The Dunk got shrunk? New study revises size estimates of fierce ancient fish

You probably know that we have a state bird and a state flower and a state tree. You might know that we also have a state invertebrate fossil. No, it is not THE trilobite — there is no such thing. There were thousands of different kinds of trilobites, only one of which, Isotelus, is THE state fossil invertebrate of Ohio.

We also have a state fossil fish, Dunkleosteus, named for Dr. David Dunkle, a paleontologist at the Smithsonian and the Cleveland Museum of Natural History from the 1940s to the 1970s. Its nickname is “The Dunk”.

Dunkleosteus lived late in the Devonian Period, about 360 million years ago, when Ohio was covered by a shallow sea. The best specimens have been found in and around Cleveland, but fossils of it (or very close relatives) have been found in the Columbus area and around the world.

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Its head and chest were covered with large plates of solid bone. It did not have teeth, but the bones of its jaws formed self-sharpening meat cleavers. It was the top predator in its ecosystem, reaching lengths of up to 30 feet.

Or did it? A recent lengthy research paper published by Russell Engleman of Case Western Reserve University has sharply reduced those estimates, which were little more than educated guesses All we’ve had to go on were the bony plates that covered the skull and the very front of the body. The rest of the skeleton was made of cartilage, the soft stuff in your ears and the tip of your nose, which doesn’t fossilize very often. That means that the shape of most of the Dunk’s body — the fins, thorax and tail — were conjecture.

To get around that, Engleman measured the distance from the front of the eye socket to the back of the skull (not including the big plate behind that) then multiplied that by 3.5 to 4.5 to estimate the total length of the body. To come up with that equation, he made over 3,000 observations on more than 900 species of fish, from sharks to halibut.

If Engleman is right, an average adult Dunkleosteus would have been only 11 feet long, and a really big one only 14 feet, a tremendous reduction in its estimated size.

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Engleman also estimated what Dunkleosteus would have weighed. The armor plates around its front indicate it had a very “chunky” body, so the largest adults would have weighed around 3,800 pounds.

I’m not changing the signs on our Dunk exhibit at the Orton Museum just yet, as I suspect other paleontologists will have something to say about the new estimates and the methods used to calculate them. Even at its smaller size, it still would have been the top predator in its ecosystem — I wouldn’t have wanted to go swimming in the Late Devonian sea.

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Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University. Contact him at gnidovec.1@osu.edu

This article originally appeared on The Columbus Dispatch: Geology: New study shrinks size of fierce ancient, armored fish