Aponileus, Hu, 1963

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 : 21-24

Aponileus ? veterokae n. sp.

Plate 16, figs 1–14, Plates 17, 18

Material. Holotype, cranidium, SUI 129316 (Pl. 13, figs 1, 5, 8, 11, 14), and assigned specimens SUI 129317–129330 , 129332–129337 , from Section J 48.1 m, Wah Wah Formation ( upper Floian ; Blackhillsian; " Pseudocybele nasuta Zone "), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Veterok.

Diagnosis. Glabella much narrower than any other species and anteriorly pointed; anterior border flat and long, not turned up as in all other species; sculpture on all sclerites more reduced than in any other species, but fine anastomosing lines present on most surfaces through most of ontogeny; librigenal lateral border furrow almost completely effaced in larger specimens; pygidium with large, long posteromedian spine.

Description. Cranidial measurements were made on the large, best preserved, holotype specimen of Pl. 17, fig. 1. Although the specimen of Pl. 17, fig. 15 is also well-preserved, it is much smaller and features such as the proportions of the glabella change during ontogeny, making it unsuitable for comparative measurement. Cranidium moderately vaulted (sag., tr.), broad, very broad across palpebral lobes (width across δ 114.8% sagittal length) and anteriorly expanded (width across Ύ 78.7% sagittal length and width across β 86.9% sagittal length), narrower posterior to palpebral lobes, with width across ɛ 81.2 % sagittal length, cranidium effaced but for line sculpture, larger specimens increasingly effaced; anterior border moderately short, 8.2% sagittal cranidial length, tapered far exsagittally by cut of facial suture, otherwise of even length (very slightly shorter medially in front of glabella), broadly anteriorly bowed, somewhat pointed medially, dorsal surface horizontal to slightly anteroventrally sloped (lateral view), also gently dorsally bowed (anterior view), with lateral edges lower than middle section, and with sculpture of fine, closely spaced subparallel ridges following curvature of border on anterior third, posterior section effaced (in large specimens; small specimen of Pl. 17, fig. 2 with ridges on entire border); doublure mainly expressed as anterior face of border, very slightly ventroposteriorly curved to make a ventral rim, narrower than width of border and narrower yet ventrally, with ventromedially angled lateral margins from cut of facial suture, about half length of dorsally visible border, with sculpture of 6–7 subparallel ridges slightly coarser than those on dorsal border; anterior border furrow short and shallow (increasingly shallow in larger specimens), broadly anteriorly bowed, with median section around anterior margin of glabella more strongly bowed and deeper than lateral sections, tangent to preglabellar furrow medially; preglabellar furrow short, moderately deep, very strongly anteriorly bowed; frontal areas small, wedge-shaped, very short adaxially and only a little longer than anterior border at maximum abaxial length, fairly steeply anteroventrally sloped, about 35–40º below horizontal; anterior branch of facial suture long, very strongly laterally bowed around frontal areas; glabella large, long and broad, anteriorly expanded from SO to maximum width approximately even with β (except very slight waisting about halfway between SO and S1), then tapered along very strongly anteriorly bowed anterior margin, with sagittal length 76.6% cranidial length, maximum width 83.2% sagittal length, and minimum width across L1 75.8% maximum width, gently vaulted (sag., tr.), convexity less posteriorly, LF slopes down to anterior border at about 50–60º below horizontal, glabella with fine anastomosing line sculpture following curve of LF (Pl. 17, figs 2, 10) or effaced (larger specimens); lateral lobes and furrows poorly defined dorsally on large specimens, but better visible ventrally and on small specimens, all furrows moderately long and shallow, S1 occurs approximately even with half-length of palpebral lobe, posteromedially directed and widest, reaches almost to 1/3 glabellar width, S2 and S3 anteromedially directed, S3 occurs even with Ύ, very narrow, barely extends beyond axial furrow, S2 slightly closer to S3 than S1, about half width of S1; glabellar lobes not independently inflated, lateral lobes ill-defined by furrows, but L1 very long, L2 longer medially and shorter laterally due to opposite directions of S1 and S2, L3 a little shorter than maximum length of L2, LF very large, sub-semi-circular, median lobe very wide; SO moderately long and fairly shallow, shallower in larger specimens, very shallow near axial furrows (Pl. 17, fig. 2), course mainly transverse, far abaxial portions slightly anteriorly curved, with line sculpture like that of LO at least in small specimens (Pl. 17, figs 2, 15); LO moderately long, longest medially and tapered laterally, somewhat longer than anterior border, 16.3% sagittal length of cranidium, broad, with width across intersection of axial furrows and cranidial posterior margin 47.0% sagittal length of cranidium, LO only very slightly independently inflated, effaced (large specimens) or with fine anastomosing line sculpture and small median node located at mid-length (small specimens); doublure not well preserved except on single small specimen (Pl. 17, fig. 19), lens-shaped, long medially and pinched out laterally; axial furrows narrow, very shallow posteriorly, somewhat deeper from approximately S1 forward, deeper at lateral glabellar furrows, separated from preglabellar furrow by small, short, lightly swollen, oblique ridge located just in front of S3 (best seen on Pl. 17, figs 2, 10), course anteriorly divergent; fixigena almost all interocular, posterior fixigena just a short strip behind palpebral lobe and extending along posterior border, interocular fixigena narrow anteriorly, expanded posteriorly due to set of palpebral lobes and expansion of glabella, roughly horizontal opposite lobes, ends gently antero- or posteroventrally sloped; palpebral lobes gently anteroventrally sloped (exsag.), slightly elevated from horizontal (tr.), very large, long, with exsagittal length 33.1% cranidial sagittal length, irregularly sub-semi-circular in shape, wide, with maximum width occurring at about 1/3 length, tapered anteriorly and posteriorly, lobe with slightly more inflated lateral rim, bordered adaxially by slightly depressed, moderately narrow, laterally crescentic region bordering slightly raised, adaxial, semilunate area; palpebral furrows almost totally effaced dorsally, somewhat better visible ventrally, furrow located between raised semilunate area and crescentic depressed area, very narrow and very shallow, moderately laterally bowed; posterior border furrow, posterior border, and its doublure extremely poorly known, border furrow moderately long and deep, shallow near glabella; posterior border short, very slightly inflated, higher posteriorly; preserved doublure only rim-like.

