Early euarthropod evolution involved a major transition from lobopodian-like taxa to organisms featuring a segmented, well-sclerotized trunk (arthrodization) and limbs (arthropodization). However, the precise origin of a completely arthrodized trunk and arthropodized ventral biramous appendages remain controversial, as well as the early onset of anterior-posterior limb differentiation in stem-group euarthropods. New fossil material and micro-computed tomography inform the detailed morphology of the arthropodized biramous appendages in the carapace-bearing euarthropod Isoxys curvirostratus from the early Cambrian Chengjiang biota. In addition to well-developed grasping frontal appendages. I. curvirostratus possesses two batches of morphologically and functionally distinct biramous limbs. The first batch consists of four pairs of short cephalic appendages with robust endites with a feeding function, whereas the second batch has more elongate trunk appendages for locomotion. Critically, our new material shows that the trunk of I. curvirostratus was not arthrodized. The results of our phylogenetic analyses recover isoxyids as some of the earliest branching sclerotized euarthropods, and strenghens the hypothesis that arthropodized biramous appendage evolved before full body arthrodization.

Anatomical data are collected by photos from a Keyence VHX 6000 and Leica M205AF stereomicroscope, a GE Phoenix Nanotom cone beam scanner and a Zeiss Xradia 520 Versa X-ray microscope.

And we analysed it in MrBayes v.3.2.7.

Nexus (.nex) is a common file type for phylogenetic data, and the required runfile to use MrBayes for phylogenetic analysis.

.p are sampled parameter files from MrBayes analysis, output for each run specigied.

Newick (.tre) files are phylogenetic tree outputs, which can be visualized directly e.g. with Figtree.

.tnt is a file type for phylogenetic data, specifically the required runfile to conduct maximum parsimony phylogenetic analysis in TNT.