Ibexaspis brevis ( Young, 1973 )

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 : 11-15

Ibexaspis brevis ( Young, 1973)

Plates 1–6

1953 Protopliomerops quattuor Hintze , p. 209, pl. 21, fig. 11 [only; pl. 21, figs 9, 12–14 = Panisaspis quattuor ( Hintze, 1953) ; pl. 21, fig. 10 = Deltapliomera eppersoni n. sp.]. 1973 Protopliomerops quattuor brevis Young , p. 106, pl. 3, figs 1, 5–7, 9, 10.

1973 Kanoshia (?) depressus Young (partim), p. 102, pl. 3, figs 25–27 [only; pl. 3, figs 21–24 = Cybelopsis depressa (Young) ].

1985 Ibexaspis brevis ( Young, 1973) ; Přibyl et al., p. 119, text-figs 22, 23.

1990 Ibexaspis brevis (Young) ; Peng, p. 112.

1997 Protopliomerops ? quattuor brevis Young ; Ross et al., pp. 20, 44.

2003 Ibexaspis brevis ( Young, 1973) ; Jell and Adrain, p. 387.

2009 Ibexaspis brevis ( Young, 1973) ; Adrain et al., p. 570, fig. 17L, M.

2011b Ibexaspis brevis ( Young, 1973) ; McAdams and Adrain, pp. 4, 17, 21.

Material. Assigned specimens SUI 115330, 115331, 129762–129804, from Section H 186.2–191.7 m, Fillmore Formation (Blackhillsian; Bathyurina plicolabeona Zone ), southern Confusion Range, Ibex area, Millard County, western Utah, USA.

Diagnosis. Glabellar sculpture very fine and dense; glabellar lobes weakly expressed, glabellar furrows S2 and S3 narrow; librigenal field narrow; pygidial axial ring median nodes pronounced.

