Artist’s reconstruction of Gangtoucunia aspera as seen in Cambrian seabed life about 514 million years ago. In the individual in the foreground, part of the skeleton is removed to show the soft polyp inside the skeleton. Credit: Reconstruction by Xiaodong Wang
Scientists have finally solved a centuries-old puzzle about the evolution of life on Earth, revealing what the first animals to develop a skeleton looked like. The discovery was made possible by an exceptionally well-preserved collection of fossils discovered in Yunnan province in eastern China. The results of the research were published in the scientific journal on November 2 Papers of the Royal Society B.
During an event called the Cambrian explosion, about 550 to 520 million years ago, the first animals to build hard, robust skeletons suddenly appeared in the fossil record in a geological blink of an eye. Many of these early fossils are simple hollow tubes ranging in length from a few millimeters to many centimeters. However, it was almost entirely unknown what kind of animal produced these skeletons, as they lack the preservation of soft parts needed to identify them as belonging to the major groups of animals still alive today.
Fossil specimen (left) and diagram (right) of Gangtoucunia aspera preserves soft tissues including gut and tentacles. Credit: Luke Parry and Guangxu Zhang
Four examples gangtoucunia aspera included in a new collection of 514-million-year-old fossils with soft tissues still intact, including gut and mouthparts. These indicate that this species has a fringed mouth with a ring of smooth, branchless tentacles about 5 mm (0.2 in) long. These were likely used to sting and capture prey such as small arthropods. Fossils also show this gangtoucunia had a blind-ended gut (open at only one end) that filled the length of the tube, divided into internal cavities.
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 create the hard skeletons that make up most of the known fossil record.
According to researchers, gangtoucunia resembled modern scyphozoan medusae polyps with a rigid tubular structure attached to the underlying substrate. The mouth of the tentacle could extend beyond the tube, but retract inside the tube to avoid predators. Unlike living jellyfish polyps, the tube gangtoucunia made from calcium phosphate, a hard mineral that makes up our own teeth and bones. The use of this material to build skeletons has become rarer among animals over time.
Close-up photo of mouth region of Gangtoucunia aspera showing tentacles used to capture prey. Credit: 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.
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
