Tsaidamaspis diarmatus 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 : 685-688

Tsaidamaspis diarmatus Chang and Fan, 1960

Figs. 3 , 4 .

1960 Plesiomegalaspis (Tsaidamaspis) diarmatus subgen. nov. sp. nov.; Chang and Fan 1960: 123, pl. 6: 1–5, 8, non 6, 7.

1960 Isotelus tsinghaiensis sp. nov.; Chang and Fan 1960: 124, pl. 6: 12.

1963 Tsaidamaspis diarmatus Chang and Fan ; Lu et al. 1963: 39, pl. 9: 3, 5, 6, non 4.

1965 Megalaspidella (Tsaidamaspis) diarmatus Chang and Fan ; Lu et al. 1965: 497, pl. 100: 10, 11, 13, non 12.

1979 Megalaspidella (Tsaidamaspis) diarmatus Chang and Fan ; Zhu et al. 1979: 111, pl. 45: 13–16.

2008 Tsaidamaspis diarmatus Chang and Fan ; Zhou and Zhou 2008: 239.

Holotype: NIGP 10109 , pygidium, see Chang and Fan (1960: 123, pl.

6: 4).

Type locality: Shihuigou, northwestern Qinghai Province, China.

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

Material.— An incomplete cranidium, two librigenae, one hypostome, and four pygidia ( NIGP 203156–203163 ), all from the type locality and horizon .

Description.—Cranidium is gently convex, with flattened anterior border, the width (tr.) of which is equal to about 70% of the cranidial length (sag.), and the length (sag.) of which occupies 30% of the cranidial length (sag.). Glabella is gently convex, and slightly narrowed at the level of S2, rounded anteriorly, 58% as wide as long. Preglabellar furrow is deepened laterally, medially shallow into the V-shaped median furrow on the frontal glabellar lobe. A prominent median tubercle is adjacent to occipital furrow. Bacculae are low, elongately oval in outline, and isolated by S1 and deep axial furrows. There are five pairs of shallow glabellar furrows; S1 extends rearwards and inward from the rear of constriction of glabella to connect at about 10% of occipital furrow; S2 is short (tr.) and suboval in outline, running backwards directed slightly; S3 is longer than S2 and almost transversely directed; S4 is long, suboval in outline, and forwards directed at 40° to the sagittal line; S5 is short and faint, extending forwards at 70° to the sagittal line. Axial furrow is deep, becoming relatively shallower forwards. Occipital ring is poorly preserved, about 10% of the glabellar length, defined by a broad (sag. and exsag.) and shallow occipital furrow. Palpebral lobe is small and semicircular, occupying about 10% of cranidial length (sag.). Anterior branches of facial sutures are divergent forward, curving inward at a rounded angle, then running rapidly forward and meeting at a pointed angle; posterior branches are long, extending posterolaterally. Palpebral area of fixigena is narrow (tr.) and strongly convex. Posterior area of fixigena is broad (tr.). Librigena is broad (tr.) and gently convex, with broad-based genal spine directed rearward and outward. Lateral border of librigena is flattened, narrowing rearwards. Posterior border of librigena is faintly defined. Eye is raised above the level of the rest of the librigena.

Hypostome is forked, with a broadly rounded median notch, about 80% as wide as long. Median body is oval, convex (tr.), and defined by shallow and broad lateral bor- der furrows. Middle furrow is shallow and broad with a pair of ridge-like maculae. Lateral border widening posteriorly. Posterior border is broadly based, bluntly pointed posteriorly. Anterior wing is rounded triangular and sloping downwards laterally. Surface is covered with terrace lines subparallel to lateral and posterior borders.

Pygidium is triangular, about 93–96% as long as wide. Axis is long and conical, narrow and convex (tr.), tapering gradually rearwards, and defined by shallow axial furrows, occupying about 73–77% of the pygidial length and 24% of the anterior pygidial width. There are at least 20 axial rings and a rounded terminal piece, which are separated by shallow ring furrows. Articulating half ring is narrow (sag.), and rounded anteriorly. Pleural field is moderately convex, curving downwards laterally and posteriorly to border. Pleural field has at least 14 pleural furrows, which extend rearward and outward to border furrow. Triangular facets present at anterolateral corners of pygidium. Pygidial border is flattened and defined by faint border furrow. Pygidial border extended rearwards as a pair of short and stout posterior border spines, which occupy about 9% of the pygidial length. Surface of pygidium is covered with irregular lines.

