Paleobiology of Argyrolagus (Marsupialia, Argyrolagidae): An astonishing case of bipedalism among South American mammals

By: María Alejandra Abello & Adriana Magdalena Candela

Summarized by: Mason Woods

Mason Woods studies geology and landscape photography; currently, he is a senior. He is a budding naturalist, finding purpose through a wide range of interests, including paleontology, biology, hydrology and philosophy. When he isn’t studying, he’s sharpening his skills in diving, climbing and hiking, leaving no stone left unturned on his path to understanding and experiencing the natural world.

What data were used?: Two scientists examined multiple specimens of Argyrolagus, a mouse-like marsupial who lived in deserts of Argentina, in order to determine the way that early marsupials might have interacted with the world and evolved. They are potentially one of the first marsupials to have assumed an upright stance and move on two legs instead of four.

Methods: These scientists studied the shape and positioning of the bones to compare this species to other marsupials still alive today in order to determine the true function of the body parts of the animal, through a process called morphofunctional analysis. If the animal moved on four limbs, its body structure would compare to other animals alive today who also move on four limbs. There are clear differences in the length, density, and structure of an animal’s body, depending on how they adapted to interact with their surroundings.

Results: The upper half of Argyrolagus appears to be that of a digger, an animal who burrowed for food, or perhaps to create shelter. Its humerus is short, with muscles that seem to have been allocated towards arm retraction, which indicates it had a strong digging ability. The elbow joint is compact and stable, indicating an adaptive response to the pressures placed on the body when jumping. The forearm bones have muscle attachments which are like that of other digging mammals, and scarring from muscles related to digging, providing further evidence of a burrowing lifestyle . The digits – what we would call fingers – on the animal’s body are like that of other mammals which also were known diggers. The evidence points to the animal moving around by jumping, and probably using its robust upper body for digging as well.

The lower half seems to be that of a jumper. The pelvis is not exactly screaming “jumper” but it does have some adaptations that would help in digging with powerful hip flexion. The hip joint is the clearest evidence, with restricted motion in certain directions, creating a stabilized joint. The femur is adapted well for digging. The knee joint is stable and is shaped for pulling action, like other leaping animals. The ankle joints are strong and with restricted motion, to assist with stability in leaping. Restricted motion prevents injury and the need for muscles to stabilize certain positions and allows the animal to put more energy towards increasing the strength of necessary muscles. 

Why is this study important?: If we can demonstrate that these animals were likely bipedal, it’s possible that we can determine the evolutionary origin of bipedalism in marsupials, which helps us to complete the picture of how descendants of this animal came to be. We can also use this information to better inform evolutionary analyses and draw conclusions about the close relatives of Argyrolagus. 

The big picture: Argyrolagus was likely a bipedal jumper, which is an atypical gait for most mammals. Jumping has been shown to be efficient for animals of this size, which would have adapted the animal well for life in an arid environment, allowing them greater ranges of motion to escape predators in areas with less vegetation in which to conceal themselves. Each piece of the puzzle gets us closer to seeing the big picture of how evolution really affects life on Earth, answering questions such as: how gradual is evolution? How much can we learn from our ancestor and what does that tell us about the future?

Citation: Abello, M. A., & Candela, A. M. (2019). Paleobiology of Argyrolagus (Marsupialia, Argyrolagidae): an astonishing case of bipedalism among South American mammals. Journal of Mammalian Evolution, 1-26.