Mendoparabolina nyensis (Taylor, 1976)

Taylor, John F., Loch, James D. & Repetski, John E., 2024, Taxonomy and stratigraphic distribution of Lotagnostus (Agnostida: Agnostidae) and associated trilobites and conodonts in the Upper Cambrian (Furongian) of Laurentia, Zootaxa 5422 (1), pp. 1-66 : 30-33

Mendoparabolina nyensis (Taylor, 1976)

( Plate 5 ; Plate 16.1, 16.10, 16.11 )

1976 Plicatolina nyensis Taylor , p. 687–688, pl. 3, figs 16–18.

Diagnosis. A species of Mendoparabolina with a short, horizontal, caecate preglabellar field; prominent constriction of the glabella where impingement by eye ridge offsets axial furrow inward directly in front of S4; a pronounced axial notch in the front of the glabella, S3 glabellar furrows oriented perpendicular to the midline; and a transverse pygidium with a strongly bilobed posterior margin in larger specimens.

Material and occurrence. Lotagnostus nolani Fauna : collection 5/22/08B (136-64-5) and D3362-CO (6-7- 2) from Ninemile Canyon, Nevada; Hedinaspis-Charchaqia Fauna: collections D7129-CO (cephalon and partial thorax), D7130-CO (4-2-0), and D7133-CO (4-0-0) from the Hales Limestone in the Hot Creek section, Nevada .

Description. Cranidium large among associated sclerites, up to 1cm in length; subtrapezoidal in outline. Prominent glabella; subrectangular in outline, glabellar length exclusive of occipital ring 60-70% of cranidial length (sag.), glabellar width (tr.) at occipital furrow equals 85-90% of glabellar length exclusive of occipital lobe; moderately convex transversely with weakly developed keel; very weakly convex from posterior margin to S2, moderately convex (sag.) forward of S2 as frontal lobe descends to preglabellar field. Axial furrows moderately impressed, mostly straight, subparallel to slightly divergent, tightly sinuous at intersection with eye ridge; preglabellar furrow moderately impressed, broadly curved to merge evenly with axial furrows, slightly recurved sagittally, slightly lengthening the preglabellar field in front of shallow notch in front of the glabella. Four pairs of lateral glabellar furrows: S1 moderately impressed, deepest near mid-length (tr.); sinuous; distal half gently curved and directed slightly posteriorly adaxially, inner half anteriorly concave, extends 75% of the way from axial furrow toward midline; S2 moderately impressed, slightly curved, anteriorly convex, intersection with axial furrow nearly perpendicular, terminates adaxially at same distance from axial furrow as S1; S3 short (tr.), slightly to moderately impressed, straight to very gently curved, oriented perpendicular to midline, opposite or slightly behind intersection of eye ridge with axial furrow; proximal end in line (exsag.) with adaxial termination of S2, extending abaxially half way to axial furrow; S4 lightly impressed, narrow, and straight, anteriorly directed at 65° angle from intersection with axial furrow at posterior edge of eye ridge; extends half way to midline. Occipital ring subcrescentic in outline; long (sag.), 17-19% of cranidial length, longest medially where occipital furrow recurves; low, slightly elongate occipital node occupies posterior half of ring, very faintly impressed furrow extends from anterior end of node diagonally toward posterolateral corners of ring. Occipital furrow moderately to deeply impressed distally, faintly impressed medially; sinuous, directed slightly posteriorly from intersection with axial furrow, extends 75% of way to midline where it is recurved to cross axial line in anteriorly convex arc; most specimens exhibit bifurcation of furrow at distal ends of the arc to form faintly impressed, posteriorly directed furrows that fail to reach posterior margin or axial furrows. Anterior margin as broad even curve. Anterior border a narrow (sag.), upturned rim that tapers gently abaxially, disappearing in front of preocular area where suture descends into border furrow. Border furrow narrow (sag.) and moderately impressed in smaller specimens, shallower and broader in larger sclerites. Anterior branch of facial suture trends slightly posteriorly from midline, traverses anterior border at oblique angle, and turns inward in tight curve to intersect distal end of eye ridge; posterior branch of facial suture diverges from posterior edge of palpebral lobe at 45° angle, broadly curved to intersect posterior margin at 85°. Preglabellar field short (sag.), 10-20% of cranidial length, longer abaxially although slightly lengthened at midline due to curvature in preglabellar furrow; subhorizontal in orientation. Preocular area subhorizontal, moderate in length (exsag.), wide (tr.), extending beyond palpebral lobes. Radiating genal caecae cover preglabellar field and preocular areas. Eye ridge well developed, directed slightly posteriorly from axial furrow, merging distally with short palpebral lobes that extend from distal end of S4 to slightly behind intersection of S2 with axial furrow. Palpebral areas narrow (tr.), subhorizontal; postocular areas long, extend to approximately cranidial midlength between S1 and S2; slightly convex, subhorizontal adaxially to slightly declined distally; finely granulate prosopon on some specimens; moderately impressed posterior border furrow widens and shallows distally.