Remarks.—An incomplete hypostome figured by Chang and Fan (1960: 123, pl. 6: 6, 7) from the Duoquanshan Formation of Shihuigou, northwestern Qinghai Province, was assigned to Tsaidamaspis diarmatus by Chang and Fan (1960), and this view was followed by Lu et al. (1963: 39, pl. 9: 4; 1965: 497, pl. 100: 12). However, this hypostome exhibits a shallow median notch and a pair of rounded posterior borders, which agree well with species of the new genus Zhiyia (see below), and should be excluded from T. diarmatus . Another hypostome collected from the same horizon by Chang and Fan (1960: 124, pl. 6: 12, as Isotelus tsinghaiensis ) is reassigned herein to T. diarmatus based on a deep median notch and forked posterior border.

Tsaidamaspis ? sp. was recorded by Chang and Fan (1960: 124, pl. 5: 13, 14) based on one poorly preserved pygidium from the Floian strata (Lower Ordovician) from Yumen of western Gansu Province, showing the outline of triangular pygidium with a narrow (tr.) pygidial axis and a great number of axial rings and pleural furrows. As the specimen is poorly preserved, further materials are required to ascertain whether it ought to be assigned to Tsaidamaspis .

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

Genus Zhiyia nov.

Zoobank LSID: urn:lsid:zoobank.org:act:BEBEF30D-A413-4684-A27E-62FCA94B7BBF

Etymology: In memory of the late Professor Zhou Zhiyi (1939–2022), of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences ( NIGPAS), who have made great contributions to the research of trilobites in China.

Type species: Isotelus tsinghaiensis Chang and Fan, 1960 ; Floian, Low- er Ordovician, Duoquanshan Formation, northwestern Qinghai Province, China.

Species included: Type species, Niobe (Niobella) obscura Zhou and Zhou, 2019 . Zhiyia sp. nov. (i.e., Asaphid gen. et sp. indet. of Chen et al. 2023).

Diagnosis.—Isoteline genus with almost obsolete cephalic and pygidial axial furrows. Anterior border of cranidium is flattened and pointed forward, defined by obvious preglabellar furrow. Occipital ring is very narrow (sag. and exsag.). Hypostome is oval in outline, with a shallow median notch and a small triangular median projection between the bilobed posterior projections. Pygidium is subsemicircular in outline, with a broad (tr.) pygidial axis. Pygidial border is wide (sag.), defined by a broad and shallow border furrow. Surface of cephalic and pygidial borders is smooth or covered with transverse terrace lines.

Remarks.—The diagnostic features listed above are based mainly on the specimens of the late Floian (Early Ordovician) species Isotelus tsinghaiensis Chang and Fan, 1960 , and Niobe (Niobella) obscura Zhou and Zhou, 2019 , from middle Floian (Lower Ordovician), Madaoyu Formation, Liexi, western Hunan Province. The middle Floian Asaphid gen. et sp. indet. of Chen et al. (2023: 107–109, figs. 6–12) from the Madaoyu Formation of western Hunan Province is also considered to belong to the new genus on the basis of its almost obsolete cephalic and pygidial axial furrows, a flattened anterior border of the cranidium, a narrow occipital ring, an oval hypostome with a shallow median notch and bilobed posterior projections, and a subsemicircular pygidium with a wide border.

