Of all the fish to ever swim in the seas, Leedsichthys problematicus may be the record-holder for the world's largest. But as the Jurassic plankton-feeder's species name suggests, Leedsichthys is a problematic fish.

Working with bits and pieces of incomplete skeletons, scientists have had a hard time figuring out the precise dimensions of the enormous creature. Now it seems that Leedsichthys, which swam the seas 165 million years ago, may have been smaller than previously believed-roughly half as big as earlier estimates, in fact.

Even so, it was probably a little bigger than today's plankton-feeding whale sharks, and its standing as the biggest bony fish ever is still intact.

The new evidence about the enormous fish is part of a study presented by University of Bristol paleontologist Jeff Liston this week at the 61st annual Symposium on Vertebrate Palaeontology and Comparative Anatomy in Edinburgh, Scotland.

Ever since Leedsichthys was first described by the British paleontologist Arthur Smith Woodward in 1889, researchers have understood that it was gigantic. The proportions of partial fossil remains made that clear-the bony gill rakers that Leedsichthys used to strain plankton from the water were about three inches long, over three times the size of those found in skipjack tuna, for instance. The gill rakers were so big, in fact, that they have sometimes been confused as the bones of other prehistoric animals, such as the jaw of a leathery-winged pterosaur.

Citing the large "gill basket" and other fossils attributed to the creature, paleontologist David Martill in 1986 dubbed Leedsichthys "the world's largest fish." He estimated that Leedsichthys might have been over 90 feet long, approaching the size of a blue whale. (Martill could not immediately be reached for comment.)

But no fully intact skeleton of the Leedsichthys has ever been found, meaning scientists have had to estimate its size according to bits and pieces of evidence from various individual creatures.

Estimating Size

In order to more thoroughly investigate the probable size of the fish, Liston and his colleagues turned to a wide collection of Leedsichthys remains-including a huge tail, bony parts of the gills, a pectoral fin, and skeletons of closely related fish for comparison-which had never been studied before as a whole. The inclusion of a greater variety of bones was critical, Liston said, because individual parts can make the fish seem larger than it truly was.

Among the noteworthy fossils in the lot was a partial skeleton called the Ariston specimen, which was excavated between 2001 and 2003. While most of the back half of this Leedsichthys was missing, Liston said, "It is certainly the most complete individual ever found, by a long way."

This relatively small individual included pectoral fins and paired skull bones that are rarely found together. "Basically," Liston said, "Ariston provided the Rosetta stone to link all the previous individuals and get an idea of what each of these specimens really represented."

Size and bulk were not the only aspects that intrigued Liston and his collaborators. The researchers also cut into sections of gill rakers and fin rays to examine lines of growth inside that serve as indicators of how old the Leedsichthys individuals were when they died.

The fossils examined by Liston and his collaborators represented five distinct individuals, and each yielded different size estimates. One fossil nicknamed Big Meg represented a fish between 37 and 44 feet long, while a huge tail came from a smaller Leedsichthys about 29 feet long. In all, the specimens in the sample were estimated to be between 26 and 55 feet long-more modest than earlier visions of a 90-foot fish.

Biggest Ever?

Despite the downsizing, though, Leedsicthys still seems to be a record holder. Based on these five individuals alone, Liston said, "it looks like Leedsichthys was the biggest fish that we have any evidence for." And there may have been larger Leedsichthys whose remains are still to be found.

"In terms of how long it could have grown," Liston said, "there was absolutely no sign of the growth curve of the largest individual plateauing, [and] therefore no sign that it was slowing down because it had reached any maximum size." That means other, as-yet-undiscovered Leedsichthys could have kept growing to become even larger than the biggest individual examined by Liston.

"I consider their size and growth estimates to be as good as they can be based on the present fossil record," said DePaul University paleontologist Kenshu Shimada.

Liston and colleagues also found some clues about the life of Leedsichthys. Based on indicators of growth inside sectioned bones, they estimate that the five Leedsichthys in the study were 19 to 40 years old. They also determined that, while even the biggest specimen was still growing when it died, the fish grew fastest during the first year or two of their lives.

Leedsichthys had good reason to grow quickly. A variety of large predators patrolled the Jurassic seas, including enormous marine reptiles with fearsome jaws, such as the four-paddled pliosaurs. For young Leedsichthys to grow to their enormous potential, they had to survive this gauntlet of hungry reptiles looking for a fish dinner, which meant they had to grow fast.

A Lasting Mystery

Even so, paleontologists are left with the mystery of why Leedsichthys grew to such impressive sizes at all. Never before were there filter feeders so large, Liston said, which "indicates that something profound happened in the water column."

Some major change in the kind of plankton that was available at the time might hold an explanation, Liston said. At the very least, Shimada notes, the existence of huge filter feeders "indicates that small organisms such as plankton were abundant enough to support them, and they may even indicate the presence of other plankton species such as krill not known in the fossil record but [which may have] evolved about the same time based on molecular data of modern krill."

For now, however, Leedsichthys is a charismatic indicator of a major shift that otherwise remains mysterious.