Basidiomycota
The oldest basidiomycetous fossils occur in a structurally preserved fern stem from the Upper Mississippian (~331 Ma) of France, and comprise septate hyphae with clamp connections that pass from cell to cell (Krings et al., 2011a). The clamp-bearing hyphae co-occur with elongate callosities in several of the host cells (Fig. 2A); however, hyphae and callosities have not been observed physically connected. If the callosities in fact formed in response to invading clamp-bearing hyphae, then this host response would be evidence of a parasitic interaction, which would support the hypothesis that ancestral Basidiomycota were parasites (Oberwinkler, 2012). Hyphae with clamp connections co-occurring with callosities have also been described in an Early Permian Psaronius tree fern root mantle from Germany (Krings et al., 2017a).
Fossilized wood-rot also provides possible evidence of basidiomycetous parasitism and pathogenicity. Tracheids and vessels of extant plants have a diversity of passive defense mechanisms to ward off or contain microbial attacks (Blanchette, 1992), while living cells may also employ active mechanical defences in the form of appositions, tyloses, and chemical defence strategies (Schwarze and Baum, 2000). Silicified Glossopteris wood from the upper Permian (~255 Ma) of Antarctica show irregular areas lacking cells, and septate hyphae with clamp connections (Harper et al., 2016). The decay pattern in this fossil is comparable to present-day rots caused by Basidiomycota. Moreover, the lumina of some of the tracheids are sealed by opaque matter, while the cell walls of other tracheids are swollen and partially occlude the lumen (Fig. 2B). Both types of occlusion could have developed to contain antagonistic fungal expansion. Basidiomycota have also been identified as the causal agents for decay in conifer wood from the Jurassic (~160 Ma) and Cretaceous (~75 Ma) of Argentina (Sagasti et al., 2019; Greppi et al., 2022) and the Cretaceous (~120 Ma) of China (Tian et al., 2020), either directly based on the presence of clamp-bearing hyphae in the decayed areas of the wood, or indirectly based on micro-patterns that are consistent with patterns generated by xylophagous Basidiomycota in present-day conifer wood.
Another structural detail of fossil woods that has been discussed in connection with fungal infection is tyloses (Decombeix et al., 2022). Harper et al. (2012) describe a Jurassic permineralized conifer axis from Antarctica (~180 Ma) in which tylosis formation co-occurs with abundant fungal remains, suggesting that the tyloses served as mechanical barriers against the advancing hyphae (Fig. 2C). However, the fact that the hyphae also occur within and around the tyloses, as well as in the rays and phloem, suggests that the fungus was able to surmount this barrier. A similar interpretation has been offered by Khan et al. (2018) for Plio-Pleistocene (~2.5 Ma) angiosperm wood from Tibet that also contains both tyloses and fungal remains.
Ascomycota
The bulk of evidence of fossil parasitic Ascomycota comes from amber fossils and cuticle preparations of compressed leaves. However, the hitherto oldest putatively parasitic Ascomycota, Paleopyrenomycites devonicus, is from the Lower Devonian Rhynie chert, and is preserved in leaf-like enations, stems, and rhizomes of the early lycophyte Asteroxylon mackiei, where it produced ostiolate perithecia in substomatal chambers just beneath stomatal pores (Taylor et al., 1999, 2005). Necrotic areas in the host suggest that the fungus was a parasite. Hyphae and spores of another fungus, probably a mycoparasite, occur in several of the perithecia.
Amber fossils, and a permineralization
Fossils of parasitic fungi associated with animals primarily come from specimens enshrined in Cretaceous and Cenozoic amber. Perhaps the most spectacular example is a fungus described as Paleoophiocordyceps coccophagus, which was a parasite of scale insects and morphologically similar to present-day Ophiocordyceps (Sung et al., 2008). This fossil occurs in mid-Cretaceous Kachin amber (~100 Ma) from Myanmar in the form of synnemata emerging from the head of the insect (Fig. 2D). Another possible fungal parasite has recently been described from a Camponotus ant in Baltic amber (~45 Ma) as Allocordyceps baltica (Hypocreales: Clavicipitaceae) (Poinar and Maltier, 2021). The fungus is characterized by a stalked, cup-shaped ascoma with partially immersed perithecia that emerges from the rectum of the ant, two separate stromata with septate mycelium that emerge from the base of the neck and the abdomen of the ant, respectively, and free-standing putative perithecia bearing putative asci with multicellular ascospores. A third remarkable amber fossil preserving a fungus–animal interaction is a springtail (Collembola) covered in hyphae and conidiophores of an Aspergillus species that is also preserved in Eocene Baltic amber (Dörfelt and Schmidt, 2005). Most conidiophores extend directly from the host surface. The fungus may have penetrated and parasitized the living organism, and may have sporulated after the host had become entangled in liquid resin. The dominance of a single insecticolous fungus, together with the excellent preservation of the springtail, suggest a parasitic mode of life of the fungus (Fig. 2E).
