The sea fountains started from the bottom, and now there it is

In adulthood, sea squirt Ciona intestinalis It resembles a linen gut, and the translucent tubes ooze out like a ghastly bouquet. But a kid Siona The sea squirt looks more like a tadpole – it swims freely in open water until it finds a difficult place to attach itself. The larva’s spongy body is extended by a notochord—a primitive, flexible backbone shared by all chordates, a group that includes all vertebrates. Once larvae Siona It finds an ideal home, and its body morphs, sucking its notochord and becoming a tube-like adult.

Sea fountains like Siona It is one of two types of marine invertebrates JacketThey are our closest invertebrate relatives. While sea squirts remain stationary, rooting in difficult places, the other type of sliders hop around the open seas, and include salps (sometimes called sea grapes), hormega (sometimes called sea pickles), and grubs (don’t have a cute title but build Houses made of mucus). These different lifestyles have led scientists to ask: Were the first freestyle tunics or bottom-dwellers?

A group of researchers has now found a 500-million-year-old fossil that looks exactly like modern sea spray, which they describe in a recent paper published in the journal Nature Communications. The fossil is dubbed Megasiphon thylakos, indicates that the ancestors of tunicates were bottom dwellers. The discovery provides new evidence for the evolutionary history of shots, pushing back the origin of the basic vertebrate body plan by 50 million years, according to Chen Kao, a Princeton researcher who was not involved in the research.

fossil Megasiphon tylakos on a piece of clay. Credit: James C. Weaver

“I have to admit, it’s quite an unusual discovery,” said Federico de Brown, an evolutionary developmental biologist at the University of São Paulo who was not involved in the new paper. “Tunic fossils are extremely rare and many published fossils have been highly examined,” Brown said, adding that Megaphone It is the most sea spray-like fossil reported to date.

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Javier Ortega-Hernandez and Rudy LeRosi-Obriel, both Harvard invertebrate paleontologists and authors on the paper, first came across the fossil in the spring of 2019 while excavating the collections of the Museum of Natural History in Utah. Ortega-Hernández works primarily with arthropods, whose hard exoskeletons are often finely ossified. But he pulled out a piece of mudstone, and saw a fossil that, in essence, looked like a tunic: like a vessel with two holes. “How strange,” Ortega Hernandez thought to himself.

He passed the fossil on to his colleague Karma Nanglo, a paleontologist at the Harvard University Museum of Comparative Zoology and an author on the paper. Nanglu describes himself as a “worm guy” but will be the first to tell you that worms aren’t real, at least as far as classification is concerned. (The creatures we call worms belong to several groups, but they share the classic body plan of a tube without limbs.) Unlike most of their colleagues, worm paleontologists have very few fossils to study. “They disappear most of the time,” said Nangluo. “You are decomposing at the speed of light.” So Nanglu had to get acquainted with other underappreciated groups of invertebrates, such as tunicates. “Like a worm,” he said, “you have to find some sort of interstitial space for yourself that isn’t already taken.”

Photo of cion intestinalis on a dark background.  The jacket is a white barrel with two straws
accident Ciona intestinalis Sea squirt. Credit: Karma Nanglo

When Nanglo heard that Ortega-Hernandez might have found the tunica fossil, he wanted to remain skeptical. “There are a lot of things that are like a lot of things,” said Nangluo.

The fossil record of citrate is hardly a record at all. Ausia fenestrata, a colander-like fossil from the Ediacaran period, has been inconclusively interpreted as coral, a sponge, and a tunic. The puzzled Scottish flipper-shaped fossil Your eyes are ozonated It was initially thought to be a type of tunic until it was re-described as an arthropod. And most of the putative fossils don’t preserve the whole body, just small, hard skeletal parts called spicules. In 2003, a group of researchers selected fossil Shankoclava anningenseIt was discovered in southern China as an early funnel mountain. “But if you look at the fossil, it doesn’t look much like a modern jacket,” said Nangluo. “It doesn’t look much like a jacket at all.”

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MegaphoneHowever, it looks just like a modern jacket. Similarities alone are not evidence, so Nanglu examined the fossil under a microscope and considered alternative identities for the fossil. Could it be part of another creature? branching algae? The burrow or excretory tube of an ancient worm? But no Cambrian creature had such a body part, and the fossil bore no resemblance to known Cambrian algae. In addition, it has preserved soft tissues, which means it wasn’t just a terrier. Nangluo saw that the fossil was streaked in dark bands extending from the base of the fossil to the tips of the siphonine, which he recognized as muscles similar to the modern tunic. Ciona intestinalis. He searched the literature for pictures of modern muscle tunics for comparison but found none.

“Karma has taken it upon himself to go like, ‘Okay, let’s order some longshirts from Woods Hole and we’ll do bad things for them,'” Ortega-Hernandez said grimly. Nanglu dissected the tunicates and found beautiful groups of muscles arranged just like the ganglia of fossils. “They kind of separate from individual fibers when they reach the tips of the siphons, and you can see they’re like fingers, arranged almost like a pitchfork,” Nanglu said.

Ciona's anatomy jacket
Anatomy of the Banded Muscle in Nanglu’s Shaped Vest (RIP!) Credit: Karma Nanglu

Other muscles were not preserved as well in the fossil, such as the annular muscles that surround the siphons of modern tunics, Brown said, adding that with only one specimen, he doesn’t think the fossil can be definitively identified as a tunic.

But for Cao, Megaphone“Unprecedented and well-preserved soft tissues including delicate muscular structures and the two swords clearly represent the tunicate modern.” Cao suggested that the authors examine the fossil with advanced microscopy techniques to study any chemical elements that may be preserved in the tunica albuginea. Tunicates can absorb trace elements from seawater, making them excellent indicators of their environment. Such a study, Kao said, “may help shed light on their living place in the Cambrian world.”

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Megaphone It provides new evidence that the earliest tunics were bottom-dwelling like modern sea squirts. said Melissa DeBase, a project scientist at the University of California, Merced, who was not involved with the paper. “Evolution is not always linear, and traits can be gained and lost in complex ways.”

But this will also open up a new mystery. If magma existed 500 million years ago, where are all the fossils? The Utah Formation that buried this tunica—likely via a sudden underwater mudslide—preserved many other soft-bodied animals. As Hernandez Ortega continues to sort through the museum’s hundreds of fossils, he has yet to see anything else that looks like a tunic. “We think this is really a unique individual, and you know, these things can be incredibly rare,” he said. Unfortunately for paleontologists, the Cambrian period may have been a lonely time to live. Fortunately for tunicates, the lack of a true brain meant they probably weren’t so sad about it.

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