Xandarellida Chen, Ramsköld, Edgecombe, and Zhou

Zhu, Yuyan, Zeng, Han, Liu, Yao & Zhao, Fangchen, 2023, New artiopodan euarthropods from the Chengjiang fauna (Cambrian, Stage 3) at Malong, Yunnan, China, Acta Palaeontologica Polonica 68 (3), pp. 427-440 : 428-431

Order Xandarellida Chen, Ramsköld, Edgecombe, and Zhou in Chen et al., 1996

Family uncertain

Genus Zhugeia nov.

ZooBank LSID: urn:lsid:zoobank.org:act:10C13356-3E01-4721-9C8E-5E30E3097946

Type species: Zhugeia acuticaudata gen. et sp. nov.; see below, by monotypy.

Etymology: For the Zhuge Mountain where the Kuangshan section is located and where the respected prime minister Liang Zhuge of Shu- Han once camped at about 225 A.D. during the Three Kingdoms Period of ancient China.

Diagnosis.—Semi-elliptical cephalon with a pair of genal spines. A pair of ovoid lateral eyes near anterior margin of cephalon. Cephalon posteriorly covering two thoracic tergites. Nine partially overlapping tergites with pleural spines. Micropygous pygidium with two small lateral spines and an elongated needle-like median spine.

Remarks.—The order Xandarellida is diagnosed by the semicircular cephalic shield with stalk lateral eyes which extend posteriorly to cover multiple thoracic tergites and pygidium with broad median spine ( Chen et al. 1996; Ramsköld et al. 1997; Chen et al. 2019). Zhugeia complies with the diagnosis of Xandarellida , such as a cephalic shield covering at least two thoracic tergites and pygidium with a median spine. Zhugeia differs from the other xandarellids in various aspects (see more details in the Remarks of species). Thus, Zhugeia is a new genus of Xandarellida .

Zhugeia acuticaudata gen. et sp. nov.

Fig. 1.

ZooBank LSID: urn:lsid:zoobank.org:act:224B5712-741A-44A1-B750-1D6EDFAF5705

Etymology: From Latin acuti spinous and caudata, tail.

Holotype: NIGP 200049 , specimen preserving most part of exoskeleton, part only (Fig. 1).

Type locality: Kuangshan section, Malong, Yunnan, China.

Type horizon: Maotianshan Shale Member, Yu’anshan Formation, Wutingaspis –Eoredlichia Trilobite Zone, Cambrian Series 2, Stage 3.

Material.— Type material only.

Diagnosis.—As for the genus.

Description.—Suboval exoskeleton consists of cephalon, thorax, and pygidium, measuring 49 mm long (including the median spine on pygidium) and 23 mm wide.

Cephalon is semi-circular. Anterior margin of cephalon is rounded, and posterior margin is approximately straight (Fig. 1A 3: ce). A pair of broad genal spines reach posteriorly to the third thoracic tergite, measuring 19 mm long and 23 mm wide (Fig. 1A 3, D: gs). Two long ovoid lateral eyes are situated near the anterior margin of cephalon (Fig. 1A 3, B, C: e). Cephalon extends posteriorly to cover two anteriormost thoracic tergites (Fig. 1A 3, E: ct1, ct2). Cephalon excluding genal angles occupies approximately one-third of whole-body length.

Except for two anteriormost thoracic tergites covered by cephalon, thoracic region, measuring 17 mm long, consists of nine partially overlapping tergites. Tergites of thorax have overlapping areas. The average overlapping area accounts for about one-fourth to one-third of the sagittal length of the tergite. The first six thoracic tergites, which are almost equal in sagittal length, have nearly straight posterior margins (Fig. 1E: t1–t6). The last three thoracic tergites, which are progressively longer in sagittal length, are curved posteriorly (Fig. 1F: t7–t9, Table 1). Anterior thoracic tergites are almost as wide as cephalon. The fourth thoracic tergite is the broadest with maximum width of 19 mm, and following tergites narrow toward the rear. Lengths of pleural spines of thoracic tergites increase towards pygidium.

Pygidium is situated within a notch formed by pleural spines of posteriormost thoracic tergite. Teardrop-shaped pygidium, measuring 16 mm long including pygidial spine, bears a pair of triangular posterolateral pygidial spines and an elongated needle-like median pygidial spine. Median pygidial spine is slightly longer than main part of pygidial tergite (Fig. 1A 3, G: ms; Fig. 1A 3, H: ls).

Remarks.—Four genera have been erected in the order Xandarellida , including Xandarella Hou, Ramsköld, and Bergström, 1991 , Cindarella Chen, Ramsköld, Edgecombe, and Zhou in Chen et al., 1996, Luohuilinella Zhang, Fu, and Dai, 2012 , and Sinoburius Hou, Ramsköld, and Bergström, 1991 .

