The specimen was found above the South Amboy Fire Clay, part of the Raritan Formation, suggesting that it is Turonian in age (Upper Cretaceous, about 90 to 94 million years ago).

The specific epithet "leggetti" was coined by the authors in honor of J.J. Leggett and company, who first discovered the amber nodule and donated it to the AMNH.

[1][2] The extinct Agaricomycetes species Quatsinoporites cranhamii, found in marine calcareous concretions on Vancouver Island, Canada, and dating to about 130–125 Mya, is probably in the Hymenochaetales or the Polyporales.

The top of the pileus is exposed on a fracture plain, and to prevent oxidation, the area was coated in a fine layer of synthetic resin, which also resulted in slightly improved visibility of the mushroom.

Though the researchers had not intended to perform destructive analysis on the sample, this fracturing warranted the sacrifice of some parts of the specimen for structural and molecular study.

Small pieces of the specimen were mounted directly on scanning electron microscopy (SEM) stubs and sputter coated with a gold/palladium alloy.

It is possible that the fractures and flowlines which split AMNH NJ-90Z during initial preparation had already penetrated into the fossil and destroyed the hermetic seal which would have been needed for preserving the organic matter of the mushroom.

[1] The combined characters of Archaeomarasmius indicate a relation to the modern family Tricholomataceae, with a close similarity to the genera Marasmius and Marasmiellus.

Of the approximately forty insect inclusions present, flies in the families Ceratopogonidae and Chironomidae, together with caddisflies of the order Trichoptera, suggest that the mushroom was growing near fresh water.

Modern Marasmiaceae members are saprobic—obtaining nutrients by breaking down organic matter—and specimens included with Archaeomarasmius indicate a similar habit for the mushroom.