Parallelora sp.
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publication ID |
https://doi.org/ 10.4202/app.2010.0106 |
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persistent identifier |
https://treatment.plazi.org/id/03BB9544-FF8F-A65F-FF7F-F9BE994CF97A |
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treatment provided by |
Felipe |
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scientific name |
Parallelora sp. |
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Figs. 6E View Fig , 26A–J View Fig .
Material.—More than 50 fragments of dorsal and 150 fragments of ventral valves from samples GB and GT. Although all specimens are highly crushed they preserve details of surface ornamentation in great detail.
Description.—Shell medium sized for the genus, subpentagonal to semicircular in outline; juvenile shells up to about 10 mm in length, less than twice to three times as wide as long, extended subtrapezoid in outline, and mucronate; adults less transverse; greatest width at the hinge line, cardinal extremities form short but distinct mucrons.
http://dx.doi.org/10.4202/app.2010.0106
Ventral valve with massive umbo and strongly incurved beak; beak ridges sharp, subparallel to the hinge line; interarea apsacline, slightly concave to flat at the hinge and strongly concave near the umbo, with curvature reaching about 180 °, usually subrectangular ( Fig. 26H View Fig 1 View Fig ), rarely lowtriangular, wide, weakly striated longitudinally; delthyrium wide and open, with the delthyrial angle attaining 48–67 °, in large specimens apically closed by callosity of thickened posterior regions of the dental adminicula; the callosity may form a more or less subdelthyrial triangular plate below the level of the interarea or may be convex with distinct median ridge or thickening ( Fig. 26E View Fig 2, E 3, F, G 2, I 2); flanks moderately convex, in posterior view flattened to concave, especially near the cardinal extremities; sulcus moderately wide, evident, deep and grove−like in the umbonal region, shallower anteriorly, bordered by a pair of clearly wider costae.
Dorsal valve transverse subelliptical to semicircular in outline, with low fold, well delimited by deep fold−bounding intercostal furrows; interarea rectangular, usually up to 1.3 mm high; umbo small, low; flanks convex but in posterior profile flattened to slightly concave near cardinal extremities.
Interior of ventral valve with strong extrasinal, anteriorly widely divergent dental adminicula, thickened and fused posteriorly in adults, but with sharp anterior edges ( Fig. 26D View Fig 2, F, G 2, I 2); in large specimens with high and concave interarea the teeth are supported distally only by dental ridges; teeth well marked but rather small, triangular in cross−section extending about 1 mm beyond the hinge line; secondary denticulation delicate but well visible along the hinge line except the nearest 2 mm from the teeth ( Fig. 26H View Fig 1 View Fig , H 2, J); muscle scars suboval in outline, deeply impressed posteriorly and posterolaterally between thickened dental adminicula, weakly delimited anteriorly, frequently divided medially by slightly elevated myophragm ( Fig. 26F, I View Fig 2); ontogenetically older valves markedly thickened posteriorly with space between dental adminicula infilled noticeably with callus ( Fig. 26D View Fig 2, E 3, G 2, I 2).
Interior of dorsal valve with rather deep but narrow sockets diverging at 125–138 °; hinge plates subrectangular, converging with the wide base of the cardinal process; crural plates not developed; cardinal process longitudinally striated ( Fig. 26A View Fig 2, A 3, B 2, C 2); adductor scars narrow, elongate, with sharp median ridge or myophragm ( Fig. 26A View Fig 2, B 2).
Shell ornamented by 14–17 costae on each flank; costae usually simple, but sporadically bifurcate on one out of three or four specimens, rounded in cross−section, widen markedly anteriorly, separated by narrow interspaces; sulcus bordering costae bifurcate 2–3 times; median costa usually present, appearing within a few mm from the beak; about 6 bifurcating costae on fold. External micro−ornamentation of fine, sinuous, imbricating lamellae with frequency of about 4 per mm; very faint radial capillae densely packed, of about 16 ridges per mm, terminating at growth lamellae in the form of very small tubercles ( Fig. 26C View Fig 3).
Remarks.—This species is represented in our material by invariably highly crushed single valves despite their massive, thick−shelled structure. On the contrary, these specimens are characterised by surprisingly well preserved surface micro−ornamentation. This is in striking contrast with equivalent spiriferid species Eochoristites neipentaiensis Chu, 1933 from sample MH2 which is represented by mainly well preserved complete single valves but with completely worn out micro−ornamentation as well as partly abraded surface costation. These preservational characteristics between different samples are most likely a result of complex hydrodynamics and post depositional conditions on one hand and variability in the diagenetic (mainly silicification) processes on the other.
The shell shape and its ornamentation as well as details of the interior of both valves suggest that the described species represents a prospirinin spiriferoid. The shell proportions, its pattern of costation, capillate and imbricate micro−ornamentation, and presence of wide, usually subrectangular ventral interarea suggest generic affiliation with Parallelora . Unfortunately, the very fragmented material lacks the characters needed for specific identification. The species from Muhua differs, however, from the late Famennian Parallelora marionensis Schumard, 1855 and the early Mississippian Parallelora nupera Carter, 1988 by having slightly higher ventral interarea, more massive ventral umbonal callosity, and less numerous, thicker costae (see Carter 1988). P. marionensis was reported also from the uppermost Famennian–early Tournaisian of the Urals by Nalivkin (1979) and from the Tournaisian of Southern Tienshan Mountains, NW China by Chen and Archbold (2000). Specimens from Muhua differ clearly from Spirifer subavis Plodowski, 1968 (= Parallelora subavis ) from Étroeungt zone of Afghanistan ( Plodowski 1968) by having greater shell and much thicker costae. It seems that the specimens are similar to Parallelora obesa Xu and Yao, 1988 from the latest Famennian–earliest Mississippian of Nanbiancun section (Guanxi Province, Southern China) but differ at least in having non−carinate dorsal fold. Unfortunately, P. obesa is inadequately figured (see Xu and Yao 1988: 296–297, pl. 77: 16–19, text−fig. 96b) and its specific characteristic difficult to assess.
Feng (1989) established a new species Brachythyrina originalis Feng, 1989 from the lower part of the Muhua Formation of the Dapoushang section. He founded the species on a single fragmentary ventral valve which seems generally similar to the species here described. Unfortunately, Feng’s (1989) description and figures of B. originalis are not adequate to warrant any reliable comparison and his species should be considered as nomen dubium until better topotype materials becomes available.
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