The type species, named by Edward Drinker Cope in 1876,[1] is Paronychodon lacustris, from the Judith River Formation of Montana, dating to 75 million years ago, during the Campanian stage.

It is a tooth about one centimetre long, elongated, recurved, lacking serrations, possessing low vertical ridges and with a D-shaped cross-section, the inner side being flattened.

[2] A second species, Paronychodon caperatus, is known from the Hell Creek Formation of North Dakota, Montana, and South Dakota and Lance Formation of Wyoming (latest Maastrichtian stage, 66 million years ago) and was originally referred to the mammal genus Tripriodon by Othniel Charles Marsh in 1889,[3] but placed in Paronychodon by George Olshevsky in 1991.

These teeth of the general "Paronychodon" type have been reported from a wide variety of times and places, including the Early Cretaceous Una Formation of Spain, dating to the late Barremian age 125 million years ago.

Philip J. Currie in 1990 also concluded to a malformation, thinking the flattened side resulted from the tooth remaining attached too long to the inner wall of the tooth-socket.