Researchers may have found an answer to evolutionary riddle after studying 500 million year-old fossils
Scientists have solved a centuries-old mystery about the evolution of life on Earth by looking at a group of fossils found in China’s eastern Yunnan Province. These fossils are very well preserved, and they show what the first animals to make skeletons looked like. The results were written up in Proceedings of the Royal Society B.
Around 550–520 million years ago, during a geological blink of an eye called the Cambrian Explosion, the fossil record suddenly shows the first animals with strong, hard skeletons. Many of these early fossils are simple tubes that are hollow and range in length from a few millimeters to many centimeters. But it was almost impossible to tell what kind of animals made these skeletons because they didn’t have any soft parts that could be used to match them to major groups of animals that are still alive today.
In the new collection of fossils from 514 million years ago, there are four Gangtoucunia aspera fossils that still have the gut and mouth parts. These show that the mouth of this species was surrounded by a ring of smooth tentacles that were about 5 mm long and didn’t branch out. These were probably used to sting and catch prey, like small arthropods. The fossils also show that Gangtoucunia had a tube-shaped gut that was only open at one end. The tube was filled with internal cavities.
These are traits that are only found in modern jellyfish, anemones, and their close relatives, which are called “cnidarians.” Fossils of organisms with soft parts are very rare. The study shows that these simple animals were among the first to make the hard skeletons that make up most of the fossil record.
Researchers think that Gangtoucunia would have looked like modern scyphozoan jellyfish polyps, which have a hard, tube-like structure that is anchored to the substrate below. The mouth of the tentacle would have been outside the tube, but it could have been pulled back inside to avoid being eaten. The tube of Gangtoucunia, on the other hand, was made of calcium phosphate, a hard mineral that is found in our teeth and bones. Over time, animals have used this material less and less to make their skeletons.
“This really is a one-in-a-million find,” said Dr. Luke Parry of the University of Oxford’s Department of Earth Sciences. These strange tubes are often found in groups of hundreds, but until now, we didn’t know how to classify them, so we thought of them as “problematic” fossils. With the help of these amazing new specimens, a key piece of the puzzle of evolution has been put in place.”
The new fossils show that Gangtoucunia was not related to annelid worms (earthworms, polychaetes, and their relatives), as had been thought before for fossils that looked similar. Gangtoucunia’s body had a smooth outside and a gut that was split longitudinally. In contrast, annelids have segmented bodies that are split across the body.
The fossil was found in the Gaoloufang section of Kunming, which is in the eastern part of China’s Yunnan Province. Bacteria that normally break down soft tissues in fossils can’t live here because there isn’t enough oxygen.
The specimens were found and collected by Ph.D. student Guangxu Zhang. He said, “When I first saw the pink soft tissue on top of a Gangtoucunia tube, I was surprised and didn’t know what it was. The next month, I found three more specimens with soft tissue that was still there. This was very exciting and made me question Gangtoucunia’s relationship to other animals. Gangtoucunia’s soft tissue, especially its tentacles, show that it is not a worm-like priapulid as previous studies had said. Instead, it is more like a coral, and that’s when I realized it is a cnidarian.
Even though the fossil makes it clear that Gangtoucunia was an early jellyfish, it doesn’t mean that other early tube-fossil species didn’t look very different. The research team has already found well-preserved tube fossils in Cambrian rocks in Yunnan province that could be identified as priapulids (marine worms), lobopodians (worms with two legs, related to arthropods today), and annelids.
“A tubicolous way of life seems to have become more common in the Cambrian,” said co-corresponding author Xiaoya Ma of Yunnan University and University of Exeter. “This might be an adaptive response to increasing predation pressure in the early Cambrian.” This study shows how important it is for us to understand these ancient animals that their soft tissues are very well preserved.
Exceptional soft tissue preservation reveals a cnidarian affinity for a Cambrian phosphatic tubicolous enigma, Proceedings of the Royal Society B: Biological Sciences. DOI: 10.1098/rspb.2022.1623
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