500-million-year-old fossils reveal answer to evolutionary riddle

An exceptionally well-preserved collection of fossils discovered in eastern Yunnan province, China, has allowed scientists to solve a centuries-old puzzle in the evolution of life on Earth and reveal what the first animals to make skeletons looked like. The results have been published today in Proceedings of the Royal Society B.

The first animals to build hard and robust skeletons appear suddenly in the fossil record at a geological moment about 550-520 million years ago during an event known as the Cambrian explosion. Many of these early fossils are simple hollow tubes ranging from a few millimeters to many centimeters in length. But what kind of animal made these skeletons was almost entirely unknown, as they lack the preservation of the soft parts needed to identify them as belonging to large groups of animals still alive today.

The new collection of 514-million-year-old fossils includes four specimens of Gangtoucunia aspera with soft tissues still intact, including the gut and mouthparts. These reveal that this species had a mouth lined with a ring of smooth, unbranched tentacles about 5 mm long. It is likely that these were used to stab and catch prey, such as small arthropods. The fossils also show that Gangtoucunia had an appendix (open only at one end), divided into internal cavities, which filled the length of the tube.

These are traits found today only in modern jellyfish, anemones and their close relatives (known as cnidarians), organisms whose soft parts are extremely rare in the fossil record. The study shows that these simple animals were among the first to build the hard skeletons that make up much of the known fossil record.

According to the researchers, Gangtoucunia would have looked like modern scyphozoan jellyfish polyps, with a hard tubular structure anchored to the underlying substrate. The tentacle mouth would have extended outside the tube, but could have been retracted into the tube to avoid predators. Unlike living jellyfish polyps, however, the tube of Gangtoucunia was made of calcium phosphate, a hard mineral that makes up our own teeth and bones. Use of this material to build skeletons has become rarer among animals over time.

Corresponding author Dr. Luke Parry, Department of Earth Sciences, University of Oxford, said: “This is truly a one in a million discovery. These mysterious tubes are often found in groups of hundreds of individuals, but until now they have been regarded as ‘problematic’ fossils, because we had no way of classifying them. Thanks to these extraordinary new specimens, a key piece of the evolutionary puzzle has been put into place.”

The new specimens clearly show that Gangtoucunia was not related to annelid worms (earthworms, polychaetes and their relatives) as previously suggested for similar fossils. It is now clear that the body of Gangtoucunia had a smooth exterior and a gut divided longitudinally, while annelids have segmented bodies with transverse division of the body.

The fossil was found at a site in the Gaoloufang section of Kunming, eastern Yunnan Province, China. Here, anaerobic (oxygen-poor) conditions limit the presence of bacteria that normally break down soft tissues in fossils.

Ph.D. student Guangxu Zhang, who collected and discovered the specimens, said: “The first time I discovered the pink soft tissue on top of a Gangtoucunia tube, I was surprised and confused as to what they were. In the following month, I found three more specimens. with soft tissue preservation, which was very intriguing and made me think about the affinity of Gangtoucunia. The soft tissue of Gangtoucunia, especially the tentacles, reveals that it is indeed not a priapulid-like worm as previous studies suggested, but more like a coral, and then I realized that it is a cnidarian .”

Although the fossil clearly shows that Gangtoucunia was a primitive jellyfish, this does not rule out the possibility that other early tube fossil species looked very different. From Cambrian rocks in Yunnan province, the research team previously found well-preserved tube fossils that could be identified as priapulids (marine worms), lobopodia (worms with paired legs, closely related to arthropods today) and annelids.

Co-author Xiaoya Ma (Yunnan University and University of Exeter) said: “A tubicolous lifestyle appears to have become increasingly common in the Cambrian, which may be an adaptive response to increasing predation pressure in the early Cambrian. This study shows that exceptional preservation of soft tissues is essential to we must understand these ancient animals.”

The article “Exceptional soft tissue preservation reveals a cnidarian affinity for a Cambrian phosphatic tubicolous enigma” will be published in Proceedings of the Royal Society B on Nov. 2