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Study suggests macroscopic bilaterian animals did not appear until 555 mya


RIVERSIDE, Calif. -- The traces left behind by ancient animals may hold
the key to determining when macroscopic bilaterians -- animals that are
symmetric about a central axis, with a body divided into equivalent right
and left halves, and with an anterior-posterior polarity (e.g., this
includes worms, ants, and ranging up to humans) -- first appeared. A team
led by Dr. Mary Droser, professor of geology at the University of
California, Riverside, studied "trace" fossils, e.g., burrows, trails and
tracks left behind by the earliest bilaterian animals. Results from their
study suggest that bilaterian animals did not appear until approximately
555 million years ago.