75
have been observed in fossil plants belonging to several different lineages, including
lycophytes (Krings et al. 2009b, 2010b), sphenophytes (Taylor et al. 2012), ferns
(Krings et al. 2011a), and gymnosperms (Stubblefield et al. 1984), but have also
been recorded in fossil fungi (Hass et al. 1994). Several of the latter records even
provide evidence of a biotrophic relationship, in which the parasite was contained
to a certain extent by the callosities, but was still able to grow and extract nutrients,
while the host remained demonstrably viable for an extended period of time while
being parasitized (Krings and Harper 2018). However, not all parasites elicit host
responses, and it may therefore be difficult, if not impossible, to determine the nutritional modes of asymptomatic fossil fungi associated with intact hosts. For example, commonly present within structurally preserved plants throughout the
Phanerozoic are small fungal reproductive units (e.g., spores, sporangia, cleistothecia, pycnidia) and mycelia that are randomly distributed; no evidence of host
responses has been found (Magnus 1903; Stubblefield and Taylor 1986; LePage
et al. 1994; García Massini et al. 2012; Klymiuk et al. 2013). Some of these fungal
remains, including ascomycotan hyphae, pseudothecia, pycnidia, and hyphomycetous spores, have nonetheless been interpreted as parasites because their (presumed)
modern equivalents are parasites of plants (LePage et al. 1994; García Massini et al.
2012). Finally, many of the host responses known in extant organisms (e.g., chemical responses; see Swain 1977; Langenheim 1994) cannot be identified in fossils or
are easily mistaken for natural decay (e.g., necroses; see Van Loon et al. 2006).
Evidence of fungal parasitism in ancient ecosystems also occurs in the form of
fungal structures that are found as detached fossils (i.e. with no information on the
host available), but that can be directly compared to modern fungal taxa known to
be parasites. For example, polyporous fungi or polypores (Basidiomycota) today
thrive as saprotrophs in decaying wood or as parasites and perpetrators of diseases
in conifers and hardwoods (Blanchette 1991; Ryvarden 1991; Schwarze et al. 2000).
The Cretaceous and Cenozoic record of these fungi is quite extensive and consists
primarily of basidiocarps (conks) that usually can be assigned to modern families
and genera with some confidence based on morphology and spore structure (e.g.,
Smith et al. 2004; Fleischmann et al. 2007). The inventory of fossil polypores suggests that these fungi were widely distributed and diverse in Neogene and Quaternary
forest paleoecosystems and significant in delignification processes and as pathogens
of woody plants.
Of all the potential levels of interaction between fungi and other organisms, parasitism is perhaps the most difficult to demonstrate in the fossil record. Without a
combination of different lines of evidence obtained from both the host and fungus,
this type of interaction cannot be discriminated from saprotrophism and even mutualism when examined in fossils (Taylor et al. 2009). Excellent examples illustrating
this problem occur in the form of structurally preserved remains of Lepidodendrales
(arborescent lycophytes) from the Carboniferous of central Europe that contain
diverse assemblages of fungal mycelia and reproductive units (Krings et al. 2007a,
2009b, 2010b, 2011d; summarized in Fig. 3.2). Some of these fungi have been interpreted as parasites based on morphology and distribution (Fig. 3.2j) or the presence
of host responses (Fig. 3.2f), while others were probably mutualists (mycorrhizal
3 Fungi as Parasites: A Conspectus of the Fossil Record
have been observed in fossil plants belonging to several different lineages, including
lycophytes (Krings et al. 2009b, 2010b), sphenophytes (Taylor et al. 2012), ferns
(Krings et al. 2011a), and gymnosperms (Stubblefield et al. 1984), but have also
been recorded in fossil fungi (Hass et al. 1994). Several of the latter records even
provide evidence of a biotrophic relationship, in which the parasite was contained
to a certain extent by the callosities, but was still able to grow and extract nutrients,
while the host remained demonstrably viable for an extended period of time while
being parasitized (Krings and Harper 2018). However, not all parasites elicit host
responses, and it may therefore be difficult, if not impossible, to determine the nutritional modes of asymptomatic fossil fungi associated with intact hosts. For example, commonly present within structurally preserved plants throughout the
Phanerozoic are small fungal reproductive units (e.g., spores, sporangia, cleistothecia, pycnidia) and mycelia that are randomly distributed; no evidence of host
responses has been found (Magnus 1903; Stubblefield and Taylor 1986; LePage
et al. 1994; García Massini et al. 2012; Klymiuk et al. 2013). Some of these fungal
remains, including ascomycotan hyphae, pseudothecia, pycnidia, and hyphomycetous spores, have nonetheless been interpreted as parasites because their (presumed)
modern equivalents are parasites of plants (LePage et al. 1994; García Massini et al.
2012). Finally, many of the host responses known in extant organisms (e.g., chemical responses; see Swain 1977; Langenheim 1994) cannot be identified in fossils or
are easily mistaken for natural decay (e.g., necroses; see Van Loon et al. 2006).
Evidence of fungal parasitism in ancient ecosystems also occurs in the form of
fungal structures that are found as detached fossils (i.e. with no information on the
host available), but that can be directly compared to modern fungal taxa known to
be parasites. For example, polyporous fungi or polypores (Basidiomycota) today
thrive as saprotrophs in decaying wood or as parasites and perpetrators of diseases
in conifers and hardwoods (Blanchette 1991; Ryvarden 1991; Schwarze et al. 2000).
The Cretaceous and Cenozoic record of these fungi is quite extensive and consists
primarily of basidiocarps (conks) that usually can be assigned to modern families
and genera with some confidence based on morphology and spore structure (e.g.,
Smith et al. 2004; Fleischmann et al. 2007). The inventory of fossil polypores suggests that these fungi were widely distributed and diverse in Neogene and Quaternary
forest paleoecosystems and significant in delignification processes and as pathogens
of woody plants.
Of all the potential levels of interaction between fungi and other organisms, parasitism is perhaps the most difficult to demonstrate in the fossil record. Without a
combination of different lines of evidence obtained from both the host and fungus,
this type of interaction cannot be discriminated from saprotrophism and even mutualism when examined in fossils (Taylor et al. 2009). Excellent examples illustrating
this problem occur in the form of structurally preserved remains of Lepidodendrales
(arborescent lycophytes) from the Carboniferous of central Europe that contain
diverse assemblages of fungal mycelia and reproductive units (Krings et al. 2007a,
2009b, 2010b, 2011d; summarized in Fig. 3.2). Some of these fungi have been interpreted as parasites based on morphology and distribution (Fig. 3.2j) or the presence
of host responses (Fig. 3.2f), while others were probably mutualists (mycorrhizal
3 Fungi as Parasites: A Conspectus of the Fossil Record