Originally identified as a variety of walking fern (Asplenium rhizophyllum), it was classified as a separate species by Thomas Nuttall in 1818.

[2] The rhizome is about 1 millimeter (0.04 in) in diameter, covered with narrowly triangular scales which are dark reddish-brown or blackish in color, and strongly clathrate (bearing a lattice-like pattern).

It is covered in narrowly triangular, dark reddish-brown scales at the base, which diminish into hairs in the upper part of the stipe.

[3] The tip of the blade sometimes develops a swelling which may differentiate into a proliferous bud and, very rarely, into a plantlet, as in walking ferns.

[2] While no named varieties or forms of A. pinnatifidum have been described, an unusual population was described from Giant City State Park in southern Illinois in 1956.

In it, the leaf blade was highly reduced, barely exceeding the rachis, except for a series of stubby projections under which the sori were borne.

[7] Individual plants have also been known on occasion to develop forked leaves, which appears to be a developmental accident rather than a stable genetically-controlled trait.

In comparison, however, A. pinnatifidum is distinctly lobed when mature, tends to have longer stipes in proportion to its leaf size, and has a more upright habit.

[3] It might be confused with Countess Dalhousie's spleenwort (A. dalhousiae), of Asia and the American Southeast, but the latter has short, dull stipes with larger, toothed scales.

In addition, the basal pinnae, which may themselves be pinnatifid, lack a stalk, the leaf blade is pointed at the tip but not drawn out at length, and there are generally fewer fronds.

× kentuckiense is also fully pinnate towards the base of the blade, with four to six pairs of pinnae, and the brown color of its stipe extends up into the basal part of the rachis.

Oliver A. Farwell, observing an unusual specimen of A. pinnatifidum, was led to suggest that the species might be a hybrid between American walking fern, Camptosorus rhizophyllus (now A. rhizophyllum), and ebony spleenwort (A. platyneuron).

[15] He was correct in viewing A. pinnatifidum as a hybrid descendant of A. rhizophyllum, but incorrect in identifying the other parent, and his suggestion was not widely taken up in the literature.

[17] In 1951, Herb Wagner, while reviewing Irene Manton's Problems of Cytology and Evolution in the Pteridophyta, suggested in passing that A. pinnatifidum itself might represent a hybrid between A. montanum and A. rhizophyllum.

As A. pinnatifidum proved to be a tetraploid while A. montanum was a diploid, a hybrid between them would be a triploid, and Wagner showed that this was in fact the case for A. × trudellii.

[19] His further experiments, published the following year, strongly suggested that A. pinnatifidum is an allotetraploid, the product of hybridization between A. montanum and A. rhizophyllum to form a sterile diploid, followed by chromosome doubling that restored fertility.

[23] In 1985, an allozyme analysis confirmed the hybrid parentage of the species,[24] and revealed that A. pinnatifidum had probably originated independently through chromosome doubling at more than one locality.

[26] The ICBN's rules were relaxed in 1972, and in 1974, John Mickel published Asplenosorus pinnatifidus as a new combination for the species to allow the continued recognition of Camptosorus.

[27] Since then, phylogenetic studies have shown that Camptosorus nests within Asplenium,[28][29] and current treatments do not recognize it as a separate genus.

A sterile triploid hybrid, formed by the crossing of A. pinnatifidum with a diploid cytotype of maidenhair spleenwort (A. trichomanes ssp.

americanum) yielded peculiar specimens with a long blade, similar in texture and doubled indusia to the hart's-tongue fern, but lengthened and tapering to a point, and not lobed except for two surprisingly large auricles at the base.

[32] Native to eastern North America, A. pinnatifidum occurs in the middle and southern Appalachian Mountains, from Pennsylvania and New Jersey southwest to Alabama and the northeastern corner of Mississippi.

It is also found in the Shawnee Hills and to some extent in the Ozarks, with outlying occurrences in southeastern Oklahoma and in Iowa County, Wisconsin.