Fossils 500 million years ago may have delayed the origin of vertebrates

Paleomated fossils had a sac-like body and a pair of siphons, much like today’s masked animals. IMAGE CREDIT: NANGLU ET AL.

In a paper published today in Nature Communications, Karma Nanglu, a paleontologist at Harvard University in the United States, and co-authors report that a well-preserved 500-million-year-old fossil is very similar to some of today’s membrane animals – there are two siphons filtering organic particles in the water, and complex muscle tissue controls the siphon. The discovery provides clues to the timing and development of early capsulats, and may even delay the date of origin of vertebrates in capsulated animals, including humans.

“It looked like a membrane animal that died yesterday and just happened to fall on a rock.” Ninicholas Treen, a developmental biologist at Princeton University who was not involved in the study, said.

In 2019, a finger-sized fossil landed on Nanglu’s desk. He specialized in the Cambrian and Ordovician periods, when many of today’s animal forms appeared. This specimen sat in a drawer at the Salt Lake City Museum of Natural History for many years. Its discoverers took it from a fossil-rich Cambrian limestone layer in western Utah, and they thought it might have been a sea squirt or membrane animal — a marine invertebrate that shares a distant ancestor with all vertebrates.

“Throughout 500 million years of recorded history, this animal has essentially no fossil record.” Nanglu said excitedly.

Today, about 3,000 species of membrane animals live in almost all marine habitats. Most of their life cycle consists of two parts – a free-swimming tadpole-like larvae that settle and morph into stationary adults. The larvae of the tunementozoa have a chordate, the precursor of the spine and a defining feature of chordates, which include all vertebrates. But only a handful of cysted fossils currently exist, which paleontologists can’t explain.

However, this new specimen, whose soft body details have been preserved, is undoubtedly an angiophila. “The picture of this fossil is fantastic.” Treen said. Nanglu and colleagues named it the Great Siphon Pouch because of its huge siphon and sac-like body. Modern membrane animals use these siphons to help them filter food. Nanglu said the details of the giant siphon muscle tissue look almost identical to those of modern membrane animal stingworms.

“The giant siphon muscle means it may already have something like a heart, although its internal structure is not protected.” Treen said: “Since you can see these atrial siphon muscles, you can almost take for granted that there is a vertebrate beating heart inside this organism. ”

This finding suggests that the two-part life history and ability to deformate animals are ancestral traits of this group.

The presence of such a complete and recognizable membrane may also push back the origin of vertebrates. It is now believed that vertebrates originated about 450 million years ago. William Jeffery, a developmental biologist at the University of Maryland Park College who was not involved in the work, said: “If the body structure of the tunicles was formed 500 million years ago, and the temeantis are actually sister groups of vertebrates, then the vertebrates may also be older.” ”

Other fossils support this idea: the giant siphon likely lived with a swimming fish-like animal called Metaspriggina, which dates back 505 million years. Metaspriggina has eyes and muscle mass, and possibly a spinal cord.

Still, Nanglu said, existing fossils aren’t enough to change the story of vertebrate evolution. “Humans are naturally drawn to this question of origins, which helps develop the story, but we need more fossil evidence,” he said. (Source: Li Huiyu, China Science News)

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