Estimated to be about 541 million years old, the type species, A. velamenus demonstrates some anatomical characters reminiscent of vetulicolians, predating the first undisputed member of this clade by 20 million years.

[1] The holotype fossil of Alienum was found in the Dengying Formation of South China and described in 2024.

The specific name velamenus is derived from the Latin word velamen, meaning "veil/sail-like", relating to the sail-like shape of the anterior of the body.

Along the length of the body there is a 9 mm (0.4 in) long stripe/groove, which can be compared to the pharyngeal groove structure of vetulicolians, although in the case of Alienum, this extends diagonally backwards, whilst in vetulicolians, this is an axial groove.

The posterior body features a stalk-like structure, around 5 mm (0.2 in) in length, similar to a tail.