How ancient fish jaws built the path to modern land animals

Before there were dinosaurs, before trees covered the land, fish were already hard at work shaping Earth’s future. They did this with a single powerful innovation: jaws.

Jaws weren’t just for biting. Fish used them for all sorts of tasks – cracking shells, digging into the seafloor, defending themselves, and even carrying their young.

This one tool gave early vertebrates – including our distant ancestors – a huge advantage. But not all fish jaws evolved the same way.

Surprising twist in fish jaw evolution

Roughly 400 million years ago, fish ruled the seas. Two big groups stood out: lobe-finned fishes and ray-finned fishes.

Lobe-finned fishes had fleshy, muscular fins that helped them move in different ways. Ray-finned fishes had lighter, more flexible fins supported by thin spines.

Today, ray-finned fishes dominate the oceans, rivers, and lakes. They include about 33,000 species – from goldfish to tuna.

Lobe-finned fishes, on the other hand, are nearly extinct. Only eight species are known to exist today, including the coelacanth and the lungfish. But a new study from the University of Michigan flips the story.

The research shows that between 359 and 423 million years ago, lobe-finned fishes were the ones leading the evolutionary race.

Their jaws were changing fast, adapting in all sorts of new ways. Meanwhile, the ray-finned fishes were evolving far more slowly.

Finding answers in ancient jaw bones

The researchers used CT scans to create 3D models of 86 fossilized fish jaws from the Silurian and Devonian Periods – dating back to around 443 million years ago. That’s older than the first forests.

The team digitally mapped these jaws to study both their shape and how they worked. To measure the strength of a fish’s bite, they calculated something called mechanical advantage.

Emily Troyer is a postdoctoral researcher at the University of Michigan and lead author of the study. She explained that essentially, the higher the mechanical advantage of the jaw, the stronger the bite force.

The results showed something no one expected. “This is a really striking result, primarily because lungfish and coelacanths today are represented by only eight living species, with not much jaw diversity going on,” Troyer said.

“However, if we look back in time 400 or so million years, we see this striking inversion. During the Devonian, we have a lot more species and a lot more innovation within their jaws.”

Lungfish jaws lead the way

Lungfish jaws, in particular, went through a major shift during the Devonian Period. Their jaws became larger, thicker, and packed with powerful muscles. This likely helped them crush hard-shelled prey, like early clams and crustaceans.

“With their really hefty jaws, they were able to eat really hard food,” Troyer said. “We think these new feeding strategies might be causing jaws to need to be shaped like this, and that some of these major innovations are associated with their ecosystems during this time.”

Rafael Rivero-Vega, co-first author of the study and a recent doctoral graduate, scanned nearly every complete lobe-finned fish jaw fossil available.

He mapped out their key features to study something called adaptive radiation. That’s when animals quickly evolve into many different forms because their environment pushes them to adapt.

Rivero-Vega saw clear patterns. “Some fishes were diversifying their jaws rapidly in shape and size, only later to stay essentially unchanged once they filled a specialized niche,” he said.

“Others had similar characteristics but a wider variety of shapes and sizes, and yet others had similar form but wouldn’t change until after they had already transitioned onto land.”

Fish jaw innovation changed the future

This research shows that evolution doesn’t follow a straight line. Just because a group is successful now doesn’t mean it always was.

Lobe-finned fishes once led the way in jaw innovation, helping set the stage for major changes in vertebrate life – including the eventual rise of amphibians, reptiles, birds, and mammals.

“When you’re looking at evolution, you can learn so much from looking at the past,” Troyer said. “Without the fossil record, we would have no idea of this inverted role reversal.”

The study also highlights the importance of old bones. Even though only a few lobe-finned fishes are still around, their ancient fossils tell a story that would otherwise have been be lost.

“It’s a great example of how innovations in shape, form, and function can be explored by different fish groups at their own pace, as long as they experience the appropriate evolutionary pressures,” said Rivero-Vega.

“All of this happened hundreds of millions of years before the dinosaurs. Fishes are awesome.”

The full study was published in the journal Current Biology.

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