U-M study traces jaw innovation and evolution in a once-mighty group of fish
Study: Macroevolutionary role reversals in the earliest radiation of bony fishes
If you're reading this sentence, you might have a fish to thank.
Fish were the first animals to evolve jaws. They use their jaws primarily to eat, but also for defense, as tools-such as to burrow or to crack open hard food-and even as a form of parental care: some fish carry eggs or their young in their mouths. Jaws are a trait that scientists think fueled evolution among vertebrates, including us.
Now, a University of Michigan study has shown that a now rare group of fish called lobe-finned fishes enjoyed an explosion of diversity between about 359-423 million years ago. They were an especially diverse group containing many species with rapidly evolving jaws and new innovations in feeding modes. In contrast, the other major group of fishes at the time, ray-finned fishes, had jaws that evolved much more slowly.
This is surprising because at some point, several million years later, their evolution stalled out. The most famous of these "living fossils" is likely the coelacanth, once thought to be extinct but discovered to be living in the deep ocean in 1938 by one of the most well-known women in science, Marjorie Courtenay-Latimer. Today, just eight species of lobe-finned fishes are recognized by scientists. By contrast, ray-finned fishes comprise about 33,000 species today and include just about any fish you can think of, from goldfish to bass to seahorses.
The study, led by U-M postdoctoral researcher Emily Troyer, is published in the journal Current Biology and supported by the National Science Foundation.
Troyer said the study underscores the importance of looking to ancient fossil records to discover new information about the process of evolution. They didn't expect to see such a disparity in evolutionary might between lobe-finned fishes and ray-finned fishes-something that wouldn't have been known if not for the fossil record of Silurian and Devonian fish.
"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 age of fishes
While scientists have long suspected the role of jaws in vertebrate evolution, there was little work that compared jaw evolution among early fishes. The study authors examined 3D models from CT scan data of 86 different species of fishes from the Silurian and Devonian periods, beginning about 443 million years ago-before even trees existed. They found that the lobe-finned fishes, lungfish and coelacanth, in particular, displayed the fastest rates of change and the most innovation in jaw shape and function.
"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. 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," Troyer said.
To determine this, the research team digitally mapped each 3D model to examine both the form of the jaw and the function by determining the mechanical advantage of the jaw, or how much force the fish could exert when they bit down.
"Essentially, the higher the mechanical advantage of the jaw, the stronger the bite force," Troyer said.
The research team found that the shape of lungfish jaws really took off in the early Devonian period. Their jaws grew big and thick, with heavy muscle. This likely gave them the ability to eat hard-shelled prey such as 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 and recent U-M doctoral graduate, collected CT scan data and visited museums to create additional 3D scans of nearly every available, complete lobe-finned fish jaw fossil for his dissertation. He then mapped important characteristics of the jaws in order to test for "adaptive radiation," or the rapid diversification of animals due to changes in their environment.
Rivero-Vega was struck by the research's revelation that each fish group was experiencing "a unique evolutionary moment in their ancient past."
"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," he 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."
