A Burgess Shale mandibulate arthropod with a pygidium – a case of convergent evolution

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 ...