Hintzeia firmimarginis ( Hintze, 1953 )

Mcadams, Neo E. B. & Adrain, Jonathan M., 2011, Systematics of the Lower Ordovician pliomerid trilobite Hintzeia, with species from the Great Basin, western USA, Zootaxa 2910, pp. 1-45 : 7-10

Hintzeia firmimarginis ( Hintze, 1953)

Plates 1–4

1953 Protopliomerops firmimarginis Hintze , p. 208, pl. 22, figs 1–8.

1953 Protopliomerops aemula Hintze (= Protopliomerops contracta Ross, 1951 ); Hintze, pl. 22, fig. 9 (only; non pl.

22, figs 13–17 = Hintzeia celsaora Ross ).

1973 Hintzeia firmimarginis (Hintze) ; Demeter, p. 56.

1997 Hintzeia firmimarginis (Hintze) ; Ross et al., p. 19.

2005 Hintzeia firmimarginis (Hintze) ; Simpson et al., p. 538.

2009 Hintzeia firmimarginis (Hintze) ; Adrain et al., p. 557, fig. 8B, E.

Material. Assigned specimens SUI 115119, 115120, 125500-125595, from Section G 99.3 m and G 99T-A m, Fillmore Formation (Tulean; Psalikilus spinosum Zone ), southern Confusion Range, Ibex area, Millard County, western Utah.

Diagnosis. Cranidium with relatively long frontal areas; anterior border depressed medially over arcuate anterior curve in posterior margin (anterior view); librigena with very wide field and smoothly sloping posterior branch of facial suture along field; pygidium with relatively narrow axis and relatively wide terminal piece, with pleurae and spines dosoventrally flattened, spines long, widely splayed, with slightly dorsally recurved tips.

Description. Cranidium sub-semicircular in outline, long and broad, with sagittal length 50.3% (45.6–52.4%) maximum width across genal angles, highly vaulted (sag., tr.), with very strongly downturned fixigenae, and with dense sculpture of tiny granules all over; anterior border long, wide arc, with broadly anteriorly bowed anterior margin, posterior margin transverse in front of fixigenae and strongly arched anteriorly from glabella , forming arcuate curve slightly less than width of LF, border very strongly dorsally inflated, highest at lateral edges of inner margin arc (anterior view), decreased medially and exsagittally, densely covered in slightly coarser but still very fine granules; doublure mainly expressed as short anterior face of border, leaving bottom half of anterior surface of posterior wall of border exposed in anterior view, with only a short doublural rim ventrally; anterior border furrow (in dorsal view) short, slightly longer medially and near apodemal pits at meeting with axial furrows, overhung by border, extremely deep, broadly arcuate with lateral transverse sections similar to those of border; glabella moderately inflated (sag., tr.), long, moderately narrow, nearly parallel-sided, very slightly anteriorly tapered, subrectangular, with maximum width across L1 93.4% (88.6–95.5%) sagittal length, and with rounded anterior margin (ventral view); lateral lobes slightly independently inflated, separated by short, deeply incised sulci directed anterolaterally at about 20–30° and with slightly backturned inner ends, L1 longest, slightly narrower than L2 and L3, subtriangular, L2 and L3 subrectangular, about 1/3 total glabellar width, frontal lobe wide, short, wedge-shaped; SO shallowly W-shaped, short and moderately deep medially, lengthened laterally over very deep apodemal pits; LO long medially, tapered laterally from midpoint of apodemal pits, moderately inflated, with small median tubercle located slightly posterior of midlength; doublure lens-shaped, fairly long, reaching nearly to SO, slightly laterally tapered, smooth; axial furrows narrow, deep, confluent with all cranidial furrows except palpebrals, roughly parallel-sided, but bowed around edges of LO and expanded back laterally at base of L1; frontal areas strongly downturned (lateral view), long (anterior view), wide, triangular, with scattered small pits; ocular ridges directed posterolaterally at about 50° below transverse, weakly inflated from axial furrows to about mid-width of frontal areas, then increasingly strongly inflated posterolaterally, with slightly coarser granulose sculpture than fixigena; palpebral furrows long, narrow, sigmoid, very shallow until about mid-length of ocular ridge, then deep and incised; palpebral lobes short, narrow, arcuate, slightly dorsolaterally raised, but lower than arch of interocular fixigena (anterior and lateral views), located with furthest exsagittal point on arc of lobe even with abaxial end of S2; interocular fixigena triangular, narrower anteriorly and expanded posteriorly, broadly arched, with steep dropoff into axial furrows; posterior fixigena approximately same length as interocular fixigena and slightly more than twice as wide, very strongly downturned beginning at point even with posterolateral edge of palpebral lobe (anterior view), with gently anteriorly convex anterior margin and smoothly curved lateral margin, entire fixigena with scattered small pits except on very slightly raised ridge following course of posterior border furrow; posterior bor- der furrow short, ventrolaterally tapered, deep, incised, transverse until genal angle, then smoothly anteriorly curved; posterior border shortest adaxially, expanded ventrolaterally until maximum length of about double reached at genal angle, then very strongly anteriorly tapered to a fine point, with small, stubby spine at genal angle, and with extremely short articulating tongue running along transverse section; doublure of inner section expressed as the articulating tongue, then curved inward and expanded into short doublure along genal angle.

