The ancestors of modern whales made their transition from land to sea about 53 million years ago. At one point during the middle of this transition, whales had a mouth full of both teeth and baleen, according to a new study from San Diego State University.
Modern blue whales, humpback whales and gray whales have teeth in the womb but are born toothless. Baleen replaces the teeth for filter feeding small fish and tiny shrimp-like krill. The disappearing embryonic teeth are evidence of a time when ancient whales had teeth and consumed larger prey.
For the current investigation, the researchers used high-resolution computed tomography (CT) to scan a 25 million year-old fossilized whale skull that belonged to Aetiocetus weltoni, an evolutionary “cousin” of modern whales. The analysis revealed that Aetiocetus had both teeth and baleen at the same time in adulthood.
Baleen decomposes and is rarely preserved intact in fossils, so the scientists turned to digital reconstructions with CT imaging to search for evidence of the elastic substance in Aetiocetus. They identified grooves and holes on the roof of the mouth that were consistent with baleen in modern mysticetes.
“We have found evidence that supports a co-occurrence of teeth and baleen, indicating the tooth-to-baleen transition occurred in a stepwise manner from just teeth, to teeth and baleen, to only baleen,” said Ekdale.
“Our study provides tangible fossil evidence of a major shift in feeding behavior from a raptorial carnivorous feeding mode to a bulk filter-feeding mode for obtaining food, among the largest animals that have ever lived in earth’s oceans. Krill are around 1/600th the size of blue whales. That’s like us humans eating nothing larger than sesame seeds floating in a pool.”
Some researchers questioned how Aetiocetus managed to process its food, considering that the baleen could get in the way of teeth in the mastication process. However, the position of the holes observed in Aetiocetus suggests that the baleen was not in the “line of fire” where it would interfere with the function of the teeth.
“While the tiny holes on the palate of Aetiocetus may look similar at a superficial level to other mammals, we can clearly demonstrate that this anatomy is related to baleen in baleen whales,” said study co-author Thomas Deméré.
The study is published in the Zoological Journal of the Linnean Society.