Ambulator keanei marsupial

A team of paleontologists describe analysis of the remains of an ancient quarter-tonne marsupial that roamed Australian lands around 3.5 million years ago. These new fossils help shed light on the mysteries surrounding other, even larger, extinct marsupials. Details of the study are published in the journal Royal Society Open Science.

THE Diprotodontidae form a family of marsupials native to Australia, New Guinea and neighboring islands. The best known members are kangaroos and wallabies. Some of these animals are characterized by their adaptation to jumping locomotion, with powerful hind legs and a strong tail used for balance. Others, like wombats, are more suited to four-legged locomotion.

We know that several members of this family of mammals were once much larger. Diprotodon optatumwho would have lived in Australia there is between 1.6 million years and 40,000 years agocould indeed measure up to three meters in length and weigh up to three tons according to some estimates.

Nevertheless, most of what we know about ancient Diprotodontids comes from fossil jaws and teeth, which means we lack accurate information about them. These animals are also very distant from other marsupials. It is therefore difficult to deduce anything from living species. This is why this new study is important.

There’s nothing like it today

About six years ago, paleontologists unearthed the fuller remains of one of these marsupials on an eroded cliff face in South Australia. They then used 3D computer scans of these bones to create a model of what this marsupial might have looked like.

The analysis of these fossils 3.5 million years old revealed several interesting points. First, the newly described species, baptized ambulator keanei, had a body plan similar to that of a bear or rhinoceros. These animals probably weighed about 250kg and measured approximately a tape measure on the shoulder. Most importantly, the model suggests that this marsupial may have walked differently from similarly sized animals alive today.

Most of today’s large herbivores, such as elephants and rhinos, are digitigrade. This means that they walk on tiptoe without their heel touching the ground.“, recalls Jacob van Zoelen, of Flinders University, the main author of this work. ” Diprotodontids are what we call plantigrades, which means that their heel [calcaneus] comes into contact with the ground when walking, as humans do“. Unlike humans, it would seem that the “fingers” of this marsupial did not come into contact with the ground while walking. ” There’s nothing like it today“, continues the researcher.

This could help explain a long-standing mystery. Indeed, scientists have already found fossilized footprints belonging to D. optatum, the largest known marsupial described above. However, none of them have fingerprints. This new discovery suggests it’s because their toes never actually touched the ground.

A pair of fossilized footprints left by D. optatum. Note the absence of visible fingerprints in the print. Credits: AB Camens/Flinders University

An advantage in a rapidly changing environment

The fact that they were plantigrade also means that they conserved energy by evenly distributing their weight when walking. These efficient strides may have allowed the newly described species to travel very long distances, which would have been a huge advantage. Back then, Australia’s lush forests and grasslands indeed turned into hot and arid desertsforcing herbivorous animals to travel farther between water and food sources.

Finally, the discovery ofA. keanei might also help explain how D.opatum got so big. The even weight distribution of the new marsupial might also have occurred in this species, which could have been a key factor in its growth.