Description. Cranidial measurements were made on sufficiently complete large specimens of Plates 1–4. Measurements were doubled from the sagittal midline if necessary, e.g., for a specimen with a damaged fixigena. Cranidium roughly semicircular in outline, with sagittal length 109.5% (102.2–117.3%) width across γ, 75.5% (71.6–79.6%) width across δ, 77.9% (73.3–82.7%) width across ε, and 53.2% (50.2–56.0%) maximum width across genal angles, very highly vaulted (sag., tr.) and medially domed, with dense tuberculate sculpture; anterior border short, 7.4% (5.4–8.7%) cranidial sagittal length, relatively broad, moderately anteriorly bowed, well inflated, densely tuberculate, broadly M-shaped in anterior view, with lateral thirds dorsally arched and median third gently ventrally projecting, doublure expressed as anterior face, rim-like ventrally; anterior border furrow very short (dorsal view), deep, distinct, median section broadly anteriorly bowed, lateral sections transverse to gently anterolaterally directed, slightly overhung by glabella medially; glabella strongly vaulted (sag., tr.), oblong with maximum width across L3 81.0% (77.7–84.4%) glabellar length (sag.), slightly tapered posteriorly, tapered and rounded anteriorly, with very dense, finely tuberculate sculpture; glabellar lobes independently inflated, well defined and separated by lateral furrows; L1 short and very narrow, subtriangular, nearly isolated by S1, with slightly coarser tubercles than other lobes, L2 and L3 roughly equal in size, each longer and wider than L1, laterally rounded, with coarser tubercles on lateral edges, LF very large, long and wide, anteriorly rounded with small median anterior furrow, also with small accessory S4 furrow located far exsagittally almost at anterolateral corner on smaller specimens (seen best on Pl. 2, figs 2, 11, 14); lateral glabellar furrows all deep, fairly short and with little median extension, S1 strongly posteromedially directed across inner margin of L1, deepest at midlength, then increasingly shallow posteriorly, separated from SO by only a few tubercles, S2 and S3 roughly transverse, deep at junctions with axial furrows, then abruptly shallowed and shortened to point on glabella; SO moderately short and deep, exsagittal portions behind L1 deeper, slightly longer, and slightly posteriorly directed into apodemal pits, furrow mainly smooth, but median section with few granules in some specimens (Pl. 3, figs 1, 9); LO moderately long (sag., exsag.), length 15.0% (13.3–17.4%) cranidial sagittal length, posterior width 29.9% (27.1–32.1%) maximum cranidial width, exsagittal sections shorter and gently anteriorly directed, ring dorsally inflated with inflation higher posteriorly, and with dense sculpture of fine tubercles and small median node; doublure semilunate, laterally pinched off, effaced, long, reaches posterior of SO medially; axial furrows moderately wide and deep, roughly parallel to slightly anteriorly divergent along glabella, confluent with all but palpebro-ocular furrows and wider at these junctions; palpebro-ocular ridges mainly expressed as palpebral lobes, with poorly delineated ridge extending sagittally to junction of glabella and anterior border (seen well on Pl. 2, fig. 3), lobe fairly narrow, semi-elliptical to half-teardrop shaped, narrow anteriorly and expanded posteriorly to maximum width across from L3, strongly anteriorly downturned (sagittal profile), exsagittal edge slightly raised above horizontal (anterior view), densely tuberculate; palpebro-ocular furrows sigmoidal, but poorly expressed anteriorly on most specimens, narrow, deep, incised along lobe part of palpebro-ocular ridge, then short and shallow along ridge part, short and shallower posterior of lobe along very narrow post-ocular ridge; interocular fixigenae about half total fixigenal width and length, triangular, roughly transverse dorsally, with anterior portions strongly sloped down to anterior border; posterior fixigenae about twice as wide as long, with length slightly ventrolaterally tapered, strongly ventrolaterally sloped, both areas of fixigena with dense tuberculate sculpture somewhat coarser than that of glabella; posterior border furrow short, deep, transverse to gently anterolaterally curved adaxially, and with stronger anterolateral curve at genal angle; posterior border short adaxially, gradually expanded ventrolaterally to maximum at genal angle and then sharply tapered at cut of posterior branch of facial suture, well inflated, with very dense tuberculate sculpture similar to that of glabella, and with small, short genal spine tapered to blunt point (Pl. 2, fig. 5); doublure turned outward abaxially to fulcrum as articulating furrow, expressed ventrally and lengthened from fulcrum to maximum at genal angle, then cut by posterior facial suture, smooth.

Rostral plate unknown.

Hypostome fairly narrow and elongate, with width across shoulders 77.5% sagittal length (and approximately equal to width across anterior wings), strongly ventrally vaulted (sag., tr.); anterior border extremely short, overhung by anterior margin of middle body medially, expanded laterally; anterior wings small, strongly upturned, subtriangular, a little longer than wide, with small, deep wing process pit; border and wings granulose; anterior border furrow likewise very short and overhung medially, deep, shallowed exsagittally toward base of wings; middle body composed of large, ovoid, posteriorly tapered anterior lobe with dense sculpture of small tubercles and short, U- or V-shaped posterior lobe with similar sculpture and effaced posteromedian margin; middle body furrow short and narrow, shallow, not well impressed except anterolaterally toward junction with lateral border furrow, mainly marked by effacement of sculpture (Pl. 2, fig. 22), very strongly posteriorly bowed in course; lateral border furrows narrow, shallow anteriorly, deep posteriorly toward posterior border furrow; lateral border very narrow anteriorly, expanded to shoulders, then narrowed again, densely covered in very small tubercles, moderately ventrally downturned; posterior border furrow short, moderately shallow medially, deep exsagittally; posterior border a little longer than width of lateral border at shoulders, with similar sculpture and slightly more downturned; doublure poorly preserved, probably reaches border furrows (see posteromedian section of Pl. 2, fig. 23), shows moderately long and shallow lateral notch and small posterior wing.

