Deltapliomera heimbergi, 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 : 25-26

Deltapliomera heimbergi n. sp.

Plates 22–27

2009 Ibexaspis sp. nov. 4; Adrain et al., p. 569, fig. 16D, H.

2009 Ibexaspis sp. nov. 5 (partim); Adrain et al., p. 569, fig. 16P (only; fig. 16L = Ibexaspis sp. nov. C herein).

Material. Holotype, cranidium, SUI 129568 (Pl. 22, figs 2, 3, 6, 9, 12), from Section H 178.2 m, and assigned specimens SUI 115304 , 115305 , 115308 , 129567 , 129569–129616 , from Section H 166.2–173.2 m, Fillmore Formation (Tulean; Panisaspis sevierensis Zone), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Alex Heimberg.

Diagnosis. Cranidium with densely spaced coarse sculpture including common punctate tubercles and pitted fixigenae, but sculpture more effaced on LO and posterior border; hypostome rhombic, very broad at shoulders and very strongly posteriorly tapered; librigena with elongate anterior branch of facial suture compared to congeners; pygidial doublure elongate medially but tapered laterally, coming to a distinct point at midline.

Description. Deltapliomera heimbergi is similar enough to D. humphriesi that it is described with a list of all morphological differences. The cranidium is of similar dimensions with sagittal length 51.5% (49.1–55.2) width across genal angles, 104.5% (96.1–113.0%) width across γ, 69.4% (63.7–72.6%) width across δ, 68.2% (65.0– 70.9%) width across ε, but slightly less vaulted (tr.), and slightly more vaulted (sag.), with coarser sculpture overall; the anterior border is usually almost fully visible vs. medially overhung in D. humphriesi ; the glabella is shorter, narrower and less anteriorly expanded (sagittal length 106.6% (100.8–111.7%) width across L3).

Hypostomes of D. heimbergi differ from those of D. humphriesi in being shorter (width across shoulders 112.9% (111.3–114.8%) length), more strongly posteriorly tapered (shoulders very wide compared to posterior border), and with less densely spaced sculpture of tiny tubercles, a little finer in size on middle body and a little coarser on borders; the anterior border is less overhung medially by the middle body; the anterior wings are smaller (shorter and narrower) and turned more anterodorsally, so that the wing process pit is less visible (it is also smaller); the middle body is much less ventrally vaulted (sag., tr.), shorter and more posteriorly tapered, with a smaller, shorter anterior lobe and larger, longer posterior lobe that wraps further around the anterior lobe; the lateral and posterior border furrows are slightly deeper; the lateral border is narrower; and the posterior border is shorter.

Librigenae of D. heimbergi are narrower and much more elongate, with width under midpoint of eye 35.4% (33.7–37.9%) length along lateral border; the smooth spot at the anterolateral margin of the eye is smaller; the librigenal field is considerably narrower and a little shorter, width 33.5% (30.4–37.0%) length, with coarser and more sparsely distributed tubercles and pits; the lateral border furrow is shallower and less anteriorly widened; the lateral border is more elongate, with a much longer anterior projection, slightly broader, and has much coarser tuberculate sculpture, particularly adaxially.

Thoracic segments differ mainly in that those of D. heimbergi are coarsely tuberculate and those of D. humphriesi are nearly effaced, and also the line of tubercles on the anterior edge of the axial ring is made of fewer, larger tubercles.

Pygidia of D. heimbergi are of similar dimensions to those of D. humphriesi with sagittal length from articulating furrow 55.7% (52.5–58.4%) width across anterior pleural band of first segment, a little less strongly vaulted (sag., particularly tr.), and have much coarser tuberculate sculpture, compared to lightly tuberculate or nearly effaced; the axis is more strongly posteriorly tapered (width of fourth ring 44.6% (40.1–53.5%) width of first ring); the axial rings are slightly longer compared to their width and more inflated; the terminal piece is small (Pl. 27, figs 12, 18, 27) or absent (Pl. 27, figs 1, 13, 26); the pleurae and pleural spines are more inflated; the spines are shorter and they are less splayed, particularly the fourth pair; the doublure is shorter laterally and longer medially, coming to a point at the midline (Pl. 27, fig. 14).

Ontogeny. Ontogenetic changes in the cranidia of Deltapliomera heimbergi are well represented. Cranidia (cf. Pl. 23, figs 1, 9) become relatively shorter and narrower overall, and more strongly vaulted (tr., sag.), while the sculpture changes from granules overlain by sparse large tubercles to small densely spaced tubercles; the anterior border shortens, deflates slightly, and becomes overhung medially by the glabella, as does the anterior border furrow; the glabella inflates to become strongly convex, the lateral furrows lengthen and shallow, and the lobes become independently inflated; LO shortens and inflates, the median node shrinks dramatically, and the tubercles on the posterior margin also shrink to blend in with finely tuberculate sculpture; the axial furrows narrow and shallow; the interocular fixigenae broaden, particularly posteriorly; the palpebral furrows broaden slightly and shallow; the palpebral lobes broaden; the posterior fixigenae broaden, and the distal portions become more steeply downturned; the posterior border furrow lengthens and shallows, particularly proximally; the posterior border inflates a bit more; the genal spine shortens to a nub; and the ontogenetic tubercles on the border and the anterolateral edge of the posterior fixigenae reduce to blend in with the rest of the sculpture.

The hypostome (cf. Pl. 25, figs 1, 19) elongates relative to the width across the shoulders; the sculpture becomes slightly coarser all over; the middle body inflates, the anterior lobe becomes more strongly posteriorly tapered and the posterior lobe lengthens medially while shortening laterally and wrapping further anteriorly around the anterior lobe; the middle body furrow becomes more strongly impressed (although still only expressed anterolaterally); the posterior border furrow lengthens and deepens; and the lateral border narrows and the posterior border shortens.

Librigenae (cf. Pl. 24, figs 16, 22) broaden; the field broadens and narrows slightly, and the pits deepen; the lateral border furrow narrows and deepens; the lateral border broadens, deflates very slightly, and its tuberculate sculpture becomes finer and more densely packed, and the anterior projection lengthens. Thoracic segments are insufficiently well known to observe ontogenetic changes.

The pygidium (cf. Pl. 26, fig. 15, Pl. 27, fig. 27) broadens slightly overall and becomes more strongly vaulted (sag., tr.); the tuberculate sculpture becomes larger in size and more sparsely distributed; the axis also increases in convexity and broadens, particularly anteriorly; the axial rings each become more independently inflated; the interring furrows lengthen and shallow; the axial furrows broaden and shallow somewhat; the pleurae inflate more; the pleural spines also inflate, and spread further apart from each other.

Discussion. Deltapliomera heimbergi is compared with D. humphriesi in the differential description above, and is compared with D. inglei and D. eppersoni in discussions of those species.

Two poorly preserved cranidia (Pl. 23, figs 3, 6, 11; Pl. 26, figs 17, 19, 22) have distinctly wider and shallower axial furrows. The cranidia are also less vaulted (tr.), with a less inflated (sag., tr.) glabella. They may represent another species, or possibly a new species of Millardaspis , but the poor preservation of the palpebral lobes and sculpture limits more precise assessment.