Tsaidamaspis 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

Genus Tsaidamaspis Chang and Fan, 1960

Type species: Plesiomegalaspis (Tsaidamaspis) diarmatus Chang and Fan, 1960 ; Floian , Lower Ordovician, Duoquanshan Formation, northwestern Qinghai Province, China .

Emended diagnosis.—Cephalon is semicircular with genal spines. Glabella is subrectangular in outline, defined by deep axial furrows. There are five pairs of glabellar furrows and a V-shaped median furrow on the frontal glabellar lobe. Anterior border is broad (sag.) and flattened. Palpebral lobe is small. 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. Hypostome is forked, with a broadly rounded median notch. Pygidium is triangular with a long (sag.) and narrow (tr.) axis. Pygidial border is declined laterally and extended posteriorly as a pair of short and stout posterior border spines (modified from Chang and Fan 1960: 123).

Remarks.— Tsaidamaspis was established by Chang and Fan (1960: 123) on the basis of the type species Tsaidamaspis diarmatus from the Duoquanshan Formation of Shihuigou, northwestern Qinghai Province, and regarded as a subgenus of Plesiomegalaspis Thoral, 1946 . However, the latter was considered by Fortey and Owens (1987) to be a junior synonym of Asaphellus Callaway, 1877 , which is distinctly different from Tsaidamaspis in having a much shorter preglabellar field, the hypostome with slightly concave and rounded posterior margin, and a semicircular pygidium without a pair of posterior border spines. Therefore, herein, we follow Lu et al. (1963: 39) and Zhou and Zhou (2008: 239) in giving Tsaidamaspis generic status. In addition, as noted by Laurie (2006) and Fortey and Bruton (2013), asaphids are difficult to classify due to major problem in the phylogeny. In this paper, we temporarily use the traditional classification and place Tsaidamaspis to subfamily Isotelinae .

The taxonomy of the Baltoscandian genus Megistaspis Jaanusson, 1956 , and its allies is much complicated (see Nielsen 1995; Hoel 1999). Of them, Megistaspis (Paramegistaspis) Balashova, 1976 , and Megistaspis (Ekeraspis) Tjernvik, 1956 , are rather similar to Tsaidamaspis in cranidial morphology. Some species of M. (Paramegistaspis), such as M. (P.) planilimbata ( Angelin, 1851) from the lower Floian and M. (P.) estonica ( Tjernvik, 1956) from the upper Floian to lower Dapingian (Lower–Middle Ordovician), show the cranidium that has an obvious anterior border, the posterior margin of hypostome with a tongue-shaped median process or a shallow median notch, and the much shorter and subsemicircular pygidium without a pair of posterior border spines (see Tjernvik 1956: pl. 6), all of these features are clearly different from those of Tsaidamaspis species. Megistaspis (Ekeraspis) [see type species Plesiomegalaspis (Ekeraspis) armata Tjernvik, 1956: 242 , pl. 7: 7–13] of the lower part of Latorp Formation (upper Tremadocian, Lower Ordovician) of Västergötland, Sweden differs from Tsaidamaspis in having less divergent forwardly anterior branches of the facial suture, rounded posterior margin of hypostome, and the pygidium with only one median spine.