Maurotarion chrysion, Adrain, Jonathan M., 2009

Adrain, Jonathan M., 2009, New and revised species of the aulacopleurid trilobite Maurotarion from the Lower Devonian (Pragian) of Nevada, Zootaxa 2215, pp. 1-23 : 11-15

Maurotarion chrysion n. sp.

(Figs 5, 6.1–6.7, 6.9, 6.11, 6.12)

Type material. Holotype, dorsal exoskeleton, SUI 109036 (Figure 5.1, 5.3, 5.4, 5.5., 5.8, 5.10), from locality CR-G(a), Wenban Limestone (Pragian), Cortez Gold Mine haul road, southern Cortez Mountains, Eureka County, Nevada, USA. Assigned specimens SUI 109037, 109039, 109044, 109045, from locality CR-G(a), SUI 109038, 109040-109042, from locality CR-G(b).

Etymology. Greek chrysion , gold, in reference to the Cortez Gold Mine.

Diagnosis. Dorsal sculpture mostly lacking, except for faint tubercle rows on thoracic axial rings; glabella relatively narrow and L1 small; genal spines of only moderate length; thorax of 12 segments; pygidium with only two clearly expressed rings and segments, though third likely present but effaced.

Description. Cranidial measurements were made only on the holotype specimen (Fig. 5.1), as the others are not completely enough preserved. Cranidium with sagittal length 80.8% width across midlength of palpebral lobes; width across anterior border 79.3% width across midlength of palpebral lobes; width across midlength of palpebral lobes 74.1% maximum cranidial width across posterior projections; anterior border with sagittal length 14.3% that of cranidium, lacking dorsal sculpture, flat over rear two thirds of area, evenly curved in sagittal section in anterior third, very faint sculpture of fine raised lines on anteroventral aspect near margin; anterior border furrow shallow and relatively short (sag., exsag.), similar in depth along course except slightly shallowed near facial suture; anterior margin and anterior border furrow with even anterior curvature, not V-shaped; preglabellar field long, sagittal length 19.8% that of cranidium, with moderate independent dorsal inflation in sagittal profile; preglabellar field and frontal area entirely lacking sculpture; anterior sections of facial suture strongly anteriorly divergent in front of palpebral lobes, maximum point of divergence opposite rear third of anterior border; eye ridge discernible only as a break in slope between frontal area and interocular fixigena; interocular fixigena relatively narrow, narrower than palpebral lobe, sloped strongly downward toward axial furrow, lacking sculpture; palpebral furrow present only as a slight break in slope anteriorly and posteriorly; palpebral lobe with exsagittal length 31.1% that of sagittal length of cranidium, lateral margin strongly laterally convex, lacking dorsal sculpture except for faint pit set at half length and slightly nearer to lateral margin than to adaxial edge of lobe, lobe held nearly horizontal; axial furrows moderately wide and deep, very slightly wider posteriorly, deflected weakly around L1, gently anteriorly convergent anterior to L1, running without interruption into strongly anteriorly bowed preglabellar furrow of similar depth; glabella with sagittal length 96.6% maximum width across rear of L1; L1 of moderate size, adaxial half teardrop shaped as in congeners but lateral margin slightly dimpled; S1 narrower than axial furrow, running uninterrupted from axial furrow to SO to fully isolate L1, junction with SO abrupt, not expanded into depressed area; SO slightly posteriorly bowed medially, deflected posteriorly behind L1 but not as strongly as many congeners, moderately deep and short (sag., exsag.); median glabellar lobe lacking sculpture; L2 with slight independent inflation, S2, L3, and S3 not discernible dorsally, but defined ventrally (Fig. 5.8); LO with sagittal length 13.2% that of cranidium, slightly longer exsagittally near junction of SO and S1, shorter behind L1, with faint median node at half length and very faint transverse row of small tubercles nearly posterior margin; posterior fixigena slightly inflated, sloped strongly posteriorly from rear of palpebral lobe, lacking dorsal sculpture, extended as moderately long (exsag.) strip along posterior projection; posterior border furrow short (exsag.) and incised, with nearly transversely straight course, only slightly posteriorly deflected at fulcrum; posterior border very short proximally, lengthened steadily distal to fulcrum, lacking sculpture, with weak dorsal inflation; doublure obscure due to articulation; S2 and S3 with obvious ventral expression as small ridges; fossula not obvious.

