Baleen whales, the iconic gentle giants of the ocean, are the largest animals to have ever existed. The blue whale, which can grow up to 30 metres long and weigh around 170 tonnes, dwarfed even the biggest dinosaurs thanks to its ability to devour thousands of kilograms of krill every day.
It does this by filter feeding – it takes in large volume of water into its mouth and is able to trap and filter out millions of krill thanks to its hair-like baleen plates that form a sieve. But how exactly did baleen whales evolve?
The theory goes that around 34 million years ago, the baleen whales (Mysticeti) split from their toothed whale relatives (Odontoceti) via a transitional species known as Janjucetus. This extinct whale had both teeth and baleen in its jaw, which suggests the ancient species gradually lost their teeth through evolution to form the filter-feeding baleen system.
However, the discovery of ‘Alfred’, a 25-million-year-old whale fossil skull unveiled by Museums Victoria, has turned this theory on its head. The findings were published in the museum’s journal today.
Dr Erich Fitzgerald, a curator of vertebrate palaeontology from Museums Victoria and one of the researchers involved in the study, says Alfred has the “smoking gun” for understanding the evolutionary history of whales.
“Alfred is a fossil of one of the earliest relatives of today’s baleen whales… yet it doesn’t have baleen,” he told SBS Science.
“Furthermore, Alfred has teeth – living baleen whales are defined by their absence of teeth and possession of this remarkable and quite highly specialised hair-like structure called baleen.”
Alfred appears to be a new, unnamed species that belongs to a group of extinct whales called aetiocetids. It is estimated to have grown to around 3-4 metres in length and weighed around 200-300 kilogrames.
The evidence gleaned from Alfred’s remarkably well-preserved skull and jaw suggests that well before ancient whales lost their teeth and evolved baleen, they were suction feeding.
Fitzgerald explains that this extinct whale would open their jaws slightly to create a small opening in the mouth and then rapidly and repeatedly draw their tongues back to create suction.
“They sucked in, out of the water around them, small fish and squid, and their teeth were almost incidental to prey capture, they weren’t the key,” he says.
Over time, because the teeth were no longer necessary for feeding, they lost their teeth and developed baleen from their gum tissue to complement this new form of feeding.
This new theory is supported by the presence of tiny horizontal scratches on the inside surface of Alfred’s teeth that face the tongue, which suggests that it was not biting on prey.
These same scratches have also been shown in marine mammals like the walrus that use suction feeding to capture prey on the seabed floor. This means that hard material from prey – bones, shells and teeth – and grains of sand rub against the inside of the teeth and cause them to be excavated.
“There are several fossils of aetiocetids and as luck would have it, or as luck wouldn’t have it, the teeth were either not preserved in those other skulls or they’re broken or they’re so worn down that there’s hardly anything at the crown left,” says Fitzgerald.
“Alfred has the smoking gun for feeding behaviour that all of these transitional aetiocetid whales probably performed, and that’s suction feeding.”
Alfred was found on Washington state’s Olympic Peninsula in the US by avid fossil collectors James and Gail Goedert, who donated the fossil remains to Fitzgerald’s team at Museums Victoria.
The rest of the body parts are still being prepared and analysed, so only the skull will go on display with 3-D printed reconstructions of parts that were either missing or incomplete.