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Fish fossil yields jaw-dropping switch in animals’ evolutionary path

AFP-JIJI

The ancestor of all creatures with jaws and a backbone was not a sleek sharklike beast but a toothless armored fish, according to a study that rewrites man’s evolutionary history.

Scientists said they have found a 419-million-year-old fish fossil in China that disproves the long-held theory that modern animals with bony skeletons — collectively known as osteichthyans — evolved from a sharklike creature with a frame made of cartilage. The osteichthyan group includes most living fish plus land animals with limbs, including humans.

It had long been thought that cartilaginous fish such as sharks and rays, which form a sister group to osteichthyans, most closely represent the original jawed ancestor that gave rise to the two types.

This meant that we osteichthyans evolved our bony frames while the group that includes sharks, rays and ratfish retained their ancestral cartilage skeletons.

But the new find of a primordial fish with a complex arrangement of small skull and jaw bones revealed a missing branch on the evolutionary tree and showed that a bony skeleton was in fact the prototype for all vertebrates, a research team wrote in the journal Nature.

“This astounding discovery does throw a spanner in the works of some long-held ideas about vertebrate evolution,” said study co-author Brian Choo from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.

“The implications are clear: Ostheichthyans did not independently acquire their bony skeletons, they simply inherited them” from their ancestors — heavily armored fish known as placoderms that are accepted to be the most primitive members of the jawed vertebrate family, he said.

“Osteichthyans didn’t go through an unarmored sharklike . . . stage during their early evolution only to reacquire their bone later on; they simply kept the plates directly from their . . . ancestors,” said Choo.

This meant that sharks and rays, instead of being the archetypal vertebrates, shed the common ancestor’s bony plates as they evolved, according to the team.

The newly discovered creature, dubbed Entelognathus primordialis, meaning primordial complete jaw, was a type of placoderm that lived in the seas of China in the late Silurian period from about 423 million to 416 million years ago.

The weird-looking animal, whose near-complete fossil was dug up near Qujing in southern China, was about 20 cm long, with a heavily armored head and trunk, and a scaly tail. It had jaws — but no teeth — and tiny eyes set in large, bony goggles. It was not a direct ancestor of today’s jawed vertebrates, but an extinct “close nephew” of our common forefather that shared many of its characteristics, Choo said.

“Every now and then you are confronted with jaw-dropping specimens like Lucy the Australopithecus (an extinct, upright-walking hominid) or the first batch of Chinese feathered dinosaur, unleashing a flood of new information that greatly clarifies our view of the distant past and often forces us to rethink what we thought we knew about evolution,” he said.