image: This illustration shows an artistic reconstruction of a mother and calf of Maiabalaena nesbittae nursing offshore of Oregon during the Oligocene, about 33 million years ago. While Maiabalaena would not have been able to chew or filter feed, muscle attachments on the bones of its throat indicate it likely had strong cheeks and a retractable tongue. These traits would have enabled it to suck water into its mouth, taking up fish and small squid in the process. The ability to suction feed would have rendered teeth, whose development requires a lot of energy to grow, unnecessary. The loss of teeth, then, appears to have set the evolutionary stage for the baleen, which the scientists estimate arose about 5 to 7 million years later. view more
Credit: Alex Boersma
A study reported in Current Biology on November 29 describes a 33-million-year-old fossil whale named Maiabalaena, which means "mother whale." The ancient whale from Oregon is especially remarkable in that it had neither teeth nor baleen.
"Maiabalaena represents a surprising intermediate stage between modern filter-feeding whales and their toothed ancestors," says Carlos Mauricio Peredo of George Mason University and the National Museum of Natural History.
Instead, he adds, Maiabalaena was a suction feeder. The findings suggest that early whales lost their teeth before the evolutionary origin of comb-like baleen. Baleen works much like a sieve, allowing modern baleen whales to filter huge volumes of small prey from seawater in quantities sufficient to support their massive bodies.
"Filter feeding in baleen whales represents an innovation without precedent among any other mammals, and its origin has been a long-standing question since Darwin," Peredo says, comparing it to the transition from scales to feathers in dinosaurs and fins to limbs in tetrapods.
At 33 million years old, Maiabalaena dates back to a period of massive geological change. The age and geographic location of Maiabalaena suggested to Peredo and his team that it would shed new light on whale evolutionary history.
Their first surprise was the discovery that Maiabalaena lacked teeth, making it the oldest toothless whale known to science. But, the real surprise came when they realized that the fossilized specimen showed no evidence for baleen either. Based on its relationship to other whales, the findings suggest that whales lost teeth first. Baleen only came later.
So, how did these whales eat? The researchers say that, based on the anatomy of the oral cavity and bones in the throat, the whales were apparently effective suction feeders without the advantages of teeth or baleen.
The findings add to evidence suggesting that the loss of teeth and the origin of baleen are separate evolutionary events. They also help to shape scientists' understanding of the evolutionary origin of baleen, which remains one of the most enigmatic and unique structures in mammals.
The new specimen dates back to the Eocene-Oligocene boundary (33 million years ago). The researchers say this time period represented a critical moment for whales, when changing geology spurred major changes in feeding. They'll now continue to document the full diversity of feeding modes across this boundary to understand how changing environments triggered the origins of modern whale groups.
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Current Biology, Peredo et al.: "Tooth Loss Precedes the Origin of Baleen in Whales" https://www.cell.com/current-biology/fulltext/S0960-9822(18)31414-3
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.cell.com/current-biology. To receive Cell Press media alerts, contact press@cell.com.
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