Protopliomerella stegneri, Adrain, 2011

Adrain, Jonathan M., 2011, Revision of the Lower Ordovician (lower Floian; Tulean) pliomerid trilobite Protopliomerella, with new species from the Great Basin, western USA 3144, Zootaxa 3144, pp. 1-113 : 13-16

Protopliomerella stegneri n. sp.

( Plates 9–13)

1973 Hintzeia celsaora (Ross) ; Demeter, p. 58, pl. 3, figs 9, 10, 14 (only; pl. 3, figs 11–13 =? Hintzeia celsaora ).

Material. Holotype, cranidium, SUI 126054 , from Section G 138–139 T m, and assigned specimens SUI 126052 , 126053 , 126055–126095 , from Section G 138–139 T m and 142–144.5 T m, Fillmore Formation ( lower Floian ; Tulean ; unzoned strata beneath Psalikilopsis cuspidicauda Zone ), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Wallace Stegner.

Diagnosis. Anterior border moderately broad; S4 barely impressed; hypostomal suture broad, with anterior wings long and much wider than shoulders; posterior section of librigenal field fairly wide; pygidial pleural spines well separated.

Description. Cranidium relatively short and wide, with sagittal length 48.4% (45.6–50.6%) maximum width across genal angles, highly vaulted (tr.) mainly because of strongly downturned fixigenae, axis moderately vaulted, cranidium covered in dense, finely granulose sculpture; anterior border moderately broad, short, gently anteriorly bowed, moderately inflated, densely granulose; doublure expressed as anterior face of border, exposes ventral half of inner wall of border, with very shallow median rostral suture arc of about 1/3 total border width; anterior border furrow very short, deep, incised, somewhat anteroventrally directed underneath posterior edge of anterior border, course gently anteriorly bowed over broad glabellar section, roughly transverse on narrow lateral sections in front of palpebro-ocular ridges, with deeper, longer apodemal pits at junctions with axial furrows; glabella relatively short and wide, gradually anteriorly tapered, with maximum width across L1 100.4% (94.7–106.5%) sagittal length, mildly vaulted (sag., tr.), with convexity highest posteriorly and falling anteriorly, with three well defined independently inflated lateral lobes, and with sculpture of fine granules, somewhat effaced in centers of L1–L3, denser medially, with lobes outlined by slightly larger granules on edges of furrows; L2 and L3 very similar, subrectangular, anterolaterally oriented at about 20º above horizontal, with L2 slightly longer and wider, L1 smaller, narrower, subtriangular, LF short, narrow, wedge-shaped, with very narrow, faint trace of S4 diverging anteromedially from S3 suggesting L4 (better visible on larger specimens, e.g., Pl. 9, figs 3, 21), S1 and S2 moderately long, very deep, with L1 slightly longer with inner end slightly posteriorly elongated, S3 shorter and shallower; SO short and deep medially, longer and deeper laterally in apodemal pits behind mid-L1, then constricted toward axial furrow, abaxial portions lined by small granules on margins of L1 and LO; LO wide, somewhat longer medially and tapered laterally, with very shallowly W-shaped anterior margin indented by apodemal pits in SO, moderately inflated, highest just posterior from mid-length (lateral view), densely granulose, with small median node and slightly larger granules on lateral margins; doublure fairly short, laterally tapered, lens-shaped; axial furrows deep, fairly narrow, wider at triangular junctions with glabellar furrows, anteriorly convergent along LO, then moderately laterally bowed around base of L1, then gradually anteriorly convergent; palpebro-ocular ridges fairly long, reaching from anterior border to anterior of L2, narrow, arcuate, tapered anteriorly and posteriorly, with widest point even with mid-L3, dorsolaterally raised (anterior view); palpebro-ocular furrows arcuate to gently sigmoid, very short, deep over most of course, shallow anteriorly and far posteriorly, anteromedially directed at about 45º above horizontal; interocular fixigenae short, fairly narrow, tapered anteriorly to point at junction of glabella and palpebro-ocular ridge, roughly equilaterally triangular; posterior fixigenae slightly longer, very wide, with broadly anteriorly curved anterior margin, very strongly downturned from fulcrum, all areas of fixigenae with dense granulose sculpture and slightly less densely concentrated small pits; posterior border furrow roughly transverse until genal angle, then anteriorly curved (lateral view), deep, short, constricted adaxially, longer over most of course, then tapered anteriorly after genal angle; posterior border moderately long, expanded laterally toward maximum length at genal angle of not quite double adaxial length, then abruptly tapered to end, moderately inflated, granulose, with small nubby spine at genal angle; doublure very short adaxially, rotated posterodorsally into articulating device near axis, gradually expanded laterally to maximum just before genal angle, and terminated by cut of facial suture at and slightly anterior from genal angle.

Rostral plate unknown.

