Remopleurides xiazhenensis, 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 : 857-860

Remopleurides xiazhenensis sp. nov.

Figs. 3A , 4, 5A, B, F–K, M.

Etymology: After the Xiazhen Formation where the species occurs.

Holotype: NIGP−151979, an articulated specimen from locality 5.

Type locality: Locality 5, about 800 m south of Zhuzhai, Yushan County. GPS coordinate is 28 ° 33’57.97” N and 118 ° 20’25.87” E .

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

Material.—Four articulated exoskeletons, ten thoraco−pygidia, two thoraces, and two cranidia (repository numbers: NIGP−151978, 151979, 151981–151983, 151985–151990)

Diagnosis.—Species of Remopleurides with non−spinose, entire pygidial margin and longitudinal ridges on occipital ring and thoracic axial rings.

Description.—Dorsal exoskeleton elliptical in outline, gently tapers backwards, and moderately convex. Cephalon semi−circular in outline. Axial furrow extremely narrow and deep. Glabella sub−circular in outline with maximum (tr.) width at

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mid−palpebral point; postero−most part waisted and defined by axial furrows that are straight and run moderately convergent backwards; most glabellar area nearly flat. Glabellar tongue rapidly bends downwards and gently tapers forwards. Glabellar surface covered with fine, sub−parallel terrace lines arranged in concentric fashion that become indistinct toward center of glabella (more clearly seen in external mold). Occipital ring elongated (tr.) spindle−shaped; lateral profile mushroom−shaped (nearly flat, laterally expanded dorsal part and constricted middle part); base protruded outwards in triangular shape beyond dorsal outline of occipital ring; about 20 longitudinal ridges developed that are slightly curved outwards and become indistinct adaxially. Occipital furrow straight and deep. Palpebral lobe narrow. Preocular and palpebral area of fixigenae absent; postocular area narrow and triangular in outline. Posterior branch of facial suture sinusoidal.

Librigena with moderately long, stout genal spine; librigenal field gently slope downwards and ornamented with moderately raised terrace−ridges. Eye large and crescentic in outline with exsagittal length being 80% of glabellar sagittal length; eye socle narrow and eye socle furrow moderately deep and wide. Posterior librigenal border narrow and disappear distally. Presence of lateral librigenal border and furrow not observed.

Thorax of 11 segments. Axis gently tapers backwards; maximum width (located between second to fourth segment) about 60% of maximum thoracic width. Axial ring strongly convex and lateral profile identical to occipital ring; up to about 20 longitudinal ridges present; number of ridges progressively reduced towards posterior segments. Articulating furrow wide, steep−sided, and deep but become shallow adaxially. Articulating half ring with a pair of elongated pits; each pit located at adaxial one−quarter of axial ring; anterior margin appears to be broadly notched medially, forming dish−shaped outline. Axial spine on the eighth (from the anterior) segment projected posteriorly and narrow−based; exact length not known. Pleura rhomboidal in outline. Pleural furrow arrowhead−shaped in outline, obliquely directed, and separates pleura into two bands of unequal size. Anterior pleural band much larger than posterior one; anterior band convex, gently slopes forwards and steeply backwards; posterior band convex with equal steepness; dorsal surface of anterior band ornamented with fine terrace lines parallel to outer pleural margin, whereas no distinct lines on posterior band. Fulcral process large, strongly raised, moderately curved distally, and located adaxial one−third of pleura; fulcral socket follows outline of fulcral process. Pleural doublure ornamented with fine terrace lines that run parallel to outer pleural margin; crescentic articulating facet present on posterior half of doublure; dorsal surface of articulating facet smooth.

Pygidium small and sub−elliptical in outline. One axial ring present and divided into two strongly convex lobes by sagittal furrow. Fulcral process narrow and elongated. No pleural region present. Pleural border slightly concave. Pygidial margin non−spinose. Pygidial doublure ornamented with fine terrace lines that are parallel to pygidial margin.

Remarks.—Morphological features that distinguish Remopleurides xiazhenensis sp. nov. from other Remopleurides species are: (i) non−spinose, entire pygidial margin (see Figs. 4C 2 , 5B 2 , G), (ii) longitudinal ridges on the occipital ring and thoracic axial rings (see Fig. 4A 1 , C 3 ), and (iii) a pair of elongated pits on thoracic articulating half ring (see Fig. 4A 1 ).

(i) Non−spinose pygidial margin: Remopleurides xiazhenensis sp. nov. has an entire pygidial margin (see Fig. 3A 1 ), while most Remopleurides species have two pairs of blade−shaped spines along pygidial margin; for example, Remopleurides colbii Portlock, 1843 (see Whittington 1950: pl. 70: 1), Remopleurides eximius Whittington (1959 : pl. 15: 7), Remopleurides pattersoni Chatterton and Ludvigsen (1976 : pl. 1: 49). There are several Remopleurides species from China and Kazkhstan for which thoraco−pygidial specimens showing the similar overall morphology to R. xiazhenensis are figured: R. taliangensis Chang and Fan, 1960 from Qinghai, China; and R. qilangensis Zhang, 1981 , R. qiakuertensis Zhang, 1981 from Xinjiang, China; R. pisiformis Weber, 1948 , R. mukatchensis nordicus Koroleva, 1982 , R. mukatchensis fastigatus Koroleva, 1982 , R. giganteus Koroleva, 1965 , R. triplexus Koroleva, 1982 , R. cf. cinghizus from Kazakhstan.

