The oldest fossil mushroom

A new fossil mushroom is described and illustrated from the Lower Cretaceous Crato Formation of northeast Brazil. Gondwanagaricites magnificus gen. et sp. nov. is remarkable for its exceptional preservation as a mineralized replacement in laminated limestone, as all other fossil mushrooms are known from amber inclusions. Gondwanagaricites represents the oldest fossil mushroom to date and the first fossil mushroom from Gondwana.


Introduction
Exceptionally preserved fossils can shed important and unprecedented light on the history of life. Particularly remarkable deposits, known as Lagerstätten, yield fossils characterized by preservation of soft tissues that decay rapidly and which are not normally preserved. In many cases, large and important groups of soft-bodied organisms would be missing entirely from the fossil record if not for their exceptional preservation in Lagerstätten. Mushrooms, an ecologically important group of fungi in the order Agaricales, produce fleshy, gilled fruiting bodies (called basidiomes) that are rarely fossilized [1]. While certainly ancient, they have an extremely depauperate fossil record with only ten fossil mushrooms reported to date, all unique amber inclusions ranging from mid-Cretaceous to Early Miocene in age [2][3][4][5][6][7][8]. Here we report the discovery of a new fossil mushroom that is unique in its preservation as a mineralized replacement, and the oldest yet encountered. The specimen comes from the laminated limestones of the Crato Formation, which outcrop on the northern flanks of the Chapada do Araripe in Ceará, Brazil; a Lagerstätte famous for the exceptional preservation of its diverse Lower Cretaceous paleobiota [9][10][11].

Material and methods
The specimen comprises a single, nearly complete mushroom preserved as a primarily goethitic replacement on a small slab (approximately 50 × 60 mm) of typical, buff-colored, millimetrically-laminated limestone from the Nova Olinda Member; the lowermost unit of the Crato Formation. It is housed in the URM Herbarium at the Universidade Federal de Pernambuco in Recife, Brazil, having been repatriated from the Illinois Natural History Survey Paleontological Collection. It was studied using a Zeiss SteREO Discovery.V20 zoom stereomicroscope with a Plan-Apochromat S 0.63x f/ Reo WD = 81 mm objective. Photographs were taken using a Canon 5D Mark III and MP-E 65 mm macro lens mounted to a Cognisys Stackshot motor rail on a copy stand. Multiple high-resolution images were then stacked using Heli-conSoft's Helicon Focus 6 and subsequently stitched together as a mosaic using Photoshop CC. Scanning electron micrographs were produced using a JEOL JSM-6060LV SEM.

Nomenclature
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Systematic paleontology
Kingdom Etymology. The genus name is a combination of Gondwana, the ancient supercontinent, the Greek word agarikon, "a mushroom," and the Greek suffix -ites, denoting a fossil. The specific epithet is the Latin adjective magnificus, meaning "magnificent" or "splendid" in reference to the remarkable preservation of the holotype.
Comments. While Gondwanagaricites is without doubt a gilled mushroom in the Agaricales, familial placement is presently impossible since no evidence of basidiospores was found during SEM examination of the specimen. The general habitus of Gondwanagaricites is reminiscent of mushrooms in the family Strophariaceae and placement in this family would be supported by the small size and robust shape of the overall basidiome, the thick context of the pileus, the putative complete attachment of the gills to the central stipe, and the apparent absence of a universal and partial veil. However, a number of other mushroom families present similar basidiome morphology (e.g., Agaricaceae, Tricholomataceae, Bolbitiaceae, etc.) and can only be separated by detailed studies of basidiospore shape, ornamentation, and coloration. Thus, since the spores of Gondwanagaricites were not observed, we refrain from assigning the new genus to a family.
Gondwanagaricites magnificus represents the oldest fossil record of a gilled mushroom and is the only fossil mushroom known from a mineralized replacement. The unique specimen extends the geological range of gilled mushrooms back by approximately 14-21 million years and confirms their presence in Gondwana during the Early Cretaceous. Molecular clock estimates suggest the divergence of the Basidiomycota around 500 Ma to 1.2 billion years [26] and G. magnificus establishes the earliest calibration point so far for the Agaricales, with a new minimum age of 113-120 Ma.