Today, whale sharks are the largest fish in the sea—growing to nearly the size of a school bus. But these massive cartilaginous critters are not the largest fish to ever glide through the ocean. One contender for the title is the ancient bony fish Leedsichthys problematicus.

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Swimming around roughly 165 million years ago, a fully grown L. problematicus stretched as long as 55 feet and might have weighed up to 99,000 pounds. These behemoths belong to a class known as bony fish—critters with skeletons of bone rather than cartilage.

Many bony fish are still around. As Colin Barras reports for Science, 95 percent of modern fish still fall into this category. But truly massive bony fish like L. problematicus all died out. Its heaviest modern counterpart is the ocean sunfish, which at its largest weighs in at a modest 5,000 pounds.

Scientists have long puzzled over why the most massive members of the group died out—and some have suggested that the answer lies in their metabolism. As Barras reports, a new study puts this metabolism hypothesis to the test.

As the authors of the new study, published in the journal Palaeontology, explain, earlier analyses of bony fish and metabolism didn’t take into account the giant ancients, including L. problematicus. “Remarkably, fossil evidence has rarely been considered despite some extinct actinopterygians [or ray-finned fishes, which is a class of bony fish] reaching sizes comparable to those of the largest living sharks,” they write.

The researchers calculated the ancient fish’s metabolic requirements, using data from today’s living bony fish “as a guide,” writes Barras. The analysis suggests that L. problematicus likely swam at a zippy speed of 11 miles per hour, while still maintaining adequate amounts of oxygen throughout its tissues. For comparison, the fastest living fish swim can't clock above 18 miles per hour, Barras reports.

Overall, the results suggest that metabolism was not likely a factor in their extinction.

This latest study was only possible thanks to accurate size estimates of L. problematicus. As Brian Switek reported for National Geographic in 2013, initial estimates suggested the massive critters reached up to 90 feet long. But scientists had never found a full skeleton, making accurate size estimates a challenge. As Switek reported, comprehensive reanalysis of incomplete skeletons suggests they were only about half the initial size estimates—growing to between 26 and 55 feet long.

As Switek wrote, L. problematicus had to grow big for good reason. In prehistoric times, they had to survive large predators like the four-paddled marine reptiles known as pliosaurs.

If there’s no metabolic reason bony fish today are smaller than they used to be, what happened? The answer still isn’t clear. This and many other questions about the creatures are challenging to answer thanks to a scarcity of fossil evidence. But perhaps by finding more of these leviathans, scientists can unlock the ancient mysteries of the sea.