[4] In 1946, Charles Lamberton illustrated another femur (upper leg bone) of H. australis; the origin and current whereabouts of this specimen are unknown.

[6] H. australis and H. antimena are classified together within the exclusively Madagascan subfamily Nesomyinae of the family Nesomyidae, which includes various African rodents.

[7] Hypogeomys australis was generally similar to H. antimena,[6] the largest living rodent of Madagascar,[8] but even larger, with little if any overlap in measurements.

[6] Grandidier described the extinct species as more robust, with more prominent muscle scars on the long bones and with longer molars with more distinct crests and lobes.

[15] In general, few small animals became extinct on Madagascar, except for these rodents, two species of Plesiorycteropus, and the shrew tenrec Microgale macpheei; in contrast, large animals such as subfossil lemurs, the carnivore Cryptoprocta spelea, Malagasy hippopotamuses, Dipsochelys tortoises, and Aepyornis and Mullerornis birds all became extinct around the time that humans arrived.