With a bite that could split a shark in two and an armored mug that only a mother could love, Dunkleosteus was one of Earth’s earliest apex predators, terrorizing subtropical seas 360 million years ago during the Devonian period. By some estimates, the monster fish measured as long as a school bus.
However, a new study takes a considerable chunk out of Dunkleosteus’ estimated size. Russell Engelman, a paleontologist pursuing his Ph.D. at Case Western Reserve University, recently compared the proportions of Dunkleosteus’ armored head with the skull size of hundreds of living and fossil fish. Last month, in the journal DiversityMr. Engelman concluded that these ancient fish maxed out at only 13 feet and were shaped more like chunky tuna than slender sharks.
For the study, Engelman examined several Dunkleosteus terrelli specimens at the Cleveland Museum of Natural History. Many of these fossils were discovered nearby in rocks along the Rocky River, making specimens of the “Dunk” a prehistoric icon in the city. But little research had been done on Dunkleosteus’ size, and some of the previous measurements seemed sketchy to Mr. Angelman.
Dunkleosteus belonged to an ancient fish faction known as arthrodires that ruled the seas during the Devonian period. Because the bulk of Dunkleosteus’ body was likely composed of fragile cartilage, only the thick armor plates that encased its head and neck were preserved as fossils. Although these plates preserve the predator’s spiky jaws, they reveal little about the rest of its body. As a result, most efforts to size Dunkleosteus relied on extrapolating from the proportions of its much smaller relatives.
According to Engelman, head length is a reliable proxy for body size in fish: Short fish species generally have shorter heads and long fish species have longer heads. He focused on the area between the fish’s eye and the back of its head. “The organism can’t mess with the size of this area too much because that’s where the brain and gills are,” Engelman said. “If your gills get too small, you suffocate.”
He compared the size of this region in Dunkleosteus with the head proportions of nearly 1,000 other species of fossil and modern fish, from smallmouth bass to large sharks. After running the measurements through several models, he concluded that the average Dunkleosteus head, which measured around 24 inches, correlated with a fish slightly longer than 11 feet. The largest known Dunkleosteus topped out at about 13.5 feet. Instead of bus-sized, these fish were closer to Volkswagen Beetles, but still bugs capable of delivering bone-crushing bites.
Reducing Dunkleosteus’ length also changes its proportions. Most reconstructions depict Dunkleosteus with the elongated body of a shark. But more complete arthrodire fossils reveal that these fish had squat, cylindrical bodies. Mr. Engelman believes that Dunkleosteus probably resembled a round tuna.
This fully feathered fish was like an armored Pac-Man. It had a mouth twice the size of a great white and probably outweighed taller sharks. “People say it’s pudge, but it’s probably just solid muscle,” Engelman said.
Since the paper’s publication, several people have called the fossil fish “Chunkleosteus” on social media. But Mr. Engelman doesn’t think the new estimates take anything away from the prowess of the old predator.
“People think this is a downgrade, but this is actually an upgrade,” he said.
Far from being a slow-swimming bottom dweller, Dunkleosteus seems to have been built for fast movements in open water. And even a shorter Dunkleosteus was still the undisputed king of the Devonian seas.
Not everyone is entirely convinced that Dunkleosteus shook a dad. Caitlin Colleary, a paleontologist at the Cleveland Museum of Natural History, said it’s hard to say for sure what Dunkleosteus really looked like without more of its body. While cartilage is rarely fossilized, the Cleveland Shale has yielded the cartilaginous bodies of sharks that coexisted with Dunkleosteus.
“Don’t get me wrong, I love a thick ‘Dunk,'” said Dr. Colleary, who was not involved in the new study. “But I won’t get too attached because in science, especially paleontology, all it takes is one new discovery to change everything.”
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