Of these, the presence of a small pygidium with non−spinose margin is confirmed for R. mukatchensis fastigatus Koroleva (1982 : pl. 7: 4, 5) and R. cf. cinghizus ( Koroleva 1982: pl. 14: 4a). The pygidia of both species, however, dif−

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fer from those of Remopleurides xiazhenensis sp. nov. in having concentric terrace lines (for the first) and straight ridges (for the second species) on the doublure of marginal border; R. xiazhenensis has terrace lines that are parallel to the border ( Figs. 4C 2 , 5B 2 ). Although the specimens of other species apparently have a small pygidium, they are too poorly preserved to compare their posterior margin in detail.

(ii) Longitudinal ridges on occipital ring and thoracic axial rings: Remopleurides species show various ornaments on occipital ring and thoracic axial rings; for example, transverse terrace lines and tubercles are developed in Remopleurides perspicax Nikolaisen (1983 : see pl. 6: 6, 8) and R. eximius Whittington (1959 : pl. 15: 7), concentric terrace lines in R. amphitryonoides Lu, 1975 (see Zhou et al. 2005: pl. 2: 13, 17) and R. cf. exallos ( Edgecombe and Webby 2007: fig. 12G, H), and tubercles in R. pattersoni Chatterton and Ludvigsen (1976 : pl. 1: 49). The longitudinal ridges are present in such Kazakhstan Remopleurides species as R. pisiformis (see Weber 1948: pl. 2: 29a, b; Koroleva 1982: pl. 9: 3a, b), R. cinghizus Koroleva (1982 : pl. 14: 1, 2), and R. akdombackensis Koroleva (1982 : pl. 15: 2). In regard to glabellar morphology, R. pisiformis ( Weber 1948: pl. 2: 27–29; see also Koroleva 1982: pl. 8: 3a, pl. 9: 1) is most similar to R. xiazhenensis sp. nov.; R. cinghizus differs in having tuberculated glabellar surface ( Koroleva 1982: pl. 13: 9) and R. akdombackensis in having more elongated (sag.) glabellar tongue ( Koroleva 1982: pl. 15: 1). Cranidia of R. pisiformis differs in having terrace lines that cover the entire glabellar surface and are arranged in fingerprint−like fashion (terrace lines in R. xiazhenensis are strongly developed on anterior and lateral glabellar area and become indistinct toward the glabellar central area; see Fig. 4A 3 ), and three pairs of glabellar furrows (no glabellar furrows are recognized in R. xiazhenensis ). It cannot be determined whether or not R. pisiformis has a non−spinose, entire pygidial margin from the illustrations. However, Koroleva (1982: 71) mentioned the presence of a notch along pygidial margin as a coaptative device that is well observed in Remopleurides perspicax ( Nikolaisen 1983: pl. 5: 2, 4), which suggests the presence of a spinose posterior pygidial margin.

The two Remopleurides species which have a non−spinose pygidial margin, R. mukatchensis fastigatus Koroleva (1982) and R. cf. cinghizus ( Koroleva 1982) have transverse terrace lines (in the first species, see Koroleva 1982: pl. 7: 5) and tubercles (in the second, see Koroleva 1982: pl. 14: 4a).

(iii) A pair of elongated (tr.) pits on articulating half rings: The paired pits are observed where dorsal exoskeleton and doublure of the thoracic axial rings is partially exfoliated (see Fig. 4A 1 , C 3 ). Since the ventral surface of the axial rings is not available, it cannot be determined whether the paired pit has a corresponding protruded structure on the doublure of axial rings. Silicified specimens of Remopleurides do not have a comparable ventral projection from the thoracic axial rings (see Whittington 1959: pl. 12: 17, pl. 14: 2; Chatterton and Ludvigsen 1976: pl. 1: 17, 31). The function of this paired pit is yet to be understood.

The ventral surface of the silicified specimens shows that the anterior margin of the articulating half rings is broadly notched posteriorly. In Remopleurides xiazhenensis sp. nov., the preserved portion of the doublure of axial rings shows a sharp, dish−shaped posterior margin (see eighth thoracic segment of Fig. 4A 1 and third thoracic segment of Fig. 4C 3 ), which has a similar outline as in the silicified specimens. The paired pits are located right at the boundary of the posterior margin (see Fig. 4C 3 ), indicating that the margin may not truly reflect the notched anterior margin of the articulating half ring. However, such sharpness of the posterior margin implies that the anterior margin of the articulating half rings is notched (the exact extent is not known) but post−mortem deformation has slightly displaced the thoracic segments, resulting in the configuration as it is.

Except for the last feature which is only discernible where the specimen is properly exfoliated, none of Remopleurides species bear both non−spinose pygidial margin and longitudinal ridges on thoracic axial rings.

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.

Fig. 2. Outcrop photographs of sampling localities. Hammer in C, E, and F is about 26 cm long. A. Mudstone outcrop at locality 2; water bottle in the elipse is about 20 cm high. B. Close−up view of mudstone at locality 2; pen is about 13.5 cm long. C. Mudstone outcrop at locality 1. D. Brachiopod lag deposit supposedly underlying mudstone at locality 1; camera lens cap is about 5 cm in diameter. E. Mudstone outcrop at locality 3; a trench was dug to collect specimens. F. Mudstone outcrop at locality 4.

Fig. 1. Location of trilobite collecting localities at Zhuzhai, Jiangxi Province, China. GPS coordinate of the locality 2 is 28°34’23.27” N and 118°20’16.50” E. A. The map of China showing the sampling locality (the black rectangle). B. The magnified view of the rectangled area in A. C. Geologic map of the sampling locality.