The Australian Rodent Fauna, Flotilla's, Flotsam Or Just Fleet Footed?

Home , Chiruromys, Leggadina, Little native mouse, Mallomys, Zyzomys

The Australian rodent fauna, flotilla's, flotsam or just fleet footed?

Ho Godthelp Schoolof Biological Science. University of New South Wales. Sydney. 2052

ABSTRACT: Until recently the lack of fossil evidence has lead to a complete reliance on molecular data for hypotheses relating to the origin, evolution and paleobiogeography of the Aus- tralian rodents. This has inevitably lead to a simplistic view of these processes. Data from newly available fossil material allow us to review the existing hypotheses and propose alternative views. Fossils show us that the evolutionary process in Australia has been complex with at least three phases of immigration, two from SE Asia directly and one from New Guinea. The new in- formation allows us to make comparisons to known geological and biological events in the SE Asian region and see the Australian rodent radiation in a wider perspective.

In Australia, the "land of the marsupial", about 25% of modern land mammal species are murid rodents. Rodents have spread into all the ecosystems of Australia and are frequently the most abundant and conspicuous mammal present. All this in less than 6 million years of occupancy. But then rodents are nature's over-achievers and we shouldn't be surprised that they have suc- ceeded in Australia as they have everywhere else in the world. And yet even with this success and the numeric dominance of rodents in the Pleistocene and Quaternary vertebrate accumulations in Australia and their abundance in the contemporary envi- ronment we have an appallingly sparse fossil record for them. Of the 70+ living species nearly half of those have no fossil record and so far there are only two extinct species (Pseudomys vandycki Godthelp. 1988 and Zyzomys rackhami, Godthelp1996.) described from Australia, both from Pliocene sites in Queensland. A consequence of the paucity of rodent fossils has been a near total reliance on molecular techniques in attempts to understand the evolution of Australia's rodent fauna. This reliance has lead, in the past, to what might be considered a simplistic view of the nature of the earliest evolutionary events that have shaped our rodent fauna. Other difficulties encountered in the palaeontological study of rodents are the fragmentary nature of the material available and the inevitable reliance on dental characters that are some- times prone to convergences and parallelisms. Despite this, the single defining synapomorphy for the murinae is a dental feature (the presence of the anterio-lingual cusp Tl on the first upper molar) and dental characters are often the only ones available. Watts and Baverstock (1992,1994) using microcompliment fixation of albumin techniques have determined the Australo-New Guinean radiation to consist of 3 clades. The identification of these clades, the Hydromys, Lorentzimys and Anisomys clades prompted these authors to propose a classification containing 2 tribes. These tribes are the Hydromiini, containing the Austra- lian genera plus some of the New Guinean genera including Melomys and Uromys, and the Ani- somyini for the genera Anisomys, Chiruromys. Hyomys. Macruromys, Mallomys and Pogonomys. Within the Australian clade the most provocative grouping is that of the Mesem- briomys/Zyzomys group and the Melomys/Uromys group. Both groups have the most derived dentitions in the Australian radiation but in ways that are radically different from each other. Recent work involving amplified repetitive DNA (Pascale et al. 1990) has identified an ele- ment of the genome called Lx, which appears to be restricted to members of the true Murinae. This synapomorphy may define a monophyletic group of the Indo-Australian and African Muri- nae. Evidence of an extraordinary evolutionary event involving the rapid rise of the most suc- cessful mammalian group and the subsequent invasion of three continents. The rich and diverse fauna being recovered from the Early Pliocene Rackham's Roost Local Fauna at Riverslei~h is providin~ us with our best view of the early evolution of rodents in 320 H. GODTHELP