Rostral plate, hypostome, and thorax not identified.

Librigenae of Aponileus ? veterokae are not well known from large specimens, and none are suitable for full measurements. Anterior branch of facial suture poorly preserved, particularly along anterior projection of border, much (over two times) longer than posterior branch, steeply sloped along field, broadly curved at lateral border, cuts border at shallow angle exposing large, triangular swath of inner face of doublure; posterior branch of facial suture short, slopes steeply down field from eye, less steeply sloped across posterior border; eye with visual surface made of tiny, closely packed lenses, long and narrow, crescentic–reniform, strongly raised, with steeply sloped lateral side almost perpendicular to librigenal field; eye rimmed at base by extremely fine raised ridge (Pl. 16, fig. 14); separated from librigenal field by narrow, moderately shallow circumocular furrow; librigenal field long and fairly narrow, with width from midlength of eye to lateral border furrow approximately (based on specimen of Pl. 16, fig. 3) 25.7% exsagittal length, field slightly narrower posteriorly, weakly laterally convex (tr.), effaced except for fine caecal pitting (Pl. 16, figs 1, 6); lateral border furrow better seen internally, highly effaced, broad and shallow, weakly laterally bowed; lateral border (including indistinct furrow) wide relative to field, approximately (based on Pl. 16, fig. 3) 60.0% as wide, gently laterally bowed, moderately inflated, with sculpture of subparallel, occasionally intersecting terrace lines following curvature of border and continuing onto genal spine; spine long, full extent unknown, but at least a little longer than librigenal field (Pl. 16, fig. 11), broad-based, gradually tapered, dorsoventrally flattened, blade-like, but moderately inflated, shallowly curved, with sculpture like that of border; posterior border furrow effaced dorsally, visible internally (Pl. 16, fig. 10); posterior border short, about half as long as width of lateral border, with terrace lines only on posterior margin; lateral doublure broad, nearly reaches border furrow, posterior section of doublure short, but also nearly reaches posterior furrow, doublure with slightly raised outer rim extending not quite to facial suture anteriorly, doublure sloped dorsolaterally toward field, with sculpture of fine terrace lines more widely spaced than those on external aspect, and with small Panderian notch near cut of posterior branch of facial suture.

