Vietnamia yushanensis, Lee, 2013

Lee, Dong-Chan, 2013, Late Ordovician trilobites from the Xiazhen Formation in Zhuzhai, Jiangxi Province, China, Acta Palaeontologica Polonica 58 (4), pp. 855-882 : 868-872

Vietnamia yushanensis sp. nov.

Figs. 3C , 8A–G, I–N, P, 9B, E, H, I, N.

Etymology: After Yushan county where the sampling localities are located.

Holotype: NIGP−152032, articulated specimen from locality 2, 2 cephala, 21 cranidia,14 thoraces,six pygidia, five free cheeks, and one hypostome.

Type locality: Locality 2, about 60 m northwest of Zhuzhai, Yushan County. GPS coordinate is 28 ° 34’23.27” N and 118 ° 20’16.50” E .

+ Fig. 8. Late Ordovician trilobites from Xiazhen Formation near Zhuzhai, Yushan County, China. All specimens are from locality 2, except NIGP−152042

(L) which is from locality 1. A – G, I – N, P. Vietnamia yushanensis sp. nov. A. NIGP−152032, holotype, articulated specimen without free cheeks; dorsal ( A 1 ), lateral ( A 2 ), and anterior ( A 3 ) views. B. NIGP−152033, articulated specimen with the dislocated left free cheek; dorsal ( B 1 ), lateral ( B 2 ), and posterior ( B 3 ) views. C. NIGP−152034, articulated specimen without free cheeks, dorsal view. D. NIGP−152035, hypostome, ventral view. E. NIGP−152036, cephalo−thorax with hypostome in place; dorsal ( E 1 ) and anterior ( E 2 ) views. F. NIGP−152037, thoraco−pygidium; dorsal ( F 1 ) and posterior ( F 2 ) views .

G. NIGP−152038, articulated specimen, dorsal view. I. NIGP−152039, thoraco−pygidium; dorsal (I 1) and lateral (I 2) views. J. NIGP−152040, right free cheek, dorsal view. K. NIGP−152041, left free cheek, dorsal view. L. NIGP−152042, pygidium, dorsal view. M. NIGP−152043, thoraco−pygidium, lateral view. N. NIGP−152044, cranidium, dorsal view. P. NIGP−152045, cranidium, dorsal view. H. Vietnamia sp. E , NIGP−152029, pygidium; dorsal (H 1) and posterior (H 2) views. O. Vietnamia ? sp., NIGP−152030, incomplete cephalon; anterior (O 1) and dorsal (O 2) views; note the presence of strongly arched, transversely elongated rostral plate. Q. Vietnamia sp. A , NIGP−152031, cephalon; oblique anterior (Q 1), anterior (Q 2), and dorsal (Q 3) views; note the presence of upside down anvil−shaped rostral plate. Scale bars 2.5 mm, except A, B, C, I 5 mm; D, H, J, N 1 mm.

http://dx.doi.org/10.4202/app.2010.0036

Type horizon: Xiazhen Formation, Upper Ordovician, Dicellograptus complexus Graptolite Zone.

Material.—Seven articulated exoskeletons, four cephalo−thoraces, 12 thoraco−pygidia, two cephala, 21 cranidia, 14 thoraces, six pygidia, five free cheeks, and one hypostome (repository numbers: NIGP−152032–152045, 152050, 152052– 152054, 152056).

Diagnosis.—Species of Vietnamia with distinct pygidial lateral and posterior edge anterior to cincture, seven pygidial axial rings, anterior node in fossula, and lower−leveled area between L1.

