Maurotarion wenbanense, 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 : 15-18

Maurotarion wenbanense n. sp.

(Fig. 7)

Type material. Holotype, dorsal exoskeleton, SUI 109047, (Figure 7.2, 7.3, 7.6, 7.8, 7.10, 7.13), from a talus block collected along the Cortez Gold Mine haul road, Wenban Limestone (Pragian), southern Cortez Mountains, Eureka County, Nevada, USA. Assigned specimen SUI 109046 from locality CR-G(a).

Etymology. From the Wenban Limestone.

Diagnosis. Cranidium with moderately dense tuberculate sculpture on glabella and scattered tubercles on the frontal areas; L1 large; librigena with very short genal spines, field lacking sculpture; thorax of 13 segments; prominent transverse tubercle row on rear of LO and thoracic axial rings; pygidium with three distinctly expressed axial rings, lacking dorsal sculpture.

Description. Cranidial measurements were made only on the holotype specimen (Fig. 7.2) as the other specimen is somewhat distorted and crushed. Cranidium with sagittal length 83.8% width across midlength of palpebral lobes; width across anterior border 80.2% width across midlength of palpebral lobes; moderate dorsal inflation and convexity in sagittal profile, glabella and preglabellar field with independent inflations; anterior border with very slightly inverted V-shaped anterior margin, relatively short, sagittal length 12.5% that of cranidium, very slightly shorter exsagittally near facial suture, entirely lacking sculpture, rear part flat in profile, anterior part curved and subcylindrical in profile; anterior border furrow long (sag., exsag.) and shallow, with similar inverted V course to anterior margin, of similar length sagittally and exsagittally, slightly shallowed near facial suture; preglabellar field long, 18.7% sagittal length of cranidium, field and frontal area with sparse sculpture of widely scattered moderate sized tubercles; eye ridge very faint, expressed most clearly immediately adjacent to axial furrow, with single tubercle set atop about 40% distance abaxially; interocular fixigena broad and lacking sculpture, separated from frontal area only by subtle break in slope due to subdued eye ridge, confluent with posterior fixigena; palpebral lobe with exsagittal length 30.5% that of sagittal length of cranidium, nearly flat, with only slight downward slope toward interocular fixigena, lacking in sculpture except for small pit set abaxial to center, lateral margin with strong, semilunate lateral convexity; palpebral furrow present only as a slight break in slope, most obvious anteriorly and posteriorly; glabella with sagittal length 98.3% maximum width across rear of L1; axial furrows wide and shallow, deflected strongly around L1, so outline of glabella is bell shaped in plan view, moderately anteriorly convergent in front of L1; preglabellar furrow shorter (sag., exsag.) than axial furrow is wide, slightly deeper, with weak anterior arc; L1 relatively large, teardrop shaped, with several small tubercles; S1 narrow and deep, nearly straight with oblique course, slightly wider and deeper near contact with axial furrow, fully isolating L1; S2 present as a slight indentation of lateral margin of glabella, L2 with extremely weak independent inflation; S3 and L3 not discernible dorsally, though S3 visible as very small lateral impression ventrally (Fig. 7.3); median glabellar lobe with sculpture of fairly densely scattered mixed small and medium sized tubercles; SO long (sag., exsag.) and shallow, anterior margin sharper than posterior margin, bowed posteriorly across median portion, shorter, deeper, and deflected posteriorly behind L1; LO with sagittal length 13.1% that of cranidium, somewhat shorter laterally behind L1, with prominent median node set at half length and sculpture of an aligned row of medium sized tubercles near posterior margin, with several smaller scattered tubercles; posterior fixigena moderately inflated, lacking dorsal sculpture, extending as broad triangular strip on posterior projection; posterior border furrow very short (exsag.), moderately deep, proximal part running transversely or very slightly anteriorly, turned slightly posteriorly at fulcrum; posterior border short and dorsally convex proximally, longer and turned posteriorly distal to fulcrum, lacking sculpture except for one small tubercle just abaxial to fulcrum in the holotype (a serial homologue of similar tubercles developed on the thoracic posterior pleural bands); ventral features largely obscured in available specimens, but fossula distinct.

Librigenal measurements were made on the right librigena of the holotype (Fig. 7.3), as it is almost perfectly preserved. Librigenal field with minimum width opposite half length of eye 26.1% exsagittal length; eye large, with exsagittal length 50.0% that of field; inflated socle not obvious, visual surface bounded by narrow depressed area on field; field completely lacking sculpture; lateral border furrow very shallow, mainly present as break in slope between field and border; border relatively narrow, of similar width everywhere along course, lacking sculpture dorsally with subdued fine subparallel lines ventrolaterally; posterior border furrow deeper and narrower than lateral border furrow, meeting lateral border furrow at acute angle; posterior border similar in exsagittal length to width of lateral border, lacking sculpture, forming oblique angle with base of genal spine; genal spine very short, with blunt tip, lacking dorsal sculpture, bisected dorsally by very faint furrow running distally from junction of lateral and posterior border furrows; anterior projection long, about twice length of genal spine; ventral morphology poorly known.

Rostral plate anteriorly very wide (Fig. 7.3, 7.11, 7.12), transversely bowed; posterior margin also with considerable transverse extent; connective sutures oblique but nearly transverse, very long; ventral sculpture of many subparallel raised lines contiguous with those on ventrolateral aspect of librigenal lateral borders.

