Pseudomys vandycki

Introduction

Rodents, Australia's only native terrestrial placental mammals, make up about one-quarter of its mammal species. Pseudomys vandycki, a small murid rodent from the Pliocene of central Queensland, is one of the earliest Australian rodents and the first Cainozoic rodent described from Australia. Pseudomys species are 'old endemics', part of an early radiation of rodents that colonized Australia from the late Miocene to early Pliocene.

Identification

Most rodent species are rats or mice, although the order contains larger and more diverse species such as capybaras, beavers and porcupines. There are about 2,000 known rodent species, making up about half of living mammal species and about one-quarter of Australian mammal species.

Rodents have characteristic, recurved, ever-growing incisors (one pair of upper and one pair of lower incisors) with enamel on the front surface of the tooth and soft dentine on the back (this sharpens the incisors through wear). Rodents have no canine teeth, and most species also have no premolars. Rodent molars are modified for grinding (the molars coalesce to form a uniform occlusal surface, with numerous sharp enamel ridges for grating plant matter). There is a large caecum (a small, blind pouch in the bowel for fermenting plant matter) and a long intestinal tract, adaptations for a predominantly herbivorous lifestyle. However, many rodents are omnivorous or insectivorous.

Rodents in the family Muridae can be hard to identify, and are distinguished from other rodents mainly by detailed morphology of the molar cusps. There are almost always three upper and three lower molars in each jaw. Most murids lack premolars. The subfamily Murinae, which includes all endemic Australian rodents, have three cusp rows in each of the first and second lophs of M1/ (the first upper molar) and molars have closed roots, leading to extensive molar wear and sometimes complete removal of the crown of the tooth. Differences in cusp morphology between species is linked to differences in diet (granivory, frugivory, insectivory and even piscivory).

Pseudomys species ('native mice') are part of the Mesembriomys group of 22 endemic Australian /New Guinean genera, united by sperm morphology (accessory hooks on the head of the sperm), loss of pectoral and abdominal teats (for a total of only four), and shared molecular or genetic characters. Living species of Pseudomys are generally unspecialized, with few distinctive features that would serve to readily identify them in the field. Many species are arid-adapted. They are found Australia-wide because they are successful generalists.

The M1/ of Pseudomys vandycki differs from that of all other murid rodents in having a large, rectangular T1 that is almost perpendicular to the T2,3 complex (T numbers refer to molar cusps). Among species of Pseudomys, its teeth most closely resemble those of the living Ash-grey Mouse, Pseudomys albocinereus.

Habitat

The Chinchilla area during the Pliocene was probably open eucalypt woodland, with a seasonal climate. The late Pliocene was generally cooler than at present, marked by increasing aridity and the spread of grasslands.

Distribution

Pseudomys vandycki is only known from the Chinchilla fossil site (Chinchilla Rifle Range) in central Queensland.

Feeding and diet

Living species of Pseudomys feed on seeds (including the seeds of native grasses), other vegetation (shoots, leaves, rhizomes, berries, nuts and flowers), fungi, insects and even (in the case of the rare Shark Bay Mouse, P. fieldi), spiders. Some species switch their diets seasonally depending on availability of certain food items like seeds or insects, and some of the more arid-adapted species can survive on little or no free water for extended periods of time.

Life history cycle

Murid rodents are the most widespread and successful of any family of mammals, occurring world-wide in diverse habitats. Pseudomys species are found in a variety of habitats, including many semi-arid or arid zone regions, although they are absent from rainforest environments.

Pseudomys species reach sexual maturity at an early age, as do all rodents, and females may reproduce many times over the course of their short lives. Gestation is short, the young are born in a precocial state, develop rapidly and are weaned at an early age. Rodents have always been prey for a variety of predators. Pseudomys are taken by snakes, owls, dingos and other predators. Like mice in many parts of the world, populations of some species of Pseudomys can experience large fluctuations in size, increasing rapidly after fire or good rainfall and crashing when conditions become less favourable. Some species of Pseudomys are in fact fire-specialists, colonising freshly burnt areas in the early stages of vegetational succession.

Most species of Pseudomys construct borrows and/or nests, and many live in family groups or are otherwise social, sharing burrows (in one instance, over twenty mice shared a single burrow). Some species of Pseudomys, the pebble-mound mice, build nests of pebbles that can cover from one-half to nearly one metre, and can carry pebbles or stones weighing over half their body weight. Pseudomys are mainly nocturnal, spending the day in their nests or burrows.

Fossils description

The holotype pf Pseudomys vandycki is a right upper jaw fragment (maxilla) with M1/ and M2/ in place, collected at the Chinchilla Rifle Range at Chinchilla in southeastern Queensland in 1983. It is held by the Queensland Museum, Brisbane.

