Scientists have confirmed a fish story of epic proportions. A presentation by Professor Jeff Liston of the National Museum of Scotland and his associates at the 61st annual Symposium on Vertebrate Paleontology and Comparative Anatomy last month estimated the size and lifestyle of the most massive fish to have ever lived.

The largest fish alive today has been measured at a whopping 41 feet in length, or roughly the size of a city bus. It is known as Rhincodon typus or, more colloquially, as the Whale Shark. The second largest living fish, the Cetorhinus maximus, or Basking Shark, weighs in not far behind at an average length of 29 feet long and weighing around 5 tons. Their prehistoric counterpart, however, puts even the Whale and Basking Sharks to shame.

Leedsichthys problematicus swam the warm oceans of the Jurassic and Cretaceous periods, approximately 165 million years ago. Liston and his associates estimate Leedsichthys reached up to fifty five feet in length, roughly the same as a semi-truck.

Like today’s biggest fish, Leedsichthys fueled its massive body by filter feeding on tiny particles in the water known as plankton. Much like the modern Basking Shark, Leedsichthys was equipped with cartilage combs inside its gills, known as gill rakers. Leedsichthys’ gill rakers were three inches long and helped the fish strain enormous amounts of food from the water it gulped while it swam.

Unlike whales and basking sharks, Leedsichthys was not a shark. It was a true bony fish, with a skeleton made of bone instead of just cartilage. However, its bones were still too flimsy to fossilize easily in the sediment of the prehistoric ocean bottom. This is how Leedsichthys (discovered in 1889 by fossil collector Alfred Leeds) acquired its species name: problematicus.

To date, remains from only seven individual Leedsichthys have been found, and not a single one of these has been complete. Scientists have been forced to guess its size and shape, a problem too difficult to yield any reliable estimates until 2001 when two geology students in the UK discovered pieces of bone in the rock they were studying in Whittlesey, Cambridgeshire. This fossil, called the Ariston Specimen, was also lacking in parts, but it was the most complete Leedsichthys skeleton scientists have ever been able to study.

Armed with this Rosetta Stone of fossils, Liston and his associates had enough pieces of the puzzle to predict what a complete skeleton would have looked like. They were able to estimate the size of an average Leedsichthys: between 26 and 55 feet long.

They were also able to measure how the fish grew. They found that Leedsichthys had a major growth spurt during its first two years, allowing it to grow big enough to avoid becoming an easy meal for the large, sea bound dinosaurs. Then its growth rate slowed but it didn’t plateau or stop. The specimens Liston studied were still growing when they died. Liston believes the fish could have lived up to 70 years, and there could have been individuals that grew even larger than 55 feet long.

These huge fish lived at a time when their dinosaur contemporaries were also reaching extreme sizes. The Jurassic and Cretaceous periods also saw heavyweights like Apatosaurus, T. rex, and Triceratops. Paleontologists have dubbed this phenomenon “gigantism” but still don’t know what caused animals the world over to evolve to behemoth dimensions. Paleontologists hope that by understanding Leedsichthys, they will uncover clues about which sort of environmental factors made the difference. A big fish may just crack the mystery.

The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.

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Published On: October 8, 2013

One Comment

  • David Campbell says:

    There’s a typo in the genus for whale sharks – Rhincodon. 
    The group to which it belongs (Pachycormidae) seems to have dominated the large planktivore niche through much of the Mesozoic (Science 19 February 2010:  Vol. 327 no. 5968 pp. 990-993,  DOI: 10.1126/science.1184743. 100-Million-Year Dynasty of Giant Planktivorous Bony Fishes in the Mesozoic Seas. Matt Friedman,  Kenshu Shimada,  Larry D. Martin, Michael J. Everhart, Jeff Liston, Anthony Maltese, Michael Triebold)

    A few dinosaurs probably ate fish caught in shallow water, but most of the threats to Leedsichthys would have been predatory fish and marine reptiles such as plesiosaurs and ichthyosaurs. 

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