3D model of the embryo of a cambrian creature; the ring-shaped brain is marked in yellow within the embryo (left) and in a detailed image on its own (right)

Penis worm’s ancient cousin fossilized with its doughnut-shaped brain intact

Scientists uncovered something unexpected in the fossilized embryo of a worm-like creature from the Cambrian period: the remains of a tiny, doughnut-shaped brain in the primordial animal’s head.

The roughly 500 million-year-old fossil is an example of the marine species Markuelia hunanensis, an ancient cousin of penis worms (priapulids) and mud dragons (Kinorhyncha). To date, scientists haven’t found fossils of the worm-like weirdos in their adult form, but researchers have uncovered hundreds of pristine embryos that capture different stages of the animals’ early development. Each of these embryos measures only about half a millimeter (0.02 inch) across. 

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