Around 400 million years ago, the world was warm. Animals were beginning to emerge from the oceans, and plants began to spread across the Earth. The northern Appalachian mountains were raised as the continents of Euramerica and Gondwana met. And the fish family reached a major splitting point.

One group branched off to become the lobe-finned fishes (like the famous coelacanth), which later spun off a group called the tetrapods. That’s us—we’re tetrapods, along with all other mammals, reptiles, birds, and amphibians. The other group became what we think of as stereotypical fish: the ray-finned fishes. That’s more or less every bone-filled fish you can think of: tuna, eels, goldfish (but not sharks; they’re cartilaginous, not bony).

A paper in Nature this week reports that ray-finned fishes may be a whole 40 million years younger than previously thought—and that a number of fossil species have been placed in the wrong slot on the family tree. The finding points to how crucial this period in evolutionary history is to our understanding of the evolution of modern vertebrates. And in the process, it clarifies the history of a strange fish that still lives in Africa called the bichir.

Outlying cousins

The living ray-finned fishes are mostly unambiguously classified, but there’s a group of black-sheep relatives: the bichir and reedfish. They’re a little weird, and they have “long vexed vertebrate biologists,” write Sam Giles, a paleobiologist at Oxford University, and her colleagues. Only in the early 20th century were these weird cousins understood to belong to the ray-finned fish family, but as an outsider group.

It’s thought that the bichir group split off from the rest of the family very early on, close to 385 million years ago. But the fossil record hasn’t been fruitful in confirming this; the oldest relevant fossils are around 100 million years old.

So Giles and colleagues turned to a group of fossil fish found across Russia, China, Kyrgyzstan, and the US. These "scanilepiform" fossils are dated to around 250 million years old—around the time of the first dinosaurs. Based on physical similarity, they have been classified as close cousins of other ray-finned fishes. But they have some similarities to these hard-to-classify bichir, too.

Using three-dimensional CT scans, Giles and her colleagues examined the details of well-preserved scanilepiform skulls. They used data on physical characteristics, as well as genetic data, to re-sketch the family tree. It came out looking somewhat different to what was previously thought.

A rejigged fish family tree

Their analysis suggested that the scanilepiforms form a group with the bichir weirdos rather than the majority of living ray-finned fishes. And it bumped some fossil species out of the inner circle of ray-finned fishes, which had the effect of changing the widely understood timeline substantially. With those fossils having changed position on the family tree, it looks like ray-finned fishes evolved around 40 million years more recently than the widely held date of 385 million years ago.

The results from the analyses include a great deal of uncertainty, but the new date coincides with a sudden increase in species diversity in the ray-finned fish fossil record, which offers important support for the revised date.

Ray-finned fish species make up around half of all vertebrate species on Earth, so understanding their evolutionary history will go a long way toward helping us understand the evolution of vertebrates. The finding suggests that around 350 million years ago, important changes were afoot—and a fuller understanding of this period will fill in a lot of puzzle pieces.

Nature, 2017. DOI: 10.1038/nature23654 (About DOIs).