Aponileus laikaae, 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 : 13-18

Aponileus laikaae n. sp.

Figure 5 , Plates 3, 4, Plate 5, figs 1–15

2009 Bathyuridae gen. nov. 6 sp. nov. 1; Adrain et al., p. 574.

Material. Holotype, cranidium, SUI 129234 (Pl. 3, figs 1, 4, 8, 10), from Section H 256–261 T m, and assigned specimens SUI 129235–129254 from H 256-261 T m and H 264–267 T m, Fillmore Formation ( upper Floian ; Blackhillsian ; Presbynileus ibexensis Zone ), southern Confusion Range , Ibex area , Millard County, western Utah, USA. The species also occurs at Section H 251.4 m, in the same zone.

Etymology. After Laika.

Diagnosis. Dorsal surfaces of cranidia, librigenae, thoracic segments, and pygidia with sculpture of dense anastomosing lines and star-shaped tubercles, the ridges of which connect with the lines; eye socle retained in large librigenae; librigenal field with tubercles grouped on adaxial part only; pygidium with long, deep pleural and ring furrows and well inflated pleural bands; four pygidial segments present; faint posteromedian spine retained as small node even in large pygidia.

Description. Cranidial measurements based on most intact specimen of Pl. 3, fig. 2. Cranidium low and wide, with only gentle dorsal vaulting; maximum width across posterior projections, but comparison of width to sagittal cranidial length is not preserved intact on any specimen; anterior border short, similar in length sagittally and exsagittally, anterior margin forwardly concave laterally, bulged forward and forwardly convex medially; border raised into strong rim, with sculpture of dense, fine raised lines running slightly obliquely to anterior margin; anterior face of border tall, with transverse ventral margin, cut laterally by long, obliquely set connective sutures, with sculpture of fine raised lines slightly larger than those on dorsal part, set subparallel to one another and bowed laterally, describing shallow inverted "U" shape in anterior view; anterior border furrow deep, short, partially obscured medially by overhanging glabella; preglabellar field absent, glabella abutting rear of anterior border medially; frontal areas small and triangular with fine anastomosing line sculpture; anterior sections of facial suture strongly laterally bowed; distance across β 93.0% cranidial sagittal length and distance across Ύ 86.1% cranidial sagittal length; palpebral lobe very large, exsagittal length 46.4% cranidial sagittal length; distance across palpebral lobes 136.5% cranidial sagittal length; lobes drooped posteriorly, maximum of curvature well behind midlength; edge of palpebral lobe inflated into rim; palpebral furrow with complex morphology; true furrow is set some distance adaxially from rim (best seen in ventral view, Pl. 3, fig. 4), narrow and curved around rear margin of lobe, broader anteriorly, progressively shallower anteriorly and shallowed out posterior to anterior edge of lobe, with stronger lateral curvature than lateral margin of lobe; furrow is separated from margin by a narrow, flat, crescentic area which is separated from the rim by a shallow secondary furrow; part of lobe adaxial to true palpebral furrow is weakly dorsally inflated; palpebral lobe held at moderate angle to general plane of glabella, abaxial parts turned to lie flat and subparallel with general plane of glabella; lobe with dorsal sculpture of dense anastomosing lines and a few tiny, faint tubercles; inflated rim of lobe continued anteriorly around Ύ and onto frontal area almost to β as sutural ridge; rear of palpebral lobe with Ɛ reentrant, posterior margin of lobe overhanging anterior edge of posterior projection; eye ridge not obvious, anterior edge of palpebral lobe nearly abutting axial furrow; posterior edge of lobe well separated from glabella; distance across Ɛ almost identical to that across Ύ; posterior fixigena with very fine anastomosing line sculpture, more subdued than on other parts of cranidium, fixigena running along projection as very short (exsag.) strip with transverse row of median tubercles; posterior border furrow deep, running to contact with posterior facial suture where it is met by a slight sutural ridge; posterior border very short, shortest just proximal to fulcrum, wider distally, with sculpture of two or three transverse tubercles proximally and seven or eight tubercles on lobate distal part; glabella evenly forwardly expanding, minimum width posteriorly 72.2% maximum anterior width, which is developed opposite β; maximum glabellar width 91.6% sagittal glabellar length (excluding LO); sagittal length (excluding LO) 74.9% cranidial sagittal length; axial furrows fairly narrow and quite shallow, deflected slightly around faint lateral swellings of glabella, turned sharply into preglabellar furrow at maximum width of glabella; preglabellar furrow slightly narrower than axial furrow, most of course obscured beneath overhanging frontal lobe of glabella and tall anterior border, essentially merged in this region with anterior border furrow; glabella with sculpture of anastomosing lines more prominent than that on fixigena, and with moderate sized star-shaped tubercles connecting with intersecting lines ( Fig. 5.1 ), tubercles slightly smaller anteriorly; glabella with distinct but slight lateral inflation behind anterior edge of palpebral lobe, some specimens (e.g., Pl. 3, fig. 12) with several faint swellings and furrows, but relationship to glabellar lobation uncertain; glabella only weakly to moderately inflated in transverse and sagittal profiles; SO long (sag., exsag.) and quite deep, running almost exactly transversely but for median posterior embayment in front of LO; distance across LO 56.3% sagittal length of cranidium; sagittal length of LO 15.2% that of cranidium; LO longer medially than laterally, with line and tubercle sculpture similar to that of glabella, except tubercles slightly larger and posterior tubercles arranged in some specimens in an approximate transverse row in front of posterior margin (e.g., Pl. 3, figs 1, 2, but not Pl. 3, fig. 12); median node not obviously expressed; axial furrow bowed posterolaterally opposite LO and shallower; doublure forming large articulating surface beneath LO with sculpture of very fine, transverse raised lines; forming groove beneath proximal part of posterior border for articulation with anterior edge of first thoracic segment; fossulae not apparent.