Librigenae subcrescentic in outline; lateral margin as broad, even curve; long, gently tapering anterior extension; lateral border narrow, smooth, uniform in width (tr.), defined by inflection at distal edge of genal field; genal field traversed by radiating genal caecae that fade posteriorly, narrow genal spine roughly 75% length of genal field (exsag.).

Pygidium large in comparison to associated sclerites, up to 5.6mm in length (sag.); transverse with axial pygidial length only 50% of maximum pygidial width; slightly convex (tr.) to moderately convex (sag.); pygidial margin as even curve from anterolateral corner to axial line, interrupted in small specimens by minute, blunt marginal spines at distal tips of two anteriormost pleurae; shallow embayment in posterior margin behind axis in small specimens becomes narrower (tr.), prominent notch in larger specimens, rendering pygidum bilobed with axial length (sag.) approximately 90% of maximum length (exsag.); narrow (tr.) postaxial ridge extends forward from notch, diminishing in height anteriorly, more pronounced in larger specimens; narrow, convex border along posterior half to third of pygidium, increasing in width (sag.) behind axis in larger specimens; border furrow ill defined by distal ends of pleurae. Axis long, 80-90% of pygidial length (sag.) and broad, approximately 33% of maximum pygidial width at anterior end; slightly tapered, width at posteriormost inter-ring furrow 70-75% width at articulating furrow; composed of 3 progressively shorter (sag.) axial rings and lunate terminal piece that bears two lateral nodes separated by faint medial furrow; axial rings with faint transverse ridge on posterior half of ring. Inter-ring furrows straight, weakly impressed, shallowed axially. Axial furrows straight, slightly convergent; moderately impressed opposite anterior two axial rings, faintly impressed opposite third ring, as break in slope around terminal axial piece. Pleural fields tapered posteriorly, slightly convex (tr.), slightly declined (tr.), moderately declined post-axially. Four pairs of pleurae defined by faintly impressed interpleural furrows; anterior pair straight, nearly transverse through proximal two thirds, turned posteriorly through outer third, cross 80% of width of pleural field (tr.) terminating just inside lateral border; second interpleural furrows long, as broad curve, slightly posteriorly directed; remaining furrows shorter, posteriormost furrow strongly directed posteriorly. Anteriormost pleural furrows moderately impressed, anterior side more steeply declined than posterior; more strongly directed posteriorly than adjacent interpleural furrow, turns at two-thirds length to converge on interpleural furrow; remaining pleural furrows as broad curves, progressively shorter and more posteriorly directed.

Discussion. Taylor (1976) described Plicatolina nyensis from the Hales Limestone at Hot Creek Canyon, Nevada, illustrating three cranidia and a partial cephalon and thorax.Among the olenids recovered from the Windfall Formation at Ninemile Canyon, Nevada, is a species that conforms with P. nyensis in the length, orientation, and caecate prosopon of the preglabellar field, in the shape and orientation of the glabellar furrows, the divergent anterior branch of the facial suture, the structure of the occipital ring, and almost every other feature. The cranidia from the Hales illustrated by Taylor (1976) differ from those of the Windfall in only two respects. The S1 furrow in the former appears transglabellar, whereas S1 furrows on Windfall specimens are separated medially. Furthermore, the palpebral lobes on the Windfall specimens are slightly longer (exsag.), extending posteriorly a short distance beyond where those of the Hales terminate opposite the distal end of S2. While examination of borrowed material from the Hales confirmed the minor difference in length of the palpebral lobes, the apparent contrast in form of S1 proved to be a preservational artefact. Material from the Hales is poorly preserved with most sclerites displaying some fracturing from compaction of the enclosing lime mudstone.Additionally, many of the specimens are internal molds that do not reliably replicate the features of the dorsal surface of the sclerites. This is the case with the illustrated holotype of P. nyensis (Taylor, 1976, pl. 3, fig. 16). A latex peel of the counterpart of the holotype, illustrated herein ( Pl. 16 , fig. 1), more accurately captures the features of the dorsal surface and confirms that S1 is not transglabellar. With the differences in cranidial morphology thereby reduced to a slightly more posterior position for the back of the palpebral lobe, we consider the Windfall specimens conspecific with those of the Hales. Additional support for this conclusion was provided by a pygidium ( Pl. 16 , fig. 11), virtually identical to those associated with P. nyensis cranidia in the Windfall (e.g., Pl. 5 , figs. 12, 15), that was recovered through re-processing of the Hales collection (7130-CO) that provided the holotype.

