Muscle attachment structures on multifunctional armour of Cambrian lobopodians

Abstract

Cambrian lobopodians are pivotal to understanding panarthropod early evolution, yet the functional significance of their diverse sclerites remains poorly understood. Although these discrete sclerites have generally been accepted as protective structures, whether they served as attachment sites for lobopod muscles is still under debate. Here, we report the first description of regularly distributed pit‐like microstructures on the underside of net‐like plates of Microdictyon  sp. from the Shuijingtuo Formation (Cambrian Stage 3), Shaanxi, China. A concentric cluster of pit‐like microstructures is located on the internal surface of each node on the plate. Through comparative analysis with extant tardigrades, onychophorans and arthropods, alongside similar fossil structures interpreted as muscle attachment sites, we suggest that these pit‐like microstructures are attachment points for leg muscles. The plates might also provide protection for underlying musculature and probably housed specialized organs beneath their sealed holes. This study suggests that the plates of Microdictyon evolved through the integration of multiple functions, providing new insights into the functional morphology and ecology of Cambrian armoured lobopodians and the early evolution of panarthropods.