These ancient predators were so bizarre that scientists once mistook them for five different phyla.

Nature · Published 3 October 2026 Swipe more like this →

In short: Before they were recognized as a single, successful order of stem-arthropods, radiodonts baffled paleontologists. Isolated fossils of their heads, mouths, and claws were misidentified as jellyfish, sponges, and crustaceans. Today, we know these Cambrian giants were the apex predators of their time, using radial jaws and specialized appendages.

Radiodonta, an extinct order of stem-group arthropods, thrived globally during the Cambrian period. These animals are defined by their unique head anatomy, which includes a tripartite sclerite complex, stalked compound eyes, and a pair of segmented frontal appendages. Their bodies, which lacked a full exoskeleton, tapered toward the rear and featured lateral flaps for swimming, likely moving in wave-like motions similar to modern rays. While most were 30 to 50 cm long, some, like the Ordovician Aegirocassis benmoulai, reached up to 2 meters.

The group’s feeding strategies were remarkably diverse. Some, like the famous Anomalocaris canadensis, were raptorial predators with flexible, grasping appendages. Others, such as Hurdia and Peytoia, possessed stout, serrated appendages suited for sifting through sediment, while filter feeders like Aegirocassis used densely packed spines to strain organic matter from the water. Their mouthparts, or oral cones, were equally varied, featuring radial arrangements of tooth plates that suggest different dietary preferences, from soft-bodied prey to benthic organisms.

The history of radiodont research is a cautionary tale in paleontology. Because their body parts—such as frontal appendages, oral cones, and head carapaces—were often found disarticulated, they were historically assigned to five distinct phyla, including Porifera and Cnidaria. It was not until the 1985 revision of Burgess Shale fossils that researchers realized these disparate fossils belonged to a single, coherent animal group. This discovery fundamentally shifted our understanding of early arthropod evolution, placing radiodonts as a crucial link between lobopodian-like ancestors and the crown group of modern Euarthropoda.

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