Hypostome long and narrow, widest across anterior wings, with width across shoulders 68.6% (65.9, 71.2%) sagittal length; hypostomal suture broad, very shallowly arcuate to nearly transverse; anterior border extremely short medially, abruptly expanded laterally into large triangular anterior wings of approximately equal width and length with small, deep wing process pits, densely covered in granules; anterior border furrow strongly anteriorly bowed, short, deep, incised medially, nearly effaced posterolaterally toward meeting with lateral border furrows; middle body long, narrow, ovoid; anterior lobe large, elongate, ovoid, about 80% total length of middle body, strongly ventrally inflated, with inflation decreasing slightly posteriorly, with dense sculpture of small tubercles over at least anterior half of lobe (entire lobe in some specimens); lateral branches of middle body furrow narrow, deep, incised, posteromedially sloped and gently laterally bowed in course, intersect lateral border furrows even with lateral notch, poorly impressed medially (longer and much shallower) on most specimens, but strongly posteriorly curved; posterior lobe of middle body U-shaped, with long, slender, anteriorly tapered arms and short, narrow midsection, lobe moderately inflated, effaced; lateral border furrows narrow, deep except over tips of lateral branches of posterior lobe of middle body and along anterior wings, incised, somewhat laterally bowed posterolaterally, subparallel for most of course and then anteriorly convergent with anterior border furrow; lateral border strongly downturned and inflated, narrow, gradually expanded posteriorly to maximum width at posterolateral corners, with dense sculpture of coarse granules and some anastomosing ridges, and with small nubby spines (just rounded angles in some specimens) at shoulders and slightly posterior from shoulders, and a longer, thicker, more pointed spine at posterolateral corners; posterior border furrow long, shallow, ill-defined, strongly posteriorly convex in course; posterior border long, fairly wide, moderately steeply ventrally angled (lateral view), with background sculpture like that of lateral border, and with small, broad-based, triangular median spine; doublure short and narrow, but reaches to lateral border furrow and about halfway to posterior border furrow, with anterior parts turned outward at lateral notch to form anterior wings, doublure moderately strongly upturned, with long, shallow lateral notch.

Librigena long and moderately wide, laterally convex but not well curved; anterior branch of facial suture short along eye and field, meets lateral border at slightly greater than 90º, branch along anterior projection of lateral bor- der longer and strongly downturned; posterior branch of facial suture very long, with two major changes of slope, the first at about half length of field, changing from angle of about 45º to very gentle slope less than 10º, and the second at junction with lateral border, changing to posterodorsal curvature along posterior projection of lateral bor- der; eye elliptical, narrow and elongate, elevated above librigenal field on socle about same height as eye (ventrolateral view); librigenal field roughly wedge-shaped, fairly broad anteriorly, strongly tapered after about half field length, with width of field under midpoint of eye 22.9% (20.4–24.9%) length along lateral border furrow, moderately steeply ventrolaterally sloped, with sculpture of dense granules and fairly dense small pits concentrated under eye and near facial sutures; lateral border furrow narrow, deep, with short deeper segment at anterior end and slightly shallower toward posterior tip, curved, with slope steeper anteriorly; lateral border highly inflated, with inflation greater ventrolaterally, broad, gradually posteriorly tapered, rotated anteroventrally to form very long anterior projection with blunt termination, posterior projection short, triangular, strongly posteriorly tapered to point, with inner face of doublure protruding, border with dense sculpture of granules slightly coarser than those of field, but effaced far ventrolaterally; doublure wide, posteriorly tapered, with slight protrusion above tip in external view, bluntly terminated anteriorly even with edge of librigenal field, rotated anteroventrally outward into anterior projection of border.

Total number of thoracic segments unknown; segments highly arched axially and pleurally (varies somewhat with thoracic position), short and broad, with width of axis 34.4% (33.5, 35.2%) width across posterior pleural band, with dense finely granulose sculpture all over, including in furrows; articulating half ring short, laterally tapered, semilunate, anteriorly upturned; articulating furrow deep, long medially, sharply constricted and deeper ventrolaterally, broadly arcuate and posteriorly convex in course; axial ring narrow, about same width as inner pleurae (may change depending on thoracic position), short, transverse medially with shallowly anterolaterally curved extremities, well inflated, slightly higher posteriorly; doublure short, laterally tapered, with fine transverse ridge at midlength; axial furrows fairly narrow, widest at junction with pleural furrow, deep, anteriorly convergent; inner pleurae and outer pleurae of roughly equal widths (may vary with thoracic position), inner pleurae nearly horizontal (anterior view), joined to steeply sloped outer pleurae by fulcral angle of about 115º; anterior pleural band very short, broad, uninflated, with extremely short articulating tongue on anterior margin set off posteriorly by incised furrow; pleural furrow very short and deep, slightly shallower near adaxial and abaxial ends, transverse for most of course, anteriorly curved at ventrolateral tip; posterior pleural band about twice length of anterior band dorsally, expanded ventrolaterally toward spine, broad, well inflated, higher near axis, laterally flattened and anterolaterally rotated to form broad-based spine, spine separated into two prongs by sharp notch in some specimens (Pl. 11, fig. 22); doublure on inner pleurae posterodorsally twisted into articulating ledge on posterior margin, long and wide ventrolaterally over spine, with deep notch at pleural furrow.

