December 10, 2022

Fossilized fish reveal oldest before-known 'jaws'

Fossilized fish reveal oldest before-known ‘jaws’

I mention the ocean, and it’s hard not to think about the jaws. Deep waters have many mouths lined with teeth: bear-trap mice for sharks and dolphins, lax lips for shallow-water fish and corals, and a baleen filter gap for gigantic whales. Fish-jawed eventually crawled out of the seas millions of years ago and gave rise to the jawed vertebrates we have today.

But when did such an evolutionary innovation appear? A pair of fossils discovered in southern China suggests the answer may lie tens of millions of years deeper than previously thought. The findings — which include beautifully preserved new species of early fish, the oldest known vertebrate teeth and plenty of armored fish — were published Wednesday over the course of four years. leaves in nature.

said Matt Friedman, a University of Michigan paleontologist who was not involved in the research but wrote Perspective essay that accompanied nature papers.

Fish jaws erupted in the fossil record 419-359 million years ago during a period known as the Fish Age, or Devonian. All fish of this age have “clearly written on their bodies,” said Michael Coates, a paleobiologist at the University of Chicago who was not involved in the new papers. They include ancient groups such as jawless fish, early jawless fish subspecies called placoderms, and emerging newcomers such as cartilaginous fish and bony fish. The first fish that jumped on land.

One of the papers, said Per Ahlberg, a paleontologist at Uppsala University in Sweden and an author on the history of fossils. But until recently, it was possible to count the number of useful Silurian fossils on one hand.

A decade ago, researchers set out to systematically survey 425 million-year-old rocks in late Silurian China, said Jay Jiequn, a paleontologist at the Chinese Academy of Sciences and an author on one of the research papers. They were rewarded with complete fossils of early jawed fish.

Encouraged by this, they dug into the ancient rocks. In 2020, these fishing expeditions have a peak: a pair of sediments outside Chongqing.

The two fossil beds are separated by a few million years, and each has a different set of species.

Dr. Ahlberg said the 436-million-year-old bed contained “small fish the size of an aquarium,” measuring just a few centimeters long, representing the oldest known whole-jaw fish. Most of them are of a flat, armored type of plaque-skinned called Xiushanosteus mirabilis, which likely lived on the sea floor. There is also Shenacanthus vermiformis, a cartilaginous fish related to sharks and rays, but with armor plates similar to those of unrelated skin–a finding that suggests that early shark-like species retained the armor plates found in earlier branches of the fish family tree.

The most remarkable specimen from the site is a jawless fish called Tujiaaspis vividus, says Philip Donoghue, a University of Bristol paleontologist and author of a paper. Thousands of head shields from the species family are known from the fossil record, but Tujiaaspis retains the first known body. It comes with a surprise: a set of paired fins sticking out of the skull, which Dr. Donoghue and colleagues suggest is a possible precursor to the pectoral and pelvic fins found in gnathostomes, which gave rise to the fish’s arms and legs that moved to the ground. Previously, researchers believed that the two sets of fins evolved separately between jawless and jawless fish.

“It overturns conventional wisdom about how paired appendages arise,” said Dr. Donoghue.

The second site, which is 439 million years old, has preserved more significant fossils. One leaf describes a group of spines, scales, and head plates from an animal called Fanjingshania renovata, All of them are dead to later examples of cartilaginous fish. Another records a spiral of connected teeth – The oldest by far of the vertebrates From a fish named Qianodus dupis. Both animals belong firmly to a branch of jawed fish called chondrichthyans, a group of cartilaginous fish that includes modern sharks, rays, and ratfish. (Bony fish such as salmon and humans are the other branch.)

The presence of shark relatives at the site suggests that the split between cartilaginous and bony fish did indeed occur by the early Silurian, Dr. Friedman said. Taken together, both sites push the origin of vertebrate jaws and teeth back by about 14 million years.

“It’s a major shift from the agreed chronology,” said Dr. Friedman, which will force a radical reconsideration of early marine ecosystems.

Jawfishes now appear to have arisen as early as the great Ordovician biodiversity, a period from 485 million to 445 million years ago when marine invertebrates ruled. Dr. Coates said that the few fish known from the period were jawless and generally unimpressive. “They look like tadpoles covered in armour,” he said. “So the last thing you would expect is primitive sharks and primitive bone fish roaming around at the same time.”

As paleontologists continue to dig deeper into the early Silurian rocks in China, they have discovered more species of fish. When it comes to first jaw fish, researchers may soon discover that they will need a larger boat.

“It is very likely that there will be more discoveries,” Dr. Ahlberg said. “It’s an overused phrase, but I mean it: this promises to completely revolutionize our understanding of the first stage of jawed vertebrate evolution.”