Introduction to HMW Volume 5: Monotremes and Marsupials

Volume 5 of the Handbook of Mammals of the World (HMW) covers 21 families in eight orders. Monotremes and Marsupials have been grouped together traditionally, and we continue that by treating them together in this volume. Monotremes retain primitive characteristics such as egg-laying and are traditionally placed in a separate subclass of mammals, Prototheria. As with much of mammalian phylogeny, monotreme taxonomy is in a state of flux and some would split the order into two. However, there remains considerable support for recognizing three separate groups of living mammals, the Monotremata in Prototheria, the orders constituting the marsupials in Metatheria, and the placentals in Eutheria.

We begin the volume with a special chapter on extinct marsupials because this is a topic of considerable interest. The last century has seen a number of species go extinct, and because we do not normally include extinct species in HMW, we felt it important to consider these forms along with their living relatives.

As with almost all groups of mammals, there is continuing controversy over the classification of the orders traditionally grouped as marsupials. We follow the arrangement exemplified by the Third Edition of Mammal Species of the World (MSW), published in 2005. However, as with other volumes in the series, we have made several improvements and updates. The views of the authors, all of whom are leading authorities, have been incorporated into this volume. We have included the most recently described species, Massoia’s Lutrine Opossum (Lutreolina massoia), from the montane forests of southern Bolivia and north-western Argentina, and the Black-tailed Antechinus (Antechinus arktos), from a tiny locality in eastern Australia, both described in 2014, as well as ongoing systematic revisions, which continue to add to our knowledge of the phylogenetic relationships of the families covered by this volume. The Systematics section in each family text reviews the ongoing taxonomic work and recent research using new molecular techniques, which has revolutionized our ability to analyze evolutionary relationships.

The monotremes comprise perhaps the most unusual of mammals. Clearly, monotremes diverged from other mammals early on in evolutionary time, and some estimates would place that not long after mammals split from their reptilian ancestors. There is some support for dividing the order into two, with Platypoda for the Platypus (Ornithorhynchus anatinus) and its fossil relatives, and Tachyglossa for the four species in two genera of echidnas. Attempts at dating the split between these two groups have so far proven difficult to reconcile with the known fossil evidence. All living monotremes are restricted to Australia and New Guinea.

Marsupials are found in both Australia and South America, testifying to their Gondwanaland roots. The largest and most diverse group of South American marsupials is the family Didelphidae (order Didelphimorphia). New World opossums range from southern Canada to Argentina and occupy a wide diversity of habitats. There are forms that are terrestrial, aquatic, arboreal, and scansorial. They have extremely flexible diets, are both hunters and scavengers, and their reproductive systems allow them to be quite good at colonizing new and different habitats.

Caenolestids (order Paucituberculata), sometimes called shrew opossums, with seven species and three genera are small Andean marsupials restricted to higher elevations. Nocturnal and secretive, they are somewhat shrew-like in their habits. They feed on insects, earthworms, and small vertebrates when they can catch them.

The final South American marsupial family, the Monito del Monte of the family Microbiotheriidae (order Microbiotheria), is perhaps the most specialized. There is only a single species of living microbiotheriid, Dromiciops gliroides. Monitos de Monte are the South American group most closely related to the Australian marsupials. They are restricted to temperate rain forests of the southern Andes and frequent bamboo thickets.

The bulk of the volume is devoted to the Australian marsupials, an old and extremely diverse lineage. Once thought to comprise only a single order, we now recognize three orders of New World marsupials (those outlined above), and four orders of Australian marsupials.

Notoryctidae (order Notoryctemorphia) contains the two species of marsupial moles. A lineage dating to around 20 million years ago, they are secretive and poorly understood fossorial counterparts of our more familiar placental moles. They most closely resemble the placental family Chrysochloridae, the golden moles. Blind and lacking external ears, they spend their lives burrowing in the sand of the central Australian deserts.

The order Dasyuromorphia (carnivorous marsupials) contains two living families, plus the extinct Thylacine (Thylacinus cynocephalus). The family Myrmecobiidae contains only the single species Myrmecobius fasciatus, the Numbat, or Marsupial Anteater. Like many of its ecological counterparts among placental forms in other parts of the world, it feeds mainly on termites. Once widespread, this iconic western Australian animal is now an endangered species.

Rounding out the carnivorous marsupial families, the family Dasyuridae with 74 species and 17 genera contains a diverse assemblage of mostly small, rodent-sized carnivores, but ranging in size up to the Tasmanian Devil (Sarcophilus harrisii). With both arboreal and terrestrial forms, the family consists mostly of long-bodied, short-legged species with pointed snouts and long, well-furred tails. They occur in habitats ranging from arid areas in Australian to rainforests in New Guinea, and feed on a variety of invertebrates and small vertebrates.

