Millardaspis milsteadi, 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 : 37-40

Millardaspis milsteadi n. sp.

Plates 47–53, Plate 54, figs 1–20

? 1973 Protopliomerops sp. II; Demeter, p. 54, pl. 3, fig. 4. 2009 Ibexaspis sp. nov. 2; Adrain et al., p. 567, fig. 14K.

Material. Holotype, cranidium, SUI 135528 (Pl. 47, figs 1, 3, 4, 6, 8), and assigned specimens SUI 135529– 135563, from Section HC6 226.5 m, Garden City Formation (Tulean; Heckethornia hyndeae Zone ), west side of Hillyard Canyon, Bear River Range, Franklin County, southeastern Idaho, USA; assigned specimens SUI 115262, 129533–129543, from Section YH 128.9 m, Yellow Hill Limestone (Tulean; Heckethornia hyndeae Zone ), Yellow Hill, near Pioche, Lincoln County, Nevada, USA.

Etymology. After Glenn Milstead.

Diagnosis. Prominent punctate tubercles on cranidium, librigenae, and thorax (of articulated specimen); punctae without tubercles on thoracic and pygidial pleural spines; glabella very short; wide and somewhat shallow axial furrows in large specimens; short, roughly triangular-based genal spine extending almost laterally from genal angle; librigenal lateral border furrow approximately doubling in width from posterior to anterior ends; lateral border wide and strongly inflated; pygidium long, with strongly independently inflated axial rings separated by long inter-ring furrows; tall, spinose small tubercles in about 2 transverse rows on pygidial axial rings (effaced in larger specimens), with cluster of large tubercles at proximal end of pleurae, but effaced spines; large and wellexpressed terminal piece.

Description. Measurements were made on the large and well-preserved specimens of Plates 47–54 and were doubled from the sagittal midline if necessary due to incomplete preservation. Cranidium moderately vaulted (tr.), more strongly sagittally vaulted, broad and relatively short, sagittal length 99.1% (96.3–103.4%) width across γ, 68.2% (64.9–72.0%) width across δ, 63.5% (59.7–66.7%) width across ε, and 47.2% (44.9–49.5%) maximum width across genal angles, with fairly densely spaced sculpture of small tubercles all over dorsal surface; anterior border short, of even length all along course, fairly broad, extends in front of narrow anterior interocular fixigena to edge of palpebro-ocular ridge, broadly anteriorly bowed, well inflated, tubercles a little smaller than on rest of cranidium; doublure turned down and expressed as anterior face, just a rim ventrally; anterior border furrow moderately short, deep, incised, gently anteriorly bowed medially, roughly transverse to weakly posterolaterally directed exsagittally anterior to fixigena; palpebro-ocular ridge poorly expressed on small specimens, ridge effaced on larger specimens, leaving palpebral lobe; lobe located across from nearly S3–mid-L2, narrow anteriorly and expanded posteriorly, irregularly half-teardrop shaped, tapered into very narrow post-ocular ridge, lobes laterally raised above horizontal, strongly anteroventrally sloped, finely tuberculate; palpebral furrow narrow, deep, incised, J-shaped; interocular fixigena broad and held horizontal posteriorly, anteriorly tapered and strongly downturned, subvertical far anteriorly, with sculpture of small pits underlying tubercles, less tuberculate adjacent to axial furrows in large specimens; posterior fixigena short, very wide, portions proximal and distal to fulcrum about evenly wide, inner half gently anteroventrally sloped, outer part moderately strongly lateroventrally downturned, sculpture like interocular fixigena; glabella subrectangular, sagittal length 102.7% (96.2–108.4%) maximum width, moderately vaulted (sag., tr.), with distinctly independently inflated lateral lobes, glabellar tubercles a little larger medially, with a scattering of larger isolated tubercles present on median portion of specimens from HC6; L1 small, elliptical, anterolaterally angled, sometimes effaced on a few specimens from HC6; L2 and L3 similar in size, subquadrate, but L2 with angled posterior margin and a little more anteriorly angled, L3 more transversely directed; LF short and broad, semilunate; glabellar furrows short and deep, extend to a little less than 1/3 width of glabella, S1 posteromedially angled, S2 and S3 approximately transverse; SO moderately short, deep, incised, slightly longer posterior to adaxial end of L1, very gently anteriorly arched medially, with distal portions deflected around posterior margins of L1; LO moderately short, strongly inflated, with rim of very small tubercles on anterior margin, transverse row of slightly larger tubercles on posterior margin, and scattered small tubercles medially (effaced on some larger specimens); doublure lens-shaped, nearly reaches SO, pinched out laterally, with sculpture of fine transverse ridges; posterior border furrow moderately short, deep, roughly transverse adaxially, anterolaterally curved abaxially; posterior border short adaxially, expanded laterally to about double length at genal angle, tapered to facial suture, well inflated, with small tubercles lining anterior margin, and slightly larger, more widely spaced tubercles on posterior margin, effaced at mid-length, and with very short, broad-based spine; doublure just a rim on inner fixigena, turned out posteriorly to articulate with thorax, and with a very short, shallow transverse furrow, abruptly expanded near genal angle, then cut by facial suture anteriorly in small arc.