Librigena with large, long eye set on weakly differentiated eye platform; eye platform separated from field by deep furrow, arcuate anteriorly and posteriorly but slightly straighter in midlength of course; eye socle partially merged into field, narrow inflated rim with very subtle bounding furrow, but sculpture of field is continued over socle; field narrow anteriorly, broader posteriorly, weakly inflated adaxially, dorsally concave abaxially, with sculpture of fine anastomosing lines across entire surface and two or three more or less longitudinal rows of small star-shaped tubercles ( Fig. 5.4 ) restricted to adaxial half; lateral border furrow very broad and shallow; lateral bor- der inflated and large, with dense subparallel raised lines independent of anastomosing sculpture on field (intersecting and overriding the field sculpture along border furrow) set obliquely to margin on posterior part of border and subparallel with margin on anterior part; border with dorsal part more ridge-like anteriorly to match tall anterior border, more semi-circular in section posteriorly; posterior border short, scarcely differentiated from rear of field by weak posterior border furrow, with sculpture of a few tubercles; genal spine long and robust, blade-like with sharp dorsolateral edge arising from rear part of lateral border; dorsal aspect flattened and with anastomosing sculpture; lateral aspect with subparallel raised lines continued without interruption from lateral border; spine slightly shorter than field (exsag.), tapered to sharp distal point; anterior projection relatively short; doublure underlying lateral border forming sharp angle with ventrolateral edge of sculptured border, with a few raised lines on lateral part but progressively effaced adaxially, slightly concave, slightly narrower anteriorly, forming broad shelf; only a small portion of doublure underlying restricted course of posterior border, intruded on by Panderian notch; Panderian notch very large, bounded by small raised rim; underside of genal spine with subdued, chevronlike raised lines.

Rostral plate not identified.

