P. atropurpurea is an apogamous autotriploid, with 3n=87 (actual base number, n=29), and is one of the historical parents of the hybrid species complex, Pellaea glabella.

Apogamy, or the ability to reproduce non-sexually, is common among rock ferns in the Pteridaceae.

The upper pinnae are long, narrow, and undivided, while the lower ones are divided into 3–15 pinnules.

This plant may be distinguished from the similar Pellaea glabella by its larger form and by the presence of hairs on the leaf axis (the main leaf stem) and on the underside of the leaflet midribs, which are absent in Pellaea glabella (as reflected in its common name smooth cliffbrake).

It has become extinct in Louisiana since the limestone caprock of a salt dome at Winfield, the only location for the fern in the state, was quarried away.