Hypostome elongate and relatively narrow, with sagittal length 107.8% (measured on Pl. 54.4) maximum width across anterior wings, moderately strongly ventrally convex (sag., tr.); anterior border extremely short medially, expanded laterally toward small, triangular, posterolaterally set anterior wings with small, deep wing process pits; anterior border furrow also extremely short, incised medially, poorly expressed laterally behind raised rim of border at anterior edge of wings; middle body long, posteriorly tapered, long (sag.) densely tuberculate (Pl. 50.2) anterior lobe, with short (sag.) more effaced posterior lobe; middle body furrow poorly expressed, shallow; lateral border furrows narrow and somewhat shallow; lateral border very narrow anteriorly, expanded to maximum width a little posterior to shoulders, then tapered to posterior border, with densely tuberculate sculpture a little finer than that of anterior lobe; posterior border furrow shallow; posterior border slightly shorter (sag.) than lateral border, gently dorsally bowed (Pl. 54, fig. 8) medially; doublure smooth, reaches border furrows, with depressed lateral notch.

Rostral plate unknown.

Librigena roughly wedge-shaped, with width under midpoint of eye 34.1% (31.8–37.6%) length along lateral border; anterior branch of facial suture very long, very steep, subvertical and slightly anteriorly bowed along field, then approximately flat along anterior projection of border, slightly downturned near tip of projection; posterior branch of facial suture shorter, steeply sloped and bowed along field, less steeply sloped exsagittally, very gently posterodorsally sloped along posterior projection of border, cut across border exposes triangular sector of doublure; ocular surface ovoid, well vaulted, situated on top of raised smooth platform, platform extends anteriorly and exsagittally in effaced, slightly swollen rounded area on field; librigenal field subtrapezoidal, longer posteriorly, with width under mid-length of eye 31.6% (25.2–38.8%) exsagittal length, gently laterally convex and ventrolaterally declined, with granulose sculpture overlain by dense distribution of small tubercles concentrated posteriorly near eye, with small pits interspersed; lateral border furrow deep, narrow posteriorly and approximately doubling in width anteriorly, finely granulose; lateral border very broad, strongly inflated, with dense sculpture of small tubercles, sculpture slightly less dense, with larger and longer tubercles sagittally near border furrow and posteriorly on projection, denser and finer ventrolaterally, posterior projection of border short and tapered to blunt angle, anterior projection very long, curved anteroventrally; doublure wide, nearly reaches border furrow, rotated outward along anterior projection to form ventral surface, smooth except for fine ridges running along exsagittal margin.

