Although modern vertebrates either have skeletons made entirely of cartilage or entirely of bone, Dark Osteus had a bit of both: a bony head and a mostly cartilaginous skeleton. This peculiar anatomy means the fossil record is littered with Dark Osteus heads but no Dark Osteus behind, so the rest of the fish’s body remains a mystery. Some estimates by scientists suggested that the ancient fish reached anywhere from 16 to 33 feet in length—as long as modern whale sharks. But a new paper published in the journal Diversity suggests Dark Osteus was much thicker: “merely” an 11-foot-long fish whose body shape was less fearsome shark and more girthful tuna.
When the paper hit Twitter, many paleoartists jumped on board birbify the fossil fish and embrace its new round shape. But outside experts suggested that the cut is not actually a major departure from previous reconstructions. “It just takes actual data to derive a more realistic proportion for Dunkleosteus,” said Sarah Z. Gibson, a paleoichthyologist at the St. Cloud State University in Minnesota, which was not involved in the new paper, in an email. She added: “It’s still a big fish! We can’t take that away from it!”
“It’s still kind of an oval shape, a little more squat than previously thought,” said James Boyle, a paleobiologist at SUNY Buffalo who was not involved in the new paper. Boyle sees the new paper’s proposal as “perfectly sound”, especially given previous reconstructions of Dark Osteus was based on far less rigorous statistical analysis, he said.
The new paper’s sole author, Russell Engelman, is a graduate student at Case Western Reserve University, and his previous research focused largely on fossil mammals. But Engelman’s Cleveland roots endeared him to the local fossils — an exquisite mix of arthrodire placoderms, extinct armored fish that lived in the Devonian era more than 360 million years ago. “I have known about Dark Osteus as long as I can remember, Engelman wrote in an email.
The pandemic prevented Engelman’s lab group from conducting fieldwork abroad or examining collections in large museums, so he sought smaller projects. Fortunately, the Cleveland Museum of Natural History is home to some of the best preserved Dark Osteus skulls in the world, let him in. Engelman recalled chatting with Amanda McGee, the head of the museum’s vertebrate paleontology collections, about one of the mounted Dark Osteus heads in the museum that were suggested to come from an 18-foot-long specimen. The assessment seemed suspicious; “a fish that big couldn’t fit in this space,” Engelman thought to himself.
Although remains of the fish were first discovered in 1867, most of what scientists know about the species comes from work published in 1932 by paleontologist Anatol Heintz, Napoli added. The paper was so good, in fact, that “very few people have actually gone back and explored more about that organism because it was so well done,” Napoli said. “But now 90 years have passed.”
Engelman knew the existing estimates regarding Dark Osteus‘s size was based on parts of the fish’s mouth. He tried experimenting with models that depended on head length and expected the estimates to be similar to those produced by the mouth model. But the main estimates predicted a Dark Osteus less than half as long. Engelman decided to use a specific modeling method based on a fish’s orbit-opercular length, or OOL—roughly the distance between the eye and the gill chamber. This length is often indicative of body length; consider the pike, a long, slender fish with a long, slender skull, or the piranha, a short fish with a short skull.
There will always be exceptions to this rule, such as the long-nosed and short-bodied seahorse, Gibson said. But in general, this size estimation strategy “can be useful, especially when there is no other data to go by, as is the case with Dark Osteus.” And scientists will only be able to determine how robust this new method is when someone comes across a full-body fossil Dark Osteus with preserved cartilage, she added.
Engelman applied this model to a large data set of modern fish, and Napoli said he wished the new paper had incorporated more ancient organisms that happened to fossilize in whole-body forms. “The modern stuff is just a small snapshot of the diversity of life as it has been throughout Earth’s history,” Napoli said. “There’s been plenty of time for things to change and be weird, and maybe for the sample we have today to be quite biased in relation to the arc of the story.”
Engelman said he would have liked to have included more data from fossil fish, but the pandemic severely limited his access to it. “My bigger concern was that if the method didn’t work on the great diversity of modern fish, it probably wouldn’t work on Dark Osteus,” Engelman said. He also added that arthrodires like Dark OsteusAlthough long extinct, they have more body shapes that resemble living fish than some other groups, making them a safer test case for the method. Gibson said she would like to apply the method to the Late Triassic fishes she is researching to see if they follow the trend.
The extinct arthrodires were long dismissed as an evolutionary outgrowth—a strange group of fish that left no living relatives. But over the past 15 years, fossil discoveries in southern China have revealed lack of connections between arthrodire placoderms and modern bony fish such as sharks and rays, making the group more evolutionarily relevant than previously thought, Napoli said.
Good science is an act of revision, and as my esteemed editor Barry once wrote, the most exciting thing in science is when we find out we were wrong. It shrank Dark Osteus does not significantly change our understanding of the fossil; rather, it changes our imaginations about the most threatening version of the fish, Gibson said. Compared to other fossil fish, Dark Osteus gets mainstream attention because of its reputation as a fearsome predator, Napoli said. This also affects illustrations of the fish, which depict a fierce grin full of jagged blades. But it is unclear whether a swim Dark Osteus would even show its serrated blades. “They might look perfectly docile,” Napoli said.
Also, squat doesn’t mean sluggish. The new Dark Osteus looks a lot like a tuna, which at least one conservation group considered The Ferrari of the ocean, capable of zipping through the water as fast as 43 miles per hour. Now that’s impressive! As fast as possible Charles Leclerc running without his car? I feel happy to live at the age of Chunkleosteus– just as terrifying, twice as relatable.