Zhiyia tsinghaiensis ( Chang and Fan, 1960 )

Wei, Xin & Zhou, Zhiqiang, 2023, Floian, Early Ordovician, trilobites from the Olongbluk Terrane, northwest China, Acta Palaeontologica Polonica 68 (4), pp. 683-693 : 688-691

Zhiyia tsinghaiensis ( Chang and Fan, 1960)

Figs. 5 , 6 .

1960 Isotelus tsinghaiensis sp. nov.; Chang and Fan 1960: 124, pl. 6: 9–11, 13–15, non 12.

1960 Plesiomegalaspis (Tsaidamaspis) diarmatus subgen. nov. sp. nov.; Chang and Fan 1960: 123, pl. 6: 6, 7.

1963 Tsaidamaspis diarmatus Chang and Fan ; Lu et al. 1963: 39, pl. 9: 4.

1965 Isotelus tsinghaiensis Chang and Fan ; Lu et al. 1965: 488, pl. 97: 12–16, non 17.

1965 Megalaspidella (Tsaidamaspis) diarmatus Chang and Fan ; Lu et al. 1965: 497, pl. 100: 12.

2008 Homalopyge tsinghaiensis (Chang and Fan) ; Zhou and Zhou 2008: 237.

Holotype: NIGP 10115 , cranidium, Chang and Fan (1960: 124, pl. 6: 10); see Fig. 5A .

Type locality: Shihuigou, northwestern Qinghai Province, China.

Type horizon: Duoquanshan Formation, Floian (Lower Ordovician).

Material.— Three cranidia, one librigena, two hypostomes and seven pygidia ( NIGP 203164–203176 ). All from the type locality and horizon .

Description.—Cranidium is gently convex (tr.), and its width between outer margins of palpebral lobes is about 87–91% of the sagittal length. Anterior border is flattened, bluntly pointed medially, and about 10% of cranidial length at the sagittal line, with transverse terrace lines. Elongated glabella is convex, broadly rounded and narrowing forwards, defined by obvious preglabellar furrow and shallower posteriorly axial furrows, about 78–88% as long as the width across the posterior ends of palpebral lobes. A prominent median tubercle is located at about 26% of the glabellar length (sag.) from the posterior. Axial furrows become shallower rearwards, and then almost totally effaced after the posterior ends of palpebral lobes. Four pairs of glabellar furrows are oval in shape, and shown as faint impressions on the exfoliated surface. S1 and S2 extend rearwards and inwards, and the latter opposite the level of the mid-length of the palpebral lobes; S3 and S4 directed forwards and inwards level with the anterior part of the palpebral lobes. Occipital ring is very narrow (sag. and exsag.) and slightly arched posteriorly, well separated from the rest of the glabella by a deep and broad (sag.) occipital furrow, and occupying about 5% of glabellar length (sag.). Palpebral lobe is gently convex, semicircular in outline, length (exsag.) equal to about 23–26% of the cranidial length (sag.), with the mid-length situated 40–42% of the cranidial length (sag.) from the posterior. Anterior branches of facial suture are divergent forward, curving inward at a rounded angle and meeting at a blunt angle in front; posterior branches extend posterolaterally. Posterior area of fixigena is long (exsag.), narrowing adaxially. Librigena is wide (tr.), gently convex (tr.), with rounded genal angle. Lateral border is flattened, defined by shallow lateral border furrow, with terrace lines parallel to margin. Posterior border furrow is effaced. Eye is semicircular and raised above the level of the librigena.

Hypostome is oval in outline, with a shallow median notch, about 95% as long as wide. Median body is oval, convex (tr.), and defined by shallow and broad (tr.) lateral border furrows and deep posterior furrow. Middle furrow is shallow and broad (sag.), with a pair of oval maculae situated at about 31% of the hypostomal length (sag.) from the posterior. Anterior lobe is subrectangular; posterior lobe is ridge-like, slightly arched anteriorly. Posterior border furrow is narrow (sag.), and slightly deeper than middle furrow. Lateral border occupies about 28% of hypostomal width. Posterior border has bilobed posterior projections and broadly based, rounded posteriorly. A small triangular median projection is situated between the bilobed posterior projections. Anterior wing is small and rounded. Surface of hypostome is covered with transverse lines.

Pygidium is subsemicircular in outline, about 58–67% as long as wide, with forwardly arched anterior margin. Axis is moderately convex (tr.), faintly defined by effaced axial furrows on exfoliated surface, occupying 46–53% of pygidial width at anterior margin, and 72–78% of pygidial length, tapering rapidly rearwards. Eight or nine rings are divided by axial ring furrows on exfoliated surface. Pleural field is gently sloping down towards border, with shallow anterior pleural furrow. Articulating facet is small and triangular in outline. Border is wide (sag.), about 21–28% of sagittal length of pygidium, sloping down towards margin, defined by a broad and shallow border furrow. Surface of pygidium is smooth, except for facet and border that are covered with approximately transverse terrace lines.

Remarks.—Our materials are identical to specimens of Zhiyia tsinghaiensis (as Isotelus tsinghaiensis by Chang and Fan 1960: 124, pl. 6: 9–11, 13–15, non 12; see also Fig. 5A ) from the same horizon and locality. It is very similar to Z. obscura (as Niobe (Niobella) obscura by Zhou and Zhou 2019: 156, fig. 5A–F) from the lower part of the Dawan Formation (upper Floian) of Yichang, western Hubei Province, but differs in having a pair of the more forwardly situated S1, smaller palpebral lobes, a relatively shorter pygidium, and obvious transverse terrace lines on the anterior border of the cranidium, the lateral border of the librigena and the pygidial border.

Of the Early Ordovician (middle Floian) species from Liexi, western Hunan Province, Asaphid gen. et sp. indet. collected from the Madaoyu Formation (see Chen et al. 2023: 107–109, figs. 6–12) is comparable with Z. tsinghaiensis particularly in its almost obsolete cephalic and pygidial axial furrows, but the former has a narrower cranidium, shallower preglabellar and occipital furrows, a longer pygidium, and smooth surface of the cranidium and pygidium. As is noted above it should be assigned to the new genus Zhiyia , and distinguished from all other described species, but until additional specimens become available, a new taxon is not formally introduced.

Stratigraphic and geographic range.—Floian, Lower Ordovician; Olongbluk terrane, NW China.

Figures

Fig.5.Asaphidtrilobite Zhiyia tsinghaiensis (Chang and Fan,1960), from the middlepart ofthe Duoquanshan Formation (upper Floian,Lower Ordovician) of Shihuigou, Delingha, northern Qinghai Province, China. A. NIGP10115, holotype, cranidium (refigured from Chang and Fan 1960: 124, pl. 6: 10). B. NIGP 203164, cranidium.C. NIGP 203165, cranidium. D. NIGP 203166, cranidium.E. NIGP 203167, librigena. F. NIGP 203168, juvenile hypostome. G. NIGP 203169, hypostome.H. NIGP 203170, pygidium. I. NIGP 203171, pygidium. J. NIGP 203172, pygidium. K. NIGP 203173, pygidium. L. NIGP 203174, pygidium. M. NIGP 203175, pygidium. N. NIGP 203176, pygidium. Scale bars:A–E, G–N 5 mm; F 2 mm.

Fig. 6. Reconstruction of the asaphid trilobite Zhiyia tsinghaiensis (Chang and Fan, 1960). Not to scale.