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| Helmetia expansa Walcott, 1918 in Losso, Caron et Ortega-Hernández, 2025. Reconstruction by Marianne Collins |
Abstract
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).
Keywords: Burgess Shale, Cambrian, Conciliterga, exceptional preservation, Helmetiida, moulting
Artiopoda Hou & Bergström, 1997
Trilobitomorpha Størmer, 1944
Conciliterga Hou & Bergström, 1997
Constituent taxa: Arthroaspis bergstroemi Stein et al., 2013; Helmetiida Novozhilov, 1960.
Order Helmetiida Novozhilov, 1960
Constituent taxa: Family Tegopeltidae Simonetta & Delle Cave, 1975; Family Helmetiidae Simonetta & Delle Cave, 1975.
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| Concilitergan diversity and relationships. A, Arthroaspis bergstroemi, MGUH 30382, from the early Cambrian (Stage 3) Sirius Passet, Greenland. B, Helmetia expansa, USNM 83952, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. C, Kuamaia lata, CJHMD 00064, from early Cambrian (Stage 3) Chengjiang, China. D, Tegopelte gigas, USNM 189201, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. E, simplified topology based on results from phylogenetic analyses herein which resolved Arthroaspis bergstroemi as an early-branching concilitergan. |
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| Holotype of Helmetia expansa USNM 83952, dorsal view. A, Cross polarized light. B, Low angle light. C, Magnification of A showing cephalon. D, Magnification of A showing lamellae. Abbreviations: cep, cephalon; anl, anterolateral spine; ant, antenna; asc, anterior sclerite; ey, eye; ex, exopodite; hyp, hypostome; lm, lamellae; mey, medial eyes; mg, midgut gland; opn, optical nerve; py, pygidium; rs, reniform structure; stn, sternite number; th, thorax. |
Family Tegopeltidae Simonetta & Delle Cave, 1975
Constituent genera: Tegopelte Simonetta & Delle Cave, 1975; Skioldia Hou & Bergström, 1997; and Saperion Hou & Bergström, 1997.
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Family Helmetiidae Simonetta & Delle Cave, 1975
Emended diagnosis: Concilitergans in which the thorax has tergites with non-effaced boundaries and well-developed pleural spines, and a pygidium with variable number of posterolateral spines and a medial terminal spine with a broad base.
Constituent genera: Helmetia Walcott, 1918; Rhombicalvaria Hou, 1987; Haifengella Zhao et al., 2014; Kuamaia Hou & Bergström, 1997.
Genus Helmetia Walcott, 1918
Type species: Helmetia expansa Walcott, 1918
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| the Burgess Shale concilitergan Helmetia expansa. Reconstruction by Marianne Collins |
Conclusion:
We redescribe Helmetia expansa as one of the best-known concilitergans based on 36 specimens from the Burgess Shale. It has large anterolateral spines, a serrated margin, a six-segmented thorax and a large pygidium with two pairs of spines.
New material reveals insights into the preservation of non-biomineralized structures in H. expansa, including antennae, biramous appendages, optic structures and traces of digestive glands. Additionally, H. expansa acquired an adult exoskeletal morphology that at least doubled in size throughout ontogeny and provides rare direct evidence of moulting in a non-biomineralized Cambrian trilobitomorph.
New morphological characters and broader taxon sampling of Cambrian artiopodans clarifies the internal relationships within Conciliterga, which contains Arthroaspis bergstroemi from Sirius Passet and two major families Helmetiidae and Tegopeltidae, both represented in the Burgess Shale and Chengjiang Lagerstätten.
Sarah R. Losso, Jean-Bernard Caron and Javier Ortega-Hernández. 2025. Helmetia expansa Walcott, 1918 Revisited – New insights into the internal anatomy, moulting and phylogeny of Conciliterga. Journal of Systematic Palaeontology. 23(1); 2468195. DOI: doi.org/10.1080/14772019.2025.2468195 [04 Apr 2025]
