NORTH 02 | Ancient Animals | Leedsichthys

The first remains of Leedsichthys were identified in the nineteenth century. Especially important were the finds by the British collector Alfred Nicholson Leeds, after whom the genus was named "Leeds' fish" in 1889. The type species is Leedsichthys problematicus. Leedsichthys fossils have been found in England, France, Germany and Chile. In 1999, based on the Chilean discoveries, a second species was named Leedsichthys notocetes, but this was later shown to be indistinguishable from L. problematicus.

Leedsichthys fossils have been difficult to interpret because the skeletons were not completely made of bone. Large parts consisted of cartilage that did not fossilize. On several occasions the enigmatic large partial remains have been mistaken for stegosaurian dinosaur bones.

As the vertebrae are among the parts that have not been preserved, it is hard to determine the total body length. Estimates have varied significantly. At the beginning of the twentieth century, a length of 9 m (30 ft) was seen as plausible, but by its end Leedsichthys was sometimes claimed to have been over 30 m (98 ft) long. Recent research has lowered this to about 16.5 m (54 ft) for the largest individuals.

Skull bones have been found indicating that Leedsichthys had a large head with bosses on the skull roof. Fossilised bony fin rays show large elongated pectoral fins and a tall vertical tail fin. The gill arches were lined by gill rakers, equipped by a unique system of delicate bone plates, that filtered plankton from the sea water, the main food source.

Along with its close pachycormid relatives Bonnerichthys and Rhinconichthys, Leedsichthys is part of a lineage of large-sized filter-feeders that swam the Mesozoic seas for over 100 million years, from the middle Jurassic until the end of the Cretaceous period.

Pachycormids might represent an early branch of Teleostei, the group most modern bony fishes belong to; in that case Leedsichthys is the largest known teleost fish.