Ingestion in toxic amounts causes severe liver damage with vomiting, diarrhea, hypothermia, and eventual death if not treated rapidly.

[3] Agaricus autumnalis was described by Charles Horton Peck in 1873, and later moved to Galerina by A. H. Smith and Rolf Singer in their 1962 worldwide monograph on that genus.

Because of differences in ecology, fruit body color and spore size combined with inadequate sampling, the authors preferred to maintain G. pseudomycenopsis as a distinct species.

[7] In the fourth edition (1986) of Singer's comprehensive classification of the Agaricales, G. marginata is the type species of Galerina section Naucoriopsis, a subdivision first defined by French mycologist Robert Kühner in 1935.

[8] It includes small brown-spored mushrooms characterized by cap edges initially curved inwards, fruit bodies resembling Pholiota or Naucoria[9] and thin-walled, obtuse or acute-ended pleurocystidia that are not rounded at the top.

[10] However, as Gulden explains, this characteristic is highly variable: "Viscidity is a notoriously difficult character to assess because it varies with the age of the fruitbody and the weather conditions during its development.

[20] The cap surface remains smooth and changes colors with humidity (hygrophanous), pale to dark ochraceous tawny over the disc and yellow-ochraceous on the margin (at least when young), but fading to dull tan or darker when dry.

The flesh is pale brownish ochraceous to nearly white, thin and pliant, with an odor and taste varying from very slightly to strongly like flour (farinaceous).

The stem ranges from 2 to 8 cm (3⁄4 to 3+1⁄8 in) long,[17] 3–9 mm thick at the apex, and stays equal in width throughout or is slightly enlarged downward.

Its color is initially whitish or light brown, but usually appears a darker rusty-brown in mature specimens that have dropped spores on it.

The basidia are four-spored (rarely with a very few two-spored ones), roughly cylindrical when producing spores, but with a slightly tapered base, and measure 21–29 by 5–8.4 μm.

The cheilocystidia (cystidia on the gill edges) are similar in shape but often smaller than the pleurocystidia, abundant, with no club-shaped or abruptly tapering (mucronate) cells present.

[24] One source notes "Often, G. marginata bears an astonishing resemblance to this fungus, and it requires careful and acute powers of observation to distinguish the poisonous one from the edible one.

"[13] K. mutabilis may be distinguished by the presence of scales on the stem below the ring, the larger cap, which may reach a diameter of 6 cm (2+3⁄8 in), and spicy or aromatic odor of the flesh.

[25] A rough resemblance has also been noted with the edible Hypholoma capnoides,[13] the 'magic' mushroom Psilocybe subaeruginosa, as well as Conocybe filaris, another poisonous amatoxin-containing species.

It is known to have most of the major classes of secreted enzymes that dissolve plant cell wall polysaccharides, and has been used as a model saprobe in recent studies of ectomycorrhizal fungi.

[16] Galerina marginata is widely distributed throughout the Northern Hemisphere, found in North America, Europe, Japan, Iran,[29] continental Asia, and the Caucasus.

Amatoxins belong to a family of bicyclic octapeptide derivatives composed of an amino acid ring bridged by a sulfur atom and characterized by differences in their side groups; these compounds are responsible for more than 90% of fatal mushroom poisonings in humans.

Beyond these symptoms, toxins severely affect the liver which results in gastrointestinal bleeding, a coma, kidney failure, or even death, usually within seven days of consumption.

[39] The ability of the fungus to produce these toxins was confirmed by growing the mycelium as a liquid culture (only trace amounts of β-amanitin were found).

[43] Later experiments confirmed the occurrence of γ-amanitin and β-amanitin in German specimens of G. autumnalis and G. marginata and revealed the presence of the three amanitins in the fruit bodies of G. unicolor.

[46] Based on this value, the ingestion of 10 G. marginata fruit bodies containing about 250 μg of amanitins per gram of fresh tissue could poison a child weighing approximately 20 kilograms (44 lb).