Pliomerina tashanensis, 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 : 877-878

Pliomerina tashanensis sp. nov.

Figs. 3E , 11A–L, O, Q.

Etymology: After the Tashan village where the type section of the Xiazhen Formation is located.

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

Holotype: NIGP−152063, cranidium from locality 3.

Type locality: Locality 3, about 400 m southeast of Zhuzhai, Yushan County. GPS coordinate is 28 ° 34’14.33” N and 118 ° 20’22.41” E .

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

Material.—Two thoraco−pygidia, nine cranidia, and eleven pygidia (repository numbers: NIGP−152062–152074, 152077).

Diagnosis.—Species of Pliomerina with abaxial end of S3 located posterior to antero−lateral glabellar corner, and more strongly backwardly curved posterior margin of pygidial axial rings.

Description.—Cranidium sub−triangular in outline. Anterior cranidial border narrow and rimmed; anterior cranidial bor− der furrow moderately deep and concave; preglabellar field absent. Axial furrow deep and wide, steep−sided. Glabella elongated (sag.) pentagonal in outline; central area moderately carinated. Four pairs of glabellar furrows present; S1 elongated (tr.) triangular in outline and slightly curved backwards medially; S2 slit−like, straight, and transversely; S3 longest, obliquely directed posteriorly; adaxial end of S1/S2/S3 connected into shallow furrow to abaxially delineate carinated central area; abaxial end of S1/S2/S3 located within palpebral area of fixigenae; abaxial end of S3 posterior to antero−lateral glabellar corner; S4 weakly impressed, straight, obliquely directed, and isolated from axial furrow. L1 elongated (tr.) triangular in outline, protruded posteriorly to overhang occipital furrow, and distal end sharply curved forwards; L2 sub−rounded; L3 sub−triangular in outline and moderately convex; L4 paper fan−shaped in outline and covered with randomly−distributed tubercles while other lobes smooth. Palpebral lobe large, crescentic in outline, exsagittal length 41% of sagittal glabellar length, and posterior end overhangs posterior area of fixigenae. Palpebral furrow triangular in outline and shallow. Palpebral area of fixigenae narrow, elongated, and moderately carinated. Occipital furrow wide and gently curved forwards and shallows sagittally. Occipital ring spindle−shaped and distal end sharply curved forwards; occipital node weakly developed. Posterior cranidial border narrow and rimmed in adaxial two−thirds, and broad with rounded postero−lateral margin in abaxial one−third; posterior cranidial border furrow moderately deep and wide, becoming shallow to disappear distally. Postocular suture runs transverse and then sharply curved posteriorly.

More than 10 thoracic segments present. Axis slightly tapers backwards; axial ring narrows and curved forwards distally; axial furrow deep and wide. Pleura divided into adaxial and abaxial region by fulcral line; elongated (tr.) strongly raised ridge with slightly bulbous ends present in adaxial region; shallow furrow developed anterior and posterior to the ridge; abaxial region slightly obliquely directed backwards and diagonally divided into raised, flat band and depressed, flat articulating facet.

Pygidium semi−circular in outline. Axis gently tapers backwards with five axial rings and terminal piece; interring furrow moderately deep and become shallow and narrow adaxially, and gently curved backwards sagittally; terminal piece elongated (tr.) triangular in outline; postaxial ridge narrow (more distinct in internal moulds). Five pleural ribs present; each widens distally, and gently curved backwards and downwards distally; postero−most pair fused posteriorly and adaxially. Pleural furrow moderately deep and wide. Pygidial margin sawtooth−shaped due to distal projection of pleural ribs. No marginal border present.

Remarks.— Pliomerina tashanensis sp. nov. is most similar to Pliomerina trisulcata Edgecombe, Banks, and Banks, 1999a from Tasmania. However, P. tashanensis is readily distinguished by having the abaxial end of S3 located posterior to antero−lateral glabellar corner. Other species including P. trisulcata have the end anterior to the corner; see P. trisulcata Edgecombe et al. (1999a : fig. 10D), Pliomerina peripata Edgecombe et al. (1999b : figs. 1.1–1.5) from Argentina, Pliomerina martelli ( Reed, 1917) (see Sheng 1974: pl. 7: 5a–c) from Yunnan, and Pliomerina serrata Zhou and Zhou (2006 : fig. 4A, B) from Inner Mongolia. Posterior margins of pygidial axial rings of P. tashanensis are more strongly curved backwards (see Fig. 11O for cast of external mould), compared with P. trisulcata (see Edgecombe et al. 1999a: fig. 9F). Other species have a straight posterior margin; for example, see Pliomerina austrina Webby (1971 : pl. 114: 8), P. peripata Edgecombe et al. (1999b : figs. 1.15–1.18).

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.