It took 25 years for Kenshu Shimada, a professor of environmental science and biology, to “catch” Bonnerichthys, a giant plankton-eating fish that swam in prehistoric seas for more than 100 million years before becoming extinct at the same time as the dinosaurs.

“I collected fossils as a kid,” says Shimada. “When I was 15, I visited the office of Dr. Teruya Uyeno, a prominent fossil fish researcher in Japan – a man who would later become my mentor. On his desk was an odd specimen, about one foot long, dark brown, and consisting of many tabular bones. “I can’t identify this,” he said. “Do you have any idea?” I was thrilled and amazed by that question. Two-and-a-half decades later, I found the answer.”

As a graduate student, Shimada collected data on the giant fish species, taking particular interest in the bits and pieces of its skeleton whose exact identity had eluded scientists for nearly 140 years. “Anyone who had seen specimen of the species knew it was a big fish, but no one knew what it looked like,” he recalls. In 2007, Matt Friedman, then a graduate student at the University of Chicago, shared Shimada’s interest in the mystery and suggested a collaboration.

While their joint work was in progress the following year, Shimada was fossil hunting in Kansas — “one of my favourite places to find ocean fishes” — when he saw a fin sticking out of the rock. Subsequent excavations revealed the specimen to be the most complete skeleton ever found for this fish. “The odds of the discovery were extremely low, given that in the last 150 years fewer than 20 fragments of this species had been found in that region,” he says.

The scientists’ combined work suggested a dynasty of giant, plankton-eating, bony fish that filled the seas during the dinosaur era — a solution to a gap in the paleontological record. While a variety of plankton had evolved by then, no one had yet indentified any large vertebrates feeding on these abundant resources.

“For the first time, we knew how the bones fit together and could place the species in the right genus,” says Shimada. “We filled in a lot of blanks about the ecology of the group, mapping out the relationships of fossils from the United States, Europe, and Japan. The fossils belonged to a group of fish whose lineage was more widespread and deeper in time than anyone had realized. And I finally knew the exact identity and significance of the odd specimen on my mentor’s desk: it turned out to be the front half of the skull of Bonnerichthys.”

The research — “100-Million-Year Dynasty of Giant Planktivorous Bony Fishes in the Mesozoic Seas” published in the internationally acclaimed journal, Science and authored by Friedman and Shimada, with contributions from 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 — is expected to rewrite the history of the oceans’ ecosystems in geology and marine ecology textbooks.

Now, Shimada and his colleagues are compiling an anatomical description of each species in the fish group. “We’ll demonstrate which forms evolved first and when each characteristic was acquired,” he says. “There are still gaps in the story — ancestral forms of the group — and we’re using snapshot views to connect the dots. When we come up with a hypothesis for how the species evolved, each new specimen we find will confirm or refine it.”

Considering the larger ramifications of his research, Shimada argues that “one logical way we can talk about the future is to look into the past. How does the ecosystem change? What drives different forms to emerge? Paleontology and geology are the only ways to see the Earth’s historical patterns. Our study shows that, if resources exist, life will find a way.”

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