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

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. Reconstruction by Xiaodong Wang. Credit: Reconstruction by Xiaodong Wang.

A collection of exceptionally well-preserved fossils unearthed in China’s eastern Yunnan province has enabled scientists to solve a centuries-old puzzle in the evolution of life on Earth, revealing 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 solid, strong skeletons suddenly appear in the fossil record in the blink of a geological eye about 550-520 million years ago during an event called the Cambrian Explosion. 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.

The new collection of 514 million years of fossils includes four specimens of Gangtoucunia aspera with soft tissues still intact, including intestines and mouth parts. These reveal that this species had a mouth surrounded by a ring of smooth, unbranched tentacles about 5 mm long. It is likely that these were used to sting and capture prey, such as small arthropods. The fossils also show that Gangtoconia had a blind intestine (open only at one end), divided into internal cavities, which filled 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 shows 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, Gangtoconia would have looked similar to a modern scyphozoan jellyfish, with a rigid tubular structure attached to the underlying 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 of gangtoconia is 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.

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

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

Corresponding author Dr Luke Barry, Department of Earth Sciences, University of Oxford, said, “This is really a one-in-a-million find. These mysterious tubes are often found in groups of hundreds of individuals, but so far they have been considered ‘problematic’ fossils, because we had no way of classifying them.” Thanks to these unusual new specimens, an essential piece of the evolutionary puzzle has been put into place.”

The new specimens clearly show that ganga Toconia was not related to annelids (earthworms, polychaetes and their relatives) as previously suggested for similar fossils. It is now clear that the body of Gangtokunya had a smooth exterior and a longitudinally divided intestine, while the annular corpuscles had segmented bodies with a transverse division of the body.

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

Fossil specimen of Gangtoucunia aspera preserved soft tissues, including gut and tentacles (left and center). The graphic on the right shows the anatomical features visible in the fossil specimens. Image credit: Luke Parry and Guangxu Zhang. Credit: Luke Parry and Guangxu Zhang.

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

Ph.D. Student Guangxu Zhang, who collected and discovered the samples, said, “The first time I discovered the pink soft tissue on top of the Gangtoucunia tube, I was surprised and confused about what it was. The next month, I found three more samples with soft tissue preservation, which was very exciting. And it made me rethink the affinity of Gangtoconia. The soft tissues of the Gangtuconia reveal, especially the tentacles, that it is certainly not a worm-like tribism as previous studies had suggested, but more like a coral, and then I realized it was a cnidarian.”

Although the fossil clearly shows that Gangtoconia was a primitive jellyfish, this does not exclude the possibility that other types of early tubular fossils looked very different. From Cambrian rocks in Yunnan province, the research team found previously well-preserved tubular fossils that could be identified as priapulidae (marine worms), lobules (paired-legged worms, closely related to today’s arthropods) and annelids.

Co-author Xiaoya Ma (University of Yunnan and University of Exeter) said: “It appears that a tubular lifestyle is becoming increasingly common in the Cambrian, which may be an adaptive response to increased predation pressure in the early Cambrian. This study demonstrates that exceptional soft tissue preservation is critical. for us to understand these ancient animals.”

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

A close-up view of the mouth of the Gang’s Tuconia aspera showing the tentacles that could have been used to catch prey. Image credit: Luke Parry and Guangxu Zhang. Credit: Luke Parry and Guangxu Zhang.

The research paper, “Exceptionally Soft Tissue Conservation of Cnidarian Affinity for Cambrian Phospho-Tubular Mysteries” will be published in Proceedings of the Royal Society B In November. 2.

more information:
Exceptionally soft tissue preservation reveals a cnidarian affinity for the Cambrian phospho-tubular gas, Proceedings of the Royal Society B: Biological Sciences (2022). DOI: 10.1098/rspb.2022.1623. royalsocietypublishing.org/doi….1098/rspb.2022.1623

Provided by Oxford University

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