Two new and extinct kangaroo species were named recently. Scientists said these modern-day kangaroo ancestors didn't hop but instead, moved on all fours.

These extinct kangaroo species - Cookeroo bulwidarri and Cookeroo hortusensis - lived between 18 to 23 million years ago.

The researchers named the new genus Cookero in honor of Dr. Bernard Cooke, a Queensland Museum researcher who spearheaded a research program on Riversleigh's ancient kangaroos' evolution.

In the Aboriginal Waanyi language, bulwidarri means "white." The Cookeroo bulwidarri was named after the Riversleigh White Hunter Site where it was discovered.

In Latin, Hortusensis means "belonging to the garden." The Cookeroo hortusensis is named after the Neville's Garden Site.

Dr. Kenny Travouillon, Western Australian Museum's mammal curator, became interested in an unidentified skull at the Queensland Museum. The skull, along with other fossils, was unearthed in north-west Queensland at a Riversleigh World Heritage during the past 30 years.

Due to the lack of scientific papers on kangaroos in recent years, Travouillon found the skull belonged to a new kangaroo species.

Kaylene Butler, one of Travouillon's PH.D. students at the University of Queensland (UQ), analyzed and named the new species. During her analysis, she discovered another unnamed kangaroo species among the University of New South Wales' collections.

Butler said the new species walked on all four legs and scurried across a thick, forested habitat that is far from the dry outback in today's modern Queensland.

Travouillon said that the new and extinct kangaroo species looked similar to modern-day ones. However, they were smaller, about the size of a wallaby.

But unlike modern-day kangaroos, these extinct ancestors didn't hop, instead, they walked on all four legs. An analysis of the leg bones found no evidence that could support hopping.

"Hopping actually occurred much later in the evolution," said Travouillon, who is a former Robert Day Fellow at UQ. 

Gilbert Price from UQ and Riversleigh researchers Professor Suzanne Hand and Professor Michael Archer from the University of New South Wales were part of the research team. The discovery was published in the Journal of Vertebrate Paleontology on Feb. 17.

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