Description.—Dorsal exoskeleton elliptical in outline. Cephalon semi−circular in outline. Cranidium sub−triangular in outline with pointed (sag.) anterior margin, and sagittal length 60% of transverse width. Anterior cranidial border nearly flat and weakly arched dorsally; anterior cranidial border furrow shallow and broad, and moderately deepens distally. Preglabellar area moderately convex; anterior furrow shallow and wide, and divergent forwards at about 30 ° from sagittal line; preglabellar furrow deeper than anterior furrow. Fossula distinctively depressed, and located diagonal to S3, immediately outside axial and anterior furrows, and immediately anterior to eye ridge; anterior node small but distinct and positioned at adaxial slope of fossula. Eye ridge moderately developed, and abruptly narrows as crossing axial furrows. Axial furrow wide and deep, and progressively wider and deeper posteriorly, with maximum width and depth opposite L1; axial furrow opposite L1 steep−sided and flat−bottomed. Glabella trapezoidal in outline, anterior margin straight to slightly convex forwards, and sagittal length 63% of maximum transverse width across L1. Three pairs of glabellar furrows present; abaxial end of all three pairs positioned within palpebral area of fixigenae. S3 obliquely directed backwards, weakly incised, and shortest. S2 slightly more oblique and weakly bifurcated. S1 deeply impressed, strongly obliquely directed posteriorly, and bifurcated at mid−length adaxially; anterior branch shorter and more weakly impressed, and proximal one−quarter of posterior branch shallow and gently curved forwards. L1 elongated (tr.), moderately convex, strongly inflated abaxially, tapers adaxially, abaxial edge overhangs axial furrows, and posterior margin weakly sinuous; adaxial one−third gently slopes down and then slightly raised sagittally, and node weakly developed at adaxial one−third. L2 and L3 sub−rectangular and raised as pointed node in central area. Intermediate lobe sub−circular in outline, weakly convex, well delineated posteriorly and abaxially by posterior branch of S1, and remaining boundaries poorly defined by weakly impressed furrows. Palpebral lobe crescentic in outline and mid−point located opposite S2. Occipital furrow moderately curved forwards following posterior margin of L1 and abruptly becomes shallower sagittally from adaxial two−thirds of its course. Occipital ring progressively widens adaxially; distal end protruded as distinct node. Preocular area of fixigenae gently slopes downwards; palpebral area steeply slopes; postocular area triangular in outline and gently slopes distally. Posterior cranidial border furrow shallow and broad, and slightly curved forwards at its mid−length. Anterior branch of facial suture slightly convex laterally and moderately convergent forwards in posterior half; anterior half runs straight and parallel−sided, and then sharply curved inwards; posterior branch runs transversely and then gently curved backwards, and cuts postero−lateral cephalic corner, forming gonatoparian suture.

Librigena with blunted postero−lateral end. Eye socle weakly developed and eye socle furrow shallow. Librigenal field gently slopes downwards in adaxial two−thirds and then steeply slopes. Librigenal lateral border furrow moderately deep and continues into anterior furrow and anterior cranidial border furrow; librigenal lateral border sharply inturned.

Hypostome elliptical in outline. Anterior margin strongly convex forwards. Posterior and lateral borders moderately convex; posterior and lateral border furrows deep but shallow out at anterior end of posterior lobe; posterior border projected as spine of moderate length. Anterior lobe sub−circular in outline; posterior lobe crescentic in outline and larger than anterior lobe. Middle furrow deep and shallows adaxially.

Thorax of 13 segments. Axis gently tapers backwards and occupies about 45% of thoracic width. Axial furrow relatively wide and deep, deepening as pit at posterior end in each segment. Axial ring slightly curved forwards medially; distal end moderately curved forwards and protruded as distinct node. Articulating furrow deep and wide in distal half and shallows and narrows in proximal half. Anterior and posterior pleural band of equal width, and strongly convex; small fulcral process located at proximal one−third (or mid−length at dorsal view) of posterior band; the processes behttp://dx.doi.org/10.4202/app.2010.0036

come progressively closer to axial furrow posteriorly; pleural furrow straight, deep and wide, becoming shallow and narrow distally. Articulating facet developed from mid−length of pleural band and abruptly widens distally at distal one−third of pleural length.

