500-million-year-old fossils solve an age-old puzzle in the evolution of life on earth

500-million-year-old fossils solve an age-old puzzle in the evolution of life on earth
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Artist's Reconstruction of Gangtoucunia Aspera

Artist’s reconstruction of Gangtoucunia aspera as it emerged in life on the Cambrian seafloor about 514 million years ago. The subject in the foreground has removed part of the skeleton to show the soft polyp inside the skeleton. Credit: Reconstruction by Xiaodong Wang

Scientists have finally solved an age-old puzzle about the evolution of life on Earth, revealing what the first animals to make skeletons looked like. This discovery was made possible thanks to an exceptionally well-preserved fossil collection discovered in eastern China’s Yunnan Province. The results of the research were published in the scientific journal Nov. Proceedings of the Royal Society B.

About 550-520 million years ago, during an event called the Cambrian Explosion, the first animals to form hard and sturdy skeletons suddenly appear in the fossil record in the blink of an eye. Most of these early fossils are simple hollow tubes ranging in size from a few millimeters to a few centimeters. However, it was almost entirely unknown what kind of animals made these skeletons, because they lacked the preservation of the soft parts needed to identify them as belonging to large groups of animals still alive today.

Gangtoucunia aspera diagram

Fossil specimen (left) and diagram (right) of Gangtoucunia aspera, which protects soft tissues, including gut and tentacles. Credits: Luke Parry and Guangxu Zhang

four examples gangtoucunia aspera Included in the new 514-million-year-old fossil collection are its soft tissues, including parts of the gut and mouth, still intact. They reveal that this species has a fringed mouth with a ring of smooth, unbranched tentacles about 5 mm (0.2 in) long. It is likely that they were used to sting and capture prey such as small arthropods. Fossils also show that gangtoucunia it had a blunt-ended gut (only one open end) divided into internal cavities that filled the length of the tube.

These are features 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 make the hard skeletons that make up most of the known fossil record.

According to researchers, gangtoucunia It looked similar to modern scyphozoan jellyfish polyps with a rigid tubular structure fixed to the underlying substrate. The tentacle mouth could extend out of the tube, but retract into the tube to avoid predators. Unlike living jellyfish polyps, tube gangtoucunia It is made of calcium phosphate, a hard mineral that makes up our own teeth and bones. The use of this material to build skeletons became rarer among animals over time.

Gangtoucunia aspera Mouth Area

Close-up of the mouth region of Gangtoucunia aspera showing the tentacles used to capture prey. Credits: Luke Parry and Guangxu Zhang

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.

Gangtoucunia aspera 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

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