Pseudomys vandycki is one of only two described fossil Australian rodent species, the other being Zyzomys rackhami, from the Pliocene of Riversleigh, northwestern Queensland. Several new rodent genera have been discovered in Australia in recent years, suggesting much greater diversity; however, these new taxa are as yet undescribed. Rodent incisors are common at many fossil sites of Pliocene age or younger. These are not diagnostic, however, other than to record the presence of rodents at a fossil site.

Evolutionary relationships

Murids (Old World rats and mice) are the largest family of mammals, with over 1100 species in about 267 genera, and probably originated in Asia. Murinae, a huge but poorly known subfamily, is taxonomically problematic and currently in need of revision. Murines probably also evolved within Asia; the oldest murine, Antemus chinjiensis, is from southern Asia and is about 13.5 million years old.

Historically, native Australian rodents (all murines) were divided into 'old endemics', descended from the first wave of immigrants postulated to have arrived during the Pliocene, and 'new endemics' (species of Rattus that arrived in Australia later, in the Pleistocene). In this scenario, both groups were descended from a small number of colonists, perhaps just a single species. However, the history of rodents in Australia may be more complex. That Pseudomys vandycki belongs to a living, endemic Australian genus suggests that the Australian murine radiation was well underway by the late Pliocene and that the first rodents must have reached Australia much earlier. This is supported by molecular evidence, which proposes an early divergence of major Australian rodent groups. There is also evidence that there were multiple waves of immigration into New Guinea, the probable source for Australian rodents, and there may also have been multiple arrivals into Australia.

Rodents may have reached Australasia between 9-7 million years ago (late Miocene) and, after diversification in New Guinea, the first immigrants (ancestral 'conilurans') to Australia may have arrived between 6 and 5 million years ago (latest Miocene to early Pliocene), when sea levels were relatively low and island groups were emergent. All Australian murines are almost certainly derived from New Guinea stock. The 'conilurins' - the various lineages of the Mesembriomys group that include Pseudomys- appear to have evolved from a single immigrant line from New Guinea that arrived in Australia in the late Miocene or early Pliocene. Australian murines, along with some genera from South-East Asian islands, may be an early radiation of murines distinct from later mainland Asian groups. However, Australian murines are not closely related to these South-East Asian murines, and resemblances are considered to be convergences.

Notomys (hopping-mice) is grouped together with Pseudomys as a species complex, possibly sharing a common ancestor. Pseudomys is found almost exclusively in Australia, although one species, P. delicatulus also occurs in New Guinea.

Sadly some of Australia's unique rodents have either become extinct since European settlement (a total of seven species, including hopping-mice, one other mouse species, one tree rat, and one stick-nest rat) or are currently threatened by loss of habitat (at least a further dozen species). One third of Australia's native mammals have become extinct since European settlement, and fully one half of these are rodents. Like many Australian native rodents, particularly from arid or semi-arid areas, some species of Pseudomys are rare, threatened or even extinct (the Basalt Plains Mouse from western Victoria).

Note on classification: Murinae, as mentioned, is currently under revision. Hydromyinae may or may not remain as a subfamily of Muridae but as currently proposed, it is considered a tribe (Hydromyini) within Murinae, which is followed in this account (from Aplin 2006).

References

• Aplin, K. P. 2006. Chapter 31. Ten million years of rodent evolution in Australasia: phylogenetic evidence and a speculative historical biogeography. Pp. 707-744 in Merrick, J. R., Archer, M., Hickey, G. M. and Lee, M. S. Y. (eds) Evolution and Biogeography of Australasian Vertebrates. Australian Scientific Publishing, Oatlands.
• Godthelp, H. 1989. Pseudomys vandycki, a Tertiary murid from Australia. Memoirs of the Queensland Museum 28, 171-173.
• Jacobs, L. L. 1978. Fossil rodents (Rhizomyidae and Muridae) from Neogene Siwalik deposits, Pakistan. Bulletin of the Museum of Northern Arizona 52, 1-103.
• Michaux, J., Reyes, A. and Catzeflis, F. 2001. Evolutionary history of the most speciose mammals: molecular phylogeny of muroid rodents. Molecular Biology and Evolution 17, 280-293.
• Musser, G. G. and Carleton, M. D. 2005. Family Muridae. Pp. 501-755 in Wilson, D. E. and Reeder, D. M. (eds). Mammal Species of the World: A Taxonomic and Geographic Reference. Smithsonian Institution Press, Washington, D.C.
• Strahan, R. (ed). 1998. The Mammals of Australia. Australian Museum/Reed Holland, Sydney.

Further reading

• Breed, B. and Ford, F. 2007. Native Mice and Rats. Australian Natural History Series, CSIRO Publishing, Canberra.