Pygidium triangular in outline. Axis occupies about 54% of maximum transverse pygidial width; axial furrow straight or slightly convex laterally; axis moderately tapers backwards. Seven axial rings present; terminal piece parabolic in outline, flat, and moderately protruded upwards in posterior end; inter−ring furrow progressively shallower posteriorly; postaxial ridge moderately convex and laterally delineated by shallow furrow which is apparently continuous into axial furrows. Five pairs of pleural furrows moderately impressed; posteriormost furrow separates terminal piece and postero−most pleural rib; pleural rib abruptly bent downwards along lateral and posterior edge of triangular pleural field; the edge moderately protruded outwards, and the lower limit represents cincture; pleural field distal to the edge nearly vertical.

Dorsal exoskeleton covered with fine granules. Some specimens enrolled.

Remarks.—The concept of Vietnamia has recently been revised by Turvey (2005); most features listed in the emended diagnosis are observed in this species. Vietnamia yushanensis sp. nov. is characterized by a distally protruding lateral and posterior pygidial edge that is immediately anterior to the cincture, seven pygidial axial rings, an anterior node in the fossula, and the absence of pygidial interpleural furrows. The presence of small morphologic features such as the anterior node cannot be confirmed from the illustrations of other species. However, the pygidial features ensure that it is a new species of Vietnamia .

Zhou and Zhen (2008: 241) transferred a few species from China previously assigned to Neseuretus into Vietnamia . Neseuretus abnormis Li, 1988 ( Li 1988: pl. 1: 8, pl. 2: 4–7) from Tibet was erected based on a poorly preserved cranidium and four pygidia; Turvey (2005) concluded that it is a probable junior synonym of Vietnamia nivalis ( Salter, 1865) ; the pygidial morphology does accord with that of V. nivalis (see Turvey 2005: pl. 4: 6, 10). Liu et al. (1991: pl. 17: 6) illustrated Neseuretus (Neseuretinus) henanensis Chang from Henan. The poorly−preserved cranidium exhibits more resemblance to Vietnamia than to Neseuretinus in having a truncated anterior glabellar margin and laterally inflated L1. Assignment of Neseuretus (Neseuretinus) sp. from Xinjiang ( Lin et al. 1990: pl. 6: 1, 2) to Vietnamia appears questionable, because the pygidium is of Calymenesun or Reedocalymene in having a distinct cincture and narrower (tr.), posteriorly−tapering postaxial region, although the cranidium resembles that of V. nivalis .

Like many calymenid taxa, Vietnamia yushanensis sp. nov. has 13 thoracic segments, which is the first documentation for the genus. V. yushanensis has an anterior node in the fossula (see Fig. 8A 1, B 1, P) which is also observed in Reedocalymene and Calymenesun ( Peng et al. 2000) ; the node is smaller than that of other taxa. Vietnamia douvillei ( Mansuy, 1908) , the type species appears to have the node (see Turvey 2005: pl. 3: 5; a small node appears to be present on the left fossula).

Stratigraphic and geographic range.—Late Ordovician and Jiangxi Province, South China.

Figures

Fig. 3. Reconstruction of new species described from the Late Ordovician Xiazhen Formation; all drawings are not to scale. A. Remopleurides xiazhenensis sp. nov., dorsal view of exoskeleton (A1), and dorsal (A2) and ventral (A3) view of thoracic segments; exact extent of thoracic axial spine in A1 is not known and articulating half ring is reconstructed as if the anterior margin is broadly notched (see text for details). B. Hibbertia aodiensis sp. nov., dorsal view of exoskeleton; pits and caeca are omitted. C. Vietnamia yushanensis sp. nov., dorsal view of exoskeleton (C1) and lateral view of thoracic segments (C2); granules (see Fig. 9N) are omitted. D. Ceraurinus zhuzhaiensis sp. nov., dorsal view of exoskeleton; granules (see Fig. 10B1, B3) are omitted. E. Pliomerina tashanensis sp. nov., dorsal view of cranidium.