With this recovery, the Plicatolina nyensis cranidia in both the Hales and the Windfall are associated with a transverse pygidium that bears distally curved pleurae and a bilobed posterior margin. This species is confidently transferred from Plicatolina Shaw, 1951 to Mendoparabolina based upon the quadrate glabella; the 4 pairs of glabellar furrows, especially the sinuous S1 and S2 furrows and the S3 that fails to intersect the axial furrows; the short, caecate, horizontal preglabellar field; the divergent anterior facial sutures; and the bilobate posterior pygidial margin in moderate to larger specimens.

The Windfall pygidia exhibit two obvious ontogenetic progressions. First, minute spines present opposite the terminal axial piece on the smallest specimens ( Plate 5.15 ) are lost during ontogeny. Further, in the smallest specimens the posterior margin is an even curve, becoming progressively more embayed with a distinct bilobed appearance evident in larger specimens.

Mendoparabolina nyensis (Taylor, 1976) compares very closely with M. piriquensis ( Rusconi, 1951a; Tortello, 2014) in glabellar shape and proportions, the shape of the S1 and S2 lateral glabellar furrows, and the orientation of the preglabellar field. The cranidia of M. nyensis , however, differ from the latter in the transverse orientation of the S3 glabellar furrow and in a slightly longer preglabellar field relative to the glabellar length, although the preglabellar fields in M. piriquensis may be incomplete or incompletely prepared. The pygidia of the two species are very similar in the composition of the axes, the shape of the pleurae, the rim-like border, and the bilobed margin in the larger specimens. That of M. nyensis , however, is relatively shorter (sag.). With the length measured sagitally and the articulating half-ring excluded, the length of the M. nyensis pygidium is only 43.5% (38-44.6%) of the maximum pygidial width, as opposed to 55% (46.3-64.5%) for M. piriquensis .

The pygidium of Mendoparabolina brevicauda ( Rusconi, 1955b; Tortello, 2018, fig. 4) is similar to that of Mendoparabolina nyensis in the nature of the axis and the pleurae. The degree of embayment along the posterior margin is more significant in M. nyensis when comparing specimens of similar sizes (Tortello, 2018, fig. 4h). The cranidium of M. brevicauda , however, is markedly different from M. nyensis . The cranidium is more rounded at the anterolateral corners, exhibits an evenly curved preglabellar furrow, has S3 and S4 glabellar furrows that are more faintly impressed, a longer preglabellar field, and the anterior lobe of the glabella, when seen in lateral view, that is less convex. In these characters the cranidium assigned to M. brevicauda conforms more closely to Bienvillia and contrasts with Mendoparabolina : the association of sclerites of M. brevicauda might bear reappraisal.

The cranidium of Plicatolina dunbari Ludvigsen & Westrop (in Ludvigsen et al., 1989, p. 15–16, pl. 5, figs 4–6) closely compares to those of Mendoparabolina nyensis in the shape of the glabella and the shape and arrangement of the lateral glabellar furrows. P. dunbari differs, however, in having a longer anterior border, a shorter preglabellar field, and a nearly straight posterior branch of the facial suture. The pygidium of P. dunbari has an evenly curved posterior margin and the distal ends of the pleurae are not as sharply turned.

The pygidium of Plicatolinella ocula ( Robison & Pantoja-Alor, 1968, p. 794, pl. 103, figs 24–26, pl. 104, figs 1–4) is bilobed as in Mendoparabolina nyensis ; however, the posterior end of the axis is less well defined, lacking the post-axial ridge highlighted by Tortello (2018). The cranidium of P. ocula exhibits large, posteriorly placed palpebral lobes at the distal end of long eye ridges that result in long, broad palpebral areas which contrast with the short, narrow palpebral areas of M. nyensis . Furthermore, the S3 furrows in P. ocula intersect the axial furrows, unlike the detached S3 furrows in M. nyensis .