Librigenal field with width at half length of eye 27.2% exsagittal length; eye with exsagittal length 45.1% that of field; socle expressed only as very faint anterior lobe (Fig. 6.4) and narrow, very shallow furrow around base of visual surface; field completely lacking sculpture; lateral border furrow with gentle lateral curvature, very shallow, especially anteriorly and posteriorly, little more than a break in slope between field and border; lateral border moderately wide and inflated, slightly narrower anteriorly, lacking sculpture dorsally, but with many closely set fine raised lines laterally and ventrolaterally, with lateral margin slightly more strongly laterally bowed than lateral border furrow; posterior border furrow much deeper and more incised than lateral border furrow, with slightly sinuous oblique course; posterior border about as long as rear of lateral border is wide, lacking sculpture, forming shallow angle with genal spine; genal spine relatively short, sharply tapered, extending posteriorly to terminate opposite fourth thoracic segment, lacking dorsal sculpture; faint furrow running posteriorly from junction of lateral and posterior border furrows discernible on anterior two thirds of spine; anterior projection relatively short; doublure with larger and more widely spaced raised lines than ventrolateral aspect of lateral border, lines become effaced near base of genal spine, not visible on ventral aspect of spine, doublure turned sharply upward and inward adaxially, forming unsculptured region terminated by inner margin aligned beneath lateral border furrow.

Rostral plate known only from articulated holotype (Fig. 5.8); broad, with wide anterior extent, posterior margin not reduced to a point but occupying narrow, anteriorly arced portion of adaxial doublural margin; connective sutures running obliquely, bowed slightly laterally; ventral surface with sculpture of closely spaced fine raised lines contiguous with those on librigenal doublure.

FIGURE 5. Maurotarion chrysion n. sp., from the Wenban Limestone (Pragian), Cortez Gold Mine haul road, southern Cortez Mountains, Eureka County, Nevada, USA. 1, 3, 4, 5, 8, 10. Dorsal exoskeleton (with portion of thorax and pygidium of different individual attached), holotype, SUI 109036, dorsal, dorsal pygidial, anterior, left lateral, ventral, and oblique views, x5 (CR-G(a)). 2, 6, 7. Dorsal exoskeleton, SUI 109037, dorsal, left lateral, and posterior views, x7.5 (CR-G(a)). 9, 11, 12. Pygidium, SUI 109038, right lateral, dorsal, and posterior views, x10 (CR-G(b)).

Hypostome not recovered.

Thorax of 12 segments; widest across fifth segment; axis occupying 34% width of thorax across first segment, 30% across fifth segment, and 32% across 12th segment; fulcrum set at about half distance distally on pleurae of anterior and middle segments, closer to axis on posterior segments; axial ring of first three segments short (sag.; exsag.) those of posterior nine segments longer, and of approximatley similar length to each other; first four axial rings with faint transverse row of small tubercles aligned near posterior margin, tubercle row possibly poorly expressed on more posterior segments but difficult to discern; axial rings with very shallow W shape; ring furrow (seen on fourth segment of holotype, Fig. 5.1) short, shallow and finely incised; axial furrow shallow; anterior pleural band shorter (exsag.) than posterior band; both bands lacking dorsal sculpture; pleural tips bluntly terminated and subquadrate; pleural furrow deepest across fulcrum, very short (exsag.) and finely incised distally, running nearly to pleural tip, deflected slightly posteriorly behind facet.

