Now, researchers from the University of Bristol suggest the giant fishes could grow up to 12 and 16m long. The giant plankton-feeders in today's oceans are among the largest living vertebrate animals alive.

The first animal known to occupy this role was a large bony fish called Leedsichthys that lived in the Middle Jurassic, around 165 million years ago. 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 and whale sharks).

However, scientists were unclear just how large this large fish could be – until now. "Leedsichthys skeletons preserve poorly, often only as isolated fragments, so previous size estimates were largely historical arm-waving exercises," said Professor Jeff Liston of the University of Kunming in China and the University of Bristol's School of Earth Sciences.

"We looked at a wide range of specimens, not just the bones but also their internal growth structures - similar to the growth rings in trees - to get some idea about the ages of these animals as well as their estimated sizes," Liston said.

The researchers demonstrated that a small adult Leedsichthys of 8-9 metres could reach that length within around 20 years, whereas after 38 years it would be around 16.5 metres long - possibly even outgrowing today's massive whale sharks.

"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 the oceans of Jurassic Earth: a 'smoking gun' that something new, widespread and highly edible was around, possibly related to the first appearance of small crustaceans called copepods," Liston said.

"This has implications for our understanding of biological productivity in modern oceans, and how that productivity has changed over time," he said.


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