Australia. To date there are more than 12 taxa of rodents represented in the fauna. Among them are members of the genera Leggadina, Zyzomys and a number of species of Pseudomys. Some of the species of Pseudomys can be assigned to a range ofspecies groups present in the modern fauna which I propose to be monophyletic, on the basis of dental characteristics. These group- ings are Pseudomys gracilicaudatus-P. nanus, P. australis-P. desertor-P. shortridgei, P. deli- catulus and P. albocinereus-P. hermannsburgensis-P. novaehollandiae-P. pumilis-P. pilligaen- sis. There are also Psuedomys species present that I cannot assign to a particular species grouping. Additionally there are several new genera that show no close affinities to modern forms. Per- haps the greatest surprise has been the discovery of a Potwarmus-like "Dendromurine" in the deposit. This is to date the latest surviving member known of this primitive group that does not seem to have survived competition with the more derived murines elsewhere in Asia. Another taxon in the fauna that may have its affinities nearer to Asian murines seems to have affinities with the widespread, primitive Chiropodomys group from South East Asia. The diversity of endemic murine genera in the Rackham's Roost assemblage suggests one of two things. Murids invaded Australia much earlier than Rackham's Roost time to enable evolution of these distinctive groups on this continent. Alternatively, murids may have entered Aus- tralia already differentiated (somewhere in southeastern Asia) into at least this many groups. Considering the first hypothesis, the potential for a much earlier origin is limited by the total ab- sence of rodents in the late Miocene Alcoota and Ongeva Local Faunas of the Northern Terri- tory (8 and 7 my respectively; e.g. Megirian et al. 1994), despite years of collecting at those sites. This does not rule out a time of origin between latest late Miocene and early Pliocene (7 to 5 million years ago), a time of climatic crisis in Australia and elsewhere (Archer et al. 1995). At present, however, there is no way to test these alternative scenarios. The questions of how and when rodents entered Australia have been asked many times and there is a long list of hypotheses as a consequence. Ellerman ( 1941) suggested that murid rodents actually evolved in Australia on the basis of the known diversity .Most others, Tate (1951), Simpson (1961), Wood (1983), Watts and Aslin (1981), to name just a few, have however proposed a South East Asian source. The hypotheses concerning the timing and number of incursions into the continent have varied much more widely, with entry ranging from about 15 million years ago (Watts and Aslin, 1981) too less than one million. Most modern researchers especially those utilizing molecular data support a single major dispersal into Australia with New Guinea as a staging point, all derived from a single ancestor. Few of these researchers have had the benefit of data from the fossil record in Australia. The fossil record indicates three major periods of evolution in the rodent fauna of Australia. The first, an initial burst of speciation, at or about the beginning of the Pliocene. The next occurred near the end of the Pliocene when many of the modern species emerged and Rattus first entered the biota and finally during the later part of the Pleistocene when there was an inter- change between Australia and New Guinea. These immigrations could only have occurred during the brief interludes when there were either land connections or when the distances between landmasses in the region were significantly shorter than they are today. Work by Hall ( 1998) presents a series of tectonic reconstructions of the region over a wide temporal range. These show that on the basis of current understanding the most plausible configuration of the landmasses for terrestrial mammal migrations occurred at about 7mya. Additionally it is from this time on that the ocean currents (Kennett et al. 1985) become favorable for short water dispersals by rafting or swimming. Prior to this period rafts would have been taken out into the Indian Ocean rather than towards land. Species of Rattus in the Plio-Pliestocene deposits at Floraville, northern Queensland, are the earliest record of the genus in Australia. This record post dates an explosion of diversity in southeastern Asia (Chaimanee and Jaeger 1999). It seems probable that the Floraville Rattus lineage is part of this Southeast Asian adaptive radiation. Other endemic Australian species of Rattus appear, in contrast, to be descendants of a second wave of immigrants that arrived from New Guinea. No representatives of this group are known from pre-PI~istocene deposits any- where in Australia. The presence in the fossil record of taxa with strong Asian affinities persuade me that there is a more complex sequence of events than a single "waif' dispersal or even dispersal through a Torresean land bridge that has brought rodents into Australia. Data from Thailand regarding the THE AUSTRALIAN RODENT FAUNA 321 timing and effect of the appearance of Rattus into that area closely mirror the emerging story of the history of that group in Australia. The similarity of the sequence of events and the nature of the taxa involved suggest that little filtration occurred and that movement through the region at that time was relatively unimpeded. One is tempted to ask why other groups of mammals especially other rodent families had not then also made their way to Australia? Perhaps they did and merely failed to survive, any other group would have found themselves competing directly with a well-established and vigorous marsupial fauna not yet suffering from the ravages of the Pleis- tocene aridity and the depravations of human intervention. Only the discovery of Tertiary aged rodent fossils from the Indonesian region will enable us to delve into the earliest origins of Australian rodents.

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