Fish fossil yields a jaw-dropping clue to man’s evolution


Fri, Sep 27, 2013 - Page 7

The ancestor of all creatures with jaws and a backbone was not a sleek, shark-like beast, but a toothless, armored fish, said a study on Wednesday that rewrites evolutionary history.

Scientists said they had found a 419-million-year-old fish fossil in China which disproves the long-held theory that modern animals with bony skeletons (osteichthyans) evolved from a shark-like creature with a frame made of cartilage.

The osteichthyan group includes most living fish, humans and other land animals with limbs.

It had long been thought that modern-day cartilaginous fish like sharks and rays, a sister group to osteichthyans, most closely represent the original ancestor that gave rise to the two animal types.

This meant that we osteichthyans would have evolved our bony frames from scratch, 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 reveals a missing branch on the evolutionary tree and shows 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,” study co-author Brian Choo of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing said. “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.”

This meant that sharks and rays shed the common ancestor’s bony plates as they evolved, the team said.

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 the southern Chinese town of Qujing, was about 20cm long, had 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, Choo said.

“I was completely blown away upon seeing this fossil for the first time, even more so as the full implications started to sink in,” he said. “Every now and then you are confronted with jaw-dropping specimens like Lucy the Australopithecus [an 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. A little fish called Entelognathus now joins the ranks of these exceptional fossil discoveries.”

Paleontologists Matt Friedman and Martin Brazeau said the implications are “stunning.”

“It will take time to fully digest the implications of such a remarkable fossil, but it is clear that a major reframing of our understanding of early gnathostome [jawed vertebrate] evolution is now in full swing,” they wrote.