Deltapliomera eppersoni, 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 : 26-28

Deltapliomera eppersoni n. sp.

Plates 28–33

1953 Protopliomerops ? quattuor Hintze , pl. 21, fig. 10 [only; pl. 21, figs. 9, 12–14 = Panisaspis quattuor ( Hintze, 1953) ; pl. 10, fig. 11= Ibexaspis brevis ( Young, 1973) ]. 2009 Ibexaspis sp. nov. 6; Adrain et al., p. 570, fig. 17T, U.

Material. Holotype, cranidium, SUI 115337 (Pl. 28, figs 1, 2, 4, 6, 9)), from Section H 191.7 m, and assigned specimens SUI 115338 , 129617–129673 , from Section H 185.6–191.7 m, Fillmore Formation (Blackhillsian; Bathyurina plicolabeona Zone ), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After John Epperson.

Diagnosis. Cranidial axial furrows wide, especially adjacent to L3 and shallow compared to congeners; cranidial sculpture with infrequent punctate tubercles and lightly pitted fixigenae; librigena more strongly pitted with common punctate tubercles; space between proximal ends of pygidial pleural spines rounded, keyholeshaped.

Description. Deltapliomera eppersoni is sufficiently similar to D. humphriesi that it is described through extended and detailed comparison with that taxon. Measurements are given for better comparison to other species. Cranidia of D. eppersoni are shorter and relatively broader overall (length 51.7% (50.1–54.3%), maximum width across genal angles, 102.1% (94.6–104.3%) width across γ, 72.1% (67.8–76.4%) width across δ, 68.1% (64.3– 72.6%) width across ε); the sculpture is of coarser and more widely spaced tubercles on both the glabella and fixigenae; the anterior border is only slightly overhung by the frontal lobe of the glabella; the glabella is less vaulted (sag., especially tr.), slightly shorter, broader at the base and less anteriorly expanded (sagittal length 111.4% (103.1–117.4%) maximum width across mid-length of L3, sagittal length 79.8% (76.8–81.9%) cranidial length, maximum width 37.0% (35.5–39.5%) maximum cranidial width); L1 is slightly larger; L3 is wider; the glabellar furrows extend further sagittally; S3 is deeper but S1 and S2 are shallower, and S2 is shorter at the junction with the axial furrows; SO is slightly longer and shallower; LO is broader and slightly longer (length 15.6% (14.7–17.0%) sagittal cranidial length, maximum width 34.5% (28.3–39.6%) cranidial width), more shallowly W-shaped (longer medially and shorter behind the inner end of L1), and tuberculate, with a row of tiny tubercles on the anterior margin, slightly larger tubercles posteriorly, and a larger median node; the axial furrows are wider (particularly across from L3), shallower (moderately deep compared to very deep in D. humphriesi ), less laterally bowed around L3, and less anteriorly divergent in general; the interocular fixigenae are broader, particularly posteriorly; the palpebral lobes are located slightly further anteriorly, and the sculpture is tuberculate rather than effaced; the posterior fixigenae are a little less strongly downturned and the outer portion (exsagittal to the edge of the post-ocular ridge) is narrower; the posterior border furrow is longer, particularly adaxially, and shallower, and a little less anterolaterally curved; the posterior border is shorter at the genal angle, and the tubercles lining the anterior margin are slightly larger, while the genal spine has a smaller base, and there are small tubercles near the posterior margin and one small tubercle just posterior to the cut of the facial suture.

Hypostomes of D. eppersoni are slightly broader than those of D. humphriesi (maximum width across shoulders 92.5% (89.8–100.0%) sagittal length); they possess smaller anterior wings set further abaxially and with smaller wing process pits; the middle body is wider anteriorly, shorter, and less strongly posteriorly tapered; the anterior lobe is more rounded posteriorly, rather than tapered, it is slightly more inflated (sag.), and it has finer tuberculate sculpture; the posterior lobe is longer medially, with narrower, longer anterolateral arms; and the lateral and posterior borders are slightly wider/longer; and the lateral notch is shorter and deeper, with less upturned doublure.

Librigenae differ in being overall narrower and more elongate (width opposite midpoint of eye 39.3% (35.8– 43.1%) length along lateral border); the ocular surface is less inflated and less elevated above the field; the platform is narrower and the anterior effaced area is smaller; the librigenal field is narrower, with more tuberculate sculpture and fewer closely spaced pits; the lateral border furrow is shallower, particularly anteriorly, and more upturned anteriorly; the lateral border is slightly narrower, with a longer anterior projection, less densely spaced tuberculate sculpture, with scattered punctate tubercles, and more robust fine ridges on the ventral side of the anterior projection (ventrolateral view).

Thoracic segments are compared based on similar proportions (indicating similar position in thorax; those of Pl. 31 compared with Pl. 42, figs 4, 19, 27). They differ mainly in that those of D. eppersoni are more tuberculate, with very small tubercles lining the posterior margin of the articulating half ring, lining the anterior margin of the axial ring and on its abaxial posterior margin, and slightly larger tubercles arranged transversely across the posterior of the ring, as well as tiny tubercles on the anterior and posterior edges of the inner pleurae (mostly effaced on segments of D. humphriesi ) and increasing in size ventrally down the spine (smaller and more closely spaced on D. humphriesi ); the axis also appears to be slightly wider (with width of axis 53.0% (50.1–55.9%) width across anterior pleural band); the axial furrows are broader, deeper.

