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They say that fossilised traces of the 540-million-year-old creature are "exquisitely well preserved".
The microscopic sea animal is the earliest known step on the evolutionary path that led to fish and - eventually - to humans.
Details of the discovery from central China appear in Nature journal.
The research team says that Saccorhytus is the most primitive example of a category of animals called "deuterostomes" which are common ancestors of a broad range of species, including vertebrates (backboned animals).
Saccorhytus was about a millimetre in size, and is thought to have lived between grains of sand on the sea bed.