The 419-million-year-old fish from the placoderm family had a complex lower jaw structure..
The discovery of what may be the earliest known creature with a face has provided scientists with the missing link needed to solve an age-old mystery as to when vertebrates developed jaws.
Discovered in China, the fossilised fish preserved in three-dimensions is the most primitive vertebrate found with a modern type of jaw, including a dentary bone found in humans.
A previously unknown member of the now extinct placoderm family, the ancient fish would have been largely covered by armoured plates. But it is the complex lower jaw structure of the 419-million-year-old fish that has scientists so exhilarated.
The fossilised placoderm.
''It is very exciting news and I don't use superlatives like that very often,'' said Flinders University strategic professor in palaeontology John Long. ''This fish is pivotal in understanding the very first creatures to have jaws.''
Outlined over seven pages in the journal Nature on Thursday, the discovery solves a niggling question about the origin of modern fish. It had been thought that the most recent common ancestor of jawed vertebrates resembled modern sharks. But the new fossil turns this view on its head.
Professor Long said the fossil fish slotted into the evolutionary timeline ahead of two groups of fish: the sharks and rays, which have cartilage skeletons, and other fish such as salmon and trout that have bone skeletons.
This indicates that the fossil uncovered in China was an ancestor of both the cartilage and bone fish groups, meaning sharks are not as primitive as first thought.
The placoderm group of fish also hold the first signs of features that have survived the evolutionary process to be evident in humans, including the dentary bone in the jaw, paired hind limbs and complex reproductive organs.
''This fish is really important for understanding the deep, distant evolution of the human body plan,'' Professor Long said.
Australian Brian Choo was among the team that found the fossil in China's Yunnan province, an urban site that has yielded some of the most important fish fossils in recent years.
He has predicted the find will ''tear up the textbooks'' because it demands new interpretations of evolutionary theory.
Professor Long agreed, describing the discovery as the most exciting find since the Lucy skeleton was unearthed in Ethiopia in 1974.
Scientifically Lucy, a 3.2-million-year-old hominid, was important because the small skull capacity was akin to that of apes but the skeleton also showed evidence that Lucy walked upright like humans. The find supported the debated view that walking preceded increase in brain size in human evolution.
While Lucy filled the relatively minor gap between modern humans and apes, the discovery of the fish fossil in China fills in a huge gap between ancient placoderm fish and modern fish species.
''This is filling in one of the biggest gaps in evolution, where previously we had no intermediate forms at all,'' Professor Long said.