Hypostome (tentatively assigned; see discussion) subquadrate, with strong, scrobiculate raised line sculpture; anterior margin turned slightly dorsally, slightly anteriorly bowed but more or less transverse; anterior wing large, expanded distally, running at about 45º angle dorsolaterally; lateral margins widest at shoulder, moderately posteriorly convergent posterior to shoulder, turned into posterior margin at distinct posterolateral corner; posterior margin posteriorly bowed, very slightly irregular in very shallow "W" shape; lateral border only weakly differentiated, marked mainly by several raised lines subparallel with margin; posterior border with more independent inflation; both lateral and posterior border well inflated laterally and dorsolaterally, with prominent raised lines; lateral bor- der furrow hardly expressed, lateral border almost contiguous with middle body; posterior border furrow distinct but long (sag.; exsag.) and shallow, appearing as concave sculptureless region; anterior part of middle body strongly ventrally inflated, with irregular, scalloped line sculpture; middle furrow set about three quarters distance posteriorly, expressed as pair of deep lateral slots oriented slightly obliquely; posterior part of middle body weakly inflated, with transverse raised line sculpture; doublure running from anterior wing to small posterior wing, forming small shelf above posterior border, broader at posterior corners; lateral notch shallow.

Thoracic segments with axial lobe slightly wider (tr.) than (Pl. 4, figs 10, 17) to slightly narrower than (Pl. 4, fig. 26) pleural lobe, depending on position in thorax; axial ring with rear margin describing shallow "W" (Pl. 4, figs 10, 16) to nearly transverse (Pl. 4, fig. 26); anterior margin similar in course, but with shallower inflections; ring with about 16 prominent, star-shaped tubercles arranged approximately in two transverse rows, with background sculpture of connecting raised lines similar to that on dorsal surfaces of other sclerites ( Fig. 5.2 ); ring with very shallow transverse depression at about half length (sag; exsag) and between transverse tubercle rows; ring furrow long (sag.; exsag.), pinched distally, with distinct breaks in slope at anterior and posterior margins, flat-bottomed with very faint, anteriorly bowed, crescentic swelling set within furrow; articulating half-ring large, transversely ellipsoid, with gentle sagittal convexity, lacking sculpture; axial furrow shallow, defined as exsagittally aligned furrow posteriorly, where it cuts across posterior pleural band, but obscure anteriorly; peg-like articulating process directed anteriorly at anterior extent of axial furrow; socket directed posteriorly at rear of axial furrow (Pl. 4, figs 23, 27); pleura with sharp break in slope at fulcrum, set just over half distance distally (Pl 4, figs 10, 16) to far distally (Pl. 4, fig. 26) depending on position in thorax; segment of pleura proximal to fulcrum with very prominent anterior articulating ridge along anterior margin and posterior articulating groove along posterior margin; anterior margin with subtriangular forward projection at fulcrum, posterior margin with corresponding posterior notch; anterior pleural band strongly exsagittally convex, almost ridge-like, very short (exsag.) proximally, progressively wider distally, with sculpture of approximately four prominent tubercles and one or two much smaller tubercles, arranged in approximate transverse row beginning about half distance along portion proximal to fulcrum and continued distally behind articulating facet; posterior pleural band similar in morphology to anterior band, except of similar length (exsag.) proximally and distally and longer than anterior band across most of width, with tubercle row started immediately adjacent to axial furrow and continued distally onto surface of pleural spine; pleural furrow very prominent, long, and deep, with flat bottom, pinched and shortened (exsag.) distally, pinched out near base of pleural spine; anterior and posterior pleural bands merged around termination of pleural furrow; articulating facet with distinct different plane than remaining distal region of segment but otherwise not large and prominent, subtriangular and sliver-like, with large anterolaterally directed socket set at anterior point of triangle; short, subtriangular pleural spine directed slightly posterolaterally to nearly directly posteriorly, depending on position, short and stubby (Pl. 4, fig. 16) to more elongate (Pl. 4, fig. 26), depending on position; dorsal sculpture of anastomosing raised lines on pleural bands coalesced into loose, posteriorly-directed chevron pattern on spine; doublure consisting of crescentic articulating surface beneath posterior part of ring, slightly ventrally concave, lacking sculpture; thin shelf underlying part of posterior pleural band distal to fulcrum, longer distally; pleural tip underlain by broad doublural shelf with loosely organized, sinuous raised lines, anterolateral socket near front, small ventrally directed swelling or peg-like process developed behind and in most specimens slightly abaxial to socket, rear with prominent notch and adaxially directed process for articulation with anterolateral socket of next most posterior segment (or pygidium).

