Human evolution: an early tunicate relative found (Introduction)

by David Turell , Friday, July 07, 2023, 16:39 (295 days ago) @ David Turell

From Utah Cambrian shale:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/07_july_2023/411356...

"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.

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"Now, in a paper published in Nature Communications this week, Nanglu and his coauthors 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.

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"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."

Comment: Gould described another animal with a notochord at that time interval, the Pikaia, in his book, Wonderful Life. And later in China in 1995 more forms were found.