The dunk was chunky, but still deadly

With a bite that could split a shark in two and an armored cup only a mother could love, Dunkleosteus was one of Earth’s first predators, terrorizing the subtropical seas 360 million years ago during the Devonian period. By some estimates, a monstrous fish measures as long as a school bus.

However, a new study takes a big bite out of Dunkleosteus’ estimated size. Russell Engelman, a paleontologist pursuing a Ph.D. at Case Western Reserve University, recently compared the proportions of the armor-covered head of Dunkleosteus to the sizes of the skulls of hundreds of living and fossil fish. Last month, in the journal diversityThese ancient fish, Mr. Engelman concluded, were only 13 feet in length and looked more like plump tuna than graceful sharks.

For the study, Mr. Engelman examined several Dunkleosteus terrelli specimens at the Cleveland Museum of Natural History. Many of these fossils were discovered nearby in the cliffs along the Rocky River, making specimens of the “dunk” a symbol of the city’s prehistoric times. But little research has been done on the size of Dunkleosteus, and some of the earlier measurements seemed suspicious to Mr. Engelmann.

Dunkleosteus belongs to an ancient family of fish known as arthropods that ruled the seas during the Devonian period. Since the bulk of Dunkleosteus’ body was likely made up of fragile cartilage, only the thick armor plates that enveloped its head and neck have been preserved as fossils. While these plates keep the predator’s jaws serrated, they reveal little about the rest of its body. As a result, most efforts to determine Dunkleosteus’ size have relied on extrapolation from the pedigree of its much smaller relatives.

According to Mr. Engelmann, head length is a reliable indicator of body size in fish: short fish species generally have shorter heads and tall fish species have longer heads. Focus on the area between the fish’s eye and the back of its head. “An organism can’t mess with the size of this area too much because that’s where the brain and gills are,” said Mr. Engelman. “If your nostrils get too small, you suffocate.”

Compare the size of this region in Dunkleosteus to the head proportions of nearly 1,000 other fossil and modern fish species, which range in size from small sea bass to large sharks. After taking measurements through several models, he concluded that the average head of Dunkleosteus, which measured about 24 inches in length, correlates with a fish slightly longer than 11 feet. The largest known Dunkleosteus topped at about 13.5 feet. Rather than behemoths the size of a bus, these fish were more akin to Volkswagen Beetles, but they’re still insects that can deliver bone-crushing bites.

Reducing Dunkleosteus’ height also changes its proportions. Most reconstructions depict Dunkleosteus with the elongated body of a shark. However, more complete arthropod fossils reveal that these fish had both squamous and cylindrical bodies. Mr. Engelman thinks Dunkleosteus probably looks like a round tuna.

This fish was fully shaped like an armored Pac-Man. It was twice the size of the great white mouth and probably outnumbered the longer shark. “People say it’s candy, but maybe it’s just hard muscle,” said Mr. Engelman.

Since the paper was published, many people have called fossil fish “Chunkleosteuson social media. But Mr. Engelman doesn’t think the new estimates take anything away from the old predator’s prowess.

“People think this is a downgrade, but this is actually an upgrade,” he said.

Far from a slow bottom-dweller, Dunkleosteus appears to have been built for fast movements in open water. And even Dunkleosteus the Shorter was still the undisputed king of the Devonian seas.

Not everyone is entirely convinced that Dunkleosteus rocked Abi’s body. It’s hard to know what Dunkleustius really looked like without more of his body, said Caitlin Colery, a paleontologist at the Cleveland Museum of Natural History. While cartilage is rarely ossified, the Cleveland Shale has produced the cartilaginous bodies of sharks that lived alongside Dunkleosteus.

“Don’t get me wrong, I love a chunky ‘dunk,'” said Dr. Colery, who was not involved in the new study. “But I’m not going to get too attached because in science, especially paleontology, it takes one new discovery to change everything.”