Librigena wedge-shaped, narrow and elongate, with width under midpoint of eye 39.1% (38.0–40.1%) length along lateral border; ocular surface not preserved, raised above field by smooth platform enlarged into roughly semicircular patch on field at anterior end; anterior branch of facial suture long, with section along field shorter than section along anterior projection of border, sections meet at angle of about 115° at lateral border furrow, suture very steeply sloped along field, nearly flat along projection; posterior branch of facial suture long, fairly steeply sloped along field, flat along posterior projection, cut exposes triangle of doublure at tip of projection; librigenal field subtrapezoidal, with width under midpoint of eye 30.7% (29.6–31.7%) length along lateral border furrow, longer posteriorly, very gently laterally convex, with sculpture of small pits concentrated posteriorly, overlain by dense mix of granules and tiny and small tubercles; lateral border furrow moderately narrow and deep, deeper and narrower posteriorly, shallowed anterior of midpoint of eye; lateral border wide, slightly tapered posteriorly, strongly inflated, a little lower posteriorly, with very dense sculpture of small tubercles, smaller and more dense exsagittally; doublure broad along field, reaches lateral margin of lateral border furrow, tapered to point above posterior projection, rotated out anteroventrally along long anterior projection, smooth except for fine ridge running along outer margin.

Total number of thoracic segments unknown; thoracic segments short and broad, with width of axis 30.7% (30.3–31.1%) width across tips of pleural spines; segment strongly vaulted, with axis strongly dorsally arched and pleurae strongly downturned abaxially from fulcrum; articulating half ring semilunate to almost semicircular, long medially and laterally tapered, situated just higher than axial ring in sagittal profile, smooth except for transverse line of very fine tubercles across posterior margin; articulating furrow long, moderately deep, with anterolateral corners deepest, shallowly U-shaped, with broad, transverse midsection, line of approximately seven small tubercles situated medially and a few more smaller tubercles anteriorly (Pl. 5, fig. 20); axial ring short, about as wide as inner pleurae, mildly inflated, shallowly U-shaped like furrow, with slightly anteriorly bowed posterior margin, and with dense sculpture of small tubercles and a few tiny tubercles, exsagittal tubercles along lateral rims of ring taller and more spinose, with distinct pair of tubercles at anterolateral and posterolateral corners of ring; doublure long and broad, pinched out laterally, with anteriorly bowed anterior margin, smooth but for several fine transverse ridges at posterior margin; axial furrows moderately narrow and shallow, overhung by spinose tubercles on axial ring and posterior pleural band (anterior, posterior views), slightly anteriorly convergent in course; anterior pleural band gently inflated, very short, appears narrower than posterior band due to base of pleural spine, but is same width, effaced, and with very short articulating furrow just behind anterior margin; pleural furrow short and moderately deep, shallower at junction with axial furrows and deeper toward exsagittal end; posterior pleural band about twice the length of anterior band (same length as axial ring), well inflated, with dense sculpture of small tubercles with tiny tubercles intercalated transversely along mid-length, and with very short articulating furrow just anterior to posterior margin; pleural spine produced from posterior band and of same length, very wide, strongly downturned, with tips very slightly turned out laterally, robustly inflated, especially at base, tapered and laterally compressed toward bluntly pointed tip, with sculpture similar to posterior band on outer face, smaller, more widely spaced tubercles on posterior face, and very tiny, densely spaced tubercles on anterior face and toward tip; doublure just a rim curved in medially from tips of pleural bands, very slightly wider posteriorly.

