Before describing a few examples of fossil fungi, it is important to consider the general question of their manner of occurrence and determination. Considering the small size and delicate nature of most fungi, it is not surprising that we have but few satisfactory records of well-defined fossil forms. The large leathery sporophores of Polyporus and other genera of the Basidiomycetes, which are familiar objects as yellow or brown brackets projecting from the trunks of diseased forest trees, have been found in a fairly perfect condition in the Cambridgeshire peat-beds, and examples have been described also by continental writers[405]. As a general rule, however, we have to depend on the chance mineralisation or petrifaction of the hyphae of a fungus-mycelium which has invaded the living or dead tissues of some higher plant. In the literature on fossil plants there are numerous recorded species of fungi founded on dark coloured spots and blotches on the impression of a leaf. Most of such records are worthless; the external features being usually too imperfect to allow of accurate identification. The occurrence of recent fungi as discolourations on leaves is exceedingly common, and the characteristic perithecia or compact and more or less spherical cases enclosing a group of sporangia in certain Ascomycetous species, might be readily preserved in a fossil condition.
Ascomycetes.
Some examples of possible Ascomycetous fungi have been recently recorded by Potonié from leaves and other portions of plants of Permian age. There is a distinct superficial resemblance between the specimens he figures and the fructifications of recent Ascomycetes, but in the absence of internal structure, it would be rash to do more than suggest the probable nature of the markings he describes. For one of the fungus-like impressions Potonié proposes the generic name Rosellinites; he compares certain irregularly shaped projections on a piece of Permian wood with the perithecia of Rosellinia, a member of the Sphaeriaceae, and describes them as Rosellinites Beyschlagii Pot.[406] Various other records of similar Ascomycetes-like fossils may be found in palaeobotanical literature[407], but it is unnecessary to examine these in detail. Unless we are able to determine the nature of the supposed fungus by microscopical methods our identifications cannot in most cases be of any great value.
An example of the perithecia of a fungus (Rosellinia congregata [Beck])[408] has been recorded from the Oligocene of Saxony, which would appear to rest on a more satisfactory basis than is often the case. In this particular instance the small projections on a piece of fossil coniferous stem present a form which naturally suggests a fungus perithecium. In cases where the black spots on a fossil stem or leaf possess a definite form and structure, it is perfectly legitimate to refer them to a group of fungi; but in very many instances the forms referred to such genera as Sphaerites and others are of little or no value. Many forms of scale-insects and galls on leaves present an obvious superficial resemblance to epiphyllous fungi, and might readily be mistaken for the fructifications of certain Ascomycetous species. As examples of scale-insects simulating fungi, reference may be made to such genera of the Coccineae as Aspidiotus, Diaspis, Lecanium, Coccus, and others. The female insects lying on the surface of a leaf, if preserved as a fossil impression, might easily be mistaken for perithecia[409].
Another pitfall in fossil mycology may be illustrated by a description of a supposed fungus, Sclerotites Salisburiae[410], Mass. on a Tertiary Ginkgo leaf. The figure given by Massalongo represents a Ginkgo leaf with well marked veins, the lamina between the veins being traversed by short discontinuous and longitudinally-running lines; the latter are referred to as the fungus. In a recent Ginkgo leaf one may easily detect with the naked eye a number of short lines between and parallel to the veins, which if examined in section are found to be secretory canals. There can be little doubt that Sclerotites Salisburiae owes its existence to the preservation of these canals.
The list of fossil fungi given by Meschinelli in Saccardo’s Sylloge Fungorum[411] includes certain species which are of no botanical value, and should have no place in any list which claims to be authentic.
Among the numerous examples of fossil ‘fungi’ which have no claim to be classed with plants, there are some which are in all probability the galleries of wood-eating animals. The radiating grooves frequently found on the inner face of the bark of a pine tree made by species of the beetle Bostrychus might be mistaken for the impressions of the firm strands of mycelial tissue of some Basidiomycetous fungus.
In some notes on fossil fungi by J. F. James[412] contributed to the American Journal of Mycology in 1893, it is pointed out that a supposed fungus described by Lesquereux from the Lower Coal-Measures as Rhizomorpha Sigillariae[413], bears a strong likeness to some insect-burrows, such as those of Bostrychus.
“A new fungus from the Coal-Measures” described by Herzer in 1893[414] may probably be referred to animal agency. In any case there is no evidence as to the fungoid nature of the object represented in the figure accompanying Herzer’s description.