Thorax known from small articulated specimen (Pl. 50, figs 1, 2, 6, 8); each segment with strong independent dorsal inflation; axis of nearly equal width (tr.) from first to sixth segment, then gently tapered (tr.) posteriorly, with strong independent inflation; pleurae also progressively tapered posteriorly, distal portion from fulcrum abaxially flexed ventrally, pleurae terminate in free, bluntly tipped spines, similar to those on pygidium; pleurae largely smooth except for single row of small tubercles along anterior margin between axial furrow and fulcrum, and a patch of small tubercles present along the anterior margin of the distal tips of the pleural spines (see Pl. 50, fig. 8); axis with more prominent tubercles across median portion of each ring, middle most of these tubercles stand tall (see Pl. 50, fig. 8).

Pygidium of four segments and terminal piece, subrhombic in outline, very strongly vaulted (sag., tr.) mainly due to strongly downwardly deflected pleural spines, but also dorsally inflated axis, widest across tips of first pleural spines, sagittal length excluding articulating half ring 76.2% (70.5–82.1%) width across anterior band of first segment; articulating half ring short (sag.), slightly shorter abaxially, anterior and posterior margins gently anteriorly arched, with line of very fine granules along posterior margin; articulating furrow deep, moderately long (sag., exsag.); axis roughly equal in length to main body of pygidium, very strongly vaulted, strongly posteriorly tapered, width (tr.) of first ring 56.1% (53.2–60.6%) pygidial width across anterior band of first segment, width of fourth ring 51.0% (45.3–55.9%) width of first ring; axial rings with very strong independent inflation, short and relatively broad, posterior rings only a little shorter than first, first ring may be slightly anteriorly bowed with second–fourth rings generally transverse, anterior margins of second–fourth rings wider (tr.) than respective posterior margins so that the rings have a sub trapezoidal outline, sculpture of granules overlain by prominent spinose tubercles roughly organized transversely along anterior portion of each ring, although some specimens possess some extra tubercles intercalated medially; terminal piece small, less inflated than rings, triangular, generally smooth (see Pl. 51, fig. 13), but some specimens possess a cluster of spinose tubercles along anterior margin (see Pl. 54, fig. 11); inter-ring furrows fairly long, with median portion slightly longer, furrows progressively shorter (sag., exsag.) posteriorly, furrow overall deep, slightly shallower medially, transverse; axial furrows moderately wide and deep (shallower on larger specimen), narrower and shallower opposite terminal piece, confluent behind terminal piece, strongly posteriorly convergent; anterior band of first pleurae very short, posterolaterally directed about 30° below horizontal, moderately inflated, smooth, with short furrow at anterior rim for articulation with thorax; pleural bands strongly posterolaterally directed, strongly inflated, extended into strongly downturned spines, first and second pleurae obliquely directed in dorsal view, with third nearly subparallel to sagittal axis and fourth slightly medially convergent in dorsal view, in posterior view fourth pair subparallel to medially bowed (Pl. 52, fig. 5), in lateral view first and second pair obliquely directed (Pl. 52, fig. 7) compared to third and fourth pair which are more strongly downturned, with fourth pair most strongly downturned, cluster of small spinose tubercles on pleurae between axial furrow and fulcrum; pleural spines slightly laterally compressed, subtriangular cross-section, tapered to blunt point, with dense sculpture of very fine tubercles on anterolateral face of distal tips of some specimens; many specimens with punctae on pleural spines; interpleural furrows moderately long, posterolaterally directed like pleurae, deep; pygidial border expressed ventrally as swollen rim at base of spines (Pl. 52, fig. 11), describing broad U-shaped curve with anterolateral tips flared laterally, sculpture of fine, closely spaced, concentric ridges following curvature of margin; doublure strongly upturned and subvertical (anterior view, Pl. 52, fig. 12), longest medially and tapered anterolaterally, smooth except for a few fine transverse ridges at base like those on pygidial border.

Ontogeny. Cranidia become slightly more strongly vaulted, with posterolateral corners becoming more strongly downturned; anterior border furrow lengthens slightly; posterior border furrow and SO lengthen slightly; the axial furrows widen, and become more laterally bowed in course; the glabella becomes wider, shorter, and more rounded (especially anteriorly); S1 broadens (especially adaxially); large prominent tubercles on the glabella and LO become more subdued, with those on LO becoming nearly effaced on some larger specimens.

