The Smithsonian’s National Museum of Natural History is home to a very unique species of whale – unlike whales alive today, it has neither teeth nor baleen.

Meet the Maiabalaena nesbittae. It lived around 33 million years ago, grew to lengths of 4.6 meters (15 feet), and was about as toothless as Grampa Simpson without his dentures. Importantly, it could be the missing part of the puzzle in the whale evolution timeline, explaining how modern-day baleen whales like the minke and humpback evolved from their more toothy ancestors, according to a paper recently published in the journal Current Biology.

"When we talk about whale evolution, textbooks tend to focus on the early stages, when whales went from land to sea," the curator of marine mammals at the National Museum of Natural History said in a statement.

Unlike dolphins, orcas, and sperm whales, baleen whales benefit from a filter-feeding system that allows them to scoop up thousands of tiny prey without lifting a jaw. Rows and rows of baleen (a material similar to that of hair and nails) filter food from the ocean water, meaning gigantic animals like the blue whale can devour several tons of food every day. Whales are unique in this regard – they are the first (and only) animal to develop a baleen-like structure. And we are not really sure how.

Maiabalaena nesbittae might prove to be the answer. Until now, the problem has been that baleen doesn’t preserve well. There is little evidence of it on the fossil records and so paleontologists have usually resorted to using fossil evidence and studies on fetal-whale development in the womb. This makes it hard to establish a timeline and work out when certain groups of whales evolved to have baleen – before or after they lost their teeth.

The specimen at the Smithsonian’s National Museum of Natural History has no teeth, which makes it the oldest toothless whale we know of. But more surprisingly, the researchers say, there was no evidence to suggest it had baleen. This would imply that it only developed after whales lost their teeth and that the two developments (losing teeth and acquiring baleen) were separate evolutionary events.

You might be wondering how the Maiabalaena nesbittae ate. The anatomy of the oral cavity and throat suggests that in the absence of teeth and baleen, the whale appeared to use suction feeders. The species lived during the Eocene-Oligocene boundary, a period of major upheaval, mass-extinction, and what the researchers say represented a “critical moment for whales” as shifting geology triggered changes to their feeding behaviors.