A fossil member of the order Laboulbeniales, which are obligate ectoparasites (Haelewaters et al., 2021), is preserved on the thorax of a stalk-eyed fly in Bitterfeld amber (~24 Ma) from Germany (Rossi et al., 2005). The authors stated that the fungus is preserved in Baltic amber, but it is actually Bitterfeld amber (Perreau et al., 2021). This fossil was assigned to the extant genus Stigmatomyces, of which representatives are parasites of Diptera, and described as S. succinii. It is the oldest bona fide fossil representative of the class Laboulbeniomycetes (Fig. 2F, G). Another fossil attributed to the Laboulbeniales with confidence has recently been discovered in Miocene Dominican amber (~18 Ma) (Perreau et al., 2021). This fungus, Columnomyces electri, occurs on the leg of a leiodid beetle, and its discovery indicated that these beetles and their parasitic Laboulbeniales have coevolved at least since the Miocene (Fig. 2H, I).
A parasitic relationship involving an ascomycete has also been suggested for what has been interpreted as a Claviceps-like sclerotium, Palaeoclaviceps parasiticus, that occurs on a grass floret in Kachin amber (Fig. 2J; Poinar et al., 2015). The fossil is believed to demonstrate the existence of intricate interactions between Clavicipitaceae and the plant family Poaceae in the Cretaceous. Recently, Poinar (2020) reported on epiphyllous pycnidia, formally described as Palaeomycus epallelus, from an angiosperm leaf in Kachin amber, and suggested that, albeit no modern equivalents to these pycnidia are known, they are most similar to leaf spot-producing Coelomycetes (Fig. 2K). Finally, lichenicolous fungi of the genus Lichenostigma (Lichenostigmatales) occurring on the apothecial margin and crustose thallus of two lichen fossils preserved in Paleogene Baltic amber (~45 Ma) have been interpreted as parasites (Kaasalainen et al., 2019).
Examples of putative mycoparasitism and hypermycoparasitism have been reported from Kachin amber by Poinar and Buckley (2007). The gilled mushroom Palaeoagaracites antiquus is parasitized by an ascomycete, Mycetophagites atrebora. The mycelium of the parasite overgrows the pileus of P. antiquus and its hyphae also occur within the host tissue (Fig. 2L). A third organism involved in this interaction is Entropezites patricii, which appears to be a necrotrophic hyperparasite based on hyphae invading and apparently destroying the mycelium of M. atrebora (Fig. 2M). Another report of a fungal hyperparasite comes from the Eocene Princeton chert of Canada (Currah et al., 1998). Paleoserenomyces allenbyensis is an ascomycete preserved on permineralized leaves of the palm Uhlia allenbyensis. The fossil fungus shares certain features with extant Serenomyces (Phyllachorales), which also forms spots on palms (Hyde and Cannon, 1999). Present in some of the locules of P. allenbyensis are globose ascomata formally described as Cryptodidymosphaerites princetonensis (Melanommatales) that share morphological traits with Didymosphaeria, a genus of plant pathogens in Pleosporales.
Plant cuticles
The oldest fossil evidence in plant cuticles of a host response to the presence of a fungus comes from the Mississippian of Germany (~350 Ma) (Hübers et al., 2011). Pronounced cuticle rims bordering the margins of fungal thalli are interpreted as a host response suggestive of parasitism. A similar host response has been observed in a Jurassic Sphenobaiera (Ginkgophyta) leaf from China (~170 Ma) (Fig. 2N; Sun et al., 2015). Cuticle alterations probably linked to a fungal colonization have also been observed in Cenozoic angiosperm leaves from Australia (~30 Ma) (Tarran et al., 2016). Cuticular rims present on these leaves appear to have directed fungal hyphae away from the stomata, and thus away from the entry points into the leaves. Pteropus brachyphylli (Pleosporales) is a fossil ascomycete that occurs on leaves of a conifer from the Upper Cretaceous of Belgium (~67 Ma). Nearly all stomata of the host leaves are occupied by the fungus, suggesting that the association was parasitic rather than saprotrophic (van der Ham and Dortangs, 2005).
Compression fossils of Paleocene conifers of the Cupressaceae and Pinaceae from Russia (~60 Ma) show various types of damage, most of which were caused by Ascomycota (Maslova et al., 2021). Fungal remains obtained through cuticle preparations of the damaged areas include hyphae, chains of conidia, various types of fruiting bodies, and dispersed spores. The diversity of Ascomycota on these Paleocene conifers is consistent with previously obtained data on the existence of these plants in a temperate humid climate with a hot summer and without a dry season. There is astonishing diversity of fungal hyphae, fruiting bodies (e.g., thyriothecia, pycnidia), and hyphopodia, mostly of microthyriaceous fungi, also on Cenozoic angiosperm leaves. The vast majority of these remains have been obtained through maceration of compression fossils (e.g., Dilcher, 1965; Bannister et al., 2016). The nutritional modes of most of these fungi remain unknown; however, some authors have noted morphological similarities to present-day plant parasites and pathogens, such as Asterina, Vizella, and Trichothyrina (Taylor et al., 2015a).