Marsupial mole

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Marsupial moles[1]
Temporal range: 20–0 Ma
Miocene to Recent
Notoryctes typhlops.jpg
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: ?Marsupialia/?Dryolestoidea
Superorder: ?Australidelphia
Order: Notoryctemorphia
Kirsch, in Hunsaker, 1977
Family: Notoryctidae
Ogilby, 1892
Genus: Notoryctes
Stirling, 1891
Species

N. typhlops
N. caurinus

Marsupial moles are a family (Notoryctidae) of cladotherian mammals of the order Notoryctemorphia. They are rare and poorly understood burrowing mammals of the deserts of Western Australia, with an ancestry going back 20 million years or so. Once classified as a monotreme, they are now thought to be a marsupial, though a recent non-peer-reviewed preprint[2] suggests they may instead be a member of the Dryolestida, an otherwise extinct group of mammals. Their precise classification was for long a matter for argument, but there are considered to be only two extant species:[1]

The two species of marsupial moles are so similar to one another that they cannot be reliably told apart in the field.

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. It is debated whether or not marsupial moles dig permanent burrows or simply fill their tunnels in behind them as they move.

Characteristics[edit]

The head is cone-shaped, with a leathery shield over the muzzle, the body is tubular, and the tail is 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 it does not fill with sand, and contains just two teats, so 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.[4]

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 larvae and cossid caterpillars.[4] Their teeth are highly distinct from those of other mammals, with characteristics not seen in therians but present in more archaic mammal clades, shared specifically with dryolestoids. Their dental formula was thought to be similar to that of other marsupials -

Dentition
4.1.2.4
3.1.3.4

-, but more recent efforts show it to be distinct:

Dentition
4.1.3.3
3.1.3.3

. This formula is identical to the morphology seen in dryolestoids.[2]

Marsupial moles are unique among mammals in their brain structure. Unlike therians and like monotremes, the olfactory bulb is located in front of the cerebrum as opposed to ventro-laterally displaced from midline, and they are larger than seen in monotremes. The neopallium is smaller than any other mammal, and it forms a dorsal cap lying on the pyriform lobe, and is not separated by any fissue, a condition seen also in monotremes but not therian mammals.[5]

Marsupial moles have a true cloaca, a feature only seen in mammals among monotremes and tenrecs, a fact which it was considered unusual.[6] The urogenital sinus is much and longer than in monotremes, approaching the condition seen in therian mammals, but unlike with the latter it is undivided. The cloaca is devoid of skin glands and associated ducts, much like in monotremes. Like in monotremes, the penis is located inside the cloaca as opposed to externally like in therian mammals, while the testicles are internal and it lacks a scrotum. Only a pair of bulbo-urethral glandulae are present, much like in monotremes, unlike the presence of derived prostates and urethral glandules in therian mammals.[7][8]

Evolution[edit]

For many years, their place within the Marsupialia was hotly debated, some workers regarding them 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,[citation needed] 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 its 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 their complete absence in 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 the marsupial moles are not closely related to any of the living marsupials, and they appear to have followed a separate line of development for a very long time, at least 50 million years. Indeed, marsupial moles share only one synapomorphy with marsupials.[2]

More recently, it has been suggested that the marsupial moles aren't actually marsupials, but members of Dryolestoidea.[2]

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. Like marsupial moles, an identity as a dryolestoid has been suggested.[2]

In itself, the discovery of a Miocene marsupial mole (Naraboryctes philcreaseri) 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, spade-like 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. However, more recent findings suggest that it wasn't closely related to modern marsupial moles, and should be considered to have an incertae sedis position among marsupials.[2]

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.

References[edit]

  1. ^ a b Groves, C.P. (2005). Wilson, D.E.; Reeder, D.M., eds. Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Baltimore: Johns Hopkins University Press. p. 22. OCLC 62265494. ISBN 0-801-88221-4. 
  2. ^ a b c d e f On Dryolestid affinities [1][full citation needed]
  3. ^ "Mole Patrol". The Marsupial Society. 2004. Retrieved 2006-11-09. 
  4. ^ a b Gordon, Greg (1984). Macdonald, D., ed. The Encyclopedia of Mammals. New York: Facts on File. p. 842. ISBN 0-87196-871-1. 
  5. ^ Vaughan, T. A., Ryan, J. M. & Czaplewski, N. J. Mammalogy. 5th Edition. (Jones & Bartlett Learning, 2010).
  6. ^ Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
  7. ^ Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
  8. ^ Riedelsheimer, B., Unterberger, P., Künzle, H. and U. Welsch. 2007. Histological study of the cloacal region and associated structures in the hedgehog tenrec Echinops telfairi. Mammalian Biology 72(6): 330-341.

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