Marsupial mole

The two species of marsupial moles are rare and poorly understood burrowing mammals of the deserts of Western Australia.

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.

Characteristics
The head is cone-shaped, with a leathery shield over the muzzle, the body tubular, the tail a short, bald stub. They are between 12 and 16 cm long, weigh 40 to 60 grams, and are uniformly covered in fairly short, very fine pale cream to white hair with an iridescent golden sheen. Their pouch has evolved to face backwards so that it does not fill with sand, and contains just two teats, so that the animal cannot bear more than two young at a time.

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.

They are insectivorous, feeding primarily on beetle larve and cossid caterpillars. Their teeth have a somewhat simplified structure, but their dental formula is similar to that of other marsupials:

Evolution
For many years their place within the Marsupialia was hotly debated, some workers regarding it as an offshoot of the Diprotodontia (the order to which most living marsupials belong), others noting similarities to a variety of other creatures, and making suggestions that, in hindsight, appear bizarre. A 1989 review of the early literature, slightly paraphrased, states:


 * When Stirling (1888) initially was unable to find the epipubic bones in Marsupial Moles, speculation was rife: the Marsupial Mole was a monotreme, it was the link between monotremes and marsupials, it had it closest affinities with the (placental) golden moles, it was convergent with edentates, it was a polyprotodont diprotodont, and so on.

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.

Classification
There are thought to be two species: the Southern Marsupial Mole (Notoryctes typhlops or itjaritjari as known by the Pitjantjatjara and Yankunytjatjara people in Central Australia, and the Northern Marsupial Mole (Notoryctes caurinus), so similar to one another that they cannot be reliably told apart in the field.

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.