Pygidia of D. eppersoni are shorter and relatively wider with sagittal length from articulating furrow 53.8% (50.1–59.4%) width across anterior pleural band of first segment; the sculpture is much more tuberculate, with the entire axial ring covered and pleurae sculptured at least adaxially; the tubercles are slightly elongate and spinose, rather than flat; the axial nodes persist at larger pygidial sizes; the axis is narrower, more strongly posteriorly tapered (width of fourth ring 41.7% (34.0–53.2%) width of first ring), and more strongly vaulted (tr.); axial rings are shorter; the terminal piece is smaller and more sharply posteriorly tapered; inter-ring, pleural, and interpleural furrows are shallower; and the pleural spines are shorter, thinner, and less strongly downturned, and the space between the proximal ends of the pleural spines is rounded (space between bases of spines forms keyhole shape).

Ontogeny. Cranidia (cf. Pl. 28, figs 1, 15, Pl. 29, figs 3, 11) become wider overall, and slightly less strongly vaulted (tr., sag. to a lesser degree), and the sculpture becomes smaller and slightly more sparsely distributed on the fixigenae; the anterior border broadens and becomes medially overhung by the glabella; the glabella widens and decreases slightly in convexity (sag., tr.), the lateral furrows lengthen and shallow abaxially while deepening adaxially, and the glabellar lobes increase in individual inflation; the axial furrows widen and shallow; the interocular fixigenae widen, particularly posteriorly, pushing the posterior end of the palpebral lobes out further laterally; the posterior fixigenae widen adaxially; the posterior border furrow lengthens and shallows; and the genal spine shortens.

The hypostome (cf. Pl. 31, figs 10, 25) lengthens and becomes relatively narrower; the middle body overall lengthens and narrows (especially the anterior lobe; the posterior lobe lengthens but does not narrow as noticeably), and inflates, and the tubercles on the anterolateral edges of the anterior lobe efface away while pits develop on the posteromedian portion; the middle body furrows narrow and deepen, but the posteromedian ends efface so that the furrows end just past the shoulder; the lateral border furrows broaden and shallow far anteriorly, but deepen significantly posteriorly; the posterior border furrow shallows; the lateral border broadens considerably; the posterior border lengthens to match it; and the sculpture of both borders becomes more effaced.

Librigenae (cf. Pl. 30, figs 3, 13, 14) broaden overall; the eye enlarges; the librigenal field broadens, particularly posteriorly, and the smaller background tubercles become more effaced; the lateral border furrow shallows posteriorly; the lateral border broadens, the posterior and especially anterior projections lengthen, and the tubercles become slightly smaller. The known thoracic segments are too close in size to demonstrate ontogenetic change.

The pygidium (cf. Pl. 30, fig. 22, Pl. 33, fig. 4) becomes broader overall (including spines), but the main body of the pygidium narrows and lengthens, and it becomes slightly less vaulted (sag., tr.) overall; the axis broadens anteriorly and becomes less strongly posteriorly tapered; the axial rings slightly decrease in individual inflation, the sculpture of densely spaced tubercles becomes more clearly developed, but the median tubercles shrink and become less apparent; the inter-ring furrows lengthen (particularly the anterior ones) and shallow, as does the articulating furrow; the axial furrows narrow, and shallow far anteriorly and posteriorly; the pleurae inflate slightly, and the tubercles become slightly smaller and more closely spaced; the furrows between pleural ribs lengthen and shallow considerably; the pleural spines splay apart, and the tips become slightly more tapered and pointed.

Discussion. Compared to D. heimbergi , cranidia of D. eppersoni possess a less strongly inflated glabella, which is broader across the posterior margin and less laterally expanded around L3; the axial furrows are shallower and slightly broader; the posterior fixigenae are shorter and narrower; and they generally have a sculpture of finer tubercles, with LO and posterior border much more effaced, and a smaller median node on LO. Hypostomes of D. eppersoni are much less strongly tapered posteriorly, with broader lateral and posterior borders, and a finer tuberculate sculpture. Librigenae are slightly shorter and the tuberculate sculpture is finer with more isolated tubercles (compared to the coarse densely spaced tubercles present on D. heimbergi ), less obvious background sculpture of densely spaced granules. Thoracic segments with broader axis, and lacking prominent coarse tubercles. Pygidia of D. heimbergi and D. eppersoni are very similar, but the latter have shorter axial rings with shallower ring furrows, smaller terminal piece, thinner pleural spines, a finer tuberculate sculpture.

Some specimens represent transitory pygidia or earliest holaspid. One (Pl. 32, fig. 27) has five pairs of pleural spines and four axial rings. It likely represents a last-stage meraspid, with the anterior segment representing an as yet unreleased posteriormost thoracic segment. A specimen of similar overall size (Pl. 32, fig. 32) has four pairs of spines, but only three axial rings. It likely represents the earliest holaspid stage following release of the last thoracic segment, and prior to the development of a fourth axial ring associated with the fourth pair of spines.