Cambrian ‘bivalved’ arthropods are a polyphyletic group of carapace-bearing arthropods which include stem euarthropods, stem mandibulates and crustaceans. Here, we describe Pakucaris apatis gen et. sp. nov. a new ‘bivalved’ mandibulate from the middle Cambrian (Wuliuan Stage) Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia, Canada). Two morphotypes of this small arthropod (11.6–26.6 mm long) are recognized, differing mainly in their sizes and number of segments, possibly reflecting sexual dimorphism or different anamorphic stages. The carapace presents a dorsal crest extending anteriorly into a small recurved rostrum and two antero-lateral processes. Around 20% of the posteriormost body segments and limbs are covered by a large spine-bearing shield. The head bears a pair of eyes, a possible pair of unsegmented appendicular projections and two pairs of segmented appendages. The thorax is multisegmented, homonomous, with weakly sclerotized segments bearing biramous limbs, composed of a stenopodous endopod with c. 20 podomeres and a paddle-shaped exopod. Pakucaris is interpreted as a nektobenthic suspension feeder. A Bayesian phylogenetic analysis implies a position within Hymenocarina as stem mandibulates. The posterior shield is regarded as a pygidium, and represents a case of morpho-functional convergent evolution between a mandibulate, artiopodans and mollisoniids. Alternatively, the growing number of known pygidium-bearing arthropods may hint at a common developmental pattern across early arthropod evolution. This study not only increases our understanding of the early evolution of mandibulates but also illustrates a unique case of early evolutionary convergence during the Cambrian Explosion.

S1_AIL_JBC_2020_SupMaterial&SupFigures.pdf: contains Supplementary Figure 1 (phylogeny) as well as: List of specimens and figures, species and characters present in the phylogenetic analysis, phylogenetic constraints applied in the phylogenetic analysis and Synapomorphies of highlighted taxa. 

S2_AIL_JBC_2020_Specimens&Measurements.xlsx: contains all specimens and measurements performed

S3_AIL_JBC_2020_Phylomatrix_.xlsx: Phylogenetic matrix

S4_AIL_JBC_2020_ Nexuscode.nex: Nexus code. Includes MrBayes parameters.