[1] This lichen stands out from its counterparts within its genus, most notably Thalloloma haemographum, due to its corticate thallus and the presence of norstictic acid.

The type specimen was found by the first author in the Barrio Jiménez area (Río Grande, Puerto Rico), on the trunk of a Cecropia peltata tree, at an altitude of 917 m (3,009 ft).

The ascomata are lirellate, meaning they are elongated and slit-like, with a purplish-brown disc and a distinct, bright red margin.

However, T. rubromarginatum is unique in genus Thalloloma due to its production of norstictic acid and the presence of a distinct cortex.

It has been observed growing on the upper branches of a fallen Cecropia peltata tree, which suggests it might favour such environments.