(Newswise)–How did baleen whales, which include the largest vertebrates to ever live, evolve to feed today almost entirely on tiny saltwater crustaceans, such as krill?
Scientists have posed this question for years, but researchers at New York Institute of Technology College of Osteopathic Medicine (NYITCOM) have provided fossil evidence that may solve the prehistoric mystery.
Instead of chewing, modern cetaceans—whales, dolphins, and porpoises—consume food by one of three processes: relying on suction through the mouth, using teeth to secure prey, or opening their mouth and then thrusting the body and mouth forward. One group of cetaceans, baleen whales, lack teeth and have baleen instead, a hair-like filter feeding system that lines their mouths. Baleen whales consume their food by sifting through vast volumes of salt water to capture krill and other small organisms. As the whale pushes the water out, its bristle-like baleen, made of the same keratin found in human fingernails, strains the organisms and leaves behind a food source for the whale.
The earliest fossil cetaceans, which existed 50 million years ago had teeth, and even baleen whales in utero start, but then stop, to form teeth. Thus it has long been understood that baleen whales have toothed ancestors. However, researchers have often debated whether these whales passed through an evolutionary stage of toothless suction feeders or whether they used teeth for filtering until baleen evolved. In support of the latter theory, NYITCOM Anatomy department researchers Jonathan Geisler, Ph.D., associate professor and chair, and Brian Beatty, Ph.D., associate professor, suggest that an early species of toothed mysticete (the scientific name given to precursors to baleen whales that retained teeth) called Coronodon havensteini retains a key evolutionary stage that gave rise to today’s baleen whales.
“The transition from teeth to baleen is widely contested, but our research indicates that ancient toothed whales relied on the spaces between their complex and enormous teeth for filtering,” said Geisler. “It appears that over millions of years, the teeth were retained until baleen became sufficiently large and complex to take over the role of filter feeding.”
The ancient remains of Coronodon havensteini were inadvertently discovered in the early 2000s by a scuba diver searching for shark teeth in South Carolina’s Wando River. Instead of finding shark teeth, diver Mark Havenstein, for whom the newly found species is named, discovered the fossilized whale. The specimen now resides in the Mace Brown Museum of Natural History at the College of Charleston, where it has been studied by Geisler, Beatty, and two collaborators from that museum, Robert Boessenecker, Ph.D., and Mace Brown, Ph.D. Found in sediments originating 30 million years ago, Coronodon havensteini is a transitional species between earlier whales that snagged fish with their teeth and modern baleen whales.
The NYITCOM investigators realized this new fossil species offers evidence about the teeth-to-baleen transition. With its skull measuring nearly one meter long, Coronodon havensteini was also larger than many other toothed mysticetes of the time, such as Mammalodon and Aetiocetus, and when compared to other ancient whales, had remarkably large molars that suggested an uncommon feeding behavior. In examining the front teeth, the researchers concluded they were used to snag prey, but this was not the case with the whale’s back teeth. The wear on Coronodon havensteini’s unusually large molars, each equal in size to the palm of one’s hand, presented no evidence of shearing or cutting food, suggesting these teeth were used to filter food by ejecting water through the spaces between closed teeth. Wear on the hidden cusps bordering these openings further supported this theory.
“This study reminds us that feeding is complex,” said Beatty. “We needed multiple lines of evidence to sort this out. Animals have tried every possible use of a tool at their disposal, and so must we in order to build our understanding of how these whales evolved,” said Beatty.
Continuing their investigation, the researchers will examine closely related species from the Charleston, South Carolina region in search of further evidence.