Pygidial measurements were made on the well-preserved specimens of Pl. 18, figs 1, 12. Pygidium broad and shallowly vaulted (sag., tr.), with maximum width across posterior band of first segment (even with rear of articulating facet) 220.7% (216.5, 224.8%) sagittal length excluding articulating half ring and posterior median spine, with semilunate main body and long posterior spine, and with curvilinear sculpture of fine ridges over entire surface; articulating half ring short, 7.7% (6.5, 8.8%) sagittal length of pygidium, shorter laterally, sculptured with terrace lines; articulating furrow short, fairly shallow, transverse; axis of four rings and terminal piece, strongly vaulted, gradually less vaulted posteriorly, with steep slope from top of terminal piece to base of spine, broad, with width across first ring 33.5% (32.8, 34.2%) pygidial width, gradually posteriorly tapered, with third ring 77.5% (76.3, 78.7%) width of first ring, relatively long, with sagittal length 80.5% (79.2, 81.8%) sagittal pygidial length; axial rings poorly defined by furrows, only very slightly independently inflated, rings of nearly equal length, only slightly shorter posteriorly, second ring (possibly also third) with short, crescentic pseudo-articulating half ring; terminal piece fairly short and wide, ill-defined, with sloped posterior margin; inter-ring furrows moderately short, progressively shorter posteriorly, very shallow, better viewed ventrally (Pl. 18, fig. 4), fourth furrow basically effaced, all furrows transverse; axial furrows narrow, moderately deep, steadily anteriorly divergent; post-axial furrow long, moderately deep, effaced medially in most, but not all (Pl. 18, figs 12, 17) specimens; posterior spine very long, at least 150% (Pl. 18, fig. 12) longer than main part of pygidium (although spines of larger specimens may be relatively shorter; the spine of Pl. 12, fig. 14 is about equal in length to the pygidium), conical, strongly posteriorly tapered, posterodorsally angled about 15º above horizontal, with sculpture of discontinuous (although many wrap around to ventral face), anastomosing, generally slightly posteriorly curved ridges, somewhat resembling overlapping scales, slightly more regularly curved, more like a chevron pattern ventrally, spine protrudes from posterior border, but dorsal base possibly connected to rear of axis by narrow and faint post-axial ridge in some specimens (e.g., Pl. 18, figs 1, 5); pleurae mostly ill-defined by furrows, pleural bands slightly independently inflated, pleural bands all short, posterior bands and anterior bands of subsequent segments (starting with first segment) merged due to indistinct interpleural furrows to appearance of single long band, pleurae increasingly posteriorly angled; inner pleurae wide anteriorly, about half width of pleurae (anterior view), progressively narrower posteriorly as tail tapers, separated by fulcral angle of about 143º (anteriorly); anterior pleural band of first segment short adaxially, lengthened abaxially into large articulating facet; pleural furrows very long, longer exsagittally, moderately shallow, first furrow gently posterolaterally directed, posterior furrows increasingly posterolaterally angled, fourth furrow steep; interpleural furrows effaced, without even a ventral presence; pygidial border moderately wide, poorly independently inflated, separated from pleural bands by wide, shallow, indistinct furrow, sculpture (best seen in lateral and posterior views) of sideways "V"s, with vertices pointed anterolaterally, and with ventral line of "V" extended into subparallel ridges continuing onto doublure; doublure moderately wide/long, of equal extent (ventral view), anterolateral parts dorsomedially angled while posteromedial parts are tucked up inside posterior of tail, so that doublure appears to taper in anterior view, doublure with sculpture of subparallel ridges a little coarser than those of dorsal sculpture.

Ontogeny. Cranidia of A.? veterokae are not very well preserved, but ontogenetic changes can still be observed. The cranidium becomes broader overall; the anterior border shortens; the palpebral lobes become relatively smaller; the glabella broadens anteriorly, changing from bullet-shaped to anteriorly expanded; and all the cranidial furrows become slightly longer/wider and shallower.

Librigenae are generally poorly preserved, but the librigenal field becomes slightly wider relative to the length; the lateral border furrow becomes completely effaced; and the lateral border inflates.

Pygidia broaden overall; all furrows shallow; the lateral border furrow becomes completely effaced and the border inflates to match the pleurae; the axis broadens and loses some convexity; the posterior median spine shortens; and the anastomosing line sculpture becomes more effaced, except on border and spine.

Discussion. Aponileus ? veterokae is obviously very different from all other species included in the genus, so much so that although we consider on balance that it belongs we have opted to assign it with question. It has a "normal" flattened anterior border, a glabella that is not greatly expanded anteriorly and which comes to a blunt point anteromedially, it displays a faint oblique ridge crossing the axial furrow opposite the frontal glabellar lobe, it has smaller palpebral lobes than any other species, and most obviously it has a pygidium with a large posteromedian spine. All of these features are common within Acidiphorus and superficially it might appear this is the taxon to which the species should be assigned. However, despite these similarities, the species does not closely resemble any species of Acidiphorus known from the literature, nor the many new species awaiting description in our collections. Species of Acidiphorus almost all have a nasute anterior border, which A.? veterokae lacks, they have cylindrical or subcylindrical genal spines (where not strongly reduced) whereas those of A.? veterokae are flattened and blade-like, and they almost all have strong tuberculate sculpture on most surfaces, whereas A.? veterokae has a sculpture of very fine, subdued, anastomosing lines on most surfaces. It is hard to make any detailed case for a relationship with any species of Acidiphorus .

Despite the autapomorphic features, A.? veterokae seems to share more characters with Aponileus , and in particular the slightly older A. ugolekae . Although the glabella is narrower and the palpebral lobes are smaller, they are still generally similar in plan and A.? veterokae remains a very large-eyed species. The sculpture of most surfaces is a direct match, save for its progressive effacement in A.? veterokae. The structure of the librigena is particularly compelling, as A.? veterokae retains the same basic dimensions and particularly the blade-like genal spine characteristic of Aponileus . It particularly resembles A. ugolekae in the fashion in which the base of the genal spine is flattened and merged with the posterior border furrow into a single continuous face. Pygidia of the two species are virtually identical with the (prominent) exception of the huge spine developed in A.? veterokae, with nearly indistinguishable sculpture and expression of pleural and ring furrows.

The shared features are convincing enough, especially given the absence of any other known species with which A.? veterokae compares in detail, that we consider the best supported current hypothesis to be that it is a highly autapomorphic species of Aponileus , sister to A. ugolekae . The autapomorphies are so striking, however, that we acknowledge the issue should remain open, and assign the species only with question.