The two living families of Peramelemorphia include the Thylacomyidae and the Peramelidae. The single surviving species of Thylacomyid, the Greater Bilby (Macrotis lagotis), is a big-eared, long-tailed denizen of the desert that is basically omnivorous. As with many other formerly widespread species, the Greater Bilby is now an endangered species.

With 18 species in six genera, Peramelidae clearly is the more diverse family of peramelemorphs. Occurring in both Australia and New Guinea, they occupy a wide range of habitats ranging from very dry to very wet. Bandicoots are small omnivores, eating a variety of small vertebrates and insects, as well as plant matter.

The largest and most diverse order of marsupials is the Diprotodontia, with eleven families. The family Phascolarctidae, contains only the single species Phascolarctos cinereus, the iconic Koala. Koalas are found along the coastal regions of eastern and southern Australia, where they inhabit eucalypt forests. Arboreal herbivores, their diet consists mainly of eucalypt leaves. Their closest relatives are the wombats.

Vombatidae, the wombats, comprises three species in two genera. All are short and stocky, with short tails and small ears. They are impressive burrowers, using short, broad feet with strong, flat claws to construct complicated burrow systems. They are found in a variety of habitats mainly in south-eastern Australia, Tasmania, and Queensland.

Pygmy possums in the family Burramyidae are divided into five species in two genera. Four of the species are restricted to Australia and Tasmania, and one ranges into New Guinea. Burramys parvus, the Mountain Pygmy Possum, a high-elevation form from the mountains of New South Wales and Victoria, is the only marsupial known to hibernate. Arboreal and scansorial, pygmy possums are omnivorous, feeding on a variety of small insects, fruits, nectar, and pollen.

Phalangeridae consists of 29 species in six genera. Cuscuses and brush-tailed possums are mostly arboreal, and the diverse assemblage includes species that range from eucalypt woodlands to the rainforests of New Guinea and surrounding islands. They are mainly medium-sized, with elongated bodies, short legs, and long tails, most of which are prehensile. Omnivorous, they feed on a wide variety of both plant and animal life.

The family Pseudocheiridae, Ring-tailed Possums and Greater Gliders, consists of 20 species in five genera. They are slightly larger versions of pygmy possums, and all but one have prehensile tails. They are arboreal and scansorial, and most are specialized leaf-eaters. They are wide-ranging, found in forested regions of both Australia and New Guinea.

The twelve species in three genera of the family Petauridae, Striped Possums, Leadbeater’s Possum, and Lesser Gliders, are all striking looking possums with facial and dorsal stripes. They occur in both Australia and New Guinea in a variety of forests. Most are gliders, with enlarged gliding membranes and prehensile tails. They have a variety of feeding habits, including both the sap of trees, and insects. Some are known to extract insects in a manner similar to the Aye-aye (Daubentonia madagascariensis) in Madagascar.

The monotypic family Tarsipedidae consists only of the single species Tarsipes rostratus, the Honey Possum. This tiny arboreal glider is the sole survivor of an ancient lineage. It has three dorsal stripes, a long pointed snout, and teeth that are reduced to pegs. They are restricted to south-western Australia, where they use their brush-tipped tongue to feed on nectar and pollen of a variety of flowers.

The family Acrobatidae consists of three species in two genera, and has representatives in both Australia and New Guinea. Feather-tailed gliders and possums are specialists on pollen and nectar, like their relative Tarsipes. The name feather-tailed refers to horizontal, stiff hairs on the tail that likely help in gliding.

The family Hypsiprimnodontidae has a very restricted distribution in the rainforests of far north-eastern Australia. The single surviving species, the Musky Rat Kangaroo (Hypsiprymnodon moschatus) is a smaller, perhaps more primitive version of the larger kangaroos. Somewhat unusual in being diurnal, it feeds on a variety of fruits and insects.

The eight species in three genera belonging to the family Potoroidae are medium-sized, brown, hopping marsupials resembling small wallabies called Bettongs or Potoroos. They tend to feed mainly on tubers and underground fungi, but also take seeds and some insects from the surface as well. They are found in both Australia and Tasmania.

Finally, the family Macropodidae contains the 59 species in 13 genera of kangaroos and wallabies. This diverse and speciose group occurs in both Australia and New Guinea. These are the iconic large, hopping marsupials readily identified with the continent of Australia. Macropod means long foot, and appropriate name for these animals that use their elongated hindfeet in a specialized hopping locomotion. They also have large, muscular tails that help to provide a tripod when standing on their hindfeet.

Altogether, the assemblage contained in this volume includes an amazing variety of animals adapted to the island continent and nearby islands, as well as a few groups still found in South America. Marsupials provide the best-known examples of convergent evolution, with species of marsupials evolving to look and behave like their placental counterparts who do similar things on other continents. In addition, there are species found in this remarkable radiation that have no counterparts anywhere in the world. Such a unique fauna rightly deserves its own volume.

Don Wilson