A new euarthropod from the Cambrian Stage 4 Guanshan Biota of South China

,


Material and methods
The sole and articulated specimen (RCP 0002) of Astutuscaris bispinifer gen. sp. nov. and two specimens (RCP 0003 and RCP 0004) belonging to Guangweicaris spinatus were collected from Yiliang County which already provided exquisitely preserved fossils in the past (Jiao et al. 2016(Jiao et al. , 2021b. The fossils are housed at the Research Center of Paleobiology, Yuxi Normal University.
All the specimens were photographed with a LEICA DFC 500 digital camera mounted on a Stereoscope LEICA M205 C. The single specimen of Astutuscaris bispinifer gen. sp. nov. was photographed with a LEICA DFC7000 T monochrome digital camera attached to a LEICA M205 FA fluo-rescence stereomicroscope, and also scanned with a Zeiss Xradia 520 Versa X-ray Microscope.
Diagnosis.-As for the type species.
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 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 ).
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 showing the head shield, 11 imbricated tergites most with pleural spines, and two terminal spines (A 1 , A 2 ); possible eyes (A 3 , A 4 , A 5 ); first appendages with small spines (A 3 , A 4 , white arrows); endopod with at least five podomeres (A 6 ). B. RCP 0002b, counterpart of the sole specimen; note the 11 imbricated tergites and trunk limbs (B 1 , B 2 ); restored specimenby digitally combining the part and counterpart, the dashed line indicating the contact (B 3 ); interpretive drawing of the narrow doublures of thoracic tergites marked in dark grey (B 4 ); T7-T10 and pleural spines (B 5 ); detail of the unknown structure (B 6 ); the limb with an endopod and a possible exopod (B 7 ); single endopod under T1 and T2 respectively (B 8 ); T11 and two separate spines being marked (B 9 ). Natural light photographs (A 1 , A 3 , A 4 , B 1 , B 8 ); fluorescent photographs (A 2 , A 5 -A 6 , B 2 , B 3 , B 5 -B 7 , B 9 ). Abbreviations: en, endopod; ex, exopod; ey, eye; fa, frontal appendage; gut, digestive tract; hs, head shield; lim, limb; p1-p5, podomeres 1-5; st, stalk; T1-T11, tergites 1-11; ts, terminal spine.
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).
The incompletely preserved walking leg with conical podomeres under the head resembles the limbs of Chengjiangocaris kunmingensis from the Xiaoshiba Biota (see Yang et al. 2013: fig. 1f), but, one pair of limbs beneath each of the first three tergites and the two long terminal spines of Astutuscaris bispinifer gen. et sp. nov. support it is not a fuxianhuiid which have a thorax with ventral segmental mismatch and a terminal tailspine with paired caudal flukes (Fu et al. 2018).
The two sturdy separate spines of Astutuscaris bispinifer gen. et sp. nov. are possibly the modified, posteriorly directed appendages of the terminal somite that is a common character of vicissicaudates a major clade of artiopods (Van Roy 2005;Lerosey-Aubril et al. 2017). If this interpretation is correct, then A. bispinifer might be a vicissicaudate-like euarthropod. Nevertheless, its curving frontalmost appendages and possible anterior and middle eyes still separate it from the vicissicaudates.
The nature of the first curving appendage without obvious segments of Astutuscaris bispinifer gen. et sp. nov. is contingent on the discovery of new specimens, although it's possible to be an antenna for lacking the typical elbow joints or articulations. Together with the incomplete head, anterior trunk tergites, and limbs, the affinity of A. bispinifer among euarthropods remains unclear until new material that could provide more key taxonomic characters is found. So far, about 24 non-trilobite euarthropod species were reported from Guanshan Biota, i.e., four radiodont species (Jiao et al. 2021b), two fuxianhuiid species (Jiao et al. 2021a), 11 species of bivalved euarthropods (Hu et al. 2013;Wu and Liu 2022), five non-trilobite artiopod species (Hu et al. 2013;Zhao et al. 2020;Jiao et al. 2022), Leanchoilia sp. (Hu et al. 2013), and Lihuacaris ferox (Jiao et al. 2021a). Accordingly, this new taxon increases the species diversity of the typical Burgess Shale-type Guanshan Biota, especially that of euarthropods.