Pygidium of four segments and terminal piece, roughly semicircular in outline, with sagittal length from articulating furrow 47.1% (43.6–49.1%) width across anterior pleural band of first segment, widest across tips of first pair of spines, very strongly vaulted (sag., tr.) in axis and due to pleural spines, with dense sculpture of small tubercles all over dorsal surface; articulating half ring crescentic, moderately short medially, laterally tapered, slightly narrower than first axial ring, with transverse line of very fine tubercles on posterior margin; articulating furrow long, slightly laterally tapered and deep, with scattered fine tubercles on median section of some specimens (Pl. 5, fig 4, Pl. 6, figs 10, 16); axis long, approximately equal in length to main body of pygidium, broad and highly vaulted anteriorly, strongly tapered and decreasingly convex posteriorly, with width across first ring 33.8% (31.6–36.2%) width across anterior band of first segment, and width across fourth ring 57.3% (50.0–64.8%) width across first ring; axial rings well defined and independently inflated, median part of anterior rings anteriorly bowed, posterior rings with straight margins, posterior rings increasingly shorter and narrower than anterior rings, all with sculpture of small tubercles with intercalated tiny tubercles, and with denser cluster of sculpture on sagittal axis at mid-length or toward posterior of ring (distinct larger tubercle/median node present in this cluster on smaller specimens, e.g., Pl. 6, fig. 8); inter-ring furrows moderately long to long, anterior furrows longer, moderately deep (deeper toward posterior of each furrow and on smaller specimens), with tiny tubercles in transverse line at midlength of furrows in many specimens (e.g., Pl. 5, fig. 4, Pl. 6, figs 3, 8, 16); terminal piece triangular, short, narrow, and strongly posteriorly tapered, about half as inflated as fourth axial ring anteriorly, and deflated posteriorly to merge into upper surface of pygidial border (on most specimens, but see Pl. 5, fig. 12), absent or poorly expressed on a few specimens (Pl. 6, figs 1, 18); axial furrows moderately narrow and deep, less well impressed over fourth pleurae, strongly anteriorly divergent, confluent with fourth interpleural furrows and median furrow behind terminal piece posteriorly; anterior pleural band present only on first segment, similar to thoracic anterior bands, very short, slightly longer exsagittally, moderately inflated, with articulating furrow just posterior to anterior margin; pleural furrow deep, short, slightly longer laterally, transverse sagittally, gently posterolaterally directed exsagittally; posterior pleural bands long (sag., exsag.), expanded exsagittally into bases of spines, strongly inflated, posterolaterally directed, increasingly so posteriorly so that fourth pleurae are parallel to each other, with dense sculpture of small tubercles; spines similar to thoracic spines, long and strongly downturned, well inflated, tapered and laterally compressed ventrally to blunt point, with sculpture becoming finer and denser toward tips, and effaced on ventral/inner side of spines (ventral, anterior views); interpleural furrows long (posterior furrows shorter, median interpleural furrow behind terminal piece fairly wide in most specimens), moderately deep, deeper sagittally, increasingly posteromedially directed; pygidial border visible dorsally as swollen area at base of pleural spines crossed by interpleural furrows, but mainly expressed ventrally as moderately inflated swollen rim sagittal from base of spines, with granulose sculpture; doublure strongly upturned, subvertical, and visible only in anterior view (Pl. 5, figs 14, 17, Pl. 6, fig. 9), fairly long, longest medially, anterolaterally tapered, smooth.

Ontogeny. Cranidial ontogenetic changes are well represented in I. brevis (cf. cranidia of Pls 1, 2; Pl. 4, fig. 24). The cranidium becomes broader and slightly shorter overall, as well as more strongly vaulted (tr., especially sag.), and the tuberculate sculpture becomes more densely distributed and slightly finer in texture; the transverse branches of the anterior border furrow separating the border from the palpebro-ocular ridges lengthen and deepen considerably, while the median part of the furrow develops into the characteristic M-shaped curve (anterior view); the glabella inflates and becomes more peaked dorsomedially (anterior view), while overall it becomes shorter and considerably wider; the glabellar furrows and SO lengthen and shallow somewhat; the axial furrows broaden and shallow; the interocular fixigena lengthens and narrows slightly, while the posterior fixigenae shorten; the palpebro-ocular ridge widens and becomes more distinctly lobate; the large tubercles on the posterior rim of LO and the posterior border shrink; the tubercles at the posterior end of the postocular ridge and just anterior from the genal spine become more effaced; and the genal spine becomes much shorter and blunter.

The two known librigenae (Pl. 2, fig. 25 and Pl. 5, fig. 29) are similar in size (exsagittal length 3.52 mm and 2.95 mm respectively), but it appears that the librigenal field broadens slightly and the adaxial region lengthens, changing from a more triangular to a more trapezoidal shape overall. The hypostome of I. brevis is represented by two specimens of almost identical size; its ontogeny is therefore unknown.

