Ibexaspis coadyi, E.B & Adrain & Karim, 2018

E. B, Neo, Adrain, Jonathan M. & Karim, Talia S., 2018, The pliomerid trilobite Ibexaspis and related new genera, with species from the Early Ordovician (Floian; Tulean, Blackhillsian) of the Great Basin, western USA, Zootaxa 4525 (1), pp. 1-152 : 15-17

Ibexaspis coadyi n. sp.

Plates 7–10

2009 Ibexaspis sp. nov. 9; Adrain et al., p. 573, fig. 18L, M.

Material. Holotype, pygidium, SUI 129830 (Pl. 10, figs 1, 4, 7, 8, 11), and assigned specimens SUI 115362 , 129805–129829 , 129831–129836 , from Section H 222.1 m, Fillmore Formation (Blackhillsian; Carolinites nevadensis Zone ), southern Confusion Range , Ibex area, Millard County, western Utah, USA.

Etymology. After Aaron Coady.

Diagnosis. Cranidial axial furrows very narrow; thoracic segments with large, dense patch (not transverse line) of tiny tubercles on median portion of articulating furrows; pygidial pleural ribs and spines relatively large; interpleural furrows narrow, nearly completely blocking view of underlying pleurae.

Description. This species is similar enough to Ibexaspis leuppi that it is described via differential discussion. Cranidia of I. coadyi differ from those of I. leuppi in being wider overall, particularly axially and across the posterior fixigenae; the glabellar sculpture is slightly coarser and the other cranidial sculpture is finer, so that the textural contrast between areas is less apparent; the anterior border is a little shorter and broader and less strongly medially depressed; the anterior border furrow is shorter, particularly exsagittally, slightly shallower, and less strongly M-shaped, with narrower transverse exsagittal sections, and is more overhung by the anterior of the glabella; the interocular fixigenae are narrower anteriorly and wider posteriorly, so that the palpebral lobes are positioned further anterolaterally; the palpebral lobes are also slightly narrower and more elongate; the glabella is broader overall and more strongly sagittally vaulted; the glabellar furrows are shorter and extend a little further medially; likewise the lateral glabellar lobes are slightly wider and less independently inflated; the axial furrows are narrower, particularly across the posterior border, and are shallower except across the posterior border; tubercles from the posterior of the glabella and anterior edge of LO overhang into SO and there are granules or tiny tubercles in SO on most specimens; LO is broader, the median node is larger and more prominent, and the tubercles on the posterior edge are longer; the posterior border is longer at the genal angle and its tubercles are coarser and more spinose posteriorly; genal spine is longer.

Hypostomes of I. coadyi are more strongly posteriorly tapered; the anterior border is more overhung by the middle body; the middle body is narrower and more posteriorly tapered, and its sculpture similar, but tubercles less clearly distinct; the posterior lobe of the middle body is slightly longer; the lateral border furrows are shallower anteriorly; the lateral border is narrower; the posterior border furrow is much shorter medially and deeper; and the posterior border is longer medially.

Librigenae differ in that those of I. coadyi are wider and shorter overall; the posterior branch of the facial suture is more steeply sloped; the ocular surface is slightly smaller; the librigenal field is wider and slightly shorter, with denser tuberculate sculpture and fewer pits and granules, and the smooth patch is smaller; the lateral border furrow is slightly narrower; the lateral border is a little wider, with a slightly longer posterior projection and slightly shorter anterior projection, and with coarser sculpture.

Thoracic segments differ mainly in that those of I. coadyi possess more densely packed, finer, slightly smaller sculpture in general, with less apparent granulose underlying sculpture; the articulating furrow has a large patch of tiny tubercles medially; and the axial furrows are slightly narrower and deeper.

Pygidia of I. coadyi are a little shorter overall; the axis is narrower, particularly anteriorly, and less vaulted; likewise the axial rings are narrower, and their sculpture is less spinose (cf. Pl. 10, fig. 1; Pl. 15, fig. 1); the terminal piece is smaller; the axial furrows are wider and shallower; the pleurae are wider; the pleural spines are a little shorter (oblique), thicker, and more strongly downturned; the pygidial border is not granulose; and small pygidia have less distinct axial nodes due to the generally coarser sculpture.

