Here we describe the oldest evidence of non-marine animals from the early Cambrian Wood Canyon Formation, California, evidence created by metazoans of a variety of sizes and behaviors. Millimeter-sized vertical trace fossils, including the U-shaped burrow Arenicolites and the vertical burrow Skolithos, as well as a centimeter-scale horizontal trace fossil, occur in conglomerate and gritty arkosic sandstone bed tops within fluvial channels. These fossils demonstrate that animals were dwelling in this habitat coincident with, or possibly predating, the first trilobites, and extend the freshwater record of animals back at least 80 m.y. The development of a functioning terrestrial ecosystem was concurrent with the early Cambrian marine radiation and suggests that freshwater environments were populated early by metazoans and that ecological opportunity likely played a determining role in metazoan exploitation of non-marine habitats versus commonly assumed influences from physiological or nonbiological barriers.

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