Ibexaspis rupauli, E.B & Adrain & Karim, 2018

E. B, Neo, Adrain, Jonathan M. & Karim, Talia S., 2018, The pliomerid trilobite Ibexaspis and related new genera, with species from the Early Ordovician (Floian; Tulean, Blackhillsian) of the Great Basin, western USA, Zootaxa 4525 (1), pp. 1-152 : 21-23

Ibexaspis rupauli n. sp.

Plates 16–19

Material. Holotype, cranidium, SUI 129891 (Pl. 18, figs 3, 5, 6, 11, 14), and assigned specimens SUI 129888– 129890, 129892–129904, from Locality YH-J, Yellow Hill Limestone (Blackhillsian; " Pseudocybele nasuta Zone "), Yellow Hill, near Pioche, Lincoln County, eastern Nevada, USA. Assigned specimens SUI 129875– 129887, from Section J 28T m and J 28.1 m, Wah Wah Formation (Blackhillsian; " Pseudocybele nasuta Zone "), southern Confusion Range, Ibex area, Millard County, western Utah, USA.

Etymology. After RuPaul.

Diagnosis. Glabella strongly forwardly expanding; S4 prominently impressed; sparse, transverse row of 4–6 small spines across rear of LO; pygidium with large terminal piece; posterior margin of terminal piece well exposed due to wide median space between fourth pair of pleural spines; terminal piece with rear overhanging posterior margin in dorsal view.

Description. Cranidial measurements were made on the large and best preserved specimens of Pl. 16 and Pl. 18. Cranidium strongly vaulted (sag., tr.) due to inflation of axis and ventral curvature of fixigenae, relatively long and wide, with sagittal length 110.3% (103.3–114.3%) width across γ, 77.3% (76.3–78.8%) width across δ, 79.9% (76.4–84.3%) width across ε, and 55.3% (52.5–57.3%) maximum width across genal angles, with sculpture of coarse tubercles on top of very fine granules, most densely distributed on median part of glabella and on cranidial borders; anterior border strongly anteriorly bowed, short and fairly narrow, shallowly M-shaped in anterior view, with median portion depressed; doublure expressed as anterior face, only a rim ventrally; anterior border furrow short and deep, strongly anteriorly arcuate (dorsal view) with narrow subtransverse segments at exsagittal ends, also shallowly M-shaped in anterior view; glabella long, narrow posteriorly, strongly anteriorly expanded, with glabellar maximum width across L4 79.6% (75.8–84.7%) sagittal length, strongly vaulted (sag., tr.), with sculpture of densely spaced small tubercles concentrated medially; glabellar lobes L1–L3 each well independently inflated, L4 less well inflated but still distinct, L1 smallest and anterolaterally angled, remaining lobes slightly larger and progressively more transversely angled; glabellar lateral furrows (S1–S4) very similar, short and very deep, S4 shallower, furrows extend medially to about 1/4 glabellar width, S1 gently posteromedially directed, S2 subtransverse, S3 and S4 gently anteromedially directed; SO very shallowly W-shaped, short and deep, particularly in apodemal pits behind mid-width of L1, confluent with axial furrows; LO short and relatively wide, well inflated, with small median node possessing densely spaced tuberculate sculpture wrapped around it, fringe of 6–8 more spinose small tubercles on posterior margin; doublure of smooth ellipsoid band extending to SO and tapered out laterally; axial furrows very deep, shallowed over posterior border, narrow, wider at intersections with glabellar lateral furrows, gently anteriorly divergent, confluent with all but palpebral furrows; ocular ridges ill-defined, short and narrow; palpebral lobes semi-ellipsoid, widest opposite S3–L4, tuberculate; palpebral furrows narrow and deep, course J-shaped, with anterior part anteromedially angled and posterior part curved around posterior edge of lobe; interocular fixigenae triangular, relatively short and narrow, broader posteriorly, strongly anteriorly downturned, with dense sculpture of small tubercles, interspersed tiny tubercles, and tiny pits, tubercles smaller near axial furrows; posterior fixigenae slightly longer than interocular fixigenae, broad, strongly ventrolaterally downturned, sculpture same as interocular fixigena; posterior border furrow deep, abruptly shallowed at intersection with axial furrows, short, incised, broadly anterolaterally curved; posterior border well inflated, fairly short, with maximum length at genal angle a little shorter than LO, sculpture of moderately densely spaced small tubercles, and with moderately short, broad-based, rapidly tapered, mediolaterally flattened genal spine, and with very short articulating ridge along posterior margin set off by short furrow (e.g., Pl. 16, fig. 3); border doublure short and turned out posteriorly adaxially, slightly lengthened abaxially reaching maximum length at genal angle, smooth.

Rostral plate and hypostome unknown.

Librigenal measurements were made on the specimen of Pl. 18, fig. 20. Librigena narrow and elongate, with width under midpoint of eye 35.2% length along lateral border; anterior branch of facial suture short and very steep along field, long and gently anteroventrally sloped along border; posterior branch of facial suture steep along field, flat along border, with section along border almost twice as long as section along field; ocular surface poorly preserved, eye raised far above field (ventrolateral view); librigenal field narrow, subtrapezoidal, with extremely narrow extension posteriorly, just along border furrow, extending about half again width of main part of field, extension is part of post-ocular ridge also running along posterior edge of field as slightly swollen margin, field with width under midpoint of eye 22.9% exsagittal length (excluding posterior extension), weakly laterally convex (ventrolateral view), with sculpture of small tubercles and pits on posterior half, anterior half granulose, with effaced patch under anterior part of eye; lateral border furrow narrow and deep, narrower posteriorly, wider and shallower anteriorly; lateral border well inflated, slightly broader than field, anteriorly and posteriorly tapered along long projections, with dense sculpture of small tubercles, tubercles increasingly smaller ventrolaterally; doublure imperfectly preserved, broad and smooth, but with a few fine ridges following contour of outer margin, doublure rotated out anteroventrally along anterior projection of border, inner surface exposed in dorsal view by cut of posterior branch of facial suture at tip of posterior projection of border.