Pygidial measurements are based on the specimens of Pl. 5, figs 1, 3, 12. Pygidium with sagittal length 51.3% (50.8–51.9) maximum width; axis with maximum anterior width 87.1% (85.7–89.5) sagittal length and 37.7% (36.4–38.4) pygidial maximum width, and sagittal length 84.4% (83.1–86.1) pygidial sagittal length; axis consists of four rings and a terminal piece; entire dorsal surface with prominent background sculpture of anastomosing raised lines, expressed even in ring furrows and pleural furrows, and connected to large, star-shaped tubercles; articulating half ring large, transverse in middle part, pinched abruptly laterally, with anastomosing line sculpture expressed on posteromedian region; first ring furrow long (sag. exsag.), slightly longer laterally than medially, flat bottomed, with anastomosing line sculpture expressed on median half (tr.); first through third rings of similar length, fourth shorter; all four rings with distinct pair of large tubercles near midline and several slightly smaller but still large tubercles arranged in irregular transverse row near posterior margin, still more irregular row of smaller tubercles set anteriorly; second, third, and fourth ring furrows progressively shorter and shallower than first, but still prominently impressed; furrow between fourth ring and terminal piece much less impressed and more transverse; pseudoarticulating half ring expressed in front of second ring furrow, occupying crescentic median portion of rear of first ring, but bearing sculpture contiguous with that of first ring; pseudoarticulating half rings posterior to second segment not obviously expressed; axial furrow shallow anteriorly, beginning behind anterior articulating peg set immediately beside axis, deflected around first two axial rings, more linear in course posterior to second ring; axial furrows gently posteriorly convergent, bowed around rear of axis to meet medially and fully circumscribe axis; no post-axial ridge developed; pleurae of first segment with morphology closely similar to that of thoracic segments, anterior pleural band longer than posterior band, pleural furrow very long and deep; two or three tubercles on anterior band of first segment, mainly distal to fulcrum, posterior band with three or four tubercles arranged transversely along entire width; first interpleural furrow shallow, not a lineation but a broad trough, second interpleural furrow visible but faint, no posterior interpleural furrows expressed; second segment with anterior and posterior bands subequal in length, each bearing tubercles across width; second pleural furrow narrower than first, but only slightly shorter and shallower; anterior pleural band of third segment expressed with two or three tubercles; third pleural furrow visible but faint; posterior pleural band of third segment barely swollen, with a single tubercle expressed; pleural bands and furrows of fourth segment not differentiated; rear margin of pygidium broadly arcuate, border not independently swollen but marked by termination of pleural bands and furrows, broad, broader laterally than medially, with raised line sculpture organized into prominent evenly spaced lines set subparallel with margin; small spine base positioned medially just above border, spine itself consisting of small raised nubbin; margin with very shallow posteromedian inflection in posterior transverse profile, lateral portions broadly ventrally bowed, median portion gently dorsally flexed; doublure broad, exactly underlying dorsally expressed bor- der with inner margin aligned with distal expression of pleural bands and furrows, strongly ventrally convex over most of course with very prominent raised line sculpture, slightly flatter with shallow concave region laterally behind articulating facet and anterolateral articulating socket.

Ontogeny. Although well preserved, the available sample size of A. laikaae is relatively small, so knowledge of holaspid ontogeny is correspondingly limited. In the cranidium, there does seem to be a trend toward stronger expression of the glabellar lobes and furrows. The smallest specimen (Pl. 3, fig. 2) has them barely differentiated whereas the largest (Pl. 3, fig. 12) has very well expressed furrows and subtly independently inflated lobes, and the second largest (Pl. 3, fig. 3) also shows stronger development of these features. The axial furrows also appear to become more strongly anteriorly divergent, and the frontal lobe of the glabella somewhat broader, in the largest specimens.