Hypostome long, narrow, widest across anterior wings, subrectangular, with width across shoulders 71.5% (69.2–73.0%) sagittal length; hypostomal suture broad, very shallowly arcuate; anterior border vanishingly short medially, longer laterally and merged into large, nearly equilaterally triangular anterior wings with small, deep wing process pits; anterior border furrow strongly anteriorly bowed, very short, moderately deep medially, slightly laterally shallowed into merger with lateral branches of middle body furrow at lateral notch; middle body long, narrow, ovoid, with long, posteriorly tapered, strongly ventrally inflated (sag., tr.) anterior lobe, lobe sculpture of small, very closely spaced granules (coarser anteriorly) rimming central circular effaced area; middle body furrow with narrow, incised, posteriorly convergent lateral sections and barely impressed posteriorly bowed median section; posterior lobe short, U-shaped, with strongly anteriorly tapered lateral sections terminating at junction of middle body and lateral border furrows, weakly inflated, effaced; lateral border furrows moderately narrow, deep, but shallow over lateral branches of posterior lobe; posterior border furrow short, shallow, ill-defined; lateral border narrow at wide, shallow lateral notch, expanded to moderately wide at shoulders, constricted just posterior from shoulders, then expanded to maximum width a little posterior from midway between shoulders and posterolateral corners, moderately inflated, gently downturned (lateral and posterior views), with dense granulose sculpture and small stubby spines at posterolateral corners and at maximum width; posterior border long, moderately downturned, with somewhat effaced area near posterior border furrow, and dense granules posteriorly; doublure moderately long, reaches nearly to posterior border furrow, lateral doublure slightly narrower, effaced.

Rostral plate unknown.

Librigena approximately triangular in outline, with width of librigenal field under midpoint of eye 42.9% (38.3–48.0%) length of lateral border furrow; anterior branch of facial suture fairly short and subvertical to gently posterodorsally sloped along field, long and subhorizontal to anteroventrally sloped at about 30° along anterior projection of lateral border; posterior branch of facial suture long and steeply posteroventrally sloped (about 40°) along field, short and strongly posterodorsally curved along posterior projection of border; eye small, ovoid, moderately inflated, separated from field by short, shallow furrow far posteriorly, otherwise by break in slope, with visual surface of many tiny, closely packed lenses; librigenal field wide, moderately long, subtriangular, gently convex with convexity increasing anteriorly (ventrolateral view), with very dense sculpture of minuscule granules and with more sparsely distributed small pits concentrated anteriorly and toward the eye; lateral border furrow narrow, moderately shallow, slightly ventrolaterally curved; lateral border strongly inflated, very long, wide, gradually tapered posteriorly to a wide point at tip of posterior projection, widened and rotated posteromedially near tip of very long anterior projection (ventrolateral view), with dense granulose sculpture about twice or three times coarser than that of field, but finer and sparser on anteroventral portion of anterior projection; doublure smooth, wide, slightly tapered and bluntly truncated at about 1/3 length of anterior projection, slightly tapered along curvature of posterior projection, forming point of projection, with inner surface visible as triangle in external view.

