Before trilobite legs: Pygmaclypeatus daziensis reconsidered and the ancestral appendicular organization of Cambrian artiopods

The Cambrian Stage 3 Chengjiang biota in South China is one of the most influential Konservat-Lagerstatten 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. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=155 SRC="FIGDIR/small/456779v1_ufig1.gif" ALT="Figure 1"> View larger version (111K): org.highwire.dtl.DTLVardef@f5d7beorg.highwire.dtl.DTLVardef@271308org.highwire.dtl.DTLVardef@152797aorg.highwire.dtl.DTLVardef@1a54cc1_HPS_FORMAT_FIGEXP M_FIG C_FIG Cover 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/).