Dunk Was Chunky, but Still Deadly

With a bite that could split a shark in two and an armored mug 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 is taking a sizable bite out of Dunkleosteus’s estimated size. Russell Engelman, a paleontologist pursuing his Ph.D. at Case Western Reserve University, recently compared the proportions of Dunkleosteus’s armor-clad head to the skull sizes of hundreds of living and fossil fish. Last month, in the journal Diversity, Mr. Engelman concluded that these ancient fish maxed out at only 13 feet and were shaped more like stocky tuna than svelte 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 cliffs along the Ohio River, making specimens of the “Dunk” a prehistoric icon in the city. But little research had been done on Dunkleosteus’s size, and some of the past measurements seemed fishy to Mr. Engelman.

Dunkleosteus belonged to an ancient faction of fish known as the arthrodires that ruled the seas during the Devonian. Because the bulk of Dunkleosteus’s body was most likely composed of fragile cartilage, only the thick armor plates that encased its head and neck were preserved as fossils. While these plates preserve the predator’s jagged 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 Mr. Engelman, head length is a reliable proxy for body size in fish: Short fish species generally have shorter heads, and long fish species longer heads. He focused on the region between a 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 the gills are,” Mr. Engelman said. “If your gills get too small, you suffocate.”

He compared the size of this region in Dunkleosteus to the head proportions of nearly 1,000 other species of fossil and modern fish, ranging in size 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 to a fish slightly longer than 11 feet. The largest known Dunkleosteus topped out at around 13.5 feet. Instead of bus-size behemoths, these fish were closer to Volkswagen Beetles, but still bugs that could deliver bone-crushing bites.

Reducing Dunkleosteus’s length also alters its proportions. Most reconstructions depict Dunkleosteus with the elongated body of a shark. However, more complete arthrodire fossils reveal that these fish had squatter, cylindrical bodies. Mr. Engelman thinks Dunkleosteus probably resembled a rotund tuna.

This full-figured fish was like an armored Pac-Man. It had a mouth twice as large as a great white’s and probably outweighed longer sharks. “People say it’s pudge, but that’s probably just solid muscle,” Mr. Engelman said.

Since the paper’s publication, several people have called the fossil fish “Chunkleosteus” on social media. But Mr. Engelman does not think the new estimates take anything away from the ancient predator’s prowess.