Trilobites and trilobite-like arthropods with exceptionally preserved dorsoventral exoskeletal elements.

Anterior facing up. A. Dorsal view of pyritized specimen of the olenid Triarthrus eatoni (USNM 400935) from the Late Ordovician Beecher’s trilobite bed (USA), showing typical trilobite dorsal morphology and organization of the trilobite-like articulation. A pair of walking legs (asterisk) is located under the cephalo-thoracic articulation. B. Unmineralized ventral exoskeletal morphology of Triarthrus eatoni (USNM 400943), including the sternite series (stn) connected by intersegmental tendinous bars (tnb). C. Dorsal view of ventral internal mould of the phacopid Placoparia cambriensis (NMW 91.46G) from the Ordovician of Wales (UK), showing three-dimensional preservation of the sternite series, transverse tendinous bars, and sites for cephalic (Cl) and thoracic (Tl) walking leg attachment (original film courtesy of the Whittington archives, University of Cambridge). D. Exceptionally preserved ventral morphology of the lightly sclerotized trilobite-like nektaspidid Misszhouia longicaudata (ELRC 11555) from the Lower Cambrian Chengjiang lagerstätte (south China) (original print courtesy of GD Edgecombe). E. Ventral view of sternite series of Misszhouia longicaudata (ELRC 11590b) showing the tendinous bars and sites for walking leg attachment in detail (original print courtesy of GD Edgecombe). F. Ventral view of an incomplete trunk of the phacopid Geesops sparsinodosus from the Middle Devonian of Eifel (Germany) (reproduced from [33], with permission of the Palaeontological Association), showing the sites for muscle attachment on the visceral side of the tergite series, and morphological continuity between the articulating furrow (af) and the apodemes (apm). Scale bars: 1 mm for A; 2 mm for B–D; 1 mm for E; 0.5 mm for F.