The two thoracic segments (Pl. 5, figs 20, 25) are very similar morphologically and appear to be from the same part of the thorax, which makes ontogenetic comparison feasible. However, they are rather close in size (width across anterior pleural band 3.75 and 4.12 mm, respectively). The most noticeable change is that the spinose tubercles at the lateral and posterolateral edges of the axial ring become less prominent, and the tuberculate sculpture overall becomes finer.

Pygidia (cf. Pl. 5, figs 1, 4 with Pl. 6, fig. 24) become slightly shorter relative to width; the tuberculate sculpture becomes slightly finer overall, and the occipital nodes in particular start to blend in more with the background sculpture; the axis elongates and narrows; articulating half ring, inter-ring, and interpleural furrows lengthen and shallow; axial furrows widen and shallow; and the pleural spines lengthen (oblique), spread further apart, and become more tapered toward the tips.

Discussion. Compared to Ibexaspis coadyi , I. brevis possesses a narrower, more strongly vaulted cranidium with slightly finer glabellar sculpture and coarser fixigenal sculpture; longer, less overhung anterior border; narrower glabella with much smaller lateral lobes and narrower lateral furrows defining them, except L1, which is longer and more posteromedially directed in I. brevis , and a less distinct median furrow on the frontal lobe; slightly wider and deeper axial furrows; smaller palpebral lobes; wider interocular fixigenae, particularly anteriorly; less distinct eye ridge; less distinct median LO tubercle; narrower postocular fixigenae; and shorter genal spines. The hypostomes are very similar, but that of I. brevis is slightly narrower and more elongate; more strongly posteriorly tapered; with a more inflated middle body; much more clearly defined (very effaced) middle body furrow; coarser sculpture overall, but especially on the middle body; narrower lateral border, particularly at the shoulders; and slightly shorter posterior border. The librigenae differ mainly in that those of I. brevis are narrower overall, with a distinctly narrower field and relatively wider lateral border; the effaced patch at the anterolateral corner of the eye is smaller; the lateral border furrow is wider and shallower, particularly anteriorly; and the lateral border is slightly more strongly inflated. Thoracic segments are very similar, but those of I. brevis possess more robust sculpture overall; have longer, more spinose tubercles at the posterolateral corners of the axial ring; and the tubercles in the articulating furrow are also larger and arranged in a more linear fashion. Pygidia of I. brevis (cf. Pl. 6, fig. 16, Pl. 10, fig. 1) are narrower overall; with a relatively wider, more strongly tapered and more strongly inflated axis; more strongly independently inflated axial rings; wider axial furrows and longer furrows between pleural ribs; slightly shorter, more strongly inflated pleural ribs with longer spines; the sculpture is finer and more densely spaced, but the axial ring nodes are more pronounced; and the doublure is longer, particularly medially.

Ibexaspis brevis is compared with I. rupauli and I. leuppi in the discussions of those species.

Specimens of Ibexaspis brevis from H 186.2 m and 191.7 m may display some minor variation between the horizons. The specimens from H 191.7 m have finer tuberculate sculpture, particularly on the hypostome and the glabella. The pygidia from H 191.7 m also seem to have a less prominent or less well expressed terminal piece, but the sample size overall is rather small (four pygidia from H 186.2 m; seven from H 191.7 m). Only a single librigena is known from each horizon, but it appears that the specimen from H 191.7 m has a narrower, longer field with a less steeply sloped posterior branch of the facial suture. Intraspecific variation could thus also account for some or all of the differences noted above in the discussion of librigenal ontogenetic change.

The terminal piece of one specimen (Pl. 5, fig. 12) is unusually elongate and strongly inflated compared to those of other pygidia, and in fact is extended posteriorly as a short posteromedian spine. This is the only known pygidium from H 185.6 m, the lowest horizon in the Bathyurina plicolabeona Zone. Its significance is hence difficult to determine, but it could represent a different species.