Total number of thoracic segments unknown. Segments short, wide, highly arched pleurally and axially with spinose pleurae strongly downturned at fulcrum, and with dense sculpture of very fine granules all over; articulating half ring short, longer medially and tapered laterally; articulating furrow short, deep and incised (Pl. 3, figs 1, 23), or shallow medially (Pl. 3, figs 2, 9, 10), with deep apodemal pits adaxial from axial furrows, course very shallowly U-shaped with wide median section transverse and gradual anterior divergence of lateral ends; axial ring short, moderately narrow, with width of axis 36.8% (34.5–39.5%) width across tips of anterior pleural band, subrectangular, slightly elongated and gently anteriorly divergent near axial furrows, gently inflated medially, more strongly near axial furrows; doublure very short, laterally tapered lens, smooth except for very fine transverse ridge at half length and sigmoid ridge located far laterally, leading from anterior margin to lateral termination of doublure (Pl. 3, fig. 13); axial furrows narrow, slightly widened into triangular shape at junction with pleural furrow, deep (Pl. 3, figs 1, 23) or shallow (Pl. 3, figs 2, 9, 10), subparallel along posterior pleural band, then strongly anteriorly convergent from pleural furrow; anterior pleural band very short, about half length of posterior band, wide, reaching to base of spine, gently independently inflated, with vanishingly short articulating tongue running width of band, set off posteriorly by extremely short, shallow furrow, and with small, hook-like articulating device at ventrolateral tip; pleural furrow short, slightly expanded ventrolaterally, very deep, incised; posterior pleural band wide, moderately long, slightly longer ventrolaterally toward spine, about twice as inflated as anterior band, with rounded inner margin, with very short articulating tongue on posterior margin, tapering out just past fulcrum, set off anteriorly by short, incised furrow (longer near axis; posterior views), and with long, laterally flattened, slightly anteromedially rotated (producing broad facet seen in lateral view; e.g., Pl. 3, figs 12, 16), strongly ventrally tapered, sharply pointed, ventrally directed spine; doublure short, mainly turned out into posterior articulating tongue, but recurved inward at base of spine to merge with wide inner face of spine.

Pygidium of five spinose segments and terminal piece, highly vaulted axially and pleurally (sag., tr.; anterior view), with fulcral angle of about 105°, roughly semicircular, long and wide, with sagittal length from articulating furrow 63.5% (59.8–67.2%) width across anterior pleural band of first segment, and with dense, finely granulose sculpture, and somewhat coarser granules on axis, along edges of pleurae and spines, and on pygidial border (ventral view), sculpture increasingly effaced (beginning with main parts of spines and pleurae) on larger specimens; articulating half ring very short, laterally tapered; articulating furrow short, deep (shallower medially on larger specimens), with deep apodemal pits slightly adaxial from axial furrows; axis long, wide anteriorly, strongly tapered posteriorly to point at tip of terminal piece, with width of fifth segment about half that of first segment, strongly vaulted (sag., tr.) anteriorly, with vaulting progressively decreased posteriorly; axial rings short, wide, individually moderately inflated, subrectangular with rounded edges, slightly anteriorly bowed (in dorsal view; shape changes slightly with orientation), particularly fifth ring; terminal piece small, triangular, about as long as wide, moderately inflated, just barely enclosed by pleurae; inter-ring furrows moderately long, slightly tapered laterally, longer in larger specimens, with first furrow longest, deep, shallower medially, with deep apodemal pits near axial furrows; axial furrows narrow, with wider areas at intersections with interpleural furrows, deep, impressed over each pair of pleurae, strongly anteriorly divergent, convergent at tip of terminal piece, and continued as single median furrow separating last pair of pleurae; anterior pleural band fully expressed only on first segment, slightly inflated, very short, wide, reaches to level with base of spines, effaced, with extremely short, thinner articulating tongue running width of band (best visible in lateral view, e.g., Pl. 3, fig. 30; damaged on many specimens) set off by very short, faint furrow, and with small, anteriorly curved hook-like articulating structure at base of band (lateral view, e.g., Pl. 3, fig. 30, Pl. 4, figs 8, 18; damaged on many specimens), weakly expressed on second segment of large pygidia as very short, slightly raised, granulose stripe (Pl. 3, fig. 21, Pl. 4, figs 1, 3, 11, 17; pleural furrow very short, deep, incised, slightly shallower ventrolaterally; posterior pleural bands each moderately inflated dorsally, increasing to very strongly inflated ventrolaterally (lateral view), long, wide, slightly tapered at adaxial ends and at base of spine, section beyond fulcrum laterally flattened, pleurae increasingly posteriorly directed, with fifth pair subparallel, each with widely separated, wide, tapered, bluntly pointed, slightly flattened (dorsoventrally and laterally) spine projecting posteroventrolateraly, with tips slightly recurved dorsally, from ventrolateral margin of pleurae; interpleural furrows moderately long adaxially, narrower ventrolaterally, deep, incised, increasingly posteriorly directed in course to follow pleurae; pygidial border expressed ventrally as long and wide shelf, with granulose sculpture, and with slightly inflated, effaced inner margin set off by long, fairly shallow furrow; doublure visible only in anterior view (Pl. 4, fig. 10) short, longest at median point and strongly tapered anterolaterally, smooth.

