The largest fish to ever swim the seas was likely smaller than we thought.

Nature · Published 3 October 2026 Swipe more like this →

In short: For decades, scientists believed the prehistoric Leedsichthys reached lengths of 90 feet, rivaling the blue whale. New analysis of fossilized remains suggests a more modest reality, yet this Jurassic giant still holds the title for the biggest bony fish in history. Its true scale remains a fascinating, incomplete puzzle.

Leedsichthys problematicus swam the oceans approximately 165 million years ago. Since its initial description by Arthur Smith Woodward in 1889, the creature has been defined by its massive, fragmented remains. Because its skeleton was composed largely of cartilage rather than bone, complete specimens are nonexistent. This lack of structural integrity led early researchers to misidentify its gill rakers as dinosaur bones, and caused size estimates to fluctuate wildly, with some claims reaching 90 feet in length.

A recent study led by University of Bristol paleontologist Jeff Liston has refined these dimensions. By analyzing a broader collection of fossils—including the exceptionally complete Ariston specimen excavated between 2001 and 2003—researchers concluded that the fish likely measured between 26 and 55 feet. While this is roughly half the size of previous, more sensationalist estimates, it confirms that Leedsichthys remains the largest bony fish ever identified. The study also examined growth rings within the gill rakers and fin rays, revealing that the individuals were between 19 and 40 years old when they died.

The data suggests these giants were fast growers, likely a survival strategy to escape the predatory pliosaurs of the Jurassic period. Interestingly, the growth curves of the largest specimens showed no signs of plateauing, implying that even larger individuals may still be waiting to be discovered. The existence of such a massive filter feeder indicates a significant shift in the Jurassic water column, potentially driven by an abundance of plankton or the evolution of early krill. Ultimately, Leedsichthys serves as a pioneering example of large-scale suspension feeding, marking a profound moment in the history of marine vertebrates.

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