The small pygidium of Simuloenus quadrisulcatus Palmer, 1968 (p. 56, pl. 8, figs 1–4) is weakly bilobed and bears pleurae that are curved distally, as seen in Mendoparabolina . The pygidium of S. quadrisulcatus , however, bears only 2 axial rings. The form and details of the cranidium of S. quadrisulcatus closely resemble those of M. nyensis in the presence of the 4 pair of glabellar furrows, small palpebral lobes set close to the glabella, the short preglabellar field, and oblique furrow crossing the occipital ring. However, the axial furrows in S. quadrisulcatus are moderately convergent resulting in a subtrapezoidal glabella, unlike the subrectangular glabella of M. nyensis , and the anterior border is convex, rather than upturned.

Figures

PLATE 5. Mendoparabolina nyensis (Taylor, 1976). Each white scale bar represents 2mm. 1. Cranidium, CM 41338, from 5/22/08B, dorsal view. 2. Cranidium, CM 41339, from 5/22/08B, dorsal view. 3-5. Cranidium, CM 41340, from 5/22/08B, dorsal, left lateral, and anterior views. 6. Cranidium, CM 41341, from 5/22/08B, dorsal view. 7. Cranidium, CM 41342, from 5/22/08B, dorsal view. 8. Cranidium, CM 41343, from 5/22/08B, dorsal view. 9, 10, 14. Pygidium, CM 41344, from 5/22/08B, dorsal, lateral, and posterior views. 11. Cranidium, USMN 775719, from D3362-CO, dorsal view. 12. Pygidium, CM 41345, from 5/22/08B, dorsal view. 13. Right librigena, USMN 775720, from D3362-CO, dorsal view. 15. Pygidium, CM 41346, from 5/22/08B, dorsal view. 16. Pygidium, CM 41347, from 5/22/08B, dorsal view. 17. Pygidium, USMN 775721, from D3362-CO, dorsal view. 18. Pygidium, CM 41348, from 5/22/08B, dorsal view.

PLATE 16. Taxa from Hedinaspis-Charchaqia Fauna in the Hales Formation, Hot Creek section. Each white scale bar represents 2mm. 1. Lotagnostus nolani n. sp. Carapace (latex peel of external mold), USMN 775754, with Mendoparabolina nyensis, cephalon (counterpart of holotype), USMN 218571 (= Taylor, 1976, pl. 3, fig. 16), from D7130-CO, dorsal view, x7.8. 2, 3. Lotagnostus nolani n. sp. Cephalon, USMN 775755, from D7133-CO, dorsal and anterior views, x11.0. 4-5. Lotagnostus nolani n. sp. Cephalon, USMN 775756, from D7130-CO, dorsal and lateral views, x9.7. 6. Lotagnostus nolani n. sp. Pygidium, USMN 775757, from D7130-CO, dorsal view, x12.2. 7. Lotagnostus nolani n. sp., pygidium, USMN 775758, from D7130-CO, dorsal view, x10.3. 8. Pseudagnostus? sp., carapace, USMN 775759, from D7130-CO, dorsal view, x8.0. 9. Pseudagnostus? sp., carapace, USMN 775760, from D7133-CO, dorsal view, x8.3. 10. Mendoparabolina nyensis (Taylor, 1976), small cranidium, USMN 775761, from D7133- CO, dorsal, x14.3. 11. Mendoparabolina nyensis (Taylor, 1976), pygidium, USMN 775762, from D7130-CO, dorsal view, x7.5. 12. Lotagnostus sp., cephalon, USMN 775763, from D7130-CO, dorsal view, x16.8. 13. Ceratopygid undet., cranidium, USMN 775764, from D7129-CO, dorsal view, x3.9. 14. Lotagnostus sp., cephalon, USMN 775765, from D7130-CO, dorsal view, x8.6. 15. Lotagnostus sp., pygidium, USMN 775766, from D7131-CO, dorsal view, x4.4. 16. Lotagnostus sp., pygidium, USMN 775767, from D7130-CO, dorsal view, x6.5. 17. Ceratopygid undet., pygidium, USMN 775768, from D7129-CO, dorsal view, x2.6.