Not much is known about the onychodonts, aside from the fact that they were predators with funny-looking faces (like Qingmendous, they all had squiggly teeth at the front of their lower jaw) and that they died out some 350 million years ago.
"We basically had two stages in the evolutionary history of the lobe finned fishes," said You'an Zhu, a paleontologist at the Chinese Academy of Sciences and Uppsala University in Sweden.
"There were the very primitive ones - so primitive they didn't even have lobed fins," he continued. "And then we get complete sets of lineages we are quite familiar with: coelacanths, also lungfish, and of course our direct ancestors the tetrapods (every land vertebrate ever, plus birds, bats, dolphins and whales)."
But few fossils have been found from the creatures that lived in between those two stages, and many aren't in great shape. So it was hard to figure out how Qingmendous was related to other creatures alive at the time, not to mention everything that came after.
That's when You'an Zhu joined the team. Using CT scans, he and his colleagues digitally reconstructed the inside of Qingmendous's skull, allowing them to get a better look at who he was and how his brain worked.
"It turned out to be a mosaic of characteristics," he said. "It kind of filled in the gaps between the primitive and the modern lobe finned fishes, bridging the two stages together."
Qingmendous is not a missing link, he said - evolution is not linear. But the characteristics of his body and brain can help explain how the sea creatures made the leap onto land.
Like the ancestors of modern terrestrial creatures, Qingmendous dwelled in brackish waters of lagoons, bays and deltas - within spitting distance of land. He had the same muscular fins that his cousins would use to clamber gracelessly out of the water. His skull also shares some of the advanced features of still-living lobe finned fish (well, advanced for 409 million years ago), indicating that his brain was more modern as well.
The lobe-finned fish had the right tools for terrestrial exploration. They also had good luck and good timing, You'an Zhu said. There are plenty of fish that can survive out of the water for short periods of time today, but they don't stick around because there's too much competition and too many predators.
But 390 million years ago, plants had comfortably colonised land and insects were widespread. The continents were essentially a well-stocked, all-you-can-eat buffet - with no one else to hog the food and not a single predator large enough to make you regret sticking your head out of the sea.
"So the lobe finned fish will take their chance and venture onto land and become our ancestors," he said.
