The rapid diversification of metazoans and their organisation in modern-style marine ecosystems during the Cambrian profoundly transformed the biosphere. What initially sparked this Cambrian explosion remains passionately debated, but the establishment of a coupling between pelagic and benthic realms, a key characteristic of modern-day oceans, might represent a primary ecological cause. By allowing the transfer of biomass and energy from the euphotic zone—the locus of primary production—to the sea floor, this biological pump would have boosted diversification within the emerging metazoan-dominated benthic communities. However, little is known about Cambrian pelagic organisms and their trophic interactions. Here we describe a filter-feeding Cambrian radiodont exhibiting morphological characters that likely enabled the capture of microplankton-sized particles, including large phytoplankton. This description of a large free-swimming suspension-feeder potentially engaged in primary consumption suggests a more direct involvement of nekton in the establishment of an oceanic pelagic-benthic coupling in the Cambrian.

A recent description of paired gnathobase-like structures (GLSs) in the head region of the radiodont Amplectobelua symbrachiata raised the question of whether these appendicular structures are more widely spread within Radiodonta, putative lower stem-group euarthropods. Here we describe a new genus of Radiodonta, Ramskoeldia gen. nov., that also bears GLSs. Its two new species, Ramskoeldia platyacantha sp. nov. and R. consimilis sp. nov., are distinguished based on the morphology of their frontal appendages. The presence of three pairs of GLSs associated with reduced segments posterior to the head and the detailed morphological similarities of the GLSs suggest that Ramskoeldia is closely related to Amplectobelua Hou et al., and they are classified together in the revised Family Amplectobeluidae. Other diagnostic characters of this family include the lack of a radiallyarranged oral cone, instead sharing mouthparts composed of smooth and tuberculate plates, and a frontal appendage with three podomeres in the shaft and prominent larger endites on podomeres 4 and 8. Due to its lack of GLSs and the different morphology of its mouthparts, membership of Lyrarapax Cong et al., in Amplectobeluidae cannot be confirmed. Appraisal of available evidence indicates that the morphology of the feeding structures, including frontal appendages, the mouth apparatus, and GLSs, serves as a fundamental source of characters in the classification of radiodonts.

Euarthropoda is one of the best-preserved fossil animal groups and has been the most diverse animal phylum for over 500 million years. Fossil Konservat-Lagerstätten, such as Burgess Shale-type deposits (BSTs), show the evolution of the euarthropod stem lineage during the Cambrian from 518 million years ago (Ma). The stem lineage includes nonbiomineralized groups, such as Radiodonta (e.g., ) that provide insight into the step-by-step construction of euarthropod morphology, including the exoskeleton, biramous limbs, segmentation, and cephalic structures. Trilobites are crown group euarthropods that appear in the fossil record at 521 Ma, before the stem lineage fossils, implying a ghost lineage that needs to be constrained. These constraints come from the trace fossil record, which show the first evidence for total group Euarthropoda (e.g.,, ) at around 537 Ma. A deep Precambrian root to the euarthropod evolutionary lineage is disproven by a comparison of Ediacaran and Cambrian lagerstätten. BSTs from the latest Ediacaran Period (e.g., Miaohe biota, 550 Ma) are abundantly fossiliferous with algae but completely lack animals, which are also missing from other Ediacaran windows, such as phosphate deposits (e.g., Doushantuo, 560 Ma). This constrains the appearance of the euarthropod stem lineage to no older than 550 Ma. While each of the major types of fossil evidence (BSTs, trace fossils, and biomineralized preservation) have their limitations and are incomplete in different ways, when taken together they allow a coherent picture to emerge of the origin and subsequent radiation of total group Euarthropoda during the Cambrian.