123
chemical treatments, and gut passage (El-Matbouli and Hoffmann 1991; Hedrick
et al. 2008). Their outer layer consists of a hardened, cuticle-like, extracellular
material secreted by the underlying valve cells. This material may be elaborated as
ridges and other surface features across the spore surface (reviewed in Gruhl and
Okamura 2015). In some species a pronounced cytoskeleton inside the valve cells
adds further mechanical reinforcement, and chitin has been identified in the periphery of myxospores (Munoz et al. 1999). Myxospores in mud have been shown to
remain viable for several months (El-Matbouli and Hoffmann 1991). Thus, there is
a good chance that these could be present along with other palynomorphs in sedimentary deposits or in peat. Such preservation has been shown for other parasite
remains (e.g. nematode and trematode eggs), but recovery may be heavily influenced by the extraction method used (Dufour and Le Bailly 2013). It is unclear,
however, how easily such remains could be identified as myxospores. Although
spore shape and surface structure are of some use, the fossilization potential of the
most important diagnostic myxozoan characters, the polar capsules, is unclear.
Clusters of nematocysts (for example so-called “nematocyst batteries” in jellyfish)
are recognisable in fossil cnidarians with soft-tissue preservation (Han et al. 2016),
but the individual nematocysts are, to our knowledge, hardly ever retained. As of
yet, no experimental studies on the taphonomy of myxospores exist. Such data
would inform on the potential recognition of fossilized myxozoan spores.
Indirect fossil evidence of endocnidozoan parasitism, i.e. pathological changes
in their hosts, could theoretically be recognised, particularly if such pathologies
occur in skeletal tissues. Soft tissue pathologies such as cysts or swollen organs
require exceptional preservation, but even in such cases it will still be difficult to
unambiguously identify myxozoan infection. This is because infections by many
other organisms can produce similar disease symptoms, as exemplified by fossilized
fish skin nodules (Petit 2010; Petit and Khalloufi 2012) from the Monte Bolca and
Solnhofen deposits. In addition, many diagnostic techniques, such as histological
staining, are inapplicable to fossilized material.
Because of the inherent restrictions of the fossil record, current reconstructions
of endocnidozoan evolutionary history have to use other sources of information
(reviewed in De Baets and Littlewood 2015; Martínez-Aquino 2016; Warnock and
Engelstädter 2021). These can include extant parasite and host phylogenies, the fossil record of closely related free-living cnidarian taxa as well as that of hosts, and
paleo-environmental data. Such data can be analysed and combined in different
ways. Ancestral character states (such as host preference) can be reconstructed
using standard phylogenetic techniques. Phylogenetic bracketing, for example, postulates the occurrence of the parasite in the last common ancestor of all recent hosts
(except for those that were clearly acquired by recent host switching). Molecular
clock analyses can also provide age estimates for origins of groups that lack fossil
data when based on both a well-resolved phylogeny and a reliable fossil calibration
of a wide set of nodes (Benton et al. 2009; Parham et al. 2012). A common practice
for age estimates of a taxon is to use the oldest reliable fossil as a minimum age
estimate and either the earliest molecular clock or the stratigraphic maximum estimate as a (soft) maximum age estimate (De Baets et al. 2015; Parham et al. 2012).
4 Evolution, Origins and Diversification of Parasitic Cnidarians
chemical treatments, and gut passage (El-Matbouli and Hoffmann 1991; Hedrick
et al. 2008). Their outer layer consists of a hardened, cuticle-like, extracellular
material secreted by the underlying valve cells. This material may be elaborated as
ridges and other surface features across the spore surface (reviewed in Gruhl and
Okamura 2015). In some species a pronounced cytoskeleton inside the valve cells
adds further mechanical reinforcement, and chitin has been identified in the periphery of myxospores (Munoz et al. 1999). Myxospores in mud have been shown to
remain viable for several months (El-Matbouli and Hoffmann 1991). Thus, there is
a good chance that these could be present along with other palynomorphs in sedimentary deposits or in peat. Such preservation has been shown for other parasite
remains (e.g. nematode and trematode eggs), but recovery may be heavily influenced by the extraction method used (Dufour and Le Bailly 2013). It is unclear,
however, how easily such remains could be identified as myxospores. Although
spore shape and surface structure are of some use, the fossilization potential of the
most important diagnostic myxozoan characters, the polar capsules, is unclear.
Clusters of nematocysts (for example so-called “nematocyst batteries” in jellyfish)
are recognisable in fossil cnidarians with soft-tissue preservation (Han et al. 2016),
but the individual nematocysts are, to our knowledge, hardly ever retained. As of
yet, no experimental studies on the taphonomy of myxospores exist. Such data
would inform on the potential recognition of fossilized myxozoan spores.
Indirect fossil evidence of endocnidozoan parasitism, i.e. pathological changes
in their hosts, could theoretically be recognised, particularly if such pathologies
occur in skeletal tissues. Soft tissue pathologies such as cysts or swollen organs
require exceptional preservation, but even in such cases it will still be difficult to
unambiguously identify myxozoan infection. This is because infections by many
other organisms can produce similar disease symptoms, as exemplified by fossilized
fish skin nodules (Petit 2010; Petit and Khalloufi 2012) from the Monte Bolca and
Solnhofen deposits. In addition, many diagnostic techniques, such as histological
staining, are inapplicable to fossilized material.
Because of the inherent restrictions of the fossil record, current reconstructions
of endocnidozoan evolutionary history have to use other sources of information
(reviewed in De Baets and Littlewood 2015; Martínez-Aquino 2016; Warnock and
Engelstädter 2021). These can include extant parasite and host phylogenies, the fossil record of closely related free-living cnidarian taxa as well as that of hosts, and
paleo-environmental data. Such data can be analysed and combined in different
ways. Ancestral character states (such as host preference) can be reconstructed
using standard phylogenetic techniques. Phylogenetic bracketing, for example, postulates the occurrence of the parasite in the last common ancestor of all recent hosts
(except for those that were clearly acquired by recent host switching). Molecular
clock analyses can also provide age estimates for origins of groups that lack fossil
data when based on both a well-resolved phylogeny and a reliable fossil calibration
of a wide set of nodes (Benton et al. 2009; Parham et al. 2012). A common practice
for age estimates of a taxon is to use the oldest reliable fossil as a minimum age
estimate and either the earliest molecular clock or the stratigraphic maximum estimate as a (soft) maximum age estimate (De Baets et al. 2015; Parham et al. 2012).
4 Evolution, Origins and Diversification of Parasitic Cnidarians