A 500-million-year-old worm embryo had a doughnut-shaped brain

(ORDO NEWS) — The preservation of a half-billion-year-old invertebrate embryo fossil is an exceptional event in itself.

However, this time the scientists were doubly lucky: the embryo of the ancient worm was preserved in an unflattened form, which made it possible to restore its internal structure.

About 500 million years ago, on the territory of present-day China and Siberia, a shallow sea lapped, whose waters teemed with a variety of life forms.

Among the most numerous inhabitants of the ancient sea were distant relatives of modern kinorhynchus – worm-like animals, the largest of which grows a little more than a millimeter in length.

These ancient worms, belonging to the species Markuelia hunanensis , laid thousands of eggs every year in soft bottom mud, and thanks to this, Chinese paleontologists excavating in the Hunan province in the south of the country have obtained many fossilized embryos.

Although no adult specimens of Markuelia hunanensis were found, the structure of the developing larvae allowed scientists to get an idea of what they looked like.

Now, paleontologists have obtained a rare find – an embryo that, in the process of turning into a fossil, did not flatten out, but retained its original spherical shape, which saved its internal organs from turning into mush.

Thanks to this, the researchers were able to examine its structure in detail, including striated muscle tissue and a circular brain (it is also called the parapharyngeal nerve ring), shaped like a doughnut.

This is the first time in paleontology that the internal organs and tissues of a Cambrian worm embryo are so well preserved.

The structure of the brain emphasizes the proximity of Markuelia hunanensis to kinorhynchus and other modern members of the Scalidophora clade, which also includes priapulids and loricifers : these are rather small marine animals whose lifestyle has not changed much over the past 500 million years.

A 500 million year old worm embryo had a doughnut shaped brain 2
Modern kinorhynchus are for the most part tiny creatures that the human eye can hardly distinguish

In the future, scientists plan to study other fossils found in the hope of finding surviving internal organs in them.

Previously, such work was not carried out, since paleontologists could not even imagine the possibility of such preservation. However, now everything has changed, and many fossils that have not yet been described are waiting for their scientists.

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