DOI: 10.5061/dryad.f1vhhmgvs
title: Data from: A new species of the Cambrian bivalved euarthropod Pectocaris with axially differentiated enditic armatures

description: 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.
description: 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 <i>et
 al</i>., 2001; Hou <i>et al</i>., 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 <i>et al.</i> (2018) (see Supplementary Information
 1), into which <i>Pectocaris inopinata</i> sp. nov. is added.
 The new dataset with 44 taxa and 84 characters is analyzed under implied
 weights (<i>k</i>=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).
description: <b>Fig. S1.</b> 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.