Think of Australia’s iconic kangaroos and you’ll probably think of them hopping along with their large and muscular back legs, a method of locomotion known as saltation.
However, a recent paper published in Alcheringa: An Australasian Journal of Palaeontology explores the locomotory evolution of kangaroos and their relatives (including wallabies, wallaroos and tree-kangaroos) across the fossil record and found that the majority of the larger forms did not use the higher speed-endurance hopping of today’s large kangaroos.
“This research is a review of many years researching kangaroo evolution and a desire to show more clearly that many kangaroos in the relatively recent past were very different from those around today,” says lead author Professor Christine Janis from the University of Bristol. “In fact, modern large hopping kangaroos are the exception in kangaroo evolution.”
The team analysed the limb and ankle bones of modern and fossil species from the past 25 millions years, and found that there were a diversity of adaptations locomotion across the large kangaroos, weighing over 20kg, that emerged in the late Miocene.
The extinct short-faced kangaroos (the sthenurines) used bipedal striding, the extinct giant wallabies (the protemnodons, a genus closely related to modern kangaroos) mostly traversed on all four limbs, and the ancestors of modern kangaroos became specialised for high-speed endurance hopping.
“Hopping is only one of many gait modes employed by kangaroos both in the past and today and the fast endurance hopping of modern kangaroos should not be regarded as some ‘evolutionary pinnacle’,” adds Janis, “What makes modern endurance-hopping kangaroos appear so unusual is the geologically recent extinction of similar animals who moved in different ways.”
Kangaroo tibias are proportionally longer among larger species, but not in the range of smaller ones, suggesting a particular adaptation for hopping at large size.
“The proportionally longer calcaneal heel in larger modern kangaroos reflects the need for a long lever arm for the Achilles tendon to balance forces around the ankle joint in large hoppers,” she says. “The shorter heel in sthenurines suggests that they are not attempting to balance such forces and so were not hoppers.”
Main image: Sthenurus stirlingi, Pleistocene of Australia © Nobu Tamura (http://spinops.blogspot.com), CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