Ontogeny. Cranidia (cf. Pl. 7, fig. 1, Pl. 8, fig. 15) become broader overall, more strongly sagittally vaulted, and slightly less strongly transversely vaulted; the tuberculate sculpture becomes finer and more densely spaced (particularly noticeable on the fixigenae), with more tiny intercalating tubercles appearing on the glabella, and the spinose tubercles near the furrows become slightly shorter (cf. Pl. 7, fig. 1, Pl. 8, fig. 23); cranidial furrows all become shallower and slightly longer or wider; the anterior border elongates slightly; the palpebro-ocular ridges become differentiated into ridge and lobe, and the lobes widen; the glabella widens and becomes less strongly convex (tr.) but more strongly convex (sag.); the posterior fixigenae become longer and wider; the posterior border and LO develop a posterior fringe of elongate tubercles; and the genal spines shorten relative to overall cranidial size.

The hypostome (cf. Pl. 9, figs 15, 18) elongates and narrows overall; the lengthening occurs particularly in the anterior part of the hypostome (mainly the anterior lobe of the middle body), also resulting in elongation of the lateral notch; the tubercles become slightly larger, more closely spaced, and more effaced; the anterior wings become longer and more posteriorly directed; the lateral border furrows narrow and deepen; the posterior border furrow shortens and deepens; the middle body furrow deepens laterally; the lateral border widens slightly, but the shoulders (widest point) shift posteriorly due to elongation of the hypostome overall; and the posterior border lengthens.

The librigenae are close in size, prohibiting observation of much ontogenetic change. However, the smallest specimen (Pl. 9, fig. 2) has a relatively longer and narrower field, with a more gently sloped posterior branch of the facial suture. The thoracic segments are too similarly sized to observe any ontogenetic change.

Pygidia (cf. Pl. 10, figs 1, 25) become broader overall; the tuberculate sculpture becomes coarser and more densely spaced in general, while the prominent pair of tubercles near the axis on the pleurae of the first two segments and the median nodes on the axial rings become effaced and blend in with the rest of the sculpture; the articulating furrow lengthens and tiny tubercles develop medially on the preannulus of the first pygidial segment; the inter-ring furrows and the furrows between pleural bands lengthen and shallow; the axial furrows widen and shallow anteriorly, become very shallow over the fourth segment, and become slightly narrower and deeper along the anterior part of the terminal piece; the axis becomes a little less vaulted (lateral view); the pleural spines thicken and inflate more, except at the tips, and they spread slightly farther apart.

Discussion. Ibexaspis coadyi is compared to I. brevis in the discussion of that species. Compared to I. rupauli (cf. Pl. 7, fig. 2, Pl. 18, fig. 3), the cranidium of I. coadyi is relatively wider, and more strongly vaulted (sag., tr.), with much finer, more densely spaced sculpture; less anteriorly bowed anterior border; wider, more parallel-sided glabella with much shallower lateral furrows defining larger lobes; broader interocular fixigenae; longer, narrower palpebral lobes; broader, more strongly downturned posterior fixigenae; LO shorter and more transverse, smaller less prominent tubercles along posterior margin. The hypostome of I. rupauli is unknown for comparison. Librigenae differ primarily in the shape of the librigenal field, which is wider and much longer in I. coadyi , particularly at mid-width and along the lateral border; the librigenae of I. coadyi are wider overall; with a smaller anterior effaced region on the field; wider, deeper lateral border furrow (anteriorly); and slightly wider lateral border. The thoracic segments of I. coadyi are considerably larger than the single known thoracic segment of I. rupauli , but they seem to differ in possessing much finer sculpture and longer articulating furrows with a cluster of fine tubercles. Pygidia of I. coadyi (cf. Pl. 10, figs 3, 14, Pl. 19, fig. 2) have a broader main body, but less splayed spines, and are considerably less vaulted (sag., tr.); with shorter inter-ring furrows and furrows between pleural ribs; shorter, narrower, less vaulted axis with less strongly individually inflated axial rings; much smaller, less inflated terminal piece enclosed posteriorly by pleurae; shorter, thicker pleural spines with blunter tips; and finer, much more densely spaced sculpture.