74
Permineralized peat and coal balls are two other rock matrices that may contain
abundant fossil evidence of fungi (e.g., Williamson 1878, 1880, 1883; Stubblefield
and Taylor 1988; Harper et al. 2015, 2016; Slater et al. 2015). However, the organic
remains in these matrices are usually compacted and partially to largely degraded,
and hence render it more difficult, but not impossible, to safely identify fungal interactions (e.g., Knoll 1985; Krings et al. 2014). Various types of fungi and fungal
interactions, as well as indirect evidence of fungal activities such as microborings
and chemical traces (Golubic et al. 1975; Marynowski et al. 2013), have also been
exquisitely preserved by other preservation modes, including silicified wood, animal hard parts, and amber (Taylor et al. 2015). However, the ecological configuration of the community in which these organisms lived is often less completely known.
3.2.2 Tracing Fungal Parasitism in the Fossil Record
Parasitic fungi exploit carbon sources that are, by definition, alive at the time of
infection. However, fossils represent snap-shots in time, with no long-term and follow- up studies available to determine the condition of the host before and after
fungal colonization. This raises the question how, if at all, parasitic fungi can be
recognized as fossils and distinguished from epi-/endophytes and saprotrophs? In
other words, how can we tell if a fossil containing evidence of the presence of a
fungus was (part of) a living organism at the time of fungal colonization, and how
can we determine whether the fungus thrived at the host’s expense?
If the host organism is preserved in pristine condition, then this could mean that
it was alive at the time of fungal colonization. Conversely, a host that is tattered,
fragmented, and shows tissue disruption and disintegration might have been in the
process of decay when colonized. Nevertheless, it is not normally possible to determine whether fragmentation and tissue destruction were initiated before or after the
fungus colonized the host, or are preservation artifacts (Krings et al. 2009b, 2010b).
Somewhat more reliable is perhaps the spatial distribution of fungi within the host.
A fungal distribution pattern within an intact host that reflects forced entry and a
consistent pathway of colonization (e.g., along the vascular bundles in plants; see
Harper et al. 2019), or is restricted to certain body parts or tissue types of the host,
is suggestive of colonization of a living host, while colonization of dead and decaying matter more likely results in random fungal distribution.
Structural features suggestive of parasitism and pathogenicity in fossil organisms
include disease symptoms and host reactions such as cell and tissue alteration or
local necroses (e.g., Mendgen et al. 1996; Pearce 1996). In rare instances, the fungal
perpetrator and the disease symptom/host response even co-occur, providing additional lines of direct evidence (Taylor et al. 1992b, c; Krings and Harper 2018). For
example, a common type of host response to fungal intrusion is the formation of
callosities, which are inwardly directed projections consisting of newly synthesized
cell wall material that encase the parasite’s penetration device, and thus may reduce
or inhibit nutrient extraction from the host (Akai 1959; Aist 1976, 1977). Callosities
C. J. Harper and M. Krings
Permineralized peat and coal balls are two other rock matrices that may contain
abundant fossil evidence of fungi (e.g., Williamson 1878, 1880, 1883; Stubblefield
and Taylor 1988; Harper et al. 2015, 2016; Slater et al. 2015). However, the organic
remains in these matrices are usually compacted and partially to largely degraded,
and hence render it more difficult, but not impossible, to safely identify fungal interactions (e.g., Knoll 1985; Krings et al. 2014). Various types of fungi and fungal
interactions, as well as indirect evidence of fungal activities such as microborings
and chemical traces (Golubic et al. 1975; Marynowski et al. 2013), have also been
exquisitely preserved by other preservation modes, including silicified wood, animal hard parts, and amber (Taylor et al. 2015). However, the ecological configuration of the community in which these organisms lived is often less completely known.
3.2.2 Tracing Fungal Parasitism in the Fossil Record
Parasitic fungi exploit carbon sources that are, by definition, alive at the time of
infection. However, fossils represent snap-shots in time, with no long-term and follow- up studies available to determine the condition of the host before and after
fungal colonization. This raises the question how, if at all, parasitic fungi can be
recognized as fossils and distinguished from epi-/endophytes and saprotrophs? In
other words, how can we tell if a fossil containing evidence of the presence of a
fungus was (part of) a living organism at the time of fungal colonization, and how
can we determine whether the fungus thrived at the host’s expense?
If the host organism is preserved in pristine condition, then this could mean that
it was alive at the time of fungal colonization. Conversely, a host that is tattered,
fragmented, and shows tissue disruption and disintegration might have been in the
process of decay when colonized. Nevertheless, it is not normally possible to determine whether fragmentation and tissue destruction were initiated before or after the
fungus colonized the host, or are preservation artifacts (Krings et al. 2009b, 2010b).
Somewhat more reliable is perhaps the spatial distribution of fungi within the host.
A fungal distribution pattern within an intact host that reflects forced entry and a
consistent pathway of colonization (e.g., along the vascular bundles in plants; see
Harper et al. 2019), or is restricted to certain body parts or tissue types of the host,
is suggestive of colonization of a living host, while colonization of dead and decaying matter more likely results in random fungal distribution.
Structural features suggestive of parasitism and pathogenicity in fossil organisms
include disease symptoms and host reactions such as cell and tissue alteration or
local necroses (e.g., Mendgen et al. 1996; Pearce 1996). In rare instances, the fungal
perpetrator and the disease symptom/host response even co-occur, providing additional lines of direct evidence (Taylor et al. 1992b, c; Krings and Harper 2018). For
example, a common type of host response to fungal intrusion is the formation of
callosities, which are inwardly directed projections consisting of newly synthesized
cell wall material that encase the parasite’s penetration device, and thus may reduce
or inhibit nutrient extraction from the host (Akai 1959; Aist 1976, 1977). Callosities
C. J. Harper and M. Krings