Scientists have been able to resolve a long-standing conundrum in the history of life on Earth by determining the appearance of the earliest creatures to create skeletons thanks to an extremely well-preserved collection of fossils that were found in eastern Yunnan Province, China.
The findings were released in the Proceedings of the Royal Society B.
In a geological flash, between 550 and 520 million years ago, during the Cambrian Explosion, the first creatures to develop strong, hard bones appeared in the fossil record. Many of these early fossils are plain, hollow tubes with lengths between a few millimetres and many centimetres. Due to the lack of preservation of the soft components required to distinguish them as belonging to the main groups of animals that are still living today, it was practically impossible to determine what kind of creatures formed these skeletons.
Four specimens of Gangtoucunia aspera with soft tissues intact, including the stomach and mouthparts, are part of the new collection of 514 million-year-old fossils.
These demonstrate that this species had a mouth bordered by a ring of 5-mm-long smooth, unbranched tentacles. These were probably employed to sting and catch prey—such little arthropods. The fossils also demonstrate that Gangtoucunia had an interior cavity-partitioned blind-ended gut that spanned the length of the tube and was only open at one end.
These characteristics are only present in contemporary jellyfish, anemones, and their near cousins (known as cnidarians), which are soft-bodied animals that are exceedingly uncommon in the fossil record. The research demonstrates that these primitive creatures were among the first to develop the hard bones that make up most of the fossil record.
The scientists hypothesised that Gangtoucunia had a rigid tube structure attached to the subterranean substrate, comparable to current scyphozoan jellyfish polyps. The mouth of the tentacle would have protruded from the tube but could have been drawn back within to evade predators. The tube of Gangtoucunia was formed of calcium phosphate, a hard material that also makes up our own teeth and bones, unlike live jellyfish polyps. It has become less common for animals to use this substance to build skeletons throughout time.
According to the corresponding author, Dr. Luke Parry of the Department of Earth Sciences at the University of Oxford, this finding is really one in a million.
Due to the lack of a classification system, these unexplained tubes, sometimes found in groups of hundreds of individuals, have been referred to as “problematic” fossils.
One of the most important pieces of the evolutionary puzzle has been securely set in place thanks to these exceptional new examples.
The new specimens unequivocally show that, contrary to what had previously been hypothesised for analogous fossils, Gangtoucunia was not related to annelid worms (earthworms, polychaetes, and their cousins). It is now evident that annelids have segmented bodies with transverse body partitioning, whereas Gangtoucunia had a smooth exterior with a gut divided longitudinally.
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 bacteria that normally degrade soft tissues in 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 about what they were.
” In the following month, I found three more specimens with preserved soft tissue, 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 unequivocally demonstrates that Gangtoucunia was a prehistoric jellyfish, it is still possible that other early tube-fossil species had completely different appearances. The study team had previously discovered well-preserved tube fossils from Cambrian rocks in Yunnan province that may be classified as priapulids (marine worms), lobopodians (worms with paired legs closely linked to modern arthropods), and annelids.