), triangular in outline, maximum width across the first axial ring, vaulted anteriorly, flatter posteriorly, with moderately dense sculpture of medium sized granules; first axial ring subrectangular in outline, with sagittal length 30.9% (28.6–33.3%) pygidial sagittal length and width 36.7% (33.3–40.0%) maximum pygidial width, anterior and posterior borders slightly bowed anteriorly, dorsally vaulted, corners squared off; second axial ring similar in shape to first axial ring, slightly shorter (sag.) and narrower, anterior curvature more pronounced than first axial ring, ring merged laterally with the pleura; third axial ring similar to second axial ring, slightly shorter (sag.) and narrower, strongly curved anteriorly; terminal piece small, ovoid in outline, longer sagittally, shorter exsagittally, anterior margin bowed anteriorly, posterior margin merged with the pygidial pleural field; first inter-ring furrow relatively long (sag.) deeply incised, shallowing exsagittally, bowed anteriorly, distally continuous with interpleural furrow; second and third inter-ring furrows similar to first inter-ring furrow, slightly shorter (sag.) and narrower (tr.), more strongly bowed anteriorly, continuous with the most proximal part of interpleural furrow; axial furrow very shallow, similar to a small depression, more incised across anterior segment (e.g., Pl. 5, fig. 11), almost effaced posteriorly; first pleural segment longer (exsag.) than the rest, extended by a pleural spine; spine thick, long, conical in outline, circular in section, running posterolaterally, distally curving more posteriorly, turning dorsally in lateral view; second and third pleural segments extended into small pleural spines beyond the pygidial margin, spines very short, thorn-like; first interpleural furrow as long (exsag.) as first inter-ring furrow, proximally continuous with first inter-ring furrow, deep proximally, strongly shallowing distally; second interpleural furrow proximally continuous with second inter-ring furrow, slightly longer (sag.), deeply incised, very narrow (tr.), shallowing dramatically distally, almost effaced; third interpleural ring effaced, only most proximal part visible; pygidial border semicircular in outline, ventrally extended in a small pair of protuberances (e.g., Pl. 6, figs, 3, 5, 7, 8) located between the second and the third pygidial pleural spines, triangular in outline, directed posteroventrally; dense sculpture of medium sized tubercles; ventral side of pygidial margin with very dense sculpture of tubercles slightly smaller than those on the dorsal aspect; doublure short (sag.), directed dorsally, with median part arched dorsally and lateral area transverse (e.g., Pl. 6, fig. 9), smooth.

Discussion. In dorsal view (Pl. 5, fig. 11, Pl. 6, fig. 2), larger pygidia of H. becki appear to have four pairs of pygidial spines. This is because the pygidial border forms two ventral points that protrude posteriorly, with a U-shaped margin between them in posterior view (Pl. 5, fig. 13, pl. 6, fig. 8). The tips of these points, which as discussed above are the homologues of cylindrical spines developed in more derived taxa, are visible in dorsal view between the second and third pygidial spines.

Mainbrookia becki differs from M. akimbo in the possession of a smaller, less inflated main glabellar lobe, smaller L1, and larger fixigena that is markedly longer (exsag.) behind the palpebral lobe. The librigena of M. becki has a much wider (tr.) field than that assigned to M. akimbo . Pygidia are very similar, but that of M. akimbo appears to have a well impressed pleural furrow on the first segment ( Tripp, 1967, pl. 4, fig. 8) whereas that of M. becki is only very faintly impressed, and only proximally.

46.3 m, southern Confusion Range, Millard County, Utah, USA. 5, 7, 8. Kawina n. sp., Kanosh Formation (Dapingian), Section K 1.5T m, southern Confusion Range, Millard County, Utah, USA. 5. Pygidium, SUI 148407, dorsal view, x25. 7. Fragment of cranidium, SUI 148408, dorsal view, x6. 8. Left librigena, SUI 148409, external view, x10. 6. Pandaspinapyga salsa Esker, 1964 , pygidium and attached thoracic segments, OU 5207, dorsal pygidial view, x5, Bromide Formation (Sandbian), Carter County, Oklahoma, USA. Note that the transverse row of pits on the pleurae of last thoracic segment and first pygidial segment of this specimen are the equivalent of the pleural furrow. The perforate tubercle sculpture is visible on the dorsal surfaces of the distal spines and particularly on the axial rings.

Figures

PLATE 5

PLATE 6