Whales include the world's largest animals, but newly identified fossils reveal they were preceded by SUV-sized filter-feeding fishes that emerged during the Jurassic Period, 170 million years ago, and lived until the extinction event that wiped out dinosaurs and numerous other species.
Although the now-extinct fishes, called pachycormiforms, were not closely related to whales, their demise left an ecological niche void that whales, sharks and rays filled starting around 56 million years ago, helping to explain the top portion of today's marine food chain.
The fish fossils, described in the latest issue of Science, also prove that filter feeding emerged long before the first whales. For this method of eating, the diner suspends itself in the water, mouth agape. Water escapes through gill slits, leaving behind the filtered food.
It can help to have a big mouth, which many of these enormous fishes must have had.
Co-author Kenshu Shimada, a research associate in paleontology at the Sternberg Museum of Natural History, told Discovery News that one of the fish he and his colleagues identified, Bonnerichthys, grew to around 20 feet in length and swam through a seaway covering what is today the state of Kansas.
"A previously described species, Leedsichthys, from the Jurassic of Europe that belongs to the same lineage that includes Bonnerichthys was even larger, likely reaching up to about 30 feet, which is the most massive bony fish of all time," added Shimada, who is also an associate professor in the Environmental Science Program and Department of Biological Sciences at DePaul University.
For the study, led by University of Oxford scientist Matt Friedman, the researchers analyzed both old and new fish fossils found in England, the U.S. and Japan. The Kansas fish was previously thought to have been like a gigantic swordfish, bearing fang-like teeth on its jawbones.
"However, our close examination of the specimen showed that such a long snout and fang-like teeth were not present in the fish," Shimada said. "Rather, with a blunt massive head, the fish had long toothless jawbones and long gill-supporting bones that are characteristic of plankton-feeding fishes."
While this fish, and the other Dinosaur-Era filter feeders, enjoyed a long existence on the planet, they were no match for the K-T extinction event that killed off 70 percent of all species then living on Earth.
"The filter-feeding pachycormiforms, relying for food on small organisms low in the trophic chain, had the perfect profile of a victim and became extinct," wrote Lionel Cavin in a commentary that also appears in Science. Cavin is a curator in the Department of Geology and Paleontology at the Natural History Museum in Geneva.
Cavin added, "The tropical niche was later refilled, first with sharks and rays from around 56 million years ago and then with modern cetaceans (whales, dolphins and porpoises) from 34 million years ago."
Yet another paper in the latest Science, authored by the University of Otago's Felix Marx and George Mason University's Mark Uhen, found that diatoms, a common type of phytoplankton, along with climatic events, influenced the evolution of cetaceans once they headed into the water.
Marx and Uhen believe that "a great increase in diatom-based productivity, possibly by increasing the bioavailability of silica and other nutrients in the Southern Ocean and coastal upwelling zones around the world through deep-mixing occurring around Antarctica" drove the evolution of baleen whales, in particular.
The research sheds light on why marine mammals that can weigh over 190 short tons and grow to 108 feet in length may subsist on minuscule diatoms and other tiny, yet prevalent, water dwellers, such as krill.