Pygidium highly vaulted pleurally and axially, composed of five segments and terminal piece, relatively short and wide, with width across tips of anterior pleural band of first segment 124.2% (117.7–133.1%) sagittal length excluding articulating half ring and free tips of spines, and with dense finely granulose sculpture; articulating half ring broad, short, laterally tapered; articulating furrow long and deep; axis conical, broad anteriorly, strongly posteriorly tapered to point at tip of terminal piece, highly vaulted anteriorly, decreasing posteriorly to barely inflated terminal piece; axial rings short and broad, with fifth ring a little shorter and slightly greater than half width of first, rings highly independently inflated, lined on all margins with small tubercles and granules, with small tubercles or pitted tubercles medially; inter-ring furrows long medially, shorter ventrolaterally into apodemal pits, deep, very deep in pits; terminal piece triangular, broad anteriorly, strongly tapered, long, with narrow posterior section extending almost to base of pygidial spines on some specimens (posterior view), mildly inflated, with impression of two short furrows projecting from anterior margin; axial furrows fairly narrow, wider at intersections with interpleural furrows, deep, widely anteriorly divergent, strong over fifth segment, then merged with fifth interpleural furrows along terminal piece; anterior pleural band present only on first segment, short, as wide as posterior band excluding its spine, strongly posterolaterally curved, with slightly flared tips (dorsal view), and with extremely short articulating tongue on anterior edge (best seen in lateral view, e.g., Pl. 12, fig. 7; Pl. 13, fig. 5), and small anteriorly directed hook structure at adaxial tip; pleural furrow short, deep, incised; posterior pleural bands long, broad, strongly independently inflated, increasingly backturned, with pleurae 1–4 approaching parallel and fifth pleurae medially convergent, granulose, with larger granules concentrated on margins of pleurae, and with small pits (some with slightly raised rims) on center of pleurae, and with large pleural spine; spines fairly short (posterior view), broad, tapered to blunt point, somewhat laterally flattened and blade-like, well separated at tips, projecting posteroventrally with gently curved ventral margin (lateral view) and slightly raised ridge at margin; interpleural furrows moderately long, anterior furrows very deep, furrows 4–5 relatively shallow but still well impressed, strongly backturned like pleurae, with 5th furrows posteriorly convergent into single furrow behind terminal piece; pygidial border located ventrally, formed of fused spine bases, with slightly raised rim anterolaterally, grading into doublure medially; doublure roughly triangular, moderately long medially and strongly anterolaterally tapered.

Ontogeny. Protopliomerella stegneri is not as well known as most other species of the genus. The small sample size and similar sizes of individual specimens hinder discussion of ontogenetic changes. However, it is possible to see that the glabella narrows and elongates slightly, all the cranidial furrows deepen, the palpebro-ocular ridges increase in anterolateral curvature, the coarseness of the pits and granules increases, and the genal angle widens. The hypostomes are very close in size, but it appears that larger hypostomes have coarser middle body tubercles and shorter lateral and posterior border spines. Librigenal ontogenetic changes include widening of the field, with a relative decrease in width of the lateral border, and with a less marked change in slope of the posterior branch of the facial suture. Thoracic segments are too poorly known to be informative. The pygidium elongates slightly, the axial ring sculpture is reduced, the inter-ring and interpleural furrows lengthen and deepen, and the terminal piece develops impressions.

Discussion. Two damaged hypostomes (Pl. 11, figs 11, 15) appear to have malformed lateral and posterior borders, although the deformation could be breakage. Demeter (1973, pl. 3, figs. 9, 10, 14) figured two pygidia and a cranidium of P. stegneri as Hintzeia celsaora .

Protopliomerella stegneri is most morphologically similar to P. kerouaci , but can be distinguished by its wider anterior border, shorter and wider glabella, weakly developed S4, longer and more posterolaterally located palpebro-ocular ridges; a hypostome with much wider anterior wings, shorter border spines, shorter posterior lobe of the middle body, and coarser, more sparsely spaced tubercles on the anterior lobe of the middle body; a slightly shorter and much wider librigena with a lower eye and less raised eye socle area, longer anterior branch of the facial suture along the field, and less pitted field; and a relatively shorter and wider pygidium with a less tapered posterior margin, less posteromedially directed pleurae, wider and slightly more separated pleural spine tips, a less complex terminal piece impression, and finer sculpture.

Protopliomerella stegneri is stratigraphically closest to P. bowlesi , but the latter taxon possesses a narrower anterior border, a considerably longer and narrower glabella with a much more clearly impressed S4, narrower interocular fixigenae and more anteriorly located palpebro-ocular ridges, more steeply sloped fixigenae; a librigena with a more evenly sloped posterior branch of the facial suture and a narrower librigenal field, particularly posteriorly; and a distinctive six-segmented pygidium with less inflated axial rings and pleurae, a smaller, narrower terminal piece, and narrower, much more closely spaced pleural spines.

Protopliomerella stegneri is compared to P. seegeri in the discussion of the latter species.