Thorius pennatulus is a species of salamander in the family Plethodontidae and one of the smallest tetrapods, with adults reaching a length of about 15 to 21 millimetres (0.59 to 0.83 in).

[2] Thorius pennatulus has been found at several locations on the Gulf slope of the northern Sierra Madre de Oaxaca, between 1,000 and 1,500 meters elevation.

[1] It is a terrestrial species and during the day hides under rocks, in or under rotten logs or among leaf litter in banana plantations.

[1] At one time it was a very common species in the limited area of forest and plantation where it is found but it underwent a large, unexplained decrease in population so that by the mid-1970s it was uncommon and since then it has continued to decline.

There has been an increase in agricultural activities within its range but there is still plenty of good quality habitat remaining so its decline is a mystery.