71
been documented in the fossil record, due probably to the simple fact that it is very
difficult to recognize these life forms as fossils because of their diminutive size
(e.g., Garcia 2002). Moreover, they lack structural features that could be used to
safely identify them, with one possible exception. Xenomas (or xenoparasitic complexes; see Chatton 1920; Weissenberg 1968) are tumor-like growths caused by a
variety of parasitic protists and fungi, including Microsporidia. They can occur on
numerous organisms such as oligochaetes, insects, and crustaceans; however, they
are predominantly found on fishes (Matos et al. 2003; Lom and Dyková 2005; Weiss
and Becnel 2014). It might therefore be worth looking for such abnormal growths
also in well preserved fossil fish specimens, although it will most likely be very difficult to positively identify the actual causative agent(s) (e.g., Petit 2010; Petit and
Khalloufi 2012).
Fungal parasites can have profound influence on the performance and community structure of other organisms, and thus are important players in ecosystem functioning today (e.g., Marcogliese 2004; Skerratt et al. 2007; Sime-Ngando 2012;
Frenken et al. 2017). Knowledge of their fossil record, evolution, and the roles they
played in biological and ecological processes in the past is therefore a key component of our understanding of the complexity and functioning of ancient ecosystems.
However, the fossil record of fungi as parasites remains incomplete and is often
difficult to interpret for several reasons as explained in the following section
(Fig. 3.1a, b).
3.2 Identifying Fungal Parasitism in the Fossil Record
3.2.1 Finding Fossil Fungi
The success of finding fungi from the geologic past heavily relies on the way the
fossils are preserved (Krings et al. 2012), even more so if nutritional modes and
interactions with other organisms are to be resolved, too. Cherts certainly represent
the most important sources of evidence of fossil fungi (Taylor et al. 2015). Chert
deposits occur at various points in geologic time and typically represent an extremely
dense microcrystalline or cryptocrystalline type of sedimentary rock (Laschet 1984;
Hesse 1989). Some cherts may be fossiliferous and demonstrate not only threedimensional and structural preservation of the organisms (sometimes even in situ),
but often also details of individual cells and subcellular structures. As a result of
faithful fossil preservation, cherts provide an ideal matrix from which to extract
information about fungi and fungal interactions with other organisms. Moreover,
cherts provide the only source of direct evidence of the fungal world within the
context of ecosystem complexity, versatility, and dynamics. Foremost among the
chert deposits yielding evidence of fungi is the Lower Devonian (~410 Ma) Rhynie
chert from Aberdeenshire, Scotland (Trewin and Kerp 2017; Garwood et al. 2020),
that contains representatives of all major fungal lineages except Basidiomycota
(Taylor et al. 2004; Krings et al. 2017a). The Rhynie chert is perhaps best known
3 Fungi as Parasites: A Conspectus of the Fossil Record
been documented in the fossil record, due probably to the simple fact that it is very
difficult to recognize these life forms as fossils because of their diminutive size
(e.g., Garcia 2002). Moreover, they lack structural features that could be used to
safely identify them, with one possible exception. Xenomas (or xenoparasitic complexes; see Chatton 1920; Weissenberg 1968) are tumor-like growths caused by a
variety of parasitic protists and fungi, including Microsporidia. They can occur on
numerous organisms such as oligochaetes, insects, and crustaceans; however, they
are predominantly found on fishes (Matos et al. 2003; Lom and Dyková 2005; Weiss
and Becnel 2014). It might therefore be worth looking for such abnormal growths
also in well preserved fossil fish specimens, although it will most likely be very difficult to positively identify the actual causative agent(s) (e.g., Petit 2010; Petit and
Khalloufi 2012).
Fungal parasites can have profound influence on the performance and community structure of other organisms, and thus are important players in ecosystem functioning today (e.g., Marcogliese 2004; Skerratt et al. 2007; Sime-Ngando 2012;
Frenken et al. 2017). Knowledge of their fossil record, evolution, and the roles they
played in biological and ecological processes in the past is therefore a key component of our understanding of the complexity and functioning of ancient ecosystems.
However, the fossil record of fungi as parasites remains incomplete and is often
difficult to interpret for several reasons as explained in the following section
(Fig. 3.1a, b).
3.2 Identifying Fungal Parasitism in the Fossil Record
3.2.1 Finding Fossil Fungi
The success of finding fungi from the geologic past heavily relies on the way the
fossils are preserved (Krings et al. 2012), even more so if nutritional modes and
interactions with other organisms are to be resolved, too. Cherts certainly represent
the most important sources of evidence of fossil fungi (Taylor et al. 2015). Chert
deposits occur at various points in geologic time and typically represent an extremely
dense microcrystalline or cryptocrystalline type of sedimentary rock (Laschet 1984;
Hesse 1989). Some cherts may be fossiliferous and demonstrate not only threedimensional and structural preservation of the organisms (sometimes even in situ),
but often also details of individual cells and subcellular structures. As a result of
faithful fossil preservation, cherts provide an ideal matrix from which to extract
information about fungi and fungal interactions with other organisms. Moreover,
cherts provide the only source of direct evidence of the fungal world within the
context of ecosystem complexity, versatility, and dynamics. Foremost among the
chert deposits yielding evidence of fungi is the Lower Devonian (~410 Ma) Rhynie
chert from Aberdeenshire, Scotland (Trewin and Kerp 2017; Garwood et al. 2020),
that contains representatives of all major fungal lineages except Basidiomycota
(Taylor et al. 2004; Krings et al. 2017a). The Rhynie chert is perhaps best known
3 Fungi as Parasites: A Conspectus of the Fossil Record