While on school holidays in Busselton, children Stella and Lyla aged nine years, along with Abbey aged five years, found a whale bone on the shores of Geographe Bay.
The girls from Narrogin visit Busselton every few months to visit their grandparents, they Googled images of whale bones when they returned to their grandparent's home.
Their mum Ash said they knew it was a bone from something.
"Coming from a farming community the girls see lots of animal bones in the bush and on farms," she said.
"The girls were initially shocked then super excited and stated that they were taking it home so I said they had to carry it.
"It was quite funny walking down the road with three girls clinging to this bone and watching people's reactions as they drove past.
"Abbey the youngest said maybe we could sell it to the museum for a million dollars."
Western Australian curator of mammalogy Dr Kenny Travouillon confirmed it was indeed a whale bone.
"It is very hard to identify species from ribs, as they look identical between species," he said.
"By the size of the rib, it could be any of the large baleen whales (whales like the blue whale, southern right whale, or a sperm whale etc).
"The rib is covered in algae, so it would have only washed up recently."
Dr Kenny said it was not rare to find them, as often whale carcasses would float for some time, so bits of them eventually found their way to the beach.
"Whales are protected by legislation, so people are not supposed to take whale material home," he said.
"It is best to leave it where it was found and notify the Department of Biodiversity, Conservation, and Attractions.
"Whale material is donated to the Western Australian Museum - but the first port of call is for people to notify DBCA."
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