Aponileus belkaae, Adrain, 2012

Adrain, Jonathan M., 2012, The Lower Ordovician (upper Floian) bathyurid trilobite Aponileus Hu, with species from Utah, Texas, and Greenland 3293, Zootaxa 3293 (1), pp. 1-67 : 18-20

Aponileus belkaae n. sp.

Plate 5, figs 16–25, Plates 6-10

? 1884 Barrandia ? sp.?; Walcott, p. 96, pl. 12, fig. 6.

1953 " Barrandia ? sp."; Hintze, p. 137, pl. 26, figs 15, 16.

2000 Theamataspis sp. ; Boyce et al., p. 125.

2009 Bathyuridae gen. nov. 6 sp. nov. 2; Adrain et al., p. 575, fig. 21I, M.

Material. Holotype, cranidium, SUI 129258 (Pl. 6, figs 1, 6, 9, 12), from Section J 16.1 m, and assigned specimens SUI 115403 , 115404 , 129255–129257 , 129259–129261 , 129267–129274 , 129278 , 129282 , 129283 , 129286 , 129287–129297 from Section J 16.0 m, J 16.1 m, J 20.0 m, and J 20.1 m, all Wah Wah Formation; assigned specimens SUI 129262–129266 , 129274–129277 , 129279 , 129281 , 129284 , 129285 from Section H 294.2 m, Fillmore Formation; all from the upper Floian (Blackhillsian; " Pseudocybele nasuta Zone "), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Belka.

Diagnosis. Cranidium with prominent tuberculate sculpture restricted to posteromedian region of glabella and LO in large specimens; raised line sculpture on most dorsal surfaces coarse and relatively widely spaced; flattened dorsal face of genal spine broad; pygidium very wide relative to length; three pygidial segments present, with third ring prominent; posteromedian pygidial spine completely lost in large specimens.

Description. Cranidial measurements were made (where relevant features are preserved) on the large specimens of Pl. 6, figs 1, 3, 10 and Pl. 7, figs 1, 4. Cranidium with sagittal length 76.1% (73.1–79.2) width across palpebral lobes; width across intersection of anterior border furrow and facial suture 62.1% (60.6–64.1) width across palpebral lobes; width across β 72.0% (71.0–73.8) width across palpebral lobes; width across Ύ 67.1% (63.0–68.5) width across palpebral lobes and 94.8% (92.6–95.9) width across Ɛ; width across rear of LO 58.8% (55.8–63.8) sagittal length; anterior margin of anterior border bowed forward medially, ranging from more or less arcuate (Pl. 6, fig. 10) to more obliquely straight (Pl. 7, fig. 1); anterior border very short in dorsal view, shorter medially, but with broad anteriorly-facing area; dorsal rim-like portion of border with five or six fine, subparallel raised lines, forward-facing region with coarser, much more widely spaced subparallel raised lines; connective sutures cutting obliquely across forward face at low angle, ventral margin of forward face occupying approximately median three-fifths of width, with slight ventral convexity; anterior margin of forward face describing shallow inverted "W"; anterior border furrow about same length (exsag.) as dorsal rim of border laterally, shorter and obscured by glabella medially, shallow but distinct; anterior section of facial sutures bowed strongly laterally; frontal area with prominent raised lines set obliquely to suture; palpebral lobes very large, slung posteriorly so that posterior half of margin has much stronger curvature than anterior half; palpebral furrow very faint, creating subtle raised rim near margin of lobe; interocular fixigena rising with gradual change in slope to palpebral lobes, areas not clearly distinguished, with sculpture of subdued scrobiculate raised lines; posterior projections not well preserved on available species, proximal part crowded against rear of palpebral lobe; posterior fixigena with only small area in front of posterior projection, and occupying only very short (exsag.) strip on projection; posterior border lacking sculpture, with strong dorsal convexity; posterior border furrow very deep; glabella with maximum width across frontal lobe 96.9% (93.1–103.7) sagittal length (excluding LO) and posterior width across L1 58.8% (55.8–63.8) sagittal length; axial furrow anteriorly convergent opposite LO and very rear of L1, anteriorly divergent in front of L1 to maximum divergence opposite frontal lobe, gently deflected around subtle swellings of lateral glabellar lobes, shallow but distinctly impressed, similar to lateral definition of anterior border furrow; preglabellar furrow running without interruption from axial furrows, shallower, running into anterior border furrow as anterior part of glabella overhangs anterior border; glabella with sculpture of fingerprint-like prominent raised lines, with arcs directed anteriorly, much more prominent on frontal lobe and partially to wholly effaced posteriorly, and large tubercles, gathered more prominently posteriorly and posteromedially, smaller and much more subdued and more widely scattered anteriorly and laterally; glabellar furrows expressed as very shallow depressed areas and indentations in lateral margin of glabella; L1–L3 expressed as subtly differentiated independent swollen regions; anteromedian part of glabella completely overhanging anterior border furrow; SO long (sag., exsag.), with sharper break in slope to main part of glabella anteriorly than to LO posteriorly; LO with sagittal length 16.9% (15.9–17.8) that of cranidium, somewhat shorter laterally, with sculpture of prominent tubercles similar to that on posteromedian portion of glabella; doublure consisting of large articulating surface beneath LO, with sculpture of fine subparallel raised lines, anterior margin describing "W" shape, and articulating transverse groove beneath proximal part of posterior border (distal portion of doublure not preserved on any available specimen); fossulae not obvious.