Two species belonging to the new genus were previously assigned to the genera Isotelus , Homotelus , and Niobe (Niobella) (see Chang and Fan 1960: 124; Zhou and Zhou 2008: 237; 2019: 156). However, Zhiyia differs from Isotelus Dekay, 1824 , in having the cephalon more rounded forwardly, a shorter and subsemicircular pygidium with a shorter axis and wider border, and an oval hypostome with a longer anterior lobe and a shallower median notch with a small triangular median projection. Compared with Homotelus Raymond, 1920 , the new genus has a flattened anterior border defined by obvious preglabellar furrow and a larger palpebral lobe, while the pygidium has almost obsolete axial furrows and a wide border defined by shallow border furrow. Isoteline Zhiyia is distinguished from Niobe Niobella) Reed, 1931, and its allies of the Niobinae mainly by its almost obsolete cephalic and pygidial axial furrows, narrower (sag. and exsag.) occipital ring, shorter hypostome and the pygidium with wider axis.

With regard to the general outline and structure of the hypostome and pygidium, Zhiyia is very similar to Protopresbynileus Hintze, 1954 (= Pseudonileus Hintze, 1953 ) from the Fillmore Formation (Lower Ordovician) of western USA (see type species Pseudonileus willdeni Hintze, 1953: 224 , pl. 15: 14–17), but is distinguished by the flattened anterior border of the cranidium and wider librigena having a f lattened border, by the obvious pygidial border and lacking a median node on the pygidial doublure. In addition to Protopresbynileus , Ptyocephalus Whittington, 1948 , of the Lower Ordovician from the Pogonip of Nevada, USA (see Ptyocephalus vigilans Whittington, 1948: 567 , pl. 82: 1–7, pl. 83: 1–11) has a border to the cranidium as in Zhiyia , but the pygidium is pentagonal in outline with a longer axis.

Similar morphology of the hypostome and pygidium is also present in questionable species of Lachnostoma Ross, 1951 , i.e., Lachnostoma platypyga Fortey and Bruton (2013: 101 , fig. 39) from the Kirtonryggen Formation (Lower Ordovician) of Spitsbergen, which is easily distinguished from the new genus as it has a strongly waisted and longer glabella, a larger palpebral lobe, shorter (exsag.) posterior area of the fixigena, and a narrower (tr.) pygidial axis. However, L. platypyga is different from the type species Lachnostoma latucelsum in having a much larger, more anteriorly positioned palpebral lobe, a much less divergent anterior section of the facial sutures and a hypostome without marginal spines. Thus, this species may not be correctly assigned to Lachnostoma .

Fitzroyaspis irritans Fortey and Shergold, 1984 , was established by Fortey and Shergold (1984) from the Nora Formation (Lower Ordovician) of central Australia, and then transferred by Laurie (2006: 309) to genus Eisarkaspis Laurie, 2006 . Its cranidium as the holotype (see Fortey and Shergold 1984: 338, pl. 42: 2) is rather similar to that of Zhiyia in having almost obsolete cephalic axial furrows and the flattened anterior border, but other materials assigned to this species may be questionable because they are collected from different sections and horizons. Therefore, it is difficult to determine whether this species might be assigned to Zhiyia . In addition, Eisarkaspis from the Horn Valley Siltstone and basal Stairway Sandstone (upper Floian–middle Darriwilian) of Amadeus Basin (see type species E. etheridgei Laurie, 2006: 309 , fig. 17–19) resembles Zhiyia in the cranidial and hypostomal morphology, but the former has a deeper lateral border furrow of the librigena, a hypostome without a small triangular median projection and a pygidium with narrower and longer axis as well as well-defined border furrow.

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

Figures

Fig. 3. Asaphid trilobite Tsaidamaspis diarmatus Chang and Fan, 1960, from the middle part of the Duoquanshan Formation (upper Floian, Lower Ordovician) of Shihuigou, Delingha, northern Qinghai Province, China. A. NIGP 203156, cranidium. B. NIGP 203157, librigena. C. NIGP 203158, librigena. D. NIGP 203159, hypostome in dorsal (D1) and lateral (D2) views. E. NIGP 203160, pygidium. F. NIGP 203161, pygidium. G. NIGP 203162, pygidium. H. NIGP 203163, pygidium. Scale bars 5 mm.

Fig. 4. Reconstruction of the asaphid trilobite Tsaidamaspis diarmatus Chang and Fan, 1960. Not to scale.