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Fossilized remains of ancient microscopic worms, dating back to over 530 million years ago have been discovered in China.

These are the kinorhynch worm fossils, also called 'mud dragons'.

In their study, researchers looked at the relationship between kinorhynchs and arthropods, as well as tiny invertebrates with segmented bodies and exoskeletons.

These 'Eokinorhynchus rarus' fossils happened to be alive during the Cambrian period, about 541-485 million years ago. They had five pairs of "large bilateral spines on their trunks". Scientists feel that they had links to modern kinorhynchs.

"Although arthropod fossils date back to more than 530 million years ago, no kino fossils have ever been reported," Professor Shuhai Xiao, coauthor of the study and geobiology scientist at Virginia Tech, said in a news release. "This is a huge gap in the fossil record, with more than 540 million years of evolutionary history undocumented. Our discovery is the first report of kino fossils."

An average of 240 species of known living kinorhynch have survived through marine environments.

New fossils give a clearer picture of body segmentation and evolution of arthropods and small invertebrate animals. These fossils show a link with modern kinorhynchs, shown through ancestral and evolutionary shifts and styles.

The fossilized specimen was 0.078 inches in length and 0.02 inches in width----just about half a grain, say the researchers.

"We used electron microscopy to thoroughly image the fossils' surface features, and then the microCT to scan their interior structures, including their midguts," said Drew Muscente, a coauthor of the study. "Because this suite of data is so comprehensive, it includes pretty much everything you can know about the morphologies of the fossils."

More information on the early evolution of small invertebrates are expected to be gathered through them.

The study was published in Scientific Reports.