The whale captured meals one at a time, Fitzgerald adds, using its powerful jaws to "rip off and swallow larger chunks of flesh from its fishy prey."
The animal's "truly enormous eyes for its size" represent an adaptation for heightened underwater vision, he says, which also suggests it was an active marine predator.
But the whale's skull indicates it couldn't produce ultrasonic signals—meaning it didn't use sonar or echolocation, like some dolphins and certain whales do today.
Instead, Fitzgerald says, Janjucetus probably had to rely on its big eyes to sense potential prey in the dim light under the surface of the sea.
Its other unusual features include a broad, short snout.
"There is no other known whale or dolphin, whether fossil or living, with such a remarkable combination of attributes," Fitzgerald said.
He says the sea creature challenges the current notion that large body size and adaptations for filter-feeding and swallowing small prey in bulk were the key to the evolution of modern baleen whales.
Scientists have found fossils of other ancient baleen whales with teeth. But those species all used their teeth for an early form of filter feeding, unlike the new find.
Freak of Nature?
The new fossil is "important and provocative," agreed Ewan Fordyce, a geology professor and whale evolution expert at the University of Otago in Dunedin, New Zealand.
"Until now, our model of baleen whale history saw filter-feeding as a key adaptive feature in that lineage, arising at the beginning," he said. "We have long thought of all the whales in that lineage as 'bulk' filter-feeders."
But Janjucetus could have been something of a freak among early baleen whales, he says, possibly having evolved in isolation and with little connection to today's species.
"Perhaps Janjucetus isn't so much typical of the start of baleen whales but represents a later side branch that acquired its strange features through evolutionary 'reversion,'" Fordyce said.
Such processes—where a species reverts back to a more primitive form—have been seen in other animal groups.
Fordyce says there is little evidence of other whales living in Australian waters 20 to 30 million years ago apart from the "equally strange and probably related Mammalodon," another early, toothed baleen whale.
He adds that rocks in New Zealand dating from the same period contain plenty of fossil whales and dolphins "but no hint of Janjucetus."
"Perhaps that tiny whale lived isolated in a restricted seaway that had little contact with other waters," he said.
The new fossil find fits in with the theory that modern whale lineages originated 34 to 35 million years ago in response to changing ocean conditions, Fordyce adds.
"Rapid climate change at that time led to a modernization of ocean circulation and probably to major changes in food resources," he said, triggering the evolution of new dolphins and whales such as Janjucetus.
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