What makes a kangaroo a kangaroo? You might immediately think of their ability to hop, but according to a study published in the Journal of Vertebrate Paleontology, ancient kangaroos weren't able to hop at all. This new research also conjures up a particularly striking image: Millions of years ago, small ancestral kangaroos lived alongside crawling, fanged cousins.
Paleontologists at the University of Queensland have uncovered two new species of extinct, non-hopping kangaroos within ancient Australian fossil deposits. When they lived, they were roughly the size of a pademelon, a small, modern marsupial. The two new species, Cookeroo bulwidarri – which lived 23 million years ago – and Cookeroo hortusensis – which lived 18-20 million years ago – were recognized as distinct species based on their unique skull and teeth arrangements.
Their skeletal anatomy suggests that they were adapted to crawling on all fours, scurrying around what used to be a densely forested region. Interestingly, this extinct crawling adaptation can also found in a variety of related creatures, ones that these newly discovered animals would have co-existed with. This suggests that hopping had not yet evolved 18 million years ago.
The tiny skull of the fangless C. hortusensis. Kaylene Butler et al./Journal of Vertebrate Paleontology
One of these co-existing animals was another ancient kangaroo ancestor: the Nambaroo was about the size of a small dog, and wielded large canine fangs. This animal, discovered several years earlier, also had opposable toes and incredibly flexible feet, which some researchers interpret as tree climbing adaptations.
Its strong forearms meant that it likely galloped around on all fours, hunting for food. The Nambaroo, like other members of its group – including the Balbaroo fangaroo – was thought to have used its sharp fangs for display purposes only, scaring off competitors and wooing mates.
All of these species, including the new, fangless ones, lived in dense woodland, and so likely had a diet of fruit and fungi. As the forests gave way to grassier, arid environments around 10 to 15 million years ago, moving around on all fours was no longer advantageous. But which of these scuttling marsupials are the direct ancestors of contemporary, bipedal, hopping kangaroos – the fanged ones or the newly discovered species?
The fanged Nambaroo is thought to have lived around 25 million years ago; on the other hand, the two new species emerged at the end of the Paleogene period around 23 million years ago, a time of huge climatic change. It was at this point that humid, tropical environments were transitioning into forests and open grasslands.
Based on the ages of the fossils, it is clear that the two new species were suited to foraging and thriving in these new environments. On the other hand, their older, fanged competitors could not keep up with the pace of change, and they ultimately died out. The reasons behind this remain unclear, but it appears that these new species were likely part of the group that eventually gave rise to modern kangaroos.