500-million-year-old fossils solve an ancient riddle in the evolution of life on Earth

Scientists have finally solved an age-old riddle in the evolution of life on Earth, revealing what the first animals that made skeletons looked like. This discovery was made possible thanks to an exceptionally well-preserved collection of fossils discovered in the eastern province of Yunnan, China. The results of the study were published on November 2 in the scientific journal Proceedings of the Royal Society B.

During an event called the Cambrian explosion about 550-520 million years ago, the first animals to build hard and robust skeletons suddenly appear in the fossil record in the blink of an eye. Many of these early fossils are simple hollow tubes ranging from a few millimeters to many centimeters in length. What kind of animals made these skeletons, however, was almost completely unknown, as they lack the soft parts necessary to identify them as belonging to large groups of animals still alive today.

Four copies of Gangtoucunia aspera with soft tissues still intact, including the gut and mouthparts, are included in the new collection of 514 million-year-old fossils. This shows that this species had a mouth lined with a ring of smooth, unbranched tentacles about 5 mm (0.2 in) long. These were probably used to sting and catch prey, such as small arthropods. The fossils also show that Gangtoucunia had a cecum (open only at one end), divided into internal cavities, which filled the length of the tube.

These are features found today only in modern jellyfish, anemones, and their closest 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 resembled 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 retracted into the tube to avoid predators. Unlike live jellyfish polyps, the tube of Gangtoucunia was made of calcium phosphate, a hard mineral that makes up our own teeth and bones. Using this material to build skeletons has become rarer among animals over time.

Corresponding author Dr. Luke Parry, Department of Earth Sciences,[{” attribute=””>University of Oxford, said: “This really is a one-in-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 firmly in place.”

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

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

PhD 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 about what they were. In the following month, I found three more specimens with soft tissue preservation, which was very exciting and made me rethink the affinity of Gangtoucunia. The soft tissue of Gangtoucunia, particularly the tentacles, reveals that it is certainly not a priapulid-like worm as previous studies suggested, but more like a coral, and then I realised that it is a cnidarian.”

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

Co-corresponding author Xiaoya Ma (Yunnan University and University of Exeter) said: “A tubicolous mode of life seems to have become increasingly common in the Cambrian, which might be an adaptive response to increasing predation pressure in the early Cambrian. This study demonstrates that exceptional soft-tissue preservation is crucial for us to understand these ancient animals.”

Reference: “Exceptional soft tissue preservation reveals a cnidarian affinity for a Cambrian phosphatic tubicolous enigma” by Guangxu Zhang, Luke A. Parry, Jakob Vinther and Xiaoya Ma, 2 November 2022, Proceedings of the Royal Society B Biological Sciences.
DOI: 10.1098/rspb.2022.1623