In spite of their unrivalled ecological success, the origins of terrestrial mandibulates have long remained virtually unknown. In recent years, claims have been made based on phylogenetic results that fuxianhuiids, iconic fossils of the Chengjiang biota and purported basal euarthropods, were in fact mandibulates, allied with the problematic euthycarcinoids as the closest sister-group to Mandibulata or even stem myriapods. A reexamination of euthycarcinoid mouthparts has since brought additional support to this view. Here, we reinvestigated the anterior anatomy of the Cambrian euarthropod Fuxianhuia and its relatives. We demonstrate that the fuxianhuiid head is that of a mandibulate, sharing similarities with hymenocarines, including a limbless intercalary segment. The hypostome is a sub-triangular sclerite partially fused but anatomically independent from a large, bilobed labral plate, as observed in many extant mandibulate taxa as well as euthycarcinoids. The so-called “specialized post-antenn(ular) appendages” are therefore the mandibles, with a tripartite palp and large dented gnathal lobe partially covered by the hypostome-labrum complex. Under a “deep split” phylogenetic scenario, Fuxianhuia and its relatives are resolved as sister group to both total-group Myriapoda and Mandibulata as a whole.

Thematic collection: This article is part of the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion

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