Librigena with eye long and tall, visual surface with strong outward convexity; anterior facial suture strongly bowed; posterior facial suture with short course; eye socle not expressed, visual surface separated from field by moderately deep, narrow furrow; field long, widest behind eye, surface with moderate convexity, with sculpture of prominent raised lines, describing anteriorly directed arcs on adaxial half of field, aligned subparallel with lateral margin on abaxial half of anterior region, and mostly effaced on abaxial half of posterior region of field; posterior border furrow nearly effaced, field separated from posterior border mainly by change in slope, raised lines contiguous across both surfaces; lateral border shallow and nearly effaced posteriorly, progressively deeper anteriorly, deep and trench-like near anterior facial suture; anterior portion of lateral border with morphology similar to that of anterior border of cranidium, with dorsal rim-like portion and flattened ventrolateral face; rim-like portion with fine, crowded raised lines, ventrolateral face with coarser, more widely spaced lines; change of slope between rimlike portion and ventrolateral face continued posteriorly and onto lateral aspect of long genal spine, but more subdued on posterior part of lateral border; genal spine with dorsal surface flattened, so that lateral rim is blade-like; dorsal aspect of spine with raised line sculpture continued from rear of field and posterior border; genal spine about as long as remainder of librigena, robust, tapered to sharp point; ventral aspect of spine more inflated than dorsal aspect, with sculpture of densely spaced raised lines running subparallel to margins; ventrolateral aspect of lateral border grading into ventral aspect of genal spine; doublure turned sharply in from ventrolateral face of border, forming sharp rim; doublure lacking sculpture, slightly concave anteriorly to markedly so posteriorly, forming depressed area between base of genal spine and large Panderian notch; Panderian notch with fine raised rim; doublure broader posteriorly than anteriorly; anterior projection (not well preserved in most available specimens) relatively small and narrow.

Rostral plate, hypostome, and thorax not identified.

Pygidial measurements are based on the large, intact specimens of Pl. 9, figs 1, 2, 7, 14. Pygidium with sagittal length 42.6% (39.4–44.0) maximum width; anterior width of axis 98.5% (91.3–109.4) sagittal length of axis and 35.4% (34.2–36.4) maximum pygidial width; sagittal length of axis 84.6% (82.9–86.3) that of pygidium; articulating half ring relatively short, lacking sculpture; ring furrows all transverse, progressively shallower posteriorly; axis with three rings and terminal piece; very small lineation not connected laterally with axial furrow separating third ring from terminal piece; rings with sculpture of prominent scrobiculate raised lines overlain by medium tubercles ranging from prominently expressed on all rings (Pl. 9, fig. 7, Pl. 10, fig. 18) to largely subdued (Pl. 9, fig. 1, Pl. 10, fig. 10); pseudoarticulating half ring expressed on second segment of some specimens as thin sliver in ring furrow (Pl. 10, fig. 10), not expressed in others, pseudoarticulating half ring not expressed on third segment; terminal piece large, about as long (sag.) as first ring; axial furrows well impressed, moderately posteriorly convergent, bowed medially to meet and fully define axis posteriorly, no post-axial ridge developed; pleural field relatively narrow, particularly behind first segment; pleural region with prominent, coarse, scrobiculate raised line sculpture over entire surface, some specimens with a few very small scattered tubercles on posterior parts (Pl. 10, figs 1, 18); anterior pleural band of first segment slightly longer than posterior band, developed into large, triangular articulating facet distal to fulcrum, with anterolateral, forwardly directed point housing an articulating socket; first pleural furrow well impressed and deep, narrowed and pinched out distally, pleural bands uniting behind articulating facet; interpleural furrows barely expressed; second pleural furrow only about half the width of first and less well expressed, pleural bands of second segment not completely expressed; pleural region of third segment not differentiated from generalized pleural field; posterior margin of pygidium describing continuous, medially transversely elongate, arc; border broad, broader laterally, with sculpture on posterior rim of subparallel raised lines somewhat finer than those on dorsal pleural surface; pygidium with gentle median flexure in posterior view and median embayment; largest specimens lacking any hint of a posteromedian spine, but small nubbin present in medium sized specimens; doublure broad, with sculpture of subparallel raised lines, more closely spaced medially than laterally, ventrally convex medially but with concave lateral regions behind articulating sockets.

Ontogeny. Of the available material, ontogenetic change can be observed in the cranidia and pygidia, but the morphology of the librigenae seems more stable through the available size range. Cranidial changes in holaspid ontogeny include a relative widening of the glabella, an increasing median anterior overhang of the frontal glabellar lobe, a relative shallowing of the axial furrows, and a reduction in the prominence of the scrobiculate raised line sculpture on the glabella, particularly posteriorly.

The pygidium is much narrower relative to length in smallest specimens (Pl. 9, fig. 19); relative width increased with size. Smaller specimens have more prominent tuberculate sculpture and less prominent raised line sculpture. Tubercles generally decrease in density with size, though some large specimens retain dense axial tuberculation. The posteromedian spine is thorn-like and upturned in smallest specimens, progressively reduced to small nubbin in medium sized specimens, and lost entirely in largest specimens. The posteromedial profile of postaxial region changes from strongly upturned with spine (Pl. 9, fig. 24), to still strongly upturned after the spine is nearly lost (Pl. 10, fig. 25), to not upturned at all in large specimens (Pl. 9, fig. 4).

Discussion. As discussed above, A. belkaae is sister to A. strelkaae , sharing the distinctive apomorphy of a short, wide pygidium which is reduced from four to three segments. Aponileus belkaae was compared with A. aasei and A. laikaae above. It is compared with A. strelkaae and A. ugolekae under discussion of those species below.