Marsupial moles spend most of their time underground, coming to the surface only occasionally, probably mostly after rains. They are blind, their eyes having become reduced to vestigial lenses under the skin, and they have no external ears, just a pair of tiny holes hidden under thick hair. They do not dig permanent burrows, filling the tunnel in behind them as they move.
The limbs are very short, with reduced digits. The forefeet have two large, flat, claws on the third and fourth digits, which are used to excavate soil in front of the animal. The hindfeet are flattened, and bear three small claws; these feet are used to push soil behind the animal as it digs. In a feature unique to this animal, the neck vertebrae are fused to give the head greater rigidity during digging.
Marsupial moles provide a remarkable example of convergent evolution, with moles generally, and with the golden moles of Africa in particular. Although only related to other moles in that they are all mammals, the external similarity is an extraordinary reflection of the similar evolutionary paths they have followed.
The mystery was not helped by the complete silence of the fossil record. On the basis that marsupial moles have some characteristics in common with almost all other marsupials, they were eventually classified as an entirely separate order: the Notoryctemorphia. Molecular level analysis in the early 1980s showed that the marsupial moles are not closely related to any of the living marsupials, and that they appear to have followed a separate line of development for a very long time, at least 50 million years. However some morphological evidence suggests that they may be related to bandicoots.
In 1985, the vast newly discovered limestone fossil deposits at Riversleigh in northern Queensland yielded a major surprise: a fossil between 15 and 20 million years old named Yalkaparidon coheni with molars like a marsupial mole, diprotodont-like incisors, and a skull base similar to that of the bandicoots. These features were by no means identical to the living species but clearly related, and possibly even of a direct ancestor. In itself, the discovery of a Miocene marsupial mole presented no great mysteries. Just like the modern forms, it had many of the features that are assumed to be adaptations for a life burrowing in desert sands, in particular the powerful, spadelike forelimbs. The Riversleigh fossil deposits, however, are from an environment that was not remotely desert-like: in the Miocene, the Riversleigh area was a tropical rainforest.
One suggestion advanced was that the Miocene marsupial mole used its limbs for swimming rather than burrowing, but the mainstream view is that it probably specialised in burrowing through a thick layer of moss, roots, and fallen leaf litter on the rainforest floor, and thus, when the continent began its long, slow desertification, the marsupial moles were already equipped with the basic tools that they now use to burrow in the sand dunes of the Western Australian desert.
The marsupial mole, once classified as a monotreme, is now known to be a marsupial. Its ancestry goes back 50 million years or more, and its precise classification is still a matter for argument.