Leedsichthys

• Name: Leedsichthys (Greek for "Leeds' fish"); pronounced leeds-ICK-thiss
• Habitat: Oceans worldwide
• Historical Period: Middle-Late Jurassic (189-144 million years ago)
• Size and Weight: 30 to 70 feet long and five to 50 tons
• Diet: Plankton
• Distinguishing Characteristics: Large size; semi-cartilaginous skeleton; thousands of teeth

About Leedsichthys

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Haikouichthys

By Bob Strauss

The "last" (i.e., species) name of Leedsichthys is "problematicus," which should give you some clue about the controversy occasioned by this gigantic prehistoric fish. The problem is that, although Leedsichthys is known from dozens of fossil remains from around the world, these specimens don't consistently add up to a convincing snapshot, leading to grossly divergent size estimates: more conservative paleontologists venture guesses of about 30 feet long and 5 to 10 tons, while others maintain that superannuated Leedsichthys adults could attain lengths of over 70 feet and weights of over 50 tons.

We're on much firmer ground when it comes to Leedsichthys' feeding habits. This Jurassic fish was equipped with a whopping 40,000 teeth, which it used not to prey on the larger fish and marine reptiles of its day, but to filter-feed plankton (much like a modern Blue Whale). By opening its mouth extra-wide, Leedsichthys could gulp in hundreds of gallons of water every second, more than enough to cover its outsized dietary needs.

As with many prehistoric animals discovered in the 19th century, the fossils of Leedsichthys were an ongoing source of confusion (and competition). When the farmer Alfred Nicholson Leeds discovered the bones in a loam pit near Peterborough, England, in 1886, he forwarded them to a fellow fossil hunter, who misidentified them as the back plates of a stegosaur dinosaur. The next year, during a trip overseas, the eminent American paleontologist Othniel C. Marsh correctly diagnosed the remains as belonging to a giant prehistoric fish, at which point Leeds made a brief career of excavating additional fossils and selling them to natural history museums.

One little-appreciated fact about Leedsichthys is that it's the earliest identified filter-feeding marine animal, a category that also includes prehistoric whales, to attain giant sizes. Clearly, there was an explosion in plankton populations during the early Jurassic period, which fueled the evolution of fish like Leedsichthys, and just as clearly this giant filter-feeder went extinct when krill populations mysteriously plunged at the cusp of the ensuing Cretaceous period.