Fossils from half a billion years ago preserve first skeletons, solving centuries-old mystery

What did the first animals to have skeletons look like? The discovery of extraordinarily well-preserved fossils – over half a billion years old – has helped scientists solve this centuries-old mystery.

The ancestors of all backboned animals (including humans) suddenly appear in the fossil record 550 million years ago during the “Cambrian explosion.” That biological “big bang” saw the evolution of all the basic body plans that we see in the animal kingdom today – including exoskeletal arthropods (ancestors of spiders, insects and scorpions) and vertebrates.

The earliest known vertebrates were small fish-like organisms, no longer than 5 centimetres. One such ancient fish, Pikaia, lived 505 million years ago.

But the answer to the question of how skeletons evolved before becoming bendy backbones has long eluded scientists.

Because the soft tissue surrounding the skeleton is rarely preserved, this has remained a palaeontological riddle.

Four specimens of a species called Gangtoucunia aspera allow scientists to finally answer these questions. The 514-million-year-old fossils preserved impressions of the animals’ soft tissues – including the gut, mouthparts and the tissue surrounding the hollow-tube skeleton.

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Gangtoucunia are revealed to have a mouth fringed with smooth tentacles about 5 millimetres long. These tentacles were probably used to capture small arthropods for food. The skeletal pioneers are also shown to have a blind-ended gut – that is, a single opening performed both the function of a mouth and an anus.

The features present in Gangtoucunia are found today only in modern jellyfish, anemones, and their close relatives, the cnidarians – organisms whose soft tissue is rarely preserved in the fossil record. The research shows these animal groups were among the first to build the fossilised skeletons.

Artist’s reconstruction of Gangtoucunia aspera as it would have appeared in life on the Cambrian seafloor, circa 514 million years ago. The individual in the foreground has part of the skeleton removed to show the soft polyp inside the skeleton. Reconstruction by Xiaodong Wang. Credit: Reconstruction by Xiaodong Wang.

Gangtoucunia would have looked like the polyps of true jellyfish. The hard, tubular body anchored to the sea floor would have been topped with the retractable, tentacled mouth. Unlike modern jellies, however, the skeletal tube of Gangtoucunia was made of calcium phosphate – the same stuff that makes up our teeth and bones, but has become more rare in animal skeletons over the aeons.

“This really is a one-in-million discovery,” says author Dr Luke Parry from Britain’s Oxford University. “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 fossils are the subject of a research paper published in the journal Proceedings of the Royal Society B.

Analysing the new Gangtoucunia specimens shows that the organisms are not related to annelid worms (like earthworms) as was previously believed. The smooth exterior of Gangtoucunia and its longitudinally-partitioned gut does not correspond to the segmented and transversely-partitioned bodies of annelids.

Fossil specimen (left) and diagram (right) of Gangtoucunia aspera preserving soft tissues, including the gut and tentacle. Image credit: Luke Parry and Guangxu Zhang. Credit: Luke Parry and Guangxu Zhang.

“The first time I discovered the pink soft tissue on top of a Gangtoucunia tube, I was surprised and confused about what they were,” says first author and Yunnan University PhD student Guangxu Zhang, who collected and discovered the specimens.

“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.”

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While Gangtoucunia certainly appears to be an early jellyfish, it doesn’t rule out the possibility that other fossil tubes may have belonged to animals that looked very different. Some are still believed to have belonged to marine worms and other early worm-like organisms.

“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,” adds co-author Dr Xiaoya Ma of both Yunnan and Exeter Universities. “This study demonstrates that exceptional soft-tissue preservation is crucial for us to understand these ancient animals.”

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