Incredible discovery has finally revealed the first animals to grow a skeleton: ScienceAlert

For life on Earth exploded in diversity some 540 million years ago, the first primitive animal skeletons began to form.

Squishy-looking sea sponges from this era have been found in tubular thimble-like shapesstructured by hard, mineralized wires – specimens thought to be among the earliest assemblages of skeletal fossils.

Yet few other early skeletons remain in the fossil record, many of which lost their soft parts long ago. As a result, it’s hard to say what Earth’s first skeletal creatures ever looked like beyond hollow tubes — and even harder to classify them.

Several accidental fossils from China have defied the odds and now offer archaeologists a real glimpse into early life forms that lived about 514 million years ago.

The fossils have preserved the soft tissues of four worm-like sea creatures belonging to the genus Gangtoucunia aspera.

Initially, scientists thought this extinct genus was related to life ringworms (such as earthworms), which are segmented horizontally. However, these new results suggest: Gangtoucunia is more closely related to polyp cnidarians, such as jellyfish, sea anemones and coral.

The mouths of these tubular organisms are surrounded by retractable tentacles about 5 millimeters (0.2 in) long, which were probably used to capture prey. Meanwhile, their gut takes up most of the body and is divided into longitudinal cavities.

The actual shape of the creatures is formed on the outside by a hard mineral known as calcium phosphate, which is also found in human bones.

“This is really a one-in-a-million discovery. These mysterious tubes are often found in groups of hundreds of individuals, but until now they were considered ‘problematic’ fossils because we couldn’t classify them,” say paleobiologist Luke Parry of the University of Oxford.

“Thanks to these extraordinary new specimens, an important piece of the evolutionary puzzle has been put firmly in place.”

Researchers discovered all four fossils in China’s eastern Yunnan province, where a lack of oxygen allowed soft tissue to evade hungry bacteria.

The crown of tentacles seen on top of these primitive polyps is only found in polyp cnidarians, including jellyfish, before they develop a free-swimming stage.

As young polyps, jellyfish are shaped like vases, with one end attached to a surface and the other end open to the ocean world. Tentacles at the entrance help to catch the prey and put it in the mouth.

In light of these results, the researchers concluded that: G. rough is an ancient seafloor polyp within or near a cnidarian subclade known as medusozoa.

Most of the animals in this subclade, called true jellyfish (scyphozoa), eventually develop free-swimming abilities, but some, such as a few species of hydrozoa, remain polyps throughout their lives. Colonies of Hydrozoan Polyps can build similar skeletons to the G. rough fossils too.

“Intriguing”, the authors Remark“we do not find a close relationship between Gangtoucunia in a clade with other medusozoans with calcium phosphate exoskeletons, suggesting that tube-building materials might have a complex early evolutionary history, possibly due to convergent losses and reductions of calcium phosphate in skeletons as it became less available by the Paleozoic.

In other words, external skeletons probably didn’t arise just once, but probably evolved multiple times in multiple different lineages.

The diversification of the animal skeleton would great driver behind the Cambrian explosion itself. But the sudden appearance of structural diversity in the fossil record may also indicate how difficult it is for thin threads of biominerals to stand the test of time.

Even from the little evidence scientists have found, it’s clear that tubular animals emerged before the explosion of animal diversity that once took our lives by storm. What caused their expansion, however, remains an open question predation is a possibility.

“Tubicous way of life appears to be increasingly common in the Cambrian, which could be an adaptive response to increasing predation pressures in the Early Cambrian,” say paleobiologist and study author Xiaoya Ma of Yunnan University in China and the University of Exeter in the UK.

“This study shows that exceptional soft tissue preservation is crucial for us to understand these ancient animals.”

The study was published in the Proceedings of the Royal Society B Biological Sciences.