Although first described in 1999, the early Cambrian Pectocaris remains a poorly known euarthropod genus. As of now, two species, Pectocaris eurypetala and Pectocaris spatiosa, are known from the Chengjiang biota. Here, we report a third species, Pectocaris inopinata sp. nov., which bears a pair of stalked eyes attached to an anterior sclerite, a pair of short but stout antennae, 29–35 cephalothoracic segments each corresponding to a pair of biramous appendages, 11–12 limbless segments carrying two rows of ventral spines and one row of dorsal spines, and a pair of broad telson flukes decorated with short marginal spines. Most strikingly, the endopods of anterior trunk appendages are each axially differentiated with two rows of setulose endites proximally and claw-like paired spines distally, in contrast to the endopods of posterior trunk appendages that carry two rows of setulose endites but lack claw-like paired spines. Phylogenetic analysis shows that P. inopinata, the other two Pectocaris species, together with their close relative Jugatacaris agilis belong to the same monophyletic clade. Our study reveals considerable difference in appendicular details and therefore presumably niche differentiation among Pectocaris and Jugatacaris species, as well as some other bivalved euarthropods in the Cambrian with similar overall appearances.

The two specimens (YKLP 16221, YKLP 16222) were collected from the Jianshan section of Haikou, Kunming. Stratigraphically, they belong to the Cambrian Stage 3 Yu’anshan Member of Chiungchussu Formation. These were preserved in the yellowish mudstones from the ‘event beds’ (cf. Zhu et al., 2001; Hou et al., 2017) and both part (slab a) and counterpart (slab b) were collected for each specimen. The anterior part of the trunk of YKLP 16221b was carefully excavated with steel needles to expose the fine details of the appendages.

Fossil structures on the surface of the rock slabs were photographed with a Canon Camera equipped with a MACRO 100 mm lens (Figs 1A, C and 2A, B) and a MP-E 65 mm lens (Figs 1B, D, E, J, K and 2C, D), respectively. Some of the fine morphologies were imaged with a Leica M205C fluorescence-microscope (Figs 1F, G, H, I and 2E, F). In order to reveal morphologies buried in the rock, the specimens were scanned with a GE Phoenix Nanotom m X-ray scanner (for overview-scans) and a Zeiss Xradia 520 Versa X-ray Microscope (for high-resolution regional scans). The resulting data were processed with the software Drishti (Versions 2.4 and 2.6.4) to generate three-dimensional models for observation.

In order to map the phylogenetic relations among the Cambrian arthropods, we perform phylogenetic analysis based on a morphological matrix slightly revised from Yang et al. (2018) (see Supplementary Information 1), into which Pectocaris inopinata sp. nov. is added. The new dataset with 44 taxa and 84 characters is analyzed under implied weights (k=3) with the parsimony program TNT v1.1 (Goloboff & Catalano, 2016).

The specimens described (YKLP 16221, YKLP 16222) are deposited in the collections of the Yunnan Key Laboratory for Palaeobiology (YKLP).

Fig. S1. Phylogenetic position of Pectocaris inopinata in the context of total-group Euarthropoda. Strict consensus of three trees calculated under implied weights (k=3) by traditional search method using TBR branch switching algorithm.