Helmetia expansa Walcott, 1918 revisited – new insights into the internal anatomy, moulting and phylogeny of Conciliterga

The trilobitomorphs are a megadiverse and ecologically versatile group of Paleozoic euarthropods that include the iconic trilobites, as well as non-biomineralized clades exclusively known from Konservat-Lagersttätten. The concilitergans, defined by the presence of a broad, variably effaced and flattened dorsal exoskeleton, have received comparatively little attention. This is particularly true for Helmetia expansa from the mid-Cambrian Burgess Shale (British Columbia) – the first chronologically reported species of Conciliterga, which was originally figured (based on a single specimen) by Walcott (1918). Here, we present the first comprehensive description of H. expansa based on all material available from the Burgess Shale, totalling 36 specimens housed at the Royal Ontario Museum and the Smithsonian Institution. The non-biomineralized dorsal exoskeleton is broad and flat, with a serrated margin throughout. The body consists of a cephalon with well-developed anterolateral spines, six thoracic tergites and a large pygidium bearing two pairs of lateral spines and a single terminal spine. The preserved appendages include short uniramous antennae followed by 15 pairs of homonomous and biramous limbs composed of a gracile endopodite and an exopodite with a broad lobe and thick lamellae. The digestive system consists of a straight gut tract, including a ‘J’-shaped foregut and five paired digestive glands on the anterior half of the body expressed as small and convex oblong structures with a submillimetric lamellar ultrastructure. Two specimens of H. expansa provide the first direct evidence of moulting in concilitergans. The lack of dorsal ecdysial sutures suggests a marginal moulting strategy similar to extant Xiphosura. A revised phylogeny of Trilobitomorpha supports Arthroaspis bergstroemi from the early Cambrian Sirius Passet of Greenland as the earliest branching concilitergan and prompts a new diagnosis for the clade. Our results inform the internal relationships within Conciliterga and formalize the families Helmetiidae (Helmetia, Rhombicalvaria, Haifengella and Kuamaia) and Tegopeltidae (Tegopelte, Skioldia, Saperion).