The cephalon of Xandarellida shows marked morphological variations. Zhugeia has a pair of broad genal spines similar to Sinoburius ( Hou and Bergström 1997; Chen et al. 2019), while Xandarella and Luohuilinella have acute genal angles rather than genal spines ( Hou and Bergström 1997; Zhang et al. 2012; Hou et al. 2018), and Cindarella has rounded genal angles ( Ramsköld et al. 1997). As in Xandarella , Sinoburius , and Cindarella , Zhugeia lacks the anterolateral eye notches typical of Luohuilinella ( Zhang et al. 2012; Hou et al. 2018). Regarding eyes, Zhugeia shares anteroventral lateral eyes that are incorporated into the dorsal cephalic shield with Xandarella and Sinoburius , in contrast to a pair of lateral stalked eyes positioned at the anterolateral cephalic margin in Luohuilinella and Cindarella ( Ramsköld et al. 1997; Zhang et al. 2012; Hou et al. 2018; Chen et al. 2019). Zhugeia has no eye slits associated with eyes that are present in Xandarella ( Hou and Bergström 1997) .

Other xandarellids exhibit different thoracic morphology from that of Zhugeia . The number of thoracic tergites of Zhugeia (9 tergites) is more than that in Sinoburius (7 tergites) and less than those in Xandarella (10 tergites), Cindarella (17 tergites), and Luohuilinella (27–30 tergites) ( Hou and Bergström 1997; Ramsköld et al. 1997; Zhang et al. 2012; Hou et al. 2018; Chen et al. 2019). As in Xandarella , Luohuilinella , and Cindarella , the anteriormost thoracic tergites of Zhugeia are almost as wide as the cephalon ( Hou and Bergström 1997; Ramsköld et al. 1997; Zhang et al. 2012; Hou et al. 2018), while the anteriormost thoracic tergites of Sinoburius are narrower than the cephalon ( Hou and Bergström 1997; Chen et al. 2019). In addition, the first thoracic tergite of Zhugeia is narrower than successive tergites, like Sinoburius ( Chen et al. 2019) . Thoracic tergites of Zhugeia narrow toward the rear, as shown in Xandarella , Cindarella , and Luohuilinella ( Hou and Bergström 1997; Ramsköld et al. 1997; Zhang et al. 2012; Hou et al. 2018), while in Sinoburius first thoracic tergite is the narrowest ( Hou and Bergström 1997; Chen et al. 2019).

Although all known members of Xandarellida have micropygous pygidium, their pygidia still show distinctions. Relative to width of cephalon, pygidium of Zhugeia is similar to that of Xandarella and is narrower than that of Sinoburius ( Hou and Bergström 1997; Chen et al. 2019). Zhugeia has a dorsal needle-like spine like Xandarella instead of the broad median spine of Sinoburius ( Hou and Bergström 1997; Chen et al. 2019). The micropygous pygidia of two species of Luohuilinella are smallest among xandarellids ( Zhang et al. 2012; Hou et al. 2018). A terminal spine is recognized in L. deletres ( Hou et al. 2018) , but other pygidial morphologies of Luohuilinella are unclear ( Zhang et al. 2012; Hou et al. 2018). Among xandarellids, only Cindarella lacks median pygidial spine ( Ramsköld et al. 1997). As in Xandarella and Cindarella , Zhugeia has a pair of small lateral spines, while Sinoburius has two pairs of lateral spines ( Hou and Bergström 1997; Ramsköld et al. 1997; Chen et al. 2019).

Stratigraphic and geographic range.— Type locality and horizon only.

Order and family incertae sedis

Genus Tonglaiia nov.

ZooBank LSID: urn:lsid:zoobank.org:act:DA8A8456-AB77-4C7A-AB23-336517CD1EDE

Etymology: After Tonglai, an ancient name of Malong where the fossil was discovered.

Type species: Tonglaiia bispinosa gen. et sp. nov.; see below, by monotypy.

Diagnosis.—Elliptical cephalon with a pair of marginal cephalic spines. Two oval eyes situated near the anterior margin of cephalon. Seven partially overlapping tergites with posterolateral marginal spines. Micropygous pygidium with a pair of posteriormost triangular spines.

Remarks.—The dorsal eyes of Tonglaiia are located in an anterior position which is similar to the aglaspidid counterparts. However, the cephalon of Tonglaiia has a rounded genal area and a unique pair of lateral marginal spines, lacking the characteristic genal spines of aglaspidids. The posterior margin of pygidium is intact, showing no evidence of the diagnostic tailspine in aglaspidids. Additionally, cuticle of Tonglaiia is non-biomineralized, another obvious difference from aglaspidids. With these distinctions, Tonglaiia does not comply the diagnosis of Aglaspidida and Vicissicaudata ( Lerosey-Aubril et al. 2017). The presence of pygidium in Tonglaiia is a typical character of trilobitomorph artiopodans ( Ortega-Hernández et al. 2013), but Tonglaiia is distinguished from other trilobitomorphs in several aspects, such as a suboval cephalon with special lateral marginal spines and a micropygous pygidium with two posterior spines. Therefore, Tonglaiia represents a new genus (see details in the remarks for the type species below).