June 17, 2024


Armored Worm reveals the evolution of three animal tribes

The well-preserved fossil of a 518-million-year-old worm helps trace the origins of three animal phyla. The newly discovered fossil worm Wufengella bengtsoni had a segmented armored body with laterally protruding bristles. Anatomical studies show that it is a primitive relative of brachiopods, bryozoans and horseshoe worms – three animal phyla whose evolutionary relationships have hitherto been a mystery.

Biologists divide the animal kingdom into divisions, each of which has a number of unique characteristics that distinguish it from other animal divisions. Certain traits are found across multiple tribes. For example, there are three animal phyla that feed with the help of the so-called lophophore, a ring-shaped organ, with tentacles in which water and food particles swirl inside. These include brachiopoda, which resemble shells; bryozoa, so named because small marine organisms clump together to resemble a carpet of algae; The horseworm (Phoronida) that anchors in burrows on the sea floor. Although the evolutionary relationship of these animal lineages has not yet been fully elucidated, they have been summarized as Lophophorata due to the lophophore present in all representatives.

Early Cambrian fossil

A team led by Jin Guo from Yunnan University in China has now taken a decisive step toward the common origin of these three animal people. In the Zhengjiang Animal Society, an early Cambrian fossil site in southwest China, they discovered a fossil worm that turned out to be an ancestral relative of the three animal people. “When I first realized what this fossil was I was looking at under a microscope, I couldn’t believe my eyes,” says co-author Luke Barry of Oxford University. “We’ve speculated a lot about this fossil and hope to see it one day.”

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The worm, which researchers have dubbed Wufengella bengtsoni and is believed to be about 518 million years old, has an asymmetrical shield made up of overlapping plates. Below is a fleshy body with a series of flattened lobes protruding from the sides. Groups of hairs protrude from the body between the lobes and the carapace. In general, the length of the worm is about 16 mm. “It appears to be the unlikely offspring of a cross between a nematode and a ketone mollusk. Interestingly, it does not belong to either of these groups,” says co-author Jacob Venther of the University of Bristol.

The position of Wufengella in the Lophophorate family tree. © Luke Barry

Relationship with ringworm

Instead, it appears to be a representative of the Camenellan tommotiids, a group of invertebrates that lived in the early Cambrian period and are considered an ancestral group of theropods and horseworms. Barry explains: “Wufengella belongs to a group of Cambrian fossils that are essential to understanding the evolution of Lophophorata. Thanks to these fossils, we now understand how ancestors with many shell-like plates evolved into armopods, which contain only two shells resembling an oyster.”

Unlike today’s representatives, who establish themselves on the sea floor or on the surfaces, Wufengella was mobile. The segmented body structure clearly indicates an evolutionary relationship with the rings, which also include the earthworm today. Biologists have previously noted similarities between armopod larvae and annelids. “We can now see that these similarities stem from a common origin,” Vinther says. “The common ancestor of phosphorous and canaliculi had an anatomy more similar to rings.”

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From mobile to stable

According to the researchers, the ancestor of worm-like lophophorates evolved a sedentary lifestyle over the course of evolution and evolved a diet in which food particles circulated toward itself with water. “The tall, worm-shaped body with many body units became less useful and therefore reduced,” Finther explains.

“This discovery shows how important fossils are to reconstructing evolution,” says co-author Greg Edgecombe of the Natural History Museum in London. “We get an incomplete picture just by looking at living animals, with relatively few anatomical features common to different people. With fossils like the Wufengella, we can trace each lineage back to its roots and see that they once looked very different and had very different ways of living, sometimes Unique and sometimes shared with relatives far away.”

Source: Jin Guo (Yunnan University, China) et al., Current Biology, doi: 10.1016/j.cub.2022.09.011