500-million-year-old fossils reveal the answer to the evolutionary puzzle

500-million-year-old fossils reveal the answer to the evolutionary puzzle

500-million-year-old fossils reveal the answer to the evolutionary puzzle

An artist’s reconstruction of Gangtoucunia aspera as it appeared in life on the Cambrian sea floor, approximately 514 million years ago. The person in the foreground has part of the skeleton removed to show the soft polyp inside the skeleton. Reconstruction of Xiaodong Wang. Credit: Reconstruction by Xiaodong Wang.

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

The the first animals to build strong and robust skeletons suddenly appear in the fossil record in the blink of a geologic eye 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. However, what kind of animal made these skeletons was almost completely unknown, as they lack the preserved soft parts needed to identify them as members of the major 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 intestines and mouthparts. This reveals that this species had a mouth lined with a ring of smooth, unbranched tentacles about 5 mm long. They were probably used to sting and capture prey, such as small arthropods. Fossils also show that Gangtoucunia had a blind-ended gut (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 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 solid skeletons that make up most of what is known the fossil record.

According to the researchers, Gangtoucunia would have looked similar to modern scyphozoan jellyfish polyps, with a hard tubular structure anchored to the underlying substrate. The mouth of the tentacles would extend outside the tube, but could be retracted inside the tube to avoid predators. However, unlike living jellyfish polyps, Gangtoucunia’s tube is made of calcium phosphate, the hard mineral that makes up our teeth and bones. The use of this material to build skeletons has become less common among animals over time.

500-million-year-old fossils reveal the answer to the evolutionary puzzle

Fossil specimen (left) and diagram (right) of Gangtoucunia aspera preserving soft tissues, including gut and tentacle. Image credit: Luke Parry and Guangxu Zhang. Credit: Luke Parry and Guangxu Zhang.

Corresponding author dr. Luke Parry, Department of Earth Sciences, University of Oxford, said: “This really is a one-in-a-million discovery. These mysterious tubes are often found in groups of hundreds of individuals, but until now they were considered ‘problem’ fossils because we had no way of classifying them. Thanks to these remarkable new specimens, a key piece of the evolutionary puzzle is firmly in 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 longitudinally partitioned gut, whereas annelids have segmented bodies with a transverse body partition.

500-million-year-old fossils reveal the answer to the evolutionary puzzle

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

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 normally decomposing bacteria soft tissues in fossils.

Ph.D.Sc. student Guangxu Zhang, who collected and discovered the specimens, said: “The first time I discovered the pink soft tissue at the top of the Gangtoucunia tube, I was surprised and confused about what they were. In the following month, I found three more preserved specimens soft tissue, which was very exciting and made me rethink the affinity of Gangtoucunia. The soft tissue of Gangtoucunia, especially the tentacles, reveals that it is certainly not a priapulid-like worm as previous research had suggested, but more like a coral, and then I realized that it’s a cnidarian.”

Although the fossil clearly shows that Gangtoucunia was a primitive medusa, it does not rule out the possibility that other early tube-fossil the 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 (sea worms), lobopodia (paired-legged worms, closely related to today’s arthropods) and annelids.

Co-corresponding author Xiaoya Ma (Yunnan University and University of Exeter) said: “The tubicolous lifestyle seems to have become more common in the Cambrian, which could be an adaptive response to increasing predation pressure in the Early Cambrian. This study shows that a remarkable the preservation of soft tissues is key to understanding these ancient animals.”

500-million-year-old fossils reveal the answer to the evolutionary puzzle

A close-up photo of the mouth region of Gangtoucunia aspera showing the tentacles that would be used to capture prey. Image credit: Luke Parry and Guangxu Zhang. Credit: Luke Parry and Guangxu Zhang.

The paper “Extraordinary soft tissue preservation reveals a cnidarian affinity for the Cambrian phosphate tubiculose enigma” will be published in Proceedings of the Royal Society B he Nov. 2.

More information:
The exceptional preservation of soft tissue reveals a cnidarian preference for the Cambrian phosphate tubicolous enigma, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.1623. … .1098/rspb.2022.1623

Citation: 500-million-year-old fossils reveal answer to evolutionary puzzle (2022, November 1) Retrieved November 2, 2022, from .html

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