A new euarthropod from the Soom Shale (Ordovician) Konservat‐Lagerstätte, South Africa, with exceptional preservation of the connective endoskeleton and myoanatomy

Abstract

A new exceptionally preserved euarthropod, Keurbos susanae gen. et sp. nov. from the Upper Ordovician Soom Shale Konservat‐Lagerstätte of South Africa, is described herein. Two specimens exhibit an unusual preservation style such that the cuticular exoskeleton is preserved in low relief but retains high‐fidelity details, whereas the internal anatomy is preserved in three dimensions and includes myoanatomy and aspects of the connective inter‐ and intrasegmental endoskeleton. The trunk has 46 homonomous segments and tapers towards the posterior. The dorsal surface comprises the posterior margins of tergites that form rounded paratergal folds at their lateral margins. Sternal morphology is seen from an internal view of the fossils and includes axial sternites and associated ovoid plates, both with fringing setae along their posterior margins. Patchy, incomplete preservation of the appendages is consistent with the ‘reversed’ taphonomy of these fossils in which sclerotized elements that project beyond the body margin, such as lamellae with vascular channels and pits, are preserved, whereas those inside the body margins are absent or poorly expressed. Euarthropod characters include an arthrodized tergal and sternal exoskeleton, and a segmental connective endoskeleton. Recent depopulation of the euarthropod stem group has witnessed conspicuously arthrodized fossils formerly placed in the stem group being shifted into the crown, increasing the likelihood that <jats:italic>Keurbos</jats:italic> is likewise a crown‐group euarthropod. Comparison with a phylogenetically disparate suite of similarly homonomous, multisegmented taxa indicates no convincing synapomorphies. The grouping of relevant middle Palaeozoic exemplars as ‘enantiopod’ pancrustaceans could suggest that Keurbos might be allied.