Hypostome represented by one specimen on holotype (Fig. 7.3), but morphology obscured by underlying right librigena and rostral plate.Thorax of 13 segments. Axis occupying about 36% of width anteriorly, tapered gradually to about 26% posteriorly; maximum width achieved across third or fourth segment, roughly the same as sagittal length; fulcrum set about two thirds distance distally on pleurae on anterior segments, slightly less than half distance distally on posterior segments; axial rings shortest (sag., exsag.) anteriorly, progressively longer but narrower on posterior segments; axial furrow deeper than pleural furrow, deflected around bowed lateral margin of ring; rings describing subtle W shape, with dorsal sculpture of a transverse row of moderate sized tubercles, less well expressed posteriorly; anterior and posterior pleural bands of similar length (exsag.), posterior band with transverse row of very faint small tubercles (almost effaced in FIGURE 7. Maurotarion wenbanense n. sp., from the Wenban Limestone (Pragian), Cortez Gold Mine haul road, southern Cortez Mountains, Eureka County, Nevada, USA. 1, 4, 5, 7, 9, 11, 12. Dorsal exoskeleton, SUI 109046, dorsal thoracic, dorsal cephalic, right lateral, dorsal pygidial, left lateral, ventral views, and detail of rostral plate, x5 except 12, x7.5 (CR-G(a)). 2, 3, 6, 8, 10, 13. Dorsal exoskeleton, holotype, SUI 109047, dorsal, ventral, anterior, librigenal external, left lateral, and dorsal pygidial views, x7.5 (talus block along haul road).

larger specimen) and one more prominent tubercle just abaxial to fulcrum (again almost effaced in larger specimen); pleural furrow short (exsag.) proximally and distally, incised, slightly longer across fulcrum; anterior pleural band developed into wide (tr.) articulating facet distal to fulcrum; pleural tip blocky and subquadrate, with a few subdued tubercles along margin, and with pleural furrow continued to margin; doublure underlying pleural tip with articulatory notch; small apodemes present beneath rear of axial furrow.

Pygidial measurements were made only on the pygidium of the holotype (Fig. 7.13); because it is articulated, the sagittal length of the pygidium does not include the articulating half ring. Pygidium with sagittal length 39.4% maximum width; width across fulcrum 62.8% maximum width; axis with anterior width 31.6% maximum pygidial width and 104.3% sagittal length of axis; sagittal length of axis 76.9% that of pygidium; entire pygidium lacking dorsal sculpture; three axial rings distinct, with possibly a fourth epxressed in front of transverse terminal piece (Fig. 7.7); rings slightly longer exsagittally than sagittally; second and third rings with prominent pseudo-articulating half rings; axial furrows moderately deep, meeting posteriorly to fully circumscribe axis; pleural furrows of first and second segments well expressed, that of third faintly expressed; anterior pleural band apparently shorter (exsag.) than posterior band, interpleural furrow between first and second segment impressed as very short (exsag.) firm lineation, those between posterior segments weakly expressed; prominent moderately wide border developed at which pleural bands and furrows terminate, border widest distally, short medially.

Discussion. There are some sculptural differences between the two available specimens, but these are likely ontogenetic as the specimen from locality CR-G (Fig. 7.1, 7.4, 7.5, 7.7, 7.9, 7.11, 7.12) is just over one and a half times larger than the holotype specimen. The larger specimen has a greater number of more densely scattered tubercles on its glabella, a greater number of smaller tubercles aligned transversely on LO and the thoracic axial rings; and has smaller and more subdued isolated tubercles distal to the fulcrum on the thoracic posterior pleural bands and the cranidial posterior border. The region of the thoracic pleurae distal to the fulcrum appears wider in the larger specimen, but this is influenced by some distortion, with the right pleural folded at the fulcrum and the left side somewhat flattened. The specimens share all other features, including the highly distinctive short genal spine. A general reduction in tubercle prominence and increase in tubercle number is a common feature of aulacopleurid ontogenies ( Adrain and Chatterton, 1994, 1995a, 1995b, 1996) and there is little reason to doubt that the specimens are conspecific.

As noted above, M. wenbanense is not easily confused with the cooccurring M. chrysion . Maurotarion wenbanense has prominent glabellar tubercles and tubercles on the fixigenae and frontal areas; M. chrysion completely lacks sculpture in these areas. Maurotarion wenbanense also has prominent transverse tubercle rows on LO and the thoracic axial rings; M. wenbanense has only extremely subdued and difficult to discern tiny tubercles in these positions. Other obvious differences include the occurrence in M wenbanense of a larger and more prominent L1, a much smaller and shorter genal spine, 13 versus 12 thoracic segments, and a pygidium with three versus two fully expressed axial rings.

Maurotarion wenbanense is somewhat similar to M. periergum in the amount and distribution of tuberculate sculpture on the cranidium and in the shared possession of 13 thoracic segments with transverse tubercle rows on the axial rings. The many differences include a relatively shorter anterior border in M. wenbanense , a narrower librigenal lateral border, a much shorter genal spine lacking a dorsal tubercle row on the adaxial part, and a pygidium with only three versus four well expressed axial rings.

Maurotarion wenbanense differs from M. fooi in the presence versus absence of tubercles on the main glabellar lobe and frontal areas, the possession of a larger L1, librigenae with a broader field lacking expression of the eye socle, a narrower librigenal lateral border, and a much shorter genal spine. The species differs from M. axitiosum ( Campbell, 1977) in the presence of stronger tuberculate sculpture on the glabella, much shallower cephalic border furrows, a broader librigenal field, a much shorter genal spine, and the possession of 13 versus 11 thoracic segments.