[2] Found in Veracruz, Mexico, it was formally described as a new species in 2002 by the lichenologists María de los Ángeles Herrera-Campos and Robert Lücking.

[1] The lichen forms small patches that are rounded to irregular in outline, 0.3 to 1 mm in diameter, smooth to silvery white in colour, and bordered by a distinct black line.

The dark line appears to be a prothallus but is actually created by the intensely pigmented outer cells of the thallus cortex.

The photobiont partner of Asterothyrium atromarginatum is a species of Trebouxia (a genus of green algae); their cells are more or less spherical and measure 6–8 μm in diameter.

Asterothyrium microsporum is similar in morphology to A. atromarginatum, but its thallus lacks the black bordering line.