[3] This diploid genus (2n = 56) may be ancestral to the two unusual suspected tetraploid species Tympanoctomys barrerae and Pipanacoctomys aureus.

[4] However, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggests that amplification and dispersion of repetitive sequences best explain the large genome size.

[5] The mountain viscacha rat is a rat-like animal with a head-body length of about 11 to 18 centimetres (4.3 to 7.1 in) and weighing from 85 to 121 grams (3.0 to 4.3 oz).

[3] Unusual features of the viscacha rat include greatly enlarged auditory bullae, and the presence of numerous whiskers on the roof of the mouth behind the incisor teeth.

The exact function of the whiskers is unclear, but they may enable the animal to sense the position of food inside the mouth.