More than 500 million years ago, marine invertebrates launched a new evolutionary experiment: skeletons. But while these durable, tube-like structures have stood the test of time as fossils, the animals’ soft bodies have decayed and disappeared, erasing any trace of what these ancient animals might have looked like. Now, a recent re-examination of these ancient skeletal tubes has finally unveiled the identity of one of these mysterious organisms.
These early calcium-reinforced “skeleton” tubes date from a period known as the Cambrian Explosion (541-510 million years ago) and appear to have been an effective survival strategy, as they appeared in several groups over a relatively short period. geological time (about 50 million years). During this period, everything from the segmented ancestors of earthworms to the bizarre ancient relatives of tardigrades created protective tube-like structures.
However, tracing the evolutionary history of these early exoskeletons has proven tricky. “Soft tissue tends to break down” Xiaoya Ma, an invertebrate paleontologist at Yunnan University in China and co-author of a study describing the findings, told Live Science. For this reason, identifying fossil Cambrian tubes has been a bit like trying to guess the contents of an empty, unlabeled tin can based solely on the shape of the can – most could just as easily have contained chicken soup than creamed corn.
But scientists are shedding light on these enigmatic skeleton makers. In the new study, published November 2 in the journal Proceedings of the Royal Society B, an international team of researchers has described four incredibly well-preserved Cambrian specimens from China’s Yunnan province. These 514-million-year-old fossils of the tube-dwelling creature Gangtoucunia aspera include the soft tissue imprints left by the bodies of animals. Studying these impressions closely, the scientists determined that the tubes belonged to, among other things, an ancient skeleton-making jellyfish.
Soft-bodied invertebrates are hard to find in the fossil record, and jellyfish in particular are almost never preserved. “This fossil was a double whammy in terms of rarity,” Luke Parrypaleobiologist at the University of Oxford and co-author of the study, Live Science told Live Science in an email.
Related: The ancient armored “worm” is the Cambrian ancestor of three major groups of animals
Normally, when a marine organism dies, scavengers and bacteria quickly attack its soft tissues. But very occasionally, a wave of fine sediment covers the remains quickly enough to prevent aerobic bacteria from settling there. Smithsonian National Museum of Natural History in Washington, DC, and this is probably also how the Yunnan site was formed.
The new fossils discovered by the study’s lead author, Guangxu Zhang, Ma’s graduate student at Yunnan University, have been preserved in such detail that paleontologists have even been able to distinguish the internal organs of animals. The creatures’ mouths were surrounded by a ring of tentacles, each about 0.2 inches (5 millimeters) long. And they had a sac-like intestine with a single opening (unlike the separate mouth and anus that vertebrates are blessed with).
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These characteristics led the team to conclude that G. rough probably belonged to the phylum Cnidaria, which includes modern jellyfish, corals and sea anemones. It also ended an older theory that the creature was an annelid worm, which is defined by its segmented body and gut with two openings.
G. rough probably hung in ancient oceans with one end of its tube anchored to other members of its species or mobile creatures such as trilobites, retracting into its shell as predators passed by. It probably fed much like modern jellyfish polyps do, extending its stinging tentacles when prey was near.
Only the larvae of one group of jellyfish, Scyphozoa, today create exoskeletons. Some other cnidarians, such as corals, retain their skeletons until adulthood. However, today’s corals build their skeleton from calcium carbonate; in contrast, G. rough made its tubes from calcium phosphate, the same tough compound that makes up our tooth enamel and bones.
Why modern cnidarians changed from calcium phosphate exoskeletons to calcium carbonate exoskeletons remains a mystery. “One potential reason is that the environment before our present time was rich in phosphorus,” Ma said. But the answer could also be found in cnidarian genetics. Ma and her team hope to answer this and other questions as their research continues. “Hopefully we’ll have more for everyone in the near future,” she said.
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