Ibexaspis leuppi, 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 : 17-20

Ibexaspis leuppi n. sp.

Plates 11–15

2009 Ibexaspis sp. nov. 10; Adrain et al., p. 574, fig. 19I, J.

Material. Holotype, cranidium, SUI 129837 (Pl. 11, figs 1, 4, 5, 7), and assigned specimens SUI 115372 , 115373 , 129838–129874 , from Section H 251.4 m, Fillmore Formation (Blackhillsian; Presbynileus ibexensis Zone ), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Clinton Leupp.

Diagnosis. Sagittal glabellar height in anterior view 26.8–28.2% width across posterior projections (excluding genal spines); L2 and L3 conical in outline; interocular fixigena wide, especially anteriorly; librigena with smooth patch on anterior of field extending almost to lateral border furrow.

Description. Cranidium long and relatively narrow, with sagittal length 99.8% (98.7–101.2%) width across γ, 71.5% (69.3–74.4%) width across δ, 73.3% (70.1–75.8%) width across ε, and 51.9% (50.8–53.2%) maximum width across genal angles, very strongly convex (tr., also sag.) from axial inflation and steep fulcral vaulting, with densely granulose and tuberculate sculpture; anterior border very short medially, longer at exsagittal ends, arcuate and strongly anteriorly bowed, wraps around frontal lobe of glabella, border well inflated, both less inflated and ventrally bowed medially, forming shallow M-shaped curve (anterior view), with densely spaced sculpture of tiny tubercles; doublure expressed as anterior face of border; anterior border furrow short, deep, incised, longer exsagittally along transverse section in front of ocular ridges, median section ventrally bowed to form M-shaped curve (anterior view), also strongly anteriorly bowed (dorsal view), furrow confluent with axial furrows; ocular ridges short and narrow, weakly independently inflated, contact axial furrows between just anterior to S3 and S4, bounded anteriorly by transverse sections of anterior border furrow and posteriorly by very short, shallow transverse furrow connected to palpebral furrow, ridge sculpture of dense granules and scattered small tubercles, tubercles smaller than those of fixigena and more widely spaced; palpebral lobes long, extend from S2 to approximately level with S4, fairly broad, with maximum width a little posterior of mid-length, semi-elliptical, laterodorsally raised, with dense sculpture of granules and tubercles slightly larger and more densely spaced than those of ocular ridge but smaller than those of fixigenae; interocular fixigenae narrow anteriorly, expanded posteriorly, subtrapezoidal, strongly anteroventrally downturned; posterior fixigenae fairly long and broad, adaxial portion roughly horizontal, gently anteroventrally sloped, post-fulcral portion broader than inner part, very strongly ventrolaterally downturned, fixigenae with dense granulose sculpture overlain by fairly densely spaced small, pointed tubercles and also very small pits; posterior border furrow deep, short, longer along mid-width, then tapered anterolaterally along curvature at and anterior to genal angle; posterior border short adaxially, expanded almost doubly to maximum length at genal angle, well inflated, with sculpture like that of anterior border, short, thick-based spine extending posterolaterally from genal angle, and with very short transverse furrow on proximal part of posterior margin; doublure turned out posteriorly proximally, rapidly widened post-fulcrum to maximum at genal angle, then cut by facial suture, effaced; axial furrows very deep, except shallow over LO, moderately narrow, widened at junctions with lateral glabellar furrows, weakly anteriorly divergent, tubercles on fixigenae and glabella infringe on furrows; glabella lozenge-shaped, long and narrow, weakly anteriorly expanded, with maximum width across L4 81.3% (76.1–85.7%) sagittal length, very strongly vaulted (sag., tr.), with subvertical anterior face (lateral profile), with sculpture of granules overlain by very dense distribution of tubercles slightly smaller than those on rest of cranidium; glabellar lateral lobes small and narrow, each distinctly independently inflated; L1 smallest, ovoid, anterolaterally directed; L2 and L3 similar, irregularly semicircular, L3 a little larger; L4 weakly inflated, distinguished mostly as very short lateral bulge even with ocular furrow; LF very large, long medially, almost semicircular in outline, with short, triangular (posteriorly tapered) furrow medially (anterior view); glabellar furrows all narrow (extend only about 25% of distance from axial furrow to sagittal axis), deep (proximal ends abruptly shallowed), long at junction with axial furrows, tapered adaxially; S1 posteromedially directed, S2 and S4 roughly transverse, S3 a little anteromedially angled; SO short, moderately deep to deep, deeper exsagittally behind L1, very shallowly W-shaped, confluent with axial furrows; LO relatively short, 14.6% (13.9–15.2%) sagittal length of cranidium, shorter behind mid-width of L1, wide, 30.8% (29.5–31.6%) width across genal angles, well inflated, with sculpture like that of posterior border, and with small median node just posterior to mid-length; doublure long, reaches posterior edge of SO, hardly tapered, pinched out laterally, effaced.

