Sea sponges from that time have been found in thimble-like tubular shapes constructed of hard, mineralized filaments—specimens believed to be among the earliest assemblages of skeletal fossils. However, few other early skeletons exist in the fossil record, and many of them lost their soft parts long ago. As a result, it’s hard to tell what Earth’s first skeletal creatures once looked like beyond the hollow tubes — and even harder to classify them. Several random fossils from China have defied the odds and are now providing archaeologists with a real glimpse of early life forms that lived some 514 million years ago. The fossils have preserved the soft tissue of four worm-like sea creatures belonging to the species Gangtoucunia aspera. Originally, scientists thought this extinct genus was a relative of living ringworms (like earthworms), which are horizontally segmented. However, these new results suggest that Gangtoucunia is more closely related to polyps such as jellyfish, sea anemones and corals. The mouths of these tube-shaped organisms are ringed by retractable tentacles about 5 millimeters (0.2 in) long, which were probably used to capture prey. Meanwhile, their gut occupies most of the body and is divided into longitudinal cavities. The creatures’ actual shape is shaped externally by a hard mineral known as calcium phosphate, which is also found in human bones. “This is truly 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 to classify them,” says the paleobiologist. Luke Parry from the University of Oxford. “Thanks to these extraordinary new specimens, a key piece of the evolutionary puzzle has been firmly placed in place.” Artist’s rendering of G. aspera with the individual in the foreground cut away to show its soft interior. (Xiaodong Wang) The researchers discovered all four fossils in China’s eastern Yunnan province, where a lack of oxygen allowed the soft tissues to avoid hungry bacteria. The crown of tentacles seen at the top of these primitive polyps is known to occur only among polypous cnidarians, including jellyfish before they develop a free-swimming stage. As young polyps, jellyfish are vase-shaped, with one end attached to a surface and the other open to the ocean world. The tentacles at the entrance help to catch the prey and pop it into the mouth. In light of these results, the researchers concluded that G. aspera is an ancient seafloor polyp either within or close to a cnidarian suborder known as medusozoa. Most animals in this suborder, the so-called true jellyfish (scyphozoa), eventually develop free-swimming abilities, but some, such as some species of hydrozoa, remain polypoid throughout their lives. Hydrozoan polyp colonies can also produce skeletons similar to fossil G. aspera. “Strangely,” the authors note, “we do not recover a close relationship between Gangtoucunia in a clade with other medusozoans with calcium phosphate exoskeletons, suggesting that pipe-building materials could have a complex early evolutionary history, possibly due to convergent losses and declines in of calcium phosphate in skeletons as it became less available through the Paleozoic.” In other words, exoskeletons probably did not arise just once, but probably evolved several times over many different generations. A fossil of G. aspera and a coincident diagram showing preserved soft tissues. (Luke Parry and Guangxu Zhang) The diversification of animal skeletons may have been a large factor behind the Cambrian explosion itself. However, the sudden appearance of structural diversity in the fossil record may also speak to how difficult it is for thin filaments of biominerals to stand the test of time. Even from the scant evidence scientists have found, it’s clear that tube-shaped animals appeared before the explosion of animal diversity that once took our lives. What triggered their expansion remains an open question, although predation is a possibility. “A tubular lifestyle appears to have become increasingly common in the Cambrian, which may be an adaptive response to increasing predation pressure in the early Cambrian,” says paleobiologist and study author Xiaoya Ma of Yunnan University in China and the University of Exeter at. the UK. “This study shows that excellent soft tissue preservation is crucial to understanding these ancient animals.” The study was published in Proceedings of the Royal Society B Biological Sciences.