Changes in the librigena involve gradual effacement of the fine tubercles on the adaxial half of the field, though they remain expressed even in the largest specimens. There is a trend towards slight expansion of the width of the field, particularly earlier in ontogeny. There are no obvious ontogenetic changes among the available pygidia, but they are all fairly close to one another in size.

Discussion. Assignment of a hypostome to Aponileus laikaae is somewhat tentative, and is based on exclusion and the morphological correspondence between the robust raised line sculpture and the dorsal raised line sculpture of other sclerites. Several other bathyurids occur with A. laikaae , including undescribed species of Acidiphorus , Strigigenalis , Benthamaspis , and a species which appears to represent early Bathyurus Billings, 1859 . Bathyurid hypostomes are generally not well known, but the morphology of the present example is markedly different from those we have definitely associated with Acidiphorus group taxa, such as Acidiphorus and Strigigenalis , in our collections. Benthamaspis is small and very rare in the collection and the hypostome corresponds poorly with its effaced morphology. Of the available options, the only significant alternative is the apparent Bathyurus , as the hypostome compares closely with that assigned by Tremblay and Westrop (1991, fig. 9.35, 9.46) to B. angustus Ross, 1970 . Despite the similarity in general proportions, however, we consider our hypostome more likely to belong to A. laikaae . The hypostome is clearly conterminant, with a suture along its anterior edge. This is true also of those assigned by Tremblay and Westrop. Bathyurus angustus , like most species of Bathyurus , lacks a preglabellar field in mature specimens. The early species of Bathyurus occurring with A. laikaae , however, retains a fairly long preglabellar field in mature holaspides. This is in conflict with a secondarily docked hypostome, whereas A. laikaae completely lacks a preglabellar field and would be expected to have a conterminant hypostome. Secondarily conterminant bathyurid hypostomes may share a generally similar morphology, as that assigned by Tremblay and Westrop (1991, fig. 15.11, 15.12) to Psephosthenaspis ellipsepyga (Tremblay and Westrop) is also very similar to that of species of Bathyurus and the specimen assigned here to A. laikaae .

Aponileus laikaae is the oldest known species of the genus, and has the most spectacularly developed sculpture. Much of the external surface of the cranidium ( Fig. 5.1 ), librigena ( Fig. 5.4 ), thoracic segments ( Fig. 5.2 ), and pygidium ( Fig. 5.3 ) is covered with strongly raised, scrobiculate lines. The lines run together from multiple directions to form raised tubercles, which seen in magnification are star-shaped. The well expressed tuberculate sculpture and lack of effacement distinguish the species from all other members of the genus. Most potential sister taxa of Aponileus are strongly tuberculate and non-effaced, and A. laikaae is retrieved as the basal-most known species of the genus ( Fig. 3 ). It differs from the effaced group including the type species further in the possession of a genal spine. It differs from A. belkaae in the possession of a somewhat shorter genal spine, tubercles on the adaxial part of the librigenal field, finer scrobiculate lines on the librigenal field, anteriorly shallower librigenal lateral bor- der furrow, longer, narrower, pygidium with a small posteromedian spine retained as a nub in largest specimens versus totally effaced through ontogeny, four versus three pygidial axial rings, and well impressed versus posteriorly effaced pygidial pleural regions.

Figures

FIGURE 3. Single most parsimonious tree resulting from analysis of the matrix of Table 1. Length = 20; c.i. = 0.95; r.i. = 0.95. Open circles are characters containing homoplasy. Character optimizations are shown carried out under the ACCTRAN assumption. Upper values at nodes are Bremer support values. Lower values at nodes are bootstrapping GC scores.

FIGURE 5. Detail views of star-shaped tubercles and anastomosing line sculpture of Aponileus laikaae n. sp. 1. Medial part of glabella of holotype, SUI 129234 (see Pl. 3, figs 1, 4, 8, 10), x40. 2. Pleural region of thoracic segment, SUI 129246 (see Pl. 4, figs 10–13), x30. 3. Pygidial axis, SUI 129252 (see Pl. 5, figs 2, 3, 5, 7, 8), x40. 4. Librigenal field, SUI 129244 (see Pl. 4, fig. 8), x40.