Fil. Spec. 59. 1841
Phenology: Sporulating summer–fall.
Habitat: Calcareous cliffs and rocky slopes, usually on limestone
Elevation: 100–2500 m
Ont., Que., Ala., Ariz., Ark., Colo., Conn., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Nebr., Nev., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., W.Va., Wis., Wyo., Mexico, Central America in Guatemala.
Contrary to D. B. Lellinger's (1985) hypothesis, isozyme data indicate that neither Pellaea glabella nor P. ternifolia was involved in the origin of this apogamous triploid. Instead, it appears that P. atropurpurea is an autopolyploid derivative of a single diploid taxon that has not yet been located. A thorough survey of spore number per sporangium in this species should be undertaken to determine whether the diploid progenitor is still extant. Collections from western Canada identified as P. atropurpurea actually represent P. gastonyi, an apogamous tetraploid produced by hybridization between P. atropurpurea and diploid populations of P. glabella. Pellaea atropurpurea has also hybridized with P. wrightiana; the hybrid is a rare apogamous pentaploid known only from western Oklahoma. Pellaea lyngholmii is the apogamous tetraploid hybrid between P. atropurpurea and P. truncata. Pellaea atropurpurea is distinguished from all these hybrids by having rachises that are densely pubescent adaxially, larger ultimate segments, and spores averaging less than 62 µm in diameter.