Fossil of half-billion-year-old sea squirt could push back origins of vertebrates

Fossil of half-billion-year-old sea squirt could push back origins of vertebrates
Rare fossil “looks like it died yesterday”

6 JUL 2023


tunicate fossil (left) and reconstructionAn ancient tunicate (fossil, left; reconstruction, right) had a saclike body and twin siphons, much like tunicates today.NANGLU ET AL.

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A version of this story appeared in Science, Vol 381, Issue 6653.Download PDF

In 2019, a finger-size fossil landed on the desk of Karma Nanglu, a paleontologist at Harvard University who specializes in the Cambrian and Ordovician periods, when many of today’s animal forms made their entrance. The specimen had sat for years in the drawer of a Salt Lake City museum; its finders, who had pulled it from a fossil-rich layer of Cambrian limestone in western Utah, thought it might be a sea squirt or tunicate—a marine invertebrate that shares a distant ancestor with all vertebrates. Nanglu was excited but cautious at the prospect of a very ancient tunicate: “That’s a group for which there is essentially no fossil record for the entire 500 million years of recorded history.”

Now, in a paper published today in Nature Communications, Nanglu and his co-authors report that the exquisitely-preserved 500-million-year-old fossil is a dead ringer for some tunicates today, with two siphons to filter organic particles from the water and complex musculature controlling the siphons. “It looks like a tunicate that died yesterday and just happened to fall down on some rock,” says Nicholas Treen, a developmental biologist at Princeton University who wasn’t involved in the work. The discovery offers clues to the timing and development of early tunicates and could even push back the date for the origin of tunicates’ sister group, the vertebrates, including humans.

Today, some 3000 species of tunicate live in almost every habitat of the oceans. Most have a two-part life cycle, including a free-swimming, tadpolelike larva that settles and metamorphoses into a stationary adult. Tunicate larvae have a notochord, the precursor to a spinal column—a defining trait of the group called chordates, which includes all vertebrates. But only a handful of tunicate fossils exist, for reasons paleontologists can’t fully explain. The best-known, 520-million-year-old Shankouclava anningense, has proven controversial: It’s tunicate-ish in that it has a siphon and seemed to filter feed, but it doesn’t resemble anything alive today.

The new specimen, however, with details of its soft body preserved, is unambiguously a tunicate. “It’s really kind of glorious, the picture of this fossil,” Treen says. Nanglu and colleagues named it Megasiphon thylakos for its large siphons and saclike body (thylakos is from the Greek word for pouch). Modern tunicates use these siphons to help filter-feed and have bands of longitudinal muscles running down their bodies as well as circular muscles around the siphons to control them. The details of Megasiphon’s musculature look almost identical to those of Ciona intestinalis, a well-studied modern tunicate, Nanglu says.

As a tunicate matures, its siphon muscles develop from the same cells that form cardiac tissue in modern vertebrates. The muscles’ presence in Megasiphon means it may have already had something like a heart, even though its internal structures aren’t preserved. “Since you can see these atrial siphon muscles, you can almost take it for granted that there is a beating, vertebrate-like heart inside this organism,” Treen says.

The fossil addresses a long-unresolved question in early chordate evolution: whether the common ancestor to all tunicates was a free-swimming organism or rooted to the bottom. Megasiphon, with its resemblance to living, sessile tunicates, strongly supports the latter hypothesis. The find suggests that tunicates’ two-part life history and ability to metamorphose is an ancestral characteristic of the group.

The existence of such a complete and recognizable tunicate at this time could also push back the origin of vertebrates, now thought to be at about 450 million years ago. “If a tunicate body plan was already established [at 500 million years ago], and the tunicates are actually the sister group of vertebrates, then vertebrates may be older as well,” says William Jeffery, a developmental biologist at the University of Maryland, College Park, who was not involved in the work. Other fossils offer some support to this idea: Megasiphon likely lived alongside a swimming, fishlike animal named Metaspriggina, dated to as old as 505 million years ago. Metaspriggina had eyes and blocks of muscle, and probably a notochord. So, by about 500 million years ago, both it and Megasiphon already have many features considered as vertebrate hallmarks, suggesting an early origin for the group.

Still, Nanglu says existing fossils aren’t enough to change the story of vertebrate evolution. “Humans are naturally pulled to this question of origins, and this helps develop that story, but we need more fossil evidence,” he says. “But … answering these longstanding questions, that’s totally why we’re in it to begin with.”

doi: 10.1126/science.adj5955

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