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Description
Here comes the big one, quite literally. This animal is the biggest species known from Oxford Clay by a wide margin, weighing nearly 5 times as much as the local sauropod or giant pliosaurs , and yet, despite having a fair number of fossils to its name, this creature remains elusive, since pretty much all of its material is rather fragmentary. I'm of course talking about Leedsichthys problematicus, the largest fish from the Mesozoic and a member of the now-extinct pachycormiformes, a group of ray-finned fish that lasted throughout the Jurassic and Cretaceous, though no other known taxon grew anywhere close in size to this Jurassic leviathan.

As is evident from its name, this giant was named in honor of fossil collector Alfred Leeds, and indeed, the first ever fossils described, parts of the bony head roof, came from the Leeds Collection, though they were initially misidentified as the plates of a stegosaur , before Arthur Smith Woodward recognized them as belonging to a huge fish, which he named "Leed's problematic fish", due to its fragmentary nature making identification difficult. Subsequently, Leeds and other collectors would find many other fragments of this ancient leviathan at Oxford Clay, such as gill rakers, isolated fin fragments, tail flukes, and other bits and pieces, some of which got misidentified as skeletal material of stegosaurs a few more times. Fossils attributed to this fish would also be collected from Normandy in France, and a fragmentary skeleton would was discovered in Germany. Although around 70 specimens have since been uncovered, all are very fragmentary, since many skeletal elements, including the front of the skull and the vertebral centra, did not ossify but remained cartilage.

Though its exact appearance remains speculative, workers have been able to infer that this giant was a placid suspension feeder much like today's whale and basking shark, as well as actual baleen whales, based on the bony gill rakers that Leedsichthys used to strain its microscopic food from the water while spending its entire life cruising at a leisurely pace. Like with its modern counterparts, its passive lifestyle was the likely reason why this pachycormid managed to reach such huge sizes. How huge it was though has been a matter of debate.

Originally, Woodward estimated L. problematicus to be around 9-10 meters long, comparable in size to its closest modern counterparts (the whale shark and basking shark), however, in 1986, David Martill (who has a bit of a history with dodgy , exaggerated size estimates ) looked at a different species of pachycormid, Asthenocormus (known from the Upper Jurassic of Germany), which is known from very complete material , and based on its proportions, calculated that the biggest L. problematicus (based on large gill rakers) might have reached a max length of 27 meters, meaning it would have rivaled the largest living whales in lenght. This, however, got debunked in 2007, when Jeff Liston described "Ariston ", a partial skeleton preserving the front half of the animal, making it the most complete specimen of Leedsichthys problematicus yet found. This relatively small individual included pectoral fins and paired skull bones that are rarely found together, giving us a clearer idea of its appearance in life.

From it, it was determined that the giant fish had disproportionally large gill rakers, which shrunk down its inferred maximum size to about 15-16 meters, closer to a humpback whale than a blue whale, but this was still a colossal fish and likely weighed close to 50 tons, rivaling the famous megalodon as the largest fish ever found. Based on the growth rings in its gill rakers, the largest Leedsichthys also proved to be the oldest, showing that this titan of the deep took up to four decades to reach its full size, and it's likely that many individuals got picked off by pliosaurs and marine crocodiles before they could grow into true giants, as most specimens represent individuals under 12 meters long. At its full size, however, only giant pliosaurids like cf. Liopleurodon and the later Pliosaurus could have possibly threatened it.

Though fossils are primarily known from the Callovian-Oxfordian, we have evidence of Leedsichthys or a related animal surviving into the Late Jurassic, such as fossils from the Kimmeridgian of Normandy, as well as several fragmentary finds, including an intact gill raker, from the Vaca Muerta Formation of Argentina, which dates to the early Tithonian, the latter of which could represent a distinct species, Leedsichthys notocetes.

Anatomical Reference: www.nationalgeographic.com/sci…

Description
Here comes the big one, quite literally. This animal is the biggest species known from Oxford Clay by a wide margin, weighing nearly 5 times as much as the local sauropod or giant pliosaurs , and yet, despite having a fair number of fossils to its name, this creature remains elusive, since pretty much all of its material is rather fragmentary. I'm of course talking about Leedsichthys problematicus, the largest fish from the Mesozoic and a member of the now-extinct pachycormiformes, a group of ray-finned fish that lasted throughout the Jurassic and Cretaceous, though no other known taxon grew anywhere close in size to this Jurassic leviathan.

As is evident from its name, this giant was named in honor of fossil collector Alfred Leeds, and indeed, the first ever fossils described, parts of the bony head roof, came from the Leeds Collection, though they were initially misidentified as the plates of a stegosaur , before Arthur Smith Woodward recognized them as belonging to a huge fish, which he named "Leed's problematic fish", due to its fragmentary nature making identification difficult. Subsequently, Leeds and other collectors would find many other fragments of this ancient leviathan at Oxford Clay, such as gill rakers, isolated fin fragments, tail flukes, and other bits and pieces, some of which got misidentified as skeletal material of stegosaurs a few more times. Fossils attributed to this fish would also be collected from Normandy in France, and a fragmentary skeleton would was discovered in Germany. Although around 70 specimens have since been uncovered, all are very fragmentary, since many skeletal elements, including the front of the skull and the vertebral centra, did not ossify but remained cartilage.

Though its exact appearance remains speculative, workers have been able to infer that this giant was a placid suspension feeder much like today's whale and basking shark, as well as actual baleen whales, based on the bony gill rakers that Leedsichthys used to strain its microscopic food from the water while spending its entire life cruising at a leisurely pace. Like with its modern counterparts, its passive lifestyle was the likely reason why this pachycormid managed to reach such huge sizes. How huge it was though has been a matter of debate.

Originally, Woodward estimated L. problematicus to be around 9-10 meters long, comparable in size to its closest modern counterparts (the whale shark and basking shark), however, in 1986, David Martill (who has a bit of a history with dodgy , exaggerated size estimates ) looked at a different species of pachycormid, Asthenocormus (known from the Upper Jurassic of Germany), which is known from very complete material , and based on its proportions, calculated that the biggest L. problematicus (based on large gill rakers) might have reached a max length of 27 meters, meaning it would have rivaled the largest living whales in lenght. This, however, got debunked in 2007, when Jeff Liston described "Ariston ", a partial skeleton preserving the front half of the animal, making it the most complete specimen of Leedsichthys problematicus yet found. This relatively small individual included pectoral fins and paired skull bones that are rarely found together, giving us a clearer idea of its appearance in life.

From it, it was determined that the giant fish had disproportionally large gill rakers, which shrunk down its inferred maximum size to about 15-16 meters, closer to a humpback whale than a blue whale, but this was still a colossal fish and likely weighed close to 50 tons, rivaling the famous megalodon as the largest fish ever found. Based on the growth rings in its gill rakers, the largest Leedsichthys also proved to be the oldest, showing that this titan of the deep took up to four decades to reach its full size, and it's likely that many individuals got picked off by pliosaurs and marine crocodiles before they could grow into true giants, as most specimens represent individuals under 12 meters long. At its full size, however, only giant pliosaurids like cf. Liopleurodon and the later Pliosaurus could have possibly threatened it.

Though fossils are primarily known from the Callovian-Oxfordian, we have evidence of Leedsichthys or a related animal surviving into the Late Jurassic, such as fossils from the Kimmeridgian of Normandy, as well as several fragmentary finds, including an intact gill raker, from the Vaca Muerta Formation of Argentina, which dates to the early Tithonian, the latter of which could represent a distinct species, Leedsichthys notocetes.

Anatomical Reference: www.nationalgeographic.com/sci…