124
Molecular clock analyses, however, can be particularly challenging for parasitic
groups for many reasons (Warnock and Engelstädter 2021). First, evolutionary rates
have repeatedly been shown to be increased in parasite lineages (Bromham 2009;
Bromham et al. 2013; Paterson et al. 2010). In addition, reconstructions and calibrations of inferred parasite-host associations are only reliable if host switching is very
rare. The latter can be detected independent of molecular clock analyses, by cophylogenetic analyses, which, however have certain pitfalls (de Vienne et al. 2013;
see also discussion in later section). For example, parasites, including many endocnidozoans, can often exploit a range of hosts and this introduces complications to
co-phylogenetic analyses that assume strict host use (Banks and Paterson 2005).
Finally, any inferences of parasite evolution, including molecular clock methods
and host-parasite cophylogenies, can be highly compromised when parasite diversities are poorly known. As we argue here and elsewhere (Okamura et al. 2018)
endocnidozoan diversities are certainly grossly underestimated and pivotal taxa
may be overlooked due to lack of sampling in many environments.
An additional and important conceptual point to stress in any discussion of evidence for fossil remains is the appreciation that evolution is a continuous process.
Thus, at some period endocnidozoans will have diverged from an entirely freeliving common ancestor. However, whether this divergence was simultaneously
linked with adoption of parasitism or whether parasitism evolved subsequently is an
open question. This question is of course relevant to a timeframe when fossils of
parasites might be expected. Later we will review how initial and further hosts may
have been adopted in simple and complex parasite life cycles, but complications
arising from the continuous nature of evolution should be born in mind as we discuss the origins of endocnidozoans and of their potential early hosts in this section.
Below we review the current state of knowledge of the origins of cnidarians and
of hosts known to be used by endocnidozoans based on the fossil record and other
evidence (e.g. molecular clock analyses). We also assess the potential for recognizing myxozoan infections in host fossil material. We then critically evaluate conclusions of a recent study of the origin and evolution of endocnidozoans deriving from
molecular clock and cophylogenetic analyses.
4.5.1 Cnidarian Origins and Fossil Record
Phylogenomic analyses of multi-gene datasets across the animal kingdom have in
recent decades resolved many questions. Basal metazoan relationships, however,
including those of cnidarians, remain especially controversial and several equally
well supported hypotheses exist (discussed in Dohrmann and Wörheide 2013; Dunn
et al. 2014; Halanych 2015). The two most prominent groups of hypotheses mainly
differ in the placement of Placozoa and Ctenophora: either united with Cnidaria to
form the “Diploblastica” (e.g. Schierwater et al. 2009) or as basal metazoan branches
leaving Cnidaria as sister to bilaterians (e.g. Simion et al. 2017; Whelan et al. 2017).
Although most molecular clock analyses predict both metazoan and bilaterian
B. Okamura and A. Gruhl
Molecular clock analyses, however, can be particularly challenging for parasitic
groups for many reasons (Warnock and Engelstädter 2021). First, evolutionary rates
have repeatedly been shown to be increased in parasite lineages (Bromham 2009;
Bromham et al. 2013; Paterson et al. 2010). In addition, reconstructions and calibrations of inferred parasite-host associations are only reliable if host switching is very
rare. The latter can be detected independent of molecular clock analyses, by cophylogenetic analyses, which, however have certain pitfalls (de Vienne et al. 2013;
see also discussion in later section). For example, parasites, including many endocnidozoans, can often exploit a range of hosts and this introduces complications to
co-phylogenetic analyses that assume strict host use (Banks and Paterson 2005).
Finally, any inferences of parasite evolution, including molecular clock methods
and host-parasite cophylogenies, can be highly compromised when parasite diversities are poorly known. As we argue here and elsewhere (Okamura et al. 2018)
endocnidozoan diversities are certainly grossly underestimated and pivotal taxa
may be overlooked due to lack of sampling in many environments.
An additional and important conceptual point to stress in any discussion of evidence for fossil remains is the appreciation that evolution is a continuous process.
Thus, at some period endocnidozoans will have diverged from an entirely freeliving common ancestor. However, whether this divergence was simultaneously
linked with adoption of parasitism or whether parasitism evolved subsequently is an
open question. This question is of course relevant to a timeframe when fossils of
parasites might be expected. Later we will review how initial and further hosts may
have been adopted in simple and complex parasite life cycles, but complications
arising from the continuous nature of evolution should be born in mind as we discuss the origins of endocnidozoans and of their potential early hosts in this section.
Below we review the current state of knowledge of the origins of cnidarians and
of hosts known to be used by endocnidozoans based on the fossil record and other
evidence (e.g. molecular clock analyses). We also assess the potential for recognizing myxozoan infections in host fossil material. We then critically evaluate conclusions of a recent study of the origin and evolution of endocnidozoans deriving from
molecular clock and cophylogenetic analyses.
4.5.1 Cnidarian Origins and Fossil Record
Phylogenomic analyses of multi-gene datasets across the animal kingdom have in
recent decades resolved many questions. Basal metazoan relationships, however,
including those of cnidarians, remain especially controversial and several equally
well supported hypotheses exist (discussed in Dohrmann and Wörheide 2013; Dunn
et al. 2014; Halanych 2015). The two most prominent groups of hypotheses mainly
differ in the placement of Placozoa and Ctenophora: either united with Cnidaria to
form the “Diploblastica” (e.g. Schierwater et al. 2009) or as basal metazoan branches
leaving Cnidaria as sister to bilaterians (e.g. Simion et al. 2017; Whelan et al. 2017).
Although most molecular clock analyses predict both metazoan and bilaterian
B. Okamura and A. Gruhl