Mammalodon probably lived by sucking small animals up from the seafloor
An ancient "dwarf" whale appears to have fed by sucking small animals out of the seafloor mud with its short snout and tongue, experts say.
Researchers say the 25 million-year-old fossil is related to today's blue whales - the largest animals on Earth.
The ancient animal's mud slurping may have been a precursor to the filter feeding seen in modern baleen whales.
These whales strain huge quantities of tiny marine animals through specialised "combs" which take the place of teeth.
The research is published in the Zoological Journal of the Linnean Society.
The fossilised remains of the primitive baleen whale Mammalodon colliveri were discovered near Torquay, in Victoria, Australia.
This animal still had teeth; it had not yet evolved the baleen plates - used for filter-feeding - which characterise present-day baleen whales.
Although Mammalodon was discovered in 1932 and named in 1939, it has not been widely studied, according to Museum Victoria, which holds specimens of this group.
The study's author, Dr Erich Fitzgerald from Museum Victoria, said that his study of the fossil led him to the conclusion that Mammalodon was a bottom-feeding mud-sucker.
The idea would support Charles Darwin's observation about whale evolution in his seminal book On the Origin of Species.
In it, Darwin speculated that some of the earliest baleen whales may have been suction feeders - and that this served as a precursor to the filter feeding of today's giants of the deep.
Mammalodon probably evolved from much bigger ancestors
Mammalodon had a total body length of about 3m. But it appears to have been a bizarre evolutionary "splinter group" from the evolutionary lineage which later led to the 30m-long blue whale.
It was effectively a dwarf whale; the research suggests that Mammalodon may have evolved into a relatively tiny form from larger ancestors.
Mammalodon belongs to the same family as Janjucetus hunderi, fossils of which were also found in 25 million-year-old Oligocene rocks near Torquay in Victoria. This family appears to be unique to south-east Australia.
"Clearly the seas off southern Australia were a cradle for the evolution of a variety of tiny, weird whales that seem to have lived nowhere else," said Dr Fitzgerald.
The baleen plates which allow today's baleen whales to filter their food from water, distinguish this group from the toothed whales - a group which includes beaked whales and dolphins.
Baleen whales are a taxonomical group which includes not only the majestic blue whale, but also the right whales, fin whales and humpbacks, to name but a few.