Earless water rat

Species of rodent
Speciesbox
| name = Earless water rat
| status = LC
| status system = IUCN3.1
| status ref = It is still unclear to which species this group is related. It is one of the most aquatically adapted rodents of the world.

This animal was first described in 1907 by the British mammalogist Oldfield Thomas, based on a single specimen caught by one Mr. C. A. W. Monckton, after whom the species was named, near Brown River, Central Province, south east Papua New Guinea. Only in July 1950 a second specimen was captured. Since then, several other examples have been caught in the mountains of eastern New Guinea, but the earless water rat remains a rather rare species. The scientific name means "Monckton's fringed mouse", which refers to the collector of the original specimen (C. A. W. Monckton) and to the fringe of hairs on the tail.

The German mammalogist Hans Rümmler placed this rodent (and Parahydromys asper) in Hydromys, but that has not been accepted generally. Lidicker (1968), who studies the morphology of the phallus of New Guinea rodents, speculated that Crossomys might not be as closely related to Hydromys as was then generally thought. Later on, this was supported by the immunological study of Watts & Baverstock (1994). This study placed Crossomys closer to Leptomys, Pseudohydromys and Xeromys than to Hydromys. The American mammalogists Guy Musser and Michael Carleton, in their contribution to the authoritative Mammal Species of the World (3rd ed. ), divided the group of murine rodents that had before been called "Hydromyinae" or "Hydromyini" in two "divisions": the Xeromys Division (Leptomys, Pseudohydromys and Xeromys) and the Hydromys Division (Crossomys, Hydromys, Microhydromys, Paraleptomys and Parahydromys [Baiyankamys was added later

The back is greyish brown, and the belly is white. The fur is soft and waterproof. The top half of the tail is light grey, and the bottom half is white. The forefeet and claws are very small, but the hindfeet are very large. The upper lip contains a row of short, strong brushes, which may be used for rasping. The external ear (the pinna) is reduced to a small oval that does not or hardly protract above the fur. It is possible that the ear canal can be closed. The brain is rather large, like in many aquatic animals. The nasals are relatively small, just like the palate and the molars. The bullae are small. The rostrum is narrow. The earless water rat is a medium sized rat, about as large as its close relative Baiyankamys. The head body length is (based on four specimens), tail length is (4), hind foot length is (4), ear length is (2), and weight 165 g (5.8 ounce avoirdupois) (1).