FIGURE 6. 1–7, 9, 11, 12, Maurotarion chrysion n. sp., from the Wenban Limestone (Pragian), Cortez Gold Mine haul road, southern Cortez Mountains, Eureka County, Nevada, USA. 1, 5, 7. Cranidium, SUI 109039, dorsal, anterior, and right lateral views, x10 (CR-G(a)). 2, 6. Cranidium, SUI 109040, dorsal and left lateral views, x7.5 (CR-G(b)). 3. Left librigena, SUI 109041, external view, x7.5 (CR-G(b)). 4. Left librigena, SUI 109042, external view, x7.5 (CR-G(b)). 9, 12. Left librigena, SUI 109044, external and internal views, x10 (CR-G(a)). 11. Right librigena, SUI 109045, external view, x10 (CR-G(a)). 8, 10, 13, Maurotarion sp. indet. from the Wenban Limestone (Pragian), locality CR-C, Cortez Gold Mine haul road, southern Cortez Mountains, Eureka County, Nevada, USA, right librigena, SUI 109043, ventrolateral, external, and internal views, x10.

Pygidium with sagittal length 43.7% maximum width; axis with anterior width 36.2% maximum pygidial width; only two axial rings and pleural furrows clearly expressed, but a third is likely present though effaced and merged with a terminal piece; articulating half ring large and crescentic; dorsal sculpture of fine granules, nearly smooth on posteromedian surfaces, tuberculate sculpture completely absent; first and second ring furrows expressed only as distal notches, posterior ones not expressed; axial furrow shallow, strongly deflected around first two segments, but meeting posterior to fully circumscribe axis; fulcrum set near to axis; anterior and posterior pleural bands expressed fully only on first segment, subequal in length; first pleural furrow well expressed and finely incised, turned posterolaterally at fulcrum to run subparallel to anterior pygidial margin; first interpleural furrow shallower than first pleural furrow, but complete from axial furrow across border to margin; second pleural furrow faint but discernible; second interpleural furrow very faint; broad, flattened border present, crossed only by first interpleural furrow; border with median inflection in dorsal view, bowed dorsally behind axis in posterior view; doublure broader anteriorly that posteriorly or medially, with sculpture of subparallel fine raised lines.

Discussion. Maurotarion chrysion occurs together with M. wenbanense at locality CR-G, but the species are very distinct and there is little chance of confusion or misassociation, particularly since each is represented by articulated specimens. The species share a relatively short median part of the anterior border and shallowing of the librigenal lateral border furrow in front of the genal spine with the posterior border furrow extended along the spine. However, there are many differences. Maurotarion chrysion lacks dorsal tuberculate sculpture in large specimens, its glabella is relatively smaller, its genal spine bsae is not inflated and the spine is smaller, its thorax has 12 versus 13 segments and has a relatively narrower axis, and its pygidium has three (only two well expressed) versus four axial rings.

Maurotarion chrysion differs from M. fooi n. sp. in the complete lack of cranidial tuberculate sculpture in large specimens, versus presence on L1, a longer preglabellar field and broader frontal areas, and librigenae lacking expression of the socle, with a broader field, particularly posteriorly, narrower lateral border, and much shorter genal spine. The only other well known Laurentian Lower Devonian species is M. axitiosum ( Campbell, 1977) , from the Lochkovian Haragan Formation of Oklahoma ( Campbell, 1977, pl. 3, figs. 2–6). Maurotarion chrysion differs from the slightly older M. axitiosum in the absence versus presence of tuberculate sculpture on the cranidium and librigenal field; the possession of larger L1; a much longer preglabellar field; a shallower anterior border and librigenal lateral border; a librigenal lateral border that is shallowed in front of the genal spine, versus merged with the posterior border in front of the spine; a much shorter genal spine; 12 versus 11 thoracic segments; and narrower thorax; very subdued tubercle rows on the axial rings of some segments only versus stronger tubercle rows on the axial ring and posterior pleural bands of most segments of most specimens; and a pygidium with only two versus three fully expressed rings.