New study reveals terrifying details about deadly ancient fish 'Dunk'

A new study reported by Live Science on Thursday has revealed some terrifying details on the bony fish Dunkleosteus terrelli, nicknamed "Dunk," that lived during the Devonian period (419 million to 358 million years ago). It turns out it was not as big as previously believed but still as chunky and scary.

The discovery was made by Russell Engelman, a doctoral student at Case Western University in Cleveland, Ohio. It all began during the pandemic when Engelman looked at Dunk specimens at the Cleveland Museum of Natural History, and something just didn’t add up for him.

"Everything in biology is influenced by body size," he told Live Science. "I tried using some of the old measurements, and biologically, they just didn't make sense."

Engelman found that the skull of the fish simply did not match up with a 30-foot-long (9-meter-long) body, so he decided to find out how the original researchers determined the size of Dunk. It was then that the real issues with the fish’s proportions emerged.

"I went back through the literature, and it turned out that most previous authors who had talked about this were basically just eyeballing it," Engelman told Live Science.

So Engelman proceeded to analyze the proportions of various fish skulls as related to their bodies and concluded that instead of being 30 feet (9m) long, Dunk was likely no longer than 13 feet (4 m).

No less terrifying

Although smaller than previously conceived, this massive armored fish is no less impressive. It boasted blade-like jaws that could snap shut with 8,000 pounds (3,600 kilograms) of force and had a bony, armored skull attached to a skeleton of cartilage. 

Its skull was terrifyingly phenomenal as it measured nearly three feet (85 centimeters) tall. Experts have compared it to the main character of the film Alien. 

The first fossils of the 360 million-year-old sea monster were discovered 150 years ago along the shores of Lake Erie near the city of Cleveland. Currently,  the largest known specimen of the fish is housed in the collections of the Cleveland Museum of Natural History. 

The new study is published in the journal Diversity.

Study abstract:

Dunkleosteus terrelli, an arthrodire placoderm, is one of the most widely recognized fossil vertebrates due to its large size and status as one of the earliest vertebrate apex predators. However, the exact size of this taxon is unclear due to its head and thoracic armor being the only elements of its body regularly preserved in the fossil record. Lengths of 5–10 m are commonly cited, but these estimates are not based on rigorous statistical analysis. Here, I estimate the body size of D. terrelli using a new metric, orbit-opercular length, and a large dataset of arthrodires and extant fishes (3169 observations, 972 species). Orbit-opercular length strongly correlates with total length in fishes (r2 = 0.947, PEcf = 17.55%), and accurately predicts body size in arthrodires known from complete remains. Applying this method to Dunkleosteus terrelli results in much smaller sizes than previous studies: 3.4 m for typical adults (CMNH 5768) with the largest known individuals (CMNH 5936) reaching ~4.1 m. Arthrodires have a short, deep, and cylindrical body plan, distinctly different from either actinopterygians or elasmobranchs. Large arthrodires (Dunkleosteus, Titanichthys) were much smaller than previously thought and vertebrates likely did not reach sizes of 5 m or greater until the Carboniferous.