The Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat-Lagerstätten worldwide thanks to the fossilization of diverse non-biomineralizing organisms through pyritization. Despite their contributions to understanding the evolution of early animals, several Chengjiang species remain poorly known owing to their scarcity and/or incomplete preservation. Here, we use micro-computed tomography to reveal in detail the ventral appendage organization of the enigmatic non-trilobite artiopod Pygmaclypeatus daziensis —one of the rarest euarthropods in Chengjiang—and explore its functional ecology and broader evolutionary significance. Pygmaclypeatus daziensis possesses a set of uniramous antennae and 14 pairs of post-antennal biramous appendages, the latter of which show an unexpectedly high degree of heteronomy based on the localized differentiation of the protopodite, endopodite and exopodite along with the antero-posterior body axis. The small body size (less than 2 cm), the presence of delicate spinose endites and well-developed exopodites with multiple paddle-shaped lamellae on the appendages of P. daziensis indicate a nekto-benthic mode of life and a scavenging/detritus feeding strategy. Pygmaclypeatus daziensis shows that appendage heteronomy is phylogenetically widespread within Artiopoda—the megadiverse clade that includes trilobites and their relatives with non-biomineralizing exoskeletons—and suggests that a single exopodite lobe with paddle-like lamellae is ancestral for this clade. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.

SUMMARYThe Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat-Lagerstätten worldwide thanks to the fossilization of diverse non-biomineralizing organisms through pyritization. Despite their contributions to understanding the evolution of early animals, several Chengjiang species remain poorly known due to their scarcity and/or incomplete preservation. Here, we use micro-computed tomography to reveal in detail the ventral appendage organization of the enigmatic non-trilobite artiopod Pygmaclypeatus daziensis – one of the rarest euarthropods in Chengjiang – and explore its functional ecology and broader evolutionary significance. Pygmaclypeatus daziensis possesses a set of uniramous antennae and 14 pairs of post-antennal biramous appendages, the latter of which show an unexpectedly high degree of heteronomy based on the localized differentiation of the protopodite, endopodite and exopodite along the antero-posterior body axis. The small body size (less than 2 cm), presence of delicate spinose endites, and well-developed exopodites with multiple paddle-shaped lamellae on the appendages of P. daziensis indicate a nekto-benthic mode of life, and a scavenging/detritus feeding strategy. Pygmaclypeatus daziensis shows that appendage heteronomy is phylogenetically widespread within Artiopoda – the megadiverse clade that includes trilobites and their relatives with non-biomineralizing exoskeletons – and suggests that a single exopodite lobe with paddle-like lamellae is ancestral for this clade.Abstract FigureCover image: Morphological reconstruction of the non-trilobite artiopod Pygmaclypeatus daziensis from the early Cambrian (Stage 3) Chengjiang biota in south China. Artwork by Holly Sullivan (https://www.sulscientific.com/).

We compare the microscopic anatomy of the mouthparts of representative species of Solifugae, Pseudoscorpiones and Parasitiformes (Acari). Specifically, we focus on the epistome, the labrum, the lateral lips (= endites of the pedipalpal coxae) and the musculature of the pharyngeal suction pump. We provide evidence that the labrum is reduced in Solifugae, but present and functional in Pseudoscorpiones and Acari. The epistome constitutes the entire dorsal face of the rostrosoma in Solifugae, but is internalized into the prosoma in Pseudoscorpiones. In Acari, the epistome shows an ancestral morphology, probably close to the ground pattern of chelicerates. The lateral lips of Solifugae contribute to the ventral face of the rostrosoma and the two lips of the mouth opening. In Solifugae, the ventral rostrosoma also includes a sclerite that might derive from a tritosternum. In Pseudoscorpiones, the lateral lips remain independent of the rostrosoma, they interlock ventral to the rostrosoma forming a perioral space. Here, the rostrosoma has an unpaired ventral lip of unresolved morphological origin, which is, however, clearly distinct from the lateral lips of Solifugae. The pharyngeal suction pump differs in all three clades in attachment, number of muscles and origin of muscles. We interpret the data as evidence for independent, parallel evolution of elements of the ground pattern of the (eu)chelicerate mouth parts. Based on the morphological elements of a common euchelicerate ground plan, the rostrosoma evolved independently in the three clades. We reject earlier hypotheses that consider the rostrosoma a character to support a phylogenetic relationship of the three clades.