Paleontologists have redescribed Zygomaturus keanei, a species of marsupial that lived in Australia some 3.5 million years ago (Pliocene period), using new skeletal remains from northern South Australia and placed this quarter-ton animal in a new genus, Ambulator; the locomotory adaptations of the animal’s legs and feet would have made it well suited to roam long distances in search of food and water when compared to earlier relatives.
Diprotodon optatum, a distant relative of Ambulator keanei. Image credit: Nellie Pease / ARC Centre of Excellence for Australian Biodiversity and Heritage / CC BY-SA 4.0.
Diprotodontids are members of Diprotodontidae, an extinct family of large herbivorous marsupials.
These animals were endemic to Australia and New Guinea during the Oligocene through Pleistocene periods from 28.4 million to 40,000 years ago.
They likely had to travel much greater distances to obtain enough food and water to keep them going.
The largest species, Diprotodon optatum, grew to the size of a car, weighing up to 2.7 tons.
“Diprotodontids are distantly related to wombats — the same distance as kangaroos are to possums — so unfortunately there is nothing quite like them today,” said Flinders University Ph.D. candidate Jacob van Zoelen.
“As a result, paleontologists have had a hard time reconstructing their biology.”
During the Pliocene period, when Ambulator keanei was alive, there was an increase in grasslands and open habitat as Australia became drier.
“We don’t often think of walking as a special skill but when you’re big any movement can be energetically costly so efficiency is key,” van Zoelen said.
“Most large herbivores today such as elephants and rhinoceroses are digitigrade, meaning they walk on the tips of their toes with their heel not touching the ground.”
Using 3D-scanning technology van Zoelen and colleagues were able to compare the partial skeleton with other diprotodontid material from collections all over the world.
By CT scanning the specimen, soft tissue impressions preserving the outline of the footpad were revealed.
“Diprotodontids are what we call plantigrade, meaning their heel-bone (calcaneus) contacts the ground when they walk, similar to what humans do,” van Zoelen said.
“This stance helps distribute weight when walking but uses more energy for other activities such as running.”
“Diprotodontids display extreme plantigrady in their hands as well, by modifying a bone of the wrist, the pisiform, into a secondary heel.”
“This ‘heeled hand’ made early reconstructions of these animals look bizarre and awkward.”
“Development of the wrist and ankle for weight-bearing meant that the digits became essentially functionless and likely did not make contact with the ground while walking,”
“This may be why no finger or toe impressions are observed in the trackways of diprotodontids.”
“So, diprotodontids such as Ambulator keanei may have evolved this morphology to traverse great distances more efficiently.”
“This morphology also allowed for greater weight to be supported, allowing diprotodontids to get very big indeed.”
“Eventually, this led to the evolution of the giant and relatively well-known Diprotodon.”
The findings appear today in the Journal of Royal Society Open Science.
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Jacob D. van Zoelen et al. 2023. Redescription of the Pliocene marsupial Ambulator keanei comb. nov. (Diprotodontidae) from inland Australia and its locomotory adaptations. Journal of Royal Society Open Science, in press; doi: 10.1098/rsos.230211
