500 million years old fossils solve a centuries-old mystery in the evolution of life on Earth

Artist’s reconstruction of Gangtoucunia aspera as it would have appeared in life on the Cambrian sea floor, about 514 million years ago. Part of the individual’s skeleton in the foreground is removed to show the soft tumor within the skeleton. Credit: Reconstruction by Xiaodong Wang

Scientists have finally been able to solve a centuries-old mystery in the evolution of life on Earth, revealing what the first animals that made skeletons looked like. This discovery was made possible by the exceptionally well-preserved collection of fossils that were discovered in the eastern Yunnan Province, China. The results of the research 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 tough, strong skeletons suddenly appeared in the fossil record in the blink of a geological eye. Many of these early fossils are simple hollow tubes ranging in length from a few millimeters to several centimeters. However, the type of animal that made these skeletons was almost completely unknown, as they lack the preservation of the soft parts needed to identify them as belonging to major groups of animals still alive today.

Fossil specimen (left) and schematic diagram (right) of Gangtoconia aspera preserved soft tissues, including gut and tentacles. Credit: Luke Parry and Guangxu Zhang

Four samples of Gangtokonya aspera With soft tissues still intact, including the intestines and mouth parts, it was included in the new group of 514 million years of fossils. These reveal that this species had a mouth surrounded by a ring of smooth, unbranched claws about 5 mm (0.2 in) long. It is likely that these were used to sting and capture prey, such as small arthropods. Excavations also show that Gangtokunya He had a blind intestine (open only at one end), divided into internal cavities, filling the length of the tube.

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

According to the researchers, Gangtokunya It would have looked similar to a modern scyphozoan jellyfish, with a rigid tubular structure fixed to a basic substrate. The tentacle’s mouth extended out of the tube, but could have retracted inside the tube to avoid predators. Unlike the polyps of live jellyfish, the tube Gangtokunya Made of calcium phosphate, a hard mineral that makes up our teeth and bones. The use of these materials to build skeletons has become more rare among animals over time.

A close-up of the mouth of the Gangtokonya aspera showing tentacles that could have been used to catch prey. Credit: Luke Parry and Guangxu Zhang

Corresponding author Dr. Luke Barry, 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.

Fossil specimen of Gangtoucunia aspera preserving soft tissues, including the gut and tentacles (left and middle). The drawing at the right illustrates the visible anatomical features in the fossil specimens. Credit: Luke Parry and Guangxu Zhang

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