Ontogeny. All figured specimens of H. firmimarginis are probably holaspid, but in the cranidia it is possible to observe the lengthening of the anterior border and increasing development of the anterior curve in the posterior margin; the lengthening of the frontal areas; and elongation and narrowing of the glabella as cranidial size increases (cf. specimens of Pl. 1, figs 1–2 with those of Pl. 1, figs 11, 20 and Pl. 2, fig. 6). The hypostome elongates slightly; the middle body furrow shallows; the shoulders broaden; the lateral border broadens at the posterolateral corners; and the posterior border nearly doubles in length (cf. Pl. 2, figs 3–5). Few changes are observable in the librigenae, but the smaller specimens (Pl. 2, figs 24–25) have slightly narrower librigenal fields that are also slightly longer under the eye than the larger specimens, and the pits on the librigenal field are shallower. Ontogenetic changes cannot be observed in the thoracic segments due to the small sample size and variability depending on the position in the thorax. Overall, the pygidium becomes slightly longer, as do the spines; the inter-ring and interpleural furrows lengthen, especially the first interpleural furrow; the second anterior pleural band develops; the axis narrows and tapers more strongly posteriorly; and the spines splay out (cf. Pl. 3, fig. 1, Pl. 4, figs 1, 3 with Pl. 4, figs 17, 20).

Discussion. Specimens of H. firmimarginis show little variation. The cranidium of Pl. 1, fig. 2 has a slightly narrower and more tapered glabella than that of Pl. 1, fig. 13. The anterior border arc is wider in that of Pl. 1, fig. 11 compared to that of Pl. 11, fig. 12. The specimen of Pl. 1, fig. 3 has a mildly deformed right S3, which is more anteriorly positioned compared to the left furrow, posterolaterally bowed, and does not reach the anterolateral margin of the glabella . The hypostome of Pl. 2, fig. 4 is narrower and more elongate than that of Pl. 2, fig. 5, but this may be ontogenetic. The librigena of Pl. 2, fig. 22 has a wider field and wider lateral border with a shorter posterior projection than the others. Among pygidia, the length, width, and shape of the terminal piece is somewhat variable (cf. Pl. 4, figs 1, 11). The splay, taper, and degree of recurvature at the tip of the spines also vary slightly.

Hintzeia firmimarginis is compared to H. parafirmimarginis in the differential description of the latter taxon, and to and H. celsaora in its discussion section. Hintzeia plicamarginis Simpson et al., 2005 , is better documented than many trilobite taxa in terms of the number of specimens illustrated, but comparison is hindered by the small size of the images and lack of many alternate views. The orientations used by Simpson et al. (2005) also differ from those herein: their cranidia and hypostomes are tilted further anteroventrally in dorsal view, their librigena is ventrolaterally tilted compared to those illustrated herein, and the posterior view of the pygidium is tilted much further anteroventrally. Differing orientations can affect the perceived appearance of lengths and widths (e.g., the width of a librigenal field), and convexity (e.g., the shape of the arc of a transverse furrow), and must be considered carefully in differential discussions.

Hintzeia firmimarginis differs from H. plicamarginis in possessing a much wider, less anteriorly bowed, less laterally downturned (anterior view) anterior border with a wider, shallower, less depressed (anterior view) arcuate indentation in the posterior margin; a shorter anterior border furrow (medially and laterally in apodemal pits); a slightly wider glabella (across L1), with a larger L1 and narrower LF; narrower axial furrows; longer frontal areas; a more prominent ocular ridge; and shorter, wider posterior fixigenae. The hypostome is relatively shorter and wider; the anterior wings are broader and larger; the middle body is shorter and more strongly posteriorly tapered, with a shorter anterior lobe, much shallower middle furrow, longer posterior lobe, more granulose sculpture anteriorly, and without prominent pits; the lateral border is slightly wider, with the shoulders located more anteriorly; and the posterior border is longer, with a smaller, more rounded median spine. Librigenae are similar, but that of H. firmimarginis has a wider field with a more steeply sloped posterior branch of the facial suture and shallower pits; the lateral border is slightly narrower, with longer anterior and posterior projections; and the doublure is exposed as a wider, narrower triangle in external view. No thoracic segments of H. plicamarginis were illustrated. The pygidium of H. firmimarginis is wider; the pleurae are more strongly anterolaterally arched and splayed further apart; and the pleural spines are shorter, wider and less tapered, and more strongly ventrally directed, with a much shorter recurved tip.