Fossil 'Mud Dragons' Shed Light On the Evolution Of Body Segmentation In Animals

Fossils of kinorhynch worms, also known as mud dragons, dating back more than 530 million years were recently unearthed in South China. Researchers say these ancient, microscopic worms fill in a huge evolutionary gap. 

"Kinos [kinorhynchs] represent an animal group that is related to arthropods -- insects, shrimps, spiders, etc. -- which are the most diverse group of animals on the planet," Shuhai Xiao, one of the study researchers and a professor of geobiology at Virginia Polytechnic Institute and State University (Virginia Tech), explained in a news release. "Although arthropod fossils date back to more than 530 million years ago, no kino fossils have ever been reported. 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."

The first kino specimen was unearthed from Nanjiang, China, in 2013, however continued excavations revealed more fossils later that year and in 2014. The newly discovered creatures, subsequently dubbed Eokinorhynchus rarus -- or rare ancient mud dragon -- dates back from the Cambrian period and contains five pairs of large bilaterally placed spines on its trunk, suggesting it is related to modern kinorhynchs, researchers say. 

Based on their study, researchers say the fossils shed light on how and why body segmentation evolved several times among not only arthropods, but also several other groups of animals. Based on several similarities, researchers also believe E. rarus and living modern kinorhynchs have a close evolutionary relationship. However, E. rarus has more body segements, suggesting they are distant ancestors of modern species. 

"There are approximately 240 living kinorhynch species, all found in marine environments. The body of kinorhynchs is divided into three sections: a head, which includes a mouth cone with teeth; a neck; and a trunk with 11 segments. These creatures could provide clues to origins of body segmentation, but such efforts have been hampered by a lack of well-preserved kinorhynch fossils, until now," Xiao said in the university's release. 

The newly discovered specimen is 0.078 inches in length, 10.02 inches in width and roughly half the size of a grain of rice. An X-ray micro-CT located at the Virginia Tech Corporate Research Center was used to identify the new species. 

"We used electron microscopy to thoroughly image the fossils' surface features, and then the microCT to scan their interior structures, including their midguts," Drew Muscente, who devised an X-ray transparent plastic grid to secure the specimen during examination, added. "Because this suite of data is so comprehensive, it includes pretty much everything you can know about the morphologies of the fossils." 

Researchers expect to find more fossils, which they hope will offer important insights into the early evolutionary history of this group of tiny and elusive animals. Their findings were recently published in the journal Scientific Reports. 

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