The known librigenae and the pygidia of M. milsteadi are too similarly sized to assess ontogenetic changes.

Discussion. Compared to Millardaspis knoxi , the cranidium of M. milsteadi (cf. Pl. 53, fig. 20, Pl. 55, fig. 8) is less vaulted (sag., tr.); the anterior border is longer and broader; the glabella is slightly more elongate, less inflated and does not overhang the anterior border (cf. Pl. 47, figs 4, 5, Pl. 55, fig. 7), has more clearly incised lateral furrows (especially S3), with S1 longer, and generally possesses sculpture of finer tubercles; the axial furrows are slightly narrower and deeper; LO is shorter (sag.) relative to sagittal length of the glabella. The librigenae of M. milsteadi are slightly longer; the field is slightly narrower, but longer, with a more gradually sloped posterior margin, and with the tubercles concentrated posteromedially; the lateral border furrow is longer overall, especially anteriorly; and the lateral border is slightly narrower, with finer sculpture and lacking fine ridges along ventrolateral margin of border. No hypostomes are known for Millardaspis knoxi so a comparison cannot be made. The thoracic segments of M. milsteadi are less tuberculate with smooth pleurae; the pleurae lack a prominent anteriorly directed process located distally (cf. Pl. 50, fig. 8, Pl. 55, fig. 9). The pygidium of M. milsteadi is longer and narrower overall; the axis is longer, more strongly posteriorly tapered, with a prominent and elongate terminal piece, overall less inflated (especially anteriorly); the axial rings are smoother with finer and fewer tubercles; the axial furrows are shallower and broader; the pleurae are more strongly backswept; and the pleural spines are slightly less strongly downturned, longer, effaced but for granules, and also more laterally compressed to a sharper tip.

Millardaspis milsteadi is known from two widely separate geographic occurrences: the well preserved type material from southeastern Idaho (HC6 226.5 m) and the somewhat less well preserved sample from eastern Nevada (YH 128.9 m). There are obvious differences in the range of intrasample variation in each collection which must be addressed. The sample from YH 128.9 m is relatively small, but cranidial tuberculation is on most specimens dense and fairly coarse, in some cases with tubercles on the rear of LO posteriorly overhanging the posterior margin. Cranidia from HC6 226.5 m, comprising a much larger sample, tend to have generally smaller cranidial tubercles, and no non-juvenile specimens are known in which tubercles on LO overhang the posterior margin. Nevertheless, there is variation in the size of the tubercles, and some specimens (e.g., Pl. 49, figs 2, 8) seem directly comparable to the YH 128.9 m sample. The two pygidia known from YH 128.9 m have prominent tubercles on all of the axial rings, the terminal piece, and the adaxial parts of the pleural ribs. None of the pygidia from HC6 226.5 m have tuberculate terminal pieces and two have nearly smooth axial rings, with only tiny tubercles along the anterior margins. However one pygidium from HC6 226.5 (Pl. 51, fig. 18) has prominent tubercles on the rings, nearly as coarse as those on the YH 128.9 specimens, and another (Pl. 51, fig. 21) is approximately intermediate in sculpture. Hence, while there are quite striking differences in pairwise comparisons of certain sclerites, the situation appears to reflect one smaller sample (YH 128.9 m) with a narrowly circumscribed range of variation and a larger sample (HC6 226.5 m) with a broader range, but one which encompasses, or nearly encompasses that of the former.

A small articulated specimen (Pl. 50, figs 1, 2, 6, 8) appears to have 11 thoracic segments. This is a low number for a pliomerid. Among contemporary taxa in the faunas, Hintzeia parafirmimarginis McAdams and Adrain, 2011a , has 15 segments. Work in progress has shown that species of Lemureops McAdams and Adrain, 2009 have 14 segments, and those of Pseudocybele have 15. Given its size and low segment count, the M. milsteadi specimen is probably a meraspid. The segment count is not known, however, for any other species described herein, nor for any member of the presumptive sister group, Panisaspis.