232
eages often falls within molecular divergence times based on host co-evolution
suggesting the fossil record of helminths themselves might be just as good or at
least complementary (and less circular in justification) to calibration based on host
associations. Data also provide evidence for obvious host switches or extinctions,
which cautions against models of pure co-divergence where use of host calibrations
to constrain divergence time estimates may be considered.
Keywords Parasitic worms · Fossil record · Molecular divergence estimates ·
Host constraints
7.1 Introduction
Helminths are multicellular parasitic worms varied in size (Caira and Littlewood
2013) from the microscopic (200 μm) to the enormously long (80 m). This polyphyletic grouping includes parasitic flatworms (flukes, tapeworms), nematodes and
acanthocephalans (Rohde 2005). They can cause diseases in humans, livestock and
wildlife (e.g., helminthiases). Some authors also include tongue worms (pentastomids: e.g., McAllister et al. 2010) or even parasitic annelids (e.g., Öktener 2005)
when describing or listing helminths from particular regions. Although an artificial
grouping, which contains a number of phylogenetically distantly related clades, the
term ‘helminths’ is still used to describe a group of parasitic organisms sharing a
similar form and lifestyle.
Despite their biomedical and economic importance, the deep time evolutionary
history of parasitic worms remains poorly understood, which has been largely
attributed to their meagre, or rather poorly explored, fossil record (Littlewood and
Donovan 2003). As many helminths use vertebrate animal hosts or other hosts with
a demonstrably better fossil record, it is helminth hosts that have been used to constrain helminth origins (by phylum) where a certain degree of host specificity might
be assumed. With such approaches, some groups of nematodes and parasitic flatworms have been suggested as existing during the Cambrian–Ordovician (Littlewood
2006; Poinar 2011, 2015). In other cases, particularly where host-specificity is common, current biogeographic distribution patterns of host-parasite associations have
been used to infer the temporal origin of the interaction (Borda et al. 2008; Verneau
et al. 2009; Badets et al. 2011; Cuthill et al. 2016). Some of the most commonly
used examples for host or biogeographic constraints include those made for the
monogenean group Polystomatidae (Verneau et al. 2002, 2009; Badets et al. 2011;
Héritier et al. 2015; Tinsley and Tinsley 2016). However, both approaches might
involve circular reasoning (Kodandaramaiah 2011; De Baets et al. 2015, 2016;
Warnock and Engelstädter 2021). Fossil associations are among the only direct
sources of evidence for the existence of particular lineages in the geological past
(De Baets and Littlewood 2015). In addition, the fossil record can provide additional information on morphology, host association and/or life history.
K. De Baets et al.
eages often falls within molecular divergence times based on host co-evolution
suggesting the fossil record of helminths themselves might be just as good or at
least complementary (and less circular in justification) to calibration based on host
associations. Data also provide evidence for obvious host switches or extinctions,
which cautions against models of pure co-divergence where use of host calibrations
to constrain divergence time estimates may be considered.
Keywords Parasitic worms · Fossil record · Molecular divergence estimates ·
Host constraints
7.1 Introduction
Helminths are multicellular parasitic worms varied in size (Caira and Littlewood
2013) from the microscopic (200 μm) to the enormously long (80 m). This polyphyletic grouping includes parasitic flatworms (flukes, tapeworms), nematodes and
acanthocephalans (Rohde 2005). They can cause diseases in humans, livestock and
wildlife (e.g., helminthiases). Some authors also include tongue worms (pentastomids: e.g., McAllister et al. 2010) or even parasitic annelids (e.g., Öktener 2005)
when describing or listing helminths from particular regions. Although an artificial
grouping, which contains a number of phylogenetically distantly related clades, the
term ‘helminths’ is still used to describe a group of parasitic organisms sharing a
similar form and lifestyle.
Despite their biomedical and economic importance, the deep time evolutionary
history of parasitic worms remains poorly understood, which has been largely
attributed to their meagre, or rather poorly explored, fossil record (Littlewood and
Donovan 2003). As many helminths use vertebrate animal hosts or other hosts with
a demonstrably better fossil record, it is helminth hosts that have been used to constrain helminth origins (by phylum) where a certain degree of host specificity might
be assumed. With such approaches, some groups of nematodes and parasitic flatworms have been suggested as existing during the Cambrian–Ordovician (Littlewood
2006; Poinar 2011, 2015). In other cases, particularly where host-specificity is common, current biogeographic distribution patterns of host-parasite associations have
been used to infer the temporal origin of the interaction (Borda et al. 2008