Artist's impression of a Janjucetus whale attacking fish by
Credit: Carl Buell; Copyright Museum Victoria
The fossilised jaws of the archaic baleen whale Janjucetus hunderi.
Credit: Jon Augier; Copyright Museum Victoria
Erich Fitzgerald cradles the fossil jaws of the tiny extinct whale Janjucetus. With a total body length of only 3 m, Janjucetus would have been dwarfed by its present day relative, the blue whale, which looms in the background.
Credit: Jon Augier; Copyright Museum Victoria
CAMBRIDGE: Fossils uncovered in southeast Australia are helping to explain the evolution of the biggest mouth in the world.
In a paper published by Biology Letters today, earlier speculations that baleen whales first evolved their huge mouths for filtering plankton have been challenged. It argues that baleen whales, the biggest animals to have ever lived, originally evolved to have large mouths so they could 'bite' their prey with big, recurved teeth.
"This study demonstrates for the first time that the earliest baleen whales lacked one of the hallmarks of all living (and most fossil) baleen whales: a loose lower jaw joint," said Erich Fitzgerald, lead author of the study and palaeontologist at Museum Victoria in Melbourne.
"This is compelling evidence that these archaic baleen whales could not expand and rotate their lower jaws, which enables living baleen whales to engulf and expel huge volumes of seawater when filter-feeding on krill and other tiny animals," he said.
Epic consumption
Bulk filter-feeding in living baleen whales is one of the most compelling adaptations in the animal kingdom. When the blue whale, the largest of the baleen whales, opens its jaws, it is committing what is arguably the most powerful biomechanical event in the history of life, said Fitzgerald.
For these filter-feeding events to take place, the blue whale must consume more than its own body weight in water. This is all the more remarkable in light of the fact that that baleen whales are mammals, yet are still capable of such extreme biological innovations.
Until now, scientists have laboured under the assumption that the earliest baleen whales had an elastic, loose lower jaw joint, able to rotate independently. Fitzgerald's examination of a primitive rigid jaw joint in one of the earliest mysticete (or baleen) whale species, Janjucetus hunderi, demonstrates that this was not the case.
Jaws of Janjucetus
The 25 million-year-old Janjucetus fossils were found in the mid-1970s eroding out of a limestone cliff on a surf beach near Torquay in Victoria by amateur fossil collector, Brian Crichton.
Fitzgerald's examination of the fossilised jaws involved comparing them to the jaws of living baleen whales. In modern baleen whales, the lower jaws are not connected to the upper jaws, but are instead joined by elastic fibers. This allows the lower jaw to stretch and maximise the amount of food and water that can be consumed.
In the Janjucetus fossil, however, it appears that the lower jaw is fused to the front. What's more, the upper jaw is wider. Both of these anatomical characteristics suggest that the ancient baleen whales may have had feeding strategies similar to the pilot whale, which creates a rapid suction pull in the water before clamping down on its prey.
This finding also suggests that the wide upper jaw of early baleen whales, such as Janjucetus, may have been an exaptation (or 'pre-adaptation') for filter-feeding that originally had a very different function.
