Astutuscaris bispinifer sp. nov.

Figs. 1, 2A 1, A 2, B 1 –B 3.

Zoobank LCID: urn:lsid:zoobank.org:act: D09B7A08-1422-484C- A6AD-0204E2E9F55C

Etymology: From Latin bispinifer, two-spined; referring to its two terminal spines on the end of the body.

Holotype: RCP 0002, an articulated specimen (Fig. 1).

Type locality: The outcrop near Lihuazhuang village (see Jiao et al. 2021b: fig. 1), Yiliang, Kunming, Yunnan, China.

Type horizon: Lower part of Wulongqing Formation, Palaeolenus Biozone, Cambrian Series 2, Stage 4 (Hu et al. 2010; Jiao et al. 2021b).

Material.— Type material only.

Diagnosis.—Small elongate euarthropod with a pair of prominent and bull-horn-like first appendages without sturdy spines and elbow articulations, a wide head shield potentially possessing a pair of antero-median eyes, an elongated thorax consisting of eleven imbricated and homonomous tergites, second to eighth thoracic tergites having lateral doublures and pleural spines, and two long separate spines attached to the last tergite.

Description. —The single nearly complete specimen, dorsoventrally compressed, has a length of 8.6 mm (excluding the first appendage pair), and a width of 2.2 mm (Fig. 1A 1 –A 3, B 1 –B 4). The body is about four times as long as wide in dorsal view. The dorsal exoskeleton consists of a wide head shield and 11 imbricated tergites, and terminates with two sturdy separate spines which are maybe modified, posteriorly directed appendages of the terminal somite (Figs. 1A 1, A 2, B 1 –B 4, 2A 1, A 2, B 1 –B 3). The wide cephalon is incomplete, but it may have a semi-elliptical outline (Figs. 1A 1 –A 4, B 1 –B 4, 2A 1, A 2, B 1 –B 3). The black carbonaceous structure preserved as a circular shape and located in the anterior and middle part of head is here interpreted as an uncertain structure given that it locates on a different level than the rest of the structures (Figs. 1B 1, B 2, B 6, 2A 1, A 2, B 1 –B 3). The left frontalmost appendage seems to be an antenna although having no distinct segments, while the right one is likely to be a great appendage for some small and medial spines on its possible last podomere (Fig. 1A 1 –A 4, B 1 –B 4). Considering that the anterior sclerite of fuxianhuiids and some bivalved euarthropods is a complete structure (Fu et al. 2018; Jin et al. 2021), two disjunct, elliptical structures located anterior to the proximal articles of frontalmost appendages are likely to be the eyes for the oval structures and elongate structures which here are respectively interpreted as possible eyes and stalks (Fig. 1A 1 –A 4, B 1 –B 4).

There are 11 imbricated tergites (Tn) behind the head (Figs. 1A 1, A 2, B 1 –B 4, 2A 1, A 2, B 1 –B 3). Incomplete anterior margin of T1, having a similar curvature to that of the successive tergites, is preserved on both part and counterpart (Fig. 1A 1, A 2, B 1 –B 4, B 8). Anterior margin of T3 is straight, and posterior margins of T5–T9 and T11 are straight (Fig. 1A 1, A 2, B 1 –B 4). The tergopleura of T3–T8 curve backwards, and their pleural spines are almost posteriorly oriented (Fig. 1A 1, A 2, B 1 –B 5). The tergopleura of T2–T8 have narrow doublures (Fig. 1A 1, A 2, B 1 –B 5). T2–T10 have a similar length. T11 is slightly longer than the preceding tergites and lacks pleural spines (Figs. 1A 1, A 2, B 1 –B 4, B 9, 2A 1, A 2, B 1 –B 3). One robust endopod underneath the head shield preserves at least five conical podomeres (Fig. 1A 3, A 4, A 6). Four elongate, red structures (“en” in Figs. 1A 1, A 2, B 1 –B 4, B 7, B 8, 2A 1, A 2, B 1 –B 3), represent trunk endopods, although they do not display obvious segments. These endopods, one located underneath T1 and T2, and two beneath T3 (Fig. 2A 1, A 2, B 1 –B 3), indicate there is one pair limb belonging to each of these three tergites. The smaller structure, relative to the endopod, of the limb beneath T3 is tentatively interpreted as exopod (Figs. 1B 7, 2A 2, B 2, B 3), further clarification of its morphology is contingent on the discovery of additional specimens. The irregular line passing along the entire length of the body may represent the gut (Fig. 1A 1, A 2, B 1 –B 4).

Remarks.— The elongate exoskeleton, the possible eyes and the first pair of appendages located antero-medially on the head, and the curvature of the first appendages are also observed in the great-appendage euarthropods (Haug et al. 2012b; Aria et al. 2020; Liu et al. 2020). The morphology of the frontalmost appendages, 11 trunk tergites, the narrow doublures of tergites, and the two terminal spines of Astutuscaris contribute to distinguish this new euarthropod genus from Yohoia (Haug et al. 2012b). The morphology of the first appendage of Astutuscaris and the pair of posterior terminal spines differ from those of other megacheirans, e.g., leanchoiliids and jianfengiids (Haug et al. 2012a, b; Aria et al. 2020; Liu et al. 2020)

Astutuscaris is distinguishable from Guangweicaris by its curving frontalmost appendages, and elongate exoskeleton without medial axial spines and distinct sections (e.g., the abruptly reduced abdomen). Guangweicaris possesses a wide head sheild with a pair of long antennae, a thorax composed of three small tergites and five normal tergites, and an abruptly reduced abdomen (Fig. 2C, D) consisting of seven tergites all bearing a axial spine (Luo et al. 2007; Yang et al. 2008; Wu and Liu 2019; Chen et al. 2020). The curving first appendages of Astutuscaris are different from the long antennae (at least 21 podomeres) of Guangweicaris (Yang et al. 2008; Chen et al. 2020). Unlike the trunk of Guangweicaris including a suddenly reduced abdomen (Yang et al. 2008; Wu and Liu 2019; Chen et al. 2020), Astutuscaris has a trunk gradually tapering backward. The last tergite of Guangweicaris bears an axial spine (Yang et al. 2008; Chen et al. 2020), while, Astutuscaris has no axial spines. The posterior end of Astutuscaris composes of two isolated spines, and the tail of Guangweicaris possesses a ventral medial extension and two lateral processes (Chen et al. 2020).

Astutuscaris bispinifer gen. et sp. nov. is different from the stages 8 and 9 of Fuxianhuia protensa Hou, 1987, by its at least 11 trunk tergites, one pair endopod under normal tergites two and three respectively, and last tergite carrying two long and separate spines. The juveniles (stages 8 and 9) of F. protensa are about 11 mm to 15 mm long, and consist of eight or nine trunk tergites which have a pattern of segmental mismatch in thoracic region and a terminal telson with paired caudal flukes (Fu et al. 2018).