[3] 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.

[5] 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.

[6] The German mammalogist Hans Rümmler placed this rodent (and Parahydromys asper) in Hydromys,[7] 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.

The American mammalogists Guy Musser and Michael Carleton, in their contribution to the authoritative Mammal Species of the World (3rd ed.

They supported their opinion with an unpublished study of the Australian biologist Ken Aplin, who also placed Crossomys closer to Hydromys.

According to his data, the Crossomys-Baiyankamys group is most closely related to Hydromys and Parahydromys, though he did not give material to support his opinion.

In all these characters, it resembles the elegant water shrew (Nectogale elegans), a good example of convergent evolution.

[2] This animal lives at 1000 to 2700 meters in elevation in Central Cordillera of Papua New Guinea, including the Huon peninsula.

Some local names can be translated as "water sugar glider", which refers to the similarity of the fur of these two species.