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U-M research traces jaw development and evolution in a once-dominant group of fish

If you’re engaging with this text, you may owe a debt of gratitude to a fish.

Fishes were the inaugural animals to develop jaws. Primarily, they utilize their jaws for feeding, but also for protection, as tools—for instance, to burrow or to break hard food—and even for nurturing: certain fish transport eggs or their young within their mouths. Jaws are a characteristic scientists believe propelled evolution among vertebrates, including humans.

Recently, a University of Michigan investigation has revealed that a now uncommon group of fish known as lobe-finned fishes experienced an explosion of variety approximately 359-423 million years ago. This was an exceptionally diverse group containing many species with quickly evolving jaws and new feeding techniques. Conversely, the other principal faction of fish at that time, ray-finned fishes, had jaws that progressed at a much more gradual rate.

This revelation is unexpected, as their evolution eventually stagnated several million years later. The most well-known of these “living fossils” is likely the coelacanth, long believed to be extinct until its rediscovery in the deep sea in 1938 by the renowned scientist Marjorie Courtenay-Latimer. Presently, scientists acknowledge just eight species of lobe-finned fishes. In stark contrast, ray-finned fishes encompass roughly 33,000 species today, including nearly any fish you can envision, from goldfish to bass to seahorses.

The research, spearheaded by U-M postdoctoral scholar Emily Troyer, appears in the journal Current Biology and is funded by the National Science Foundation.

Troyer indicated that the study highlights the significance of examining ancient fossil records to uncover new insights regarding evolutionary processes. They were not anticipating a discovery of such a marked discrepancy in evolutionary prowess between lobe-finned and ray-finned fishes—an understanding that would have remained hidden without the fossil evidence from Silurian and Devonian fish.

“When engaging with evolution, you can gain extensive knowledge by examining the past,” Troyer mentioned. “Without the fossil evidence, we’d remain oblivious to this reversed role reversal.”

The era of fishes

While researchers have long acknowledged the role of jaws in vertebrate progression, little research has compared jaw evolution among early fishes. The study’s authors analyzed 3D models derived from CT scan data of 86 distinct fish species from the Silurian and Devonian periods, commencing around 443 million years ago—before trees even existed. They discovered that lobe-finned fishes, including lungfish and coelacanth, particularly showed the highest rates of transformation and the most innovations in jaw shape and function.

“This is a remarkable outcome, primarily because lungfish and coelacanths today comprise only eight extant species, exhibiting minimal jaw diversity. However, if we look back roughly 400 million years, we observe this impressive reversal. During the Devonian, a greater number of species and more innovations existed within their jaws,” Troyer explained.

To arrive at this conclusion, the research team digitally mapped each 3D model to assess both jaw morphology and functionality by evaluating the mechanical advantage of the jaw, indicating the force the fish could exert when biting.

“Essentially, the greater the mechanical advantage of the jaw, the more powerful the bite force,” Troyer noted.

The research team observed that the configuration of lungfish jaws significantly advanced during the early Devonian period. Their jaws expanded and thickened, supported by robust muscle. This likely enabled them to consume hard-shelled prey like early clams and crustaceans.

“With their substantial jaws, they could tackle tough food,” Troyer stated. “We hypothesize that these novel feeding strategies necessitated jaws to be shaped in this manner, with several major innovations tied to their ecosystems during that time.”

Rafael Rivero-Vega, co-first author and recent U-M doctoral graduate, gathered CT scan data and visited museums to compile additional 3D scans of nearly every available complete lobe-finned fish jaw fossil for his dissertation. He subsequently mapped pivotal features of the jaws to investigate “adaptive radiation,” or the swift diversification of animals attributable to environmental changes.

Rivero-Vega was astonished by the discovery that each fish group encountered “a distinct evolutionary phase in their ancient history.”

“Some fishes rapidly diversified their jaws in shape and size, only later to remain largely unchanged once they occupied a specialized niche; others displayed similar traits but with a broader variety of shapes and sizes, while yet others exhibited similar forms but did not alter until they had transitioned onto land,” he explained.

“It’s a remarkable illustration of how innovations in shape, form, and function can be explored by various fish groups at their own pace as long as they face the appropriate evolutionary pressures. And all of this occurred hundreds of millions of years prior to the dinosaurs. Fishes are fascinating.”

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