By Brad Butterfield

326 million years ago, Arkansas was underwater and teeming with an unusual, gill-covered shark, a shark that is now called Cosmoselachus mehlingi after a years-long project led by Dr. Allison Bronson, a Cal Poly Humboldt professor. The unique conditions in the Fayetteville Shale preserved the precious cartilage that was critical in digitally reconstructing an accurate image of the ancient shark, which was named to honor Carl Mehling, a senior museum specialist for the American Museum of Natural History (AMNH). The shark’s ancient origins and recent identification have been made possible by CT scans, digital reconstruction, and years of careful work carried out by Bronson alongside Alan Pradel, John Denton, and John Maisey. The findings were recently published by the journal Geodiversitas.

Cosmoselachus mehlingi is distinctly important, according to Bronson, due to the fact that its ancestors survived a catastrophic mass extinction that killed 95% of the fish population in the Devonian period.

“In the wake of these extinctions, we often have what we call an adaptive radiation where the groups that make it through can then diversify into all these niches that were left open,” Bronson said.

The acidic and anoxic environment of Arkansas’ Fayetteville Shale preserved the shark’s delicate cartilage for millions of years until the 1970’s, when Royal and Gene Mapes, the husband and wife duo, discovered the fossil. The Mapes would eventually donate the specimen to the AMNH. 

“Cartilage is really rare in the fossil record, because it’s soft and squishy, and usually microbes break it down before it can fossilize,” Bronson said.

The rare preservation of the shark’s cartilage allowed Bronson and her collaborators to use CT scanning to digitally reconstruct the ancient shark’s biological structure and identify one of its most unique attributes, gill covers.

“It’s made of little rays of cartilage that grow off of its gill arches,” Bronson said. “In our shark, unlike anything that’s been described in any cartilaginous fish, they’re all fused together.” 

The gill covers may have functioned to help close the gill openings, according to Bronson. Though gill covers are found today in ratfish, Bronson and her colleagues’ statistical testing showed that it is unlikely that ratfish are descendents of the group of fishes that includes Cosmoselachus mehlingi. Additionally, the digital reconstruction revealed that the ancient shark had widely spaced teeth with tooth-like scales in between.

  Bronson and her colleagues’ work to identify Cosmoselachus mehlingi gives scientists a valuable glimpse into our ancient world and another datapoint that aids in our understanding of evolution over time.

“Once you have an understanding of the long branching pattern of evolution, you can reconstruct the way that traits evolved over time with a group like cartilaginous fishes that’s been successful for more than 400 million years, and made it through multiple mass extinction events to now be the dominant predators of the ocean,” Bronson said. “A lot of evolution is just like, if it ain’t broke, don’t fix it.”