The species epithet refers to its characteristic caudate ascospores, i.e., tapering and elongated, with a tail-like extension at one end.

[1] Trichothelium caudatum has a continuous, smooth thallus that is pale yellowish-green in colour, and can grow up to 2 cm (0.8 in) across.

The lichen is characterized by its 7 to 15 perithecial setae, arranged in 3 to 6 irregular groups forming a horizontal crown.

The setae are bristle-shaped to narrowly acute or lanceolate, rarely fin-shaped, and measure 0.25 to 0.40 mm in length.

Ascospores are distinctly tapering to clavate-caudate with a tail-like proximal end, 9–13-septate, colourless, and measure 55–75 by 5–8 μm.