Crossomys Thomas, 1907

Helgen, Kristofer M., Leary, Tanya & Aplin, Kenneth P., 2010, A Review of Microhydromys (Rodentia: Murinae), with Description of a New Species from Southern New Guinea, American Museum Novitates 3676, pp. 1-22 : 1-2

Poche, 1906, Crossomys Thomas, 1907 , Baiyankamys Hinton, 1943, Xeromys Thomas, 1889 , Leptomys Thomas, 1897 , Paraleptomys Tate and Archbold, 1941 , Pseudohydromys Rümmler, 1934 , and Microhydromys Tate and

1 Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History; Division of Mammals , National Museum of Natural History , Smithsonian Institution, Washington, D.C. 20013-7012; Vertebrate Zoology, Bernice P. Bishop Museum, Honolulu, Hawaii ([e-mail]) .

2 Parks and Wildlife Division, Department of Environment, Climate Change and Water, G.P.O. Box 95, Parramatta, NSW 2124, Australia; World Wide Fund for Nature, Kikori Integrated Conservation and Development Project, Moro, Southern Highlands Province, Papua New Guinea.

3 Division of Vertebrate Zoology (Mammalogy), American Museum of Natural History; and Australian National Wildlife Collection, CSIRO Division of Sustainable Ecosystems, Canberra, ACT 2601 Australia.

Archbold, 1941). All of these genera occur on the island of New Guinea, with Hydromys and Xeromys found also in Australia ( Flannery, 1995; Hitchcock, 1998). Though they span a great range of body sizes, habits, and ecologies, because of similar derivations in craniodental anatomy they have long been regarded as a natural (monophyletic) group (e.g., Rümmler, 1934, 1938; Tate, 1951; Laurie and Hill, 1954; Lidicker, 1968, 1973; Menzies and Dennis, 1979; Flannery, 1995; Helgen, 2005b; cf. Breed and Aplin, 1994; Watts and Baverstock, 1994, 1996), a point recently confirmed by a molecular genetic study of mitochondrial and nuclear gene sequence data from a broad array of Old World murine rodents ( Rowe et al., 2008). As such, these genera together constitute an outstanding endemic Australo-Papuan adaptive radiation that has received minimal study. The appropriate taxonomic rank of this group of rodents requires further consideration; under current classification schemes, this group could be recognized under a single ‘‘Division’’ (sensu Musser and Carleton, 2005) or perhaps as a subtribe, Hydromyina, within a more expansive tribe of Australo-Papuan rodents classified as the Hydromyini ( Breed and Aplin, 1994; Watts and Baverstock, 1994, 1996; Rowe et al., 2008). Here we employ the subtribal descriptor ‘‘hydromyinan’’ as a precise (if cumbersome) designation for members of this diverse but monophyletic suite of rats and mice.

With a few honorable exceptions ( Mahoney, 1968; Musser and Piik, 1982; Flannery, 1989), very little taxonomic work has been focused on hydromyinans in the half century since the publication of Tate’s (1951) magnum opus on the systematics of Australo- Papuan murines and the landmark review of Wallacean and Melanesian mammal systematics published by Laurie and Hill (1954). This is now changing. Recent publications have provided comprehensive taxonomic reviews of Baiyankamys ( Helgen, 2005b) , Leptomys ( Musser et al., 2008) , and Pseudohydromys ( Helgen and Helgen, 2009; including the nominal genera Mayermys Laurie and Hill, 1954 and Neohydromys Laurie, 1952 —see Musser and Carleton, 2005), and revisions of Paraleptomys and Hydromys are in progress by Helgen and collaborators. The number of hydromyinan species documented in these revisionary works more than doubles the number of species recognized by current faunal reviews and taxonomic compendia (i.e., Flannery, 1995; Musser and Carleton, 2005). The present contribution is a revision of Microhydromys Tate and Archbold, 1941 , the aim of which is to document the diagnostic attributes, taxonomic content, and geographic distribution of the genus.

Two species have previously been recognized in Microhydromys — M. richardsoni Tate and Archbold, 1941 , and M. musseri Flannery, 1989 (Flannery, 1990, 1995; Musser and Carleton, 1993, 2005). The type species of Microhydromys , M. richardsoni , was described on the basis of a single specimen collected in the Mamberamo Basin, on the northern slopes of New Guinea’s expansive Central Cordillera, during the Third Archbold Expedition to New Guinea ( Tate and Archbold, 1941; Archbold et al., 1942). Three decades would pass before any additional samples referable to M. richardsoni were collected, and no specimens apart from the holotype were discussed in the literature until 1979, when Menzies and Dennis (1979: 55), after a review of global museum holdings, observed that ‘‘the very few records of this species are all in hill forest ranging from Sogeri in the SE to central Irian Jaya, but at lower altitudes than the other ‘moss mice,’ ’’ and George (1979: 95) wrote that ‘‘ Microhydromys richardsoni has only been collected on three occasions from widely separated localities in lowland New Guinea.’’

Flannery (1989) provided the only revision of the genus Microhydromys to date, produced within the context of describing a new species, Microhydromys musseri . In his review of the genus, Flannery (1989) discussed the single known specimen of M. musseri and documented a total of four museum specimens referable to M. richardsoni ; in a later publication (his revised Mammals of New Guinea), he discussed one additional specimen referred to M. richardsoni ( Flannery, 1995) .

We have recently revisited and studied all museum specimens previously referred to Microhydromys , collected one additional modern specimen of Microhydromys in southern Papua New Guinea, and discovered a subfossil jaw referable to Microhydromys in a Holocene cave deposit in northern Papua New Guinea. We draw on this material to define and diagnose the genus Microhydromys , argue that ‘‘ Microhydromys ’’ musseri is more appropriately referred to the genus Pseudohydromys , and describe a new species of Microhydromys . Our contribution is purely alpha taxonomic in scope; pending ongoing molecular studies, we do not defend or review the precise phylogenetic relationships of Microhydromys among hydromyinans and within the broader context of endemic of Australo-Papuan murines ( Rowe et al., 2008). Instead, we focus on documenting relevant morphological, environmental, and distributional aspects of the genus. This is our intention for Microhydromys , just as it was in recent generic-level overviews of Baiyankamys ( Helgen, 2005b) , Leptomys ( Musser et al., 2008) , and Pseudohydromys ( Helgen and Helgen, 2009) , and will be for future reports. In our view, this approach takes priority at the present time because it is this taxic level that is currently most underestimated in New Guinea mammals ( Helgen, 2007), and because taxonomic overviews are ideally requisite to direct future studies aimed toward better discerning, refining, and testing mammalian patterns of historical biogeography, phylogenetic and ecomorphological diversification, and conservation prioritization in the Melanesian region (e.g., Flannery, 1995; Heads, 2001, 2002, Amori et al., 2008; Schipper et al., 2008).