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Fossilized Fish Reveal Earliest Known Prequel of ‘Jaws’

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Mention the ocean, and it’s hard not to consider jaws. The deep waters contain many tooth-lined mouths: the bear-trap maws of sharks and dolphins, the slack lips of shoaling and reef fish, the baleen-filter gape of enormous whales. Jawed fish eventually crawled out of the seas hundreds of thousands of years ago and gave rise to the jawboning vertebrates we’re today.

But when did such an evolutionary innovation arise? A pair of fossil beds discovered in Southern China suggest that the reply may lie tens of hundreds of thousands of years deeper into the past than previously thought. The findings — which include beautifully preserved recent species of early fish, the oldest-known vertebrate teeth and numerous fish with armor — were published Wednesday across 4 papers in Nature.

“This can be a step change in where we’re desirous about these events within the chronology of vertebrate evolution,” said Matt Friedman, a paleontologist on the University of Michigan who was not involved within the research but wrote a perspective article that accompanied the Nature papers.

Jawed fish explode into the fossil record 419-359 million years ago during a period often known as the age of fish, or the Devonian. Fish of this era all have “their identities clearly written on their bodies,” said Michael Coates, a paleobiologist on the University of Chicago who was not involved in the brand new papers. They include ancient groups like jawless fish, lineages of early jawed fish called placoderms, ascendant newcomers like cartilaginous and bony fish and the primary fishes to hop onto land.

This sudden diversity of jawed fish — also called gnathostomes — has long led scientists to suspect that their origins must lie deeper within the fossil record, a period often known as the Silurian, said Per Ahlberg, a paleontologist at Uppsala University in Sweden and an writer on one in all the papers. But until recently, the variety of useful Silurian gnathostome fossils could possibly be counted on one hand.

A decade ago, researchers got down to systematically survey the 425-million-year-old rocks of the late Silurian period in China, said Gai Zhikun, a paleontologist on the Chinese Academy of Sciences and an writer on one in all the papers. They were rewarded with complete fossils of early jawed fish.

Encouraged, they delved into older rocks. In 2020 these fishing expeditions got a bite: a pair of deposits outside of Chongqing.

The 2 fossil beds are separated by a number of million years in time, each with a unique complement of species.

The 436-million-year-old bed accommodates “little fishtank-sized fishes,” Dr. Ahlberg said, only a number of centimeters long, which represent the oldest-known complete jawed fish. Most are of a flat, armored placoderm species named Xiushanosteus mirabilis, which probably lived on the seafloor. Also present is Shenacanthus vermiformis, a cartilaginous fish related to sharks and rays, but with armor plates resembling those of unrelated placoderms — a find that implies early sharklike species retained the armor-plates present in earlier branches of the fish family tree.

Philip Donoghue, a paleontologist on the University of Bristol and an writer on one in all the papers, says probably the most remarkable specimen from the location is a jawless fish called Tujiaaspis vividus. Hundreds of head shields from the species’ family are known from the fossil record, but Tujiaaspis preserves the primary known body. It comes with a surprise: a set of paired fins jutting out from the skull, which Dr. Donoghue and his colleagues suggest is a probable precursor of the pectoral and pelvic fins present in gnathostomes, which for fish that moved onto land gave rise to legs and arms. Previously, researchers believed the 2 sets of fins evolved individually between jawless and jawed fish.

“It overturns conventional wisdom on how paired appendages originated,” Dr. Donoghue said.

The second site, at 439 million years old, preserved more vital fossils. One paper describes a group of spines, scales and head-plates from an animal named Fanjingshania renovata, all of them dead-ringers for later examples of cartilaginous fish. One other records a whirl of connected teeth — the oldest yet known from a vertebrate — from a fish named Qianodus duplicis. Each animals belong firmly to the branch of jawed fish called the chondrichthyans, the group of cartilaginous fish that include modern sharks, rays and ratfish. (Bony fish like salmon and humans are the opposite branch.)

The presence of shark relatives at the location suggests that the split between cartilaginous and bony fish had already occurred by the early Silurian, Dr. Friedman said. Taken together, each sites push the origin of vertebrate jaws and teeth back by almost 14 million years.

“It’s a giant shift from the consensus chronology,” Dr. Friedman said, which is able to force a drastic reconsideration of early marine ecosystems.

Jawed fish now appear to have originated as early because the Great Ordovician Biodiversification, a period around 485 million to 445 million years ago when marine invertebrates ruled. The few known fish from that period are jawless and customarily unprepossessing, Dr. Coates said. “They give the impression of being like armor-plated tadpoles,” he said. “So the final thing you’d expect is for proto-sharks and proto-bony fish to be swanning around at the identical time.”

As paleontologists proceed to dig deeper into early Silurian rocks in China, they’ve uncovered much more fish species. In relation to the earliest jawed fish, researchers may soon find that they’ll need an even bigger boat.

“It’s highly likely that there can be more discoveries,” Dr. Ahlberg said. “It’s an overused phrase, but I mean it: This guarantees to completely revolutionize our understanding of the earliest phase of jawed vertebrate evolution.”

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