500-Million-Year-Old Fossils Reveal Answer to Evolutionary Conundrum

An exceptionally well-preserved collection of fossils discovered in the eastern province of Yunnan, China, has enabled scientists to solve an ancient riddle in the evolution of life on Earth and reveal what the first animals that made skeletons looked like. The results were published today in Proceedings of the Royal Society B.

The first animals to build hard and robust skeletons suddenly appear in the fossil record in a geological instant about 550-520 million years ago during an event called the Cambrian explosion. 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.

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. This shows that this species had a mouth lined with a ring of smooth, unbranched tentacles about 5 mm 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 characteristics 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 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 retracted into the tube to avoid predators. Unlike live jellyfish polyps, however, Gangtoucunia’s tube 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, University of Oxford, said: “This is truly a discovery of one in a million. These mysterious tubes are often found in groups of hundreds of individuals, but until now they have been considered ‘problematic’ fossils because we couldn’t classify them. Thanks to these extraordinary new specimens, an important piece of the evolutionary puzzle has been put firmly in place.”

The new specimens clearly show that Gangtoucunia was unrelated to annelids (earthworms, polychaetes, and their relatives), as previously suggested for similar fossils. It is now clear that Gangtoucunia’s body had a smooth exterior and a longitudinally divided gut, while annelids have segmented bodies with a transverse distribution of the body.

The fossil was found at a site in the Gaoloufang section in Kunming, China’s eastern Yunnan province. Here, anaerobic (oxygen-poor) conditions limit the presence of bacteria that normally degrade soft tissues at 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 as to what they were. In the following month, I found three more specimens with preservation of soft tissue, which was very exciting and made me reconsider Gangtoucunia’s affinity.The soft tissue of Gangtoucunia, especially the tentacles, reveals that it is certainly not a priapulide-like worm as previous studies suggested, but more like a coral , and then I realized it’s a cnidarian.”

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

Co-corresponding author Xiaoya Ma (Yunnan University and University of Exeter) said: “Tubicous way of life appears to be increasingly common in the Cambrian, which could be an adaptive response to increasing predation pressures in the early Cambrian. This study shows that exceptional soft tissue conservation is crucial for us to 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.