Ann Arbor Times

A recent study from the University of Michigan has found that lobe-finned fishes, a group now represented by only eight living species, once experienced a rapid burst of jaw evolution and diversity between 359 and 423 million years ago. This research, published in the journal Current Biology and supported by the National Science Foundation, highlights how this ancient group developed new feeding strategies through innovations in jaw structure.

The research team led by postdoctoral researcher Emily Troyer used 3D models generated from CT scans to examine the jaws of 86 different fish species from the Silurian and Devonian periods. Their analysis revealed that lobe-finned fishes—including early lungfish and coelacanths—displayed much faster rates of change in jaw shape and function compared to their contemporaries, ray-finned fishes.

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

At present, ray-finned fishes are far more diverse than lobe-finned fishes, making up about 33,000 known species worldwide. Lobe-finned fishes are often referred to as "living fossils," with modern examples such as the coelacanth—rediscovered alive in 1938 after being thought extinct for millions of years.

Troyer noted that scientists had not previously seen such a significant difference in evolutionary activity between these two groups during their early history. The study found that lungfish jaws especially evolved to become larger and stronger during the early Devonian period. These adaptations likely allowed them to feed on hard-shelled prey like 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.”

To measure changes in jaw strength over time, researchers examined mechanical advantage—the force exerted when biting down—and found higher values among certain ancient lobe-finned fish species.

“Essentially, the higher the mechanical advantage of the jaw, the stronger the bite force,” Troyer explained.

Rafael Rivero-Vega, co-first author and recent U-M doctoral graduate who contributed extensive CT scan data for his dissertation work on lobe-finned fish fossils, observed that each major fish group followed its own path through evolutionary history.

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

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