Before life on Earth exploded in diversity about 540 million years ago, the first skeletons of primitive animals were already beginning to form.
Fluffy-looking sea sponges from this time have been found in thimble-like tubular forms, structured by hard, mineralized threads, specimens believed to be among the earliest assemblages of skeletal fossils.
However, there are few other early skeletons 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 earliest skeletal creatures looked like beyond hollow tubes, and even harder to classify them.
Several serendipitous fossils from China have defied the odds and are now giving archaeologists a real glimpse into the first 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.
Scientists initially thought this extinct genus was a relative of living annelid worms (like earthworms), which are horizontally segmented. However, these new results suggest that Gangtoucunia is more closely related to polyp cnidarians, such as jellyfish, sea anemones and coral.
The mouths of these tube-shaped organisms are surrounded by retractable tentacles about 5 millimeters (0.2 in) long, which were probably used to capture prey. Meanwhile, its intestine occupies most of the body and is divided into longitudinal cavities.
The creatures’ actual form is molded externally by a hard mineral known as calcium phosphate, which is also found in human bones.
“This really is a discovery in a million. These mysterious tubes are often found in groups of hundreds of individuals, but until now, they had been considered ‘problematic’ fossils because we had no way to classify them,” says the paleobiologist . Luke Parry of the University of Oxford.
“Thanks to these extraordinary new specimens, a key piece of the evolutionary puzzle has been put firmly in place.”
Artist’s reconstruction of G. aspera with the individual in the foreground sectioned to show its soft interior. (Xiaodong Wang)
The researchers discovered the four fossils in Yunnan province in eastern China, where a lack of oxygen has allowed the soft tissues to evade hungry bacteria.
The crown of tentacles seen at the top of these primitive polyps is known only among cnidarian polyps, 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 open to the ocean world. The tentacles at the entrance help to hook the prey and bring it into the mouth.
In light of these results, the researchers concluded that G. aspera is an ancient seafloor polyp within or proximal to a cnidarian subclade known as Medusozoa.
Most of the animals in this subclade, the so-called true jellyfish (scyphozoans), eventually develop free-swimming abilities, but some, such as some hydrozoan species, remain polyps throughout their lives. Colonies of hydrozoan polyps can also build skeletons similar to G. aspera fossils.
“Interestingly,” the authors note, “we do not recover a close relationship of Gangtoucunia in a clade with other meduzoans with calcium phosphate exoskeletons, suggesting that tube-building materials might have a complex early evolutionary history, possibly due to of convergent loss and reduction. of calcium phosphate in skeletons as it became less available through the Paleozoic.”
In other words, exoskeletons likely did not arise just once, but likely evolved multiple times in multiple different lineages.
A fossil of G. aspera and matching diagram showing preserved soft tissues. (Luke Parry and Guangxu Zhang)
The diversification of the animal skeleton may have been a major driver of 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 strands of biominerals to stand the test of time.
Even from the little evidence that scientists have found, it’s clear that tube-shaped animals were appearing before the explosion of animal diversity that once invaded our lives. What triggered their expansion remains an open question, although predation is a possibility.
“A tubular way of life seems to have become increasingly common in the Cambrian, which could be an adaptive response to increased predation pressure in the early Cambrian,” says paleobiologist and study author Xiaoya Ma from Yunnan University in China and the University of Exeter in the UK.
“This study shows that exceptional soft tissue preservation is crucial to understanding these ancient animals.”
The study was published in Proceedings of the Royal Society B Biological Sciences.