Archaeomarasmius is an extinct genus of gilled fungus in the Agaricales family Tricholomataceae, containing the single species Archaeomarasmius leggetti. It is known from two fruit bodies recovered from amber, one consisting of a complete cap with a broken stem, the other consisting of a fragment of a cap. The cap has a diameter ranging from , while the stem is thick. Spores were also recovered from the amber, and are broadly ellipsoid to egg-shaped, measuring roughly 7.3 by 4.7 μm. The species, which resembles the extant genera Marasmius and Marasmiellus, is inferred to have been saprobic on plant litter or other forest debris. The genus is solely known from the New Jersey amber deposits along the Atlantic coastal plain in New Jersey, United States, which date from the Turonian stage (about 90–94 Mya) of the Upper Cretaceous. Archaeomarasmius is one of only five known agaric fungus species known in the fossil record, and the only one to be described from New Jersey amber. History and classification The genus is known only from the two holotype fossils, a fruit body (or mushroom) and a fragment of a mushroom, both currently residing in the American Museum of Natural History. The specimens, collected in November 1994 from the area of East Brunswick, New Jersey, by G.R. Case, P.D. Borodin, and J.J. Leggett, were found as a single clear yellow amber nodule in diameter. 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). Due to weathering, the amber specimen AMNH NJ-90 fractured into a number of chips along fractures and flow lines. The chips with the holotype specimens, AMNH NJ-90Y and AMNH NJ-90Z, were first studied by a group of researchers consisting of David Hibbett and Michael Donoghue from Harvard University with David Grimaldi of the AMNH. Hibbett and colleagues published their 1997 type description in the American Journal of Botany. The generic epithet Archaeomarasmius is a combination of the Greek archaeo- meaning "ancient" and "Marasmius", a modern genus which it resembles. 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. When first reported, Archaeomarasmius leggetti was the second extinct species of agaric fungus to be described, and it is the only species