Before life on Earth exploded in diversity about 540 million years ago, the first skeletons of primitive animals had already begun to form.
Spongy sea sponges of that time are tubular, thimble-shaped, organized with hard metal threads, specimens considered to be among the oldest groups of skeletal fossils.
However, there are several other early skeletons in the fossil record, and many of them have lost their soft parts long ago. As a result, it is difficult to determine what the first skeletal creatures on Earth later looked like outside of hollow tubes—and even more difficult to classify them.
Many random fossils from China have defied the odds and are now offering archaeologists a real-life look at early life forms that lived about 514 million years ago.
Fossils have preserved the soft tissues of four worm-like marine creatures of the genus Gangtoucunia aspera.
At first, scientists believed that this extinct genus was a close relative of living annelids (like earthworms), which were horizontally segmented. However, these new data indicate that Gangtoucunia is closely related to coelenterates such as jellyfish, sea anemones, and corals.
Their mouths are surrounded by retractable tentacles about 5 millimeters (0.2 in) long, which may have been used to capture prey. Meanwhile, her intestines occupy most of the body and are divided by longitudinal cavities.
The actual shape of these creatures is formed on the outside by a hard mineral known as calcium phosphate, which is also found in human bones.
Paleontologist Luke Barry of the University of Oxford says: “It’s really a one in a million find. These mysterious pipes are often found in groups of hundreds, but until now they’ve been considered ‘problematic’ fossils because we haven’t been able to classify them. Thanks to these with this unusual new feature, an important piece of the evolutionary puzzle has been put in place.”
The researchers found all four fossils in China’s eastern province of Yunnan, where a lack of oxygen allowed soft tissues to escape from hungry bacteria.
The crown of tentacles seen on top of this primitive slimy polyp is known to occur only between polyps, including jellyfish, before they enter the free-swimming stage.
In light of these results, the researchers concluded that G. aspera is a mucosal polyp on the seafloor, either within or adjacent to a subclade of coelenterates known as medusozoa. Most of the animals in this subclass, called true jellyfish (scyphozoans), develop the ability to swim freely over time, but some, such as several species of hydrozoa, remain myxoid polyps throughout their lives.
Exoskeletons could not only have arisen once, but they could have evolved many times in several different lineages. Perhaps the diversity of the animal’s skeleton was a big driving force behind the Cambrian explosion itself. However, the sudden appearance of structural diversity in the fossil record may also indicate how difficult it is for fragile strands of vital minerals to stand the test of time.
And even from the little evidence that scientists have found, it is clear that tubular animals appeared before the explosion of animal diversity that once took our lives by storm. What led to their expansion remains an open question, although predation is one possibility.
The study was published in a journal. Proceedings of Royal Society B Biological Sciences.
Source: Science Alert.