Total number of thoracic segments unknown. Single known segment (Pl. 19, fig. 20) with relatively wide axis 37.1% width across anterior pleural band, strongly vaulted; articulating half ring poorly preserved, raised to same height as axial ring (lateral view); articulating furrow short and deep, shallowly U-shaped, wrapping around half ring, with a few granules; axial ring relatively long compared to width, also shallowly U-shaped, anterolaterally tapered, well inflated, with moderately dense sculpture of small tubercles; doublure moderately short, lens-shaped, smooth but for fine transverse ridge near posterior margin; axial furrows shallow and fairly narrow, bowed laterally around ring; fulcral angle very steep, almost subvertical; anterior pleural band very short, inflated, with extremely short furrow setting off articulating surface on anterior edge; posterior pleural band slightly shorter than axial ring, well inflated, equal in breadth to anterior band, tuberculate; pleural spine poorly preserved, with thick base, tapered exsagittally, slightly laterally compressed, tuberculate dorsally, smooth ventrally; doublure very narrow, just curled under tips of outer pleurae.

Pygidial measurements were made on the larger and more intact specimens of Plates 17 and 19. Pygidium of four segments and terminal piece, strongly vaulted (sag., tr.) mainly from convexity of axis and pleural spines, relatively narrow and long, with sagittal length from articulating furrow 47.9% (45.7–48.8%) width across anterior pleural band, widest across tips of first pleural spines, with moderately densely spaced sculpture of small tubercles; articulating half ring semilunate, short and laterally tapered, smooth; articulating furrow moderately long and deep; axis long, equal to length of pygidium excluding spines, broad anteriorly, 35.6% (35.1–36.1%) pygidial width, strongly posteriorly tapered, strongly convex (sag., tr.), convexity decreases posteriorly; axial rings each well independently inflated, relatively broad and short, each ring slightly longer medially than laterally, with densely spaced sculpture of small tubercles, with four tubercles arranged in a rhombus at midline of second two segments (Pl. 19, fig. 2); terminal piece triangular, large, long and broad anteriorly, strongly posteriorly tapered, posterior tip extends slightly beyond rounded posterior margin; inter-ring furrows short, deep, shallower medially, fourth furrow shorter and shallower; axial furrows moderately wide and shallow (Pl. 19, fig. 2), deeper in smaller specimens, strongly anteriorly divergent; anterior pleural band present only on first segment, very short, swollen, with extremely short transverse articulating furrow along anterior margin; pleural furrow also only on first segment, deep and very short, slightly longer exsagittally; posterior pleural bands well inflated, relatively long and wide, slightly shorter near axial furrows, with sculpture of small tubercles; pleural spines long (oblique), fairly wide, strongly inflated, slightly laterally compressed, tapered to blunt point, tuberculate like pleurae dorsally, granulose ventrally; interpleural furrows moderately long and deep, posterior furrows shallower, furrows increasingly posteromedially directed; pygidial border of very short raised rim at inner base of spines; doublure strongly upturned, best seen in anterior view, moderately long, smooth, upper margin shallowly W-shaped (e.g., Pl. 19, fig. 11).

Ontogeny. Ontogenetic changes in the cranidium and pygidium are well represented in the specimens of I. rupauli , but other sclerites are unknown (the hypostome) or known only from single specimens. Cranidia (cf. Pl. 16, fig. 16, Pl. 18, figs 3, 7) become slightly broader relative to sagittal length and therefore a little less vaulted (tr.); the glabella also decreases in convexity (tr., sag. to a lesser extent), it widens slightly anteriorly to S3, and the glabellar furrows become shorter, particularly adjacent to the axial furrows; the axial furrows narrow and become less strongly impressed over LO; SO shortens and becomes more W-shaped rather than transverse in course; LO deflates slightly, lengthens medially, and its median and posterior fringe tubercles become less prominent while tubercles on the anterior portion increase in size; the palpebral lobes narrow slightly; the interocular fixigenae widen slightly, pushing the posterior end of the palpebral lobes further away from the glabella; the genal spine flattens slightly and develops a wider base; the cranidial sculpture overall becomes much more densely distributed; and the two primary ontogenetic tubercles located anterior to the genal spine and posterior to the palpebral lobe decrease in prominence.

Pygidia (cf. Pl. 17, fig. 18, Pl. 19, figs 1, 2, 19) increase in convexity (tr., also sag.), and the sculpture becomes slightly finer and more densely spaced; the axis broadens, particularly anteriorly, but also becomes less strongly posteriorly tapered, and slightly less vaulted (tr.); the terminal piece enlarges and inflates; the axial nodes weaken and blend more into the rest of the sculpture; the inter-ring furrows and the furrows between the pleural ribs become longer and deeper; the axial furrows widen and deepen; the pleural spines shorten relative to the length of the pygidium, their tips become more sharply pointed, and they splay further apart laterally.

Discussion. Ibexaspis rupauli has been compared with its congeners in the species discussions above. The species is by far the youngest member of the genus and of the overall clade treated herein. It occurs approximately 50 m above the next youngest species, I. leuppi , which is retrieved as its sister taxon in the phylogenetic analysis.