Rostral plate unknown.

Hypostome elongate and narrow, with maximum width across shoulders 83.2% (81.8–84.6%) sagittal length, strongly ventrally convex (tr.), less strongly convex sagittally; anterior border extremely short medially, overhung by middle body in some specimens (Pl. 14, fig. 13; but see Pl. 14, fig. 16), lengthened laterally toward anterior wings; anterior border furrow short, deep, incised, broadly anteriorly bowed; anterior wings roughly trapezoidal, longer than wide, with relatively large wing process pit, very finely tuberculate; middle body long and narrow, posteriorly tapered, with lower convexity posteriorly, poorly separated into lobes; anterior lobe long, wide, ovoid, with densely spaced sculpture of small tubercles and scattered small pits; posterior lobe short to very short, Ushaped, less densely tuberculate with slightly smaller tubercles; middle body furrow mostly not apparent ventrally except far exsagittally as very short, ventromedially angled narrow sections near lateral border furrows (median portion visible as short, somewhat effaced, strongly posteriorly bowed arc on Pl. 14, fig. 13), furrow generally better visible dorsally; lateral border furrow narrow, deep, incised, gently sagittally bowed at about half length of middle body; lateral border fairly narrow, gently ventrolaterally sloped, with dense sculpture of fine tubercles; posterior border furrow very long and shallow medially, abruptly very deep exsagittally where confluent with lateral border furrows, strongly posteriorly bowed; posterior border similar to lateral border, median section gently dorsally bowed (posterior view); doublure poorly preserved, with short lateral notch.

Librigena narrow and elongate, wedge-shaped, gently laterally convex; anterior branch of facial suture long, steep section along field about half length of flat section along anterior projection of border, sections meet at obtuse angle of about 120–130°; posterior branch of facial suture also long, smoothly and steeply sloped along field, slightly upturned along border, cuts posterior projection to expose triangle of doublure; ocular surface large, ovoid, well inflated, formed from numerous tightly packed lenses (most clearly seen internally, Pl. 14, fig. 11), separated from field by effaced rim, with large smooth patch extending anterolaterally almost to lateral border furrow and extending posteriorly to about 1/3 width of eye; librigenal field subtrapezoidal, narrow anteriorly and broader posteriorly, with width under midpoint of eye 25.2% (24.0–26.3%) exsagittal length, weakly laterally convex (ventrolateral view), with sculpture of granules overlain by small tubercles and small pits on posterior part of field (anterior portion covered by large smooth patch); lateral border furrow narrow, slightly wider anteriorly, very deep, shallower at anterior and posterior extremities; lateral border broad, particularly relative to width of field, strongly inflated, with dense sculpture of small tubercles, larger and less dense near border furrow, with long, anteroventrally directed and out-turned anterior projection, and with fairly long posterior projection tapered to blunt angle; doublure wide, smooth but for a few very fine ridges tracing contour of exsagittal margin, turned out anteroventrally from just a little past the anterior edge of the field to form ventral surface of anterior projection.

Total number of thoracic segments unknown. Segments short and broad, with width of axis 30.5% (28.9– 31.8%) width across anterior pleural band, strongly arched axially and due to pleural spines, with sculpture of granules overlain by tubercles; articulating half ring semilunate, long medially, laterally tapered out, raised to equal height of axial ring (lateral view), with transverse band of very tiny tubercles on posterior margin; articulating furrow broadly and shallowly U-shaped, surrounds articulating half ring, very deep, shallower at anterolateral ends, with median sculpture of tiny tubercles in rough transverse row; axial ring short and relatively broad, well inflated, very shallowly U-shaped, with weakly posteriorly bowed anterior margin and weakly anteriorly bowed median posterior margin, ring with sculpture of transverse row of small tubercles along posterior margin, row of slightly smaller tubercles across anterior margin, and intercalated tubercles medially, also with very short furrow along posterior edge above inflation of doublure (posterior view); axial furrows strongly laterally bowed, moderately wide across posterior pleural band, very narrow across anterior band, moderately deep; inner pleurae very broad, outer pleurae very narrow, strongly downturned at subvertical fulcral angle; anterior pleural band very short, equal in width to posterior band, moderately inflated, with extremely short, faint transverse furrow just behind anterior margin of inner pleurae that runs into pleural furrow at exsagittal ends, and with extremely short furrow at very anterior edge of outer pleurae (best observed on Pl. 13, fig. 9); pleural furrow short, shallow adaxially, deep for rest of transverse course; posterior pleural band a little more than twice as long as anterior band, well inflated, with very short articulating furrow setting off short articulating tongue on posterior edge, and with sculpture of transverse row of small, elongate tubercles along anterior and posterior edges which travel down onto pleural spine; pleural spine produced from posterior pleural band, with same length as band adaxially, lengthened abaxially to maximum length just before taper to bluntly pointed tip (at about 90% width of segment), strongly inflated, laterally flattened to produce ovoid cross-section and bluntly bladed anterior edge, spine strongly downturned, with sculpture of tiny tubercles along tapered tip and with small, but slightly elongate tubercles at mid-length and row of tubercles along posterior edge; doublure extremely narrow, just a rim curled medially from base of outer pleurae, slightly wider posteriorly and slightly out-turned anteriorly as part of articulating device, effaced.

Pygidial measurements were made on the specimens of Pl. 13, fig. 34, Pl. 14, figs 26, 29, 32, and Pl. 15, doubling from the sagittal midline as necessary. Pygidium with subtriangular main body, sagittal length 45.9% (43.1–51.2%) width across anterior pleural band of first segment, highly vaulted (sag., tr.), broadest across tips of first pair of pleural spines, formed from four segments with long pleural spines, with dense sculpture of granules overlain by small tubercles on all dorsal surfaces; articulating half ring short, laterally tapered, semilunate, with transverse row of tiny tubercles on posterior margin; articulating furrow long medially, abruptly shorter laterally, deep, shallower medially, with a few very small tubercles at mid-length medially, course anteriorly bowed; axis semi-conical, broad and strongly convex anteriorly, strongly tapered and decreasingly convex posteriorly, first ring 35.9% (33.8–40.1%) width across first segment, fourth ring 53.7% (50.2–58.3%) width across first ring, equal in length (sag.) to main body of pygidium; axial rings well independently inflated, relatively short and broad, transverse to slightly anteriorly bowed, with small median node toward posterior of each ring; terminal piece small, triangular, posteriorly tapered, approximately equilateral; inter-ring furrows long, moderately deep to deep, posterior part of each furrow and exsagittal sections deeper, with a few very small tubercles medially; axial furrows moderately narrow and deep, shallower posteriorly, especially over last segment, but a little deeper again along terminal piece; anterior pleural band present only on first segment, very short, moderately inflated, broad, gently posterolaterally directed, with extremely short, transverse furrow along anterior edge; pleural furrow fairly short, longer exsagittally, shallow adaxially, deeper along rest of gently posterolaterally angled course; posterior pleural bands strongly inflated, with relatively broad inner pleurae and narrow outer pleurae separated by fulcral angle of about 130°, bands narrower altogether posteriorly, increasingly posteriorly angled, with third pair subparallel and fourth pair parallel or posteromedially angled, inner end evenly rounded on anterior two pairs, tapered on posterior two pairs; pleural spines long, strongly inflated like pleurae, tapered ventrally to blunt points, strongly posteroventrally downturned, splayed with a little less than a spine-width between pairs, with effaced ventral side; interpleural furrows long, deep adaxially, shallower exsagittally, posterolaterally directed like pleurae, granulose; pygidial border expressed ventrally at base of pleural spines, as narrow swollen rim with fine tubercles; doublure strongly upturned (Pl. 15, fig. 11), longest medially, tapered anterolaterally.

Ontogeny. The cranidium (cf. Pl. 11, fig. 1, Pl. 13, fig. 7) elongates and widens slightly overall, decreases in transverse and sagittal convexity; the tuberculate sculpture becomes finer and more densely spaced, particularly on the glabella; the anterior border lengthens, particularly laterally; the anterior border furrow deepens and lengthens, particularly laterally, it develops the M-shaped curvature, and the lateral branches become more transverse rather than anterolaterally directed; the glabella elongates and decreases in convexity (sag., tr.); the lateral furrows lengthen, particularly at the distal ends, and deepen; the axial furrows widen and deepen; LO broadens; the interocular fixigenae widen especially anteriorly; the posterior fixigenae lengthen slightly and widen; the tubercles forming a fringe on the posterior of LO become more effaced, and the genal spines shorten.

The hypostome (cf. Pl. 14, figs 12, 16, 23) becomes more elongate and relatively narrower; the middle body inflates, increasing sagittal convexity; the tuberculate sculpture spreads posteriorly over the rear portion of the anterior lobe of the middle body and over the middle body furrow, obscuring the furrow, and it coarsens on the posterior lobe; the pits on the middle body develop and deepen; the anterior border furrow lengthens and deepens slightly, particularly laterally; the lateral border furrows widen and deepen; the posterior border furrow lengthens and shallows considerably medially; and the posterior border lengthens slightly.

Librigenae (cf. Pl. 14, figs 1, 3) become narrower and relatively more elongate as the librigenal field becomes narrower; the tuberculate sculpture of the field and border becomes finer; the border narrows and deflates slightly; and the anterior projection of the border elongates. The known thoracic segments are too close in size to observe ontogenetic changes.

Pygidia (cf. Pl. 13, fig. 35, Pl. 15, fig. 1) become broader and slightly longer; the tuberculate sculpture becomes slightly finer and slightly more widely spaced, and the median axial tubercles become slightly less prominent; the axis broadens anteriorly and elongates, and increases in transverse convexity, particularly medially; the axial rings become more strongly independently inflated; axial furrows widen and deepen; inter-ring, articulating, and furrows between pleural ribs lengthen considerably and deepen; the pleural spines lengthen slightly, become more tapered and pointed at the tips, and splay further apart.

Discussion. Cranidia of I. leuppi differ from those of I. brevis in that they possess a much finer and less densely spaced tuberculate sculpture; broader glabella; longer anterior border furrow, with exsagittal transverse portion of furrow especially so and furrow more strongly M-shaped in anterior view; medial portion of anterior border slightly shorter; palpebral lobes longer; interocular fixigena wider. The hypostomes of I. leuppi are wider with much wider lateral and posterior borders; the sculpture is generally finer; anterior wings are broader as is the middle body. Librigenae of I. leuppi possess less dense sculpture on field with more obvious pits, sculpture on border is also less dense; field is shorter; lateral border furrow deeper; border is longer; smooth patch on anterior portion of field larger. Thoracic segments of I. leuppi have a shorter articulating half ring; posterior margin of axial ring is more strongly anteriorly arched; tubercles are slightly taller and more spine like. Ibexaspis leuppi pygidia possess shorter ring furrows; larger terminal axial piece; coarser tuberculate sculpture; pleural spines more gently declined with less obvious break in slope (see lateral profile) and generally more slender, with fourth pair of pleural spines longer.

Cranidia of I. leuppi compare with those of I. rupauli (cf. Pl. 18, fig. 3) as follows: glabella longer and more subrectangular; S1-S4 less clearly incised in lateral profile; fixigena broader; ocular ridge less clearly defined; genal spine shorter; axial furrow, SO, and posterior border furrow generally broader; LO overall shorter and wider, with medial margins more transverse; tuberculate sculpture much finer and less densely spaced on glabella, fixed cheeks, and palpebral lobes, with fewer larger tubercles and more fine tubercles on LO. No hypostome is known for I. rupauli and a comparison cannot be made. Librigena of I. leuppi differ from those of I. rupauli by possessing a broader and longer field with more widely spaced tubercles and clear pits. Only one poorly preserved thoracic segment is known for I. rupauli , but those of I. leuppi differ from it by possessing smaller tubercles on the axial ring; a longer articulating furrow with a scattering of small tubercles. Pygidia of I. leuppi are broader with the pleural spines more widely splayed laterally; anterior portion of axis broader; fourth pair of pleurae more completely encompassing terminal axial piece; and generally possess a finer tuberculate sculpture.