Recently analyzed fossil evidence shows for the first time that giant plankton-eating fish lurked prehistoric seas for more than 100 million years before they became extinct at the same time as the dinosaurs.

In the Feb. 19 issue of the journal Science, Kenshu Shimada, associate professor of environmental and biological science at DePaul University, and an international research team describes how fossils from Asia, Europe and the United States reveal a previously unknown dynasty of giant plankton-eating bony fish that filled the seas of the dinosaur era 66 million to 172 million years ago. The study is expected to rewrite the history of ocean’s ecosystems in the world’s geology and marine ecology textbooks.

The fish, estimated to be nearly 20 feet in length, possessed narrow toothless jaws that supported a gaping mouth as well as long, gill-supporting bones, all of which were necessary to filter out enormous quantities of tiny plankton. The team named this fish Bonnerichthys, in honor of the family in Kansas which made many notable fossil discoveries in the area.

Interestingly, the ancestors of modern groups of large plankton-eaters–such as whale sharks, manta rays and baleen whales–emerged or became prominent only after the extinction of Bonnerichthys and its relatives, indicating that modern-day plankton-eaters evolved to fill the ecological niche left behind by these dinosaur-aged plankton-feeders.

“A variety of plankton had evolved by the middle of the dinosaur era, so it was a long-standing scientific mystery as to why almost no large vertebrates were feeding on these abundant resources in ancient seas,” said Shimada, who also serves as research associate at the Sternberg Museum of Natural History in Kansas. “Our study demonstrates that giant plankton-eaters were thriving through more than half of the dinosaur era spanning over 100 million years, which is much longer than any lineages of modern plankton-feeding giants.”

The team effort emerged in 2007 when two Chicago paleontologists–Matt Friedman, then a University of Chicago graduate student, and Shimada, who was experienced with extinct plankton-eating sharks–joined forces. They began to re-examine the skeletal remains of a fish species whose exact identity eluded scientists for nearly 140 years. “Large meat-eating vertebrates usually have large, easily-identifiable teeth that generally have better chances of preservation in the fossil record. But the fish we studied was largely known only by its fins and had no teeth,” noted Shimada, explaining why the fish’s identity had remained elusive for so long.

Similar giant plankton-eating fish were known to have existed for only 20 million years in Europe. “As soon as we recognized these animals existed far longer than anyone thought, I started examining museum collections and found more examples that had been overlooked or misidentified,” explained Friedman, now of Oxford University.

Luck also played a role in the breakthrough. Shimada, who through happenstance at the age of 15 learned of an important specimen from Asia and studied the fish from the United States as a graduate student in the mid-1990s, fortuitously discovered a nearly complete skeleton of Bonnerichthys in western Kansas in 2008. “The odds of the discovery were extremely low given that fewer than 20 specimens of this species–mostly fragments–had been found in the last 150 years of extensive fossil-collecting history in that region,” said Shimada. The specimen added new data for the forthcoming paper in Science, titled “100-Million-Year Dynasty of Giant Planktivorous Bony Fishes in the Mesozoic Seas.”

In addition to Friedman and Shimada, the report’s authors were Larry D. Martin of University of Kansas, Michael Everhart of the Sternberg Museum of Natural History, Jeff Liston of the University of Glasgow, and Anthony Maltese and Michael Triebold of Triebold Paleontology, and the Rocky Mountain Dinosaur Resource Center, Colorado.