True monster: An artist’s impression of Leedsichthys, the whale of its day, which lived on enormous quantities of plankton. Photo: R. Nicholls/Paleocreations.com/University of Glasgow/PA Wire

It sounds like a fisherman’s tale. But the giant Jurassic fish Leedsichthys, that lived 165 million years ago, was a true monster, scientists claim.

Calculations based on fragments of fossil skeleton suggest it grew to eight or nine metres in 20 years and reached 16.5 metres in 38 years.

Scientists believe Leedsichthys was the whale of its day, living on enormous quantities of plankton.

Jeff Liston, from the University of Glasgow, said: “The giant plankton-feeders we know to live in today’s oceans are among the largest living vertebrate animals alive. The Leedsichthys was the first animal known to occupy this role.

“What we didn’t have any clear idea of was how large this large fish really was: its skeleton preserves poorly, it is often just isolated fragments, so previous size estimates were largely historical arm-waving exercises.”

The team, including researchers from Canada, looked at various remains of the fish including a near-complete new specimen unearthed in Peterborough.

Liston added: “We sat down and looked at a wide range of specimens, not only at the bones, but their internal growth structures as well – similar to the growth rings in trees – to get some ideas about the ages of these animals, as well as their estimated sizes ...

“This fish was a pioneer for the ecological niche filled today by mammals, like blue whales, and cartilaginous fish, such as manta rays, basking sharks, whale sharks.

“Before then, vertebrate suspension-feeders did not get larger than 50 centimetres in length.

“Something important had changed. The existence of these large suspension-feeding fish at this time is highly significant, as it would seem to be clear evidence of a major change in plankton populations in earth’s oceans of Jurassic Earth.” The research shows that evolution was opting for giant size in the sea as well as on land, where some of the biggest dinosaurs roamed.

The findings appear in the journal Mesozoic Fishes 5: Global Diversity and Evolution – Proceedings of the International Meeting.

A bigger fish to fry

The name Leedsichthys means ‘Leeds’ fish’, after the fossil collector Alfred Nicholson Leeds, who discovered it in 1886 near Peterborough, England.

Leedsichthys was a filter-feeder that swam the Mesozoic seas for over 100 million years, from the middle Jurassic until the end of the Cretaceous periods.

The fossils discovered in the 19th century were believed to be so fragmented that they were difficult to identify.

This meant that researchers were unable to determine the fish’s length.

However, by piecing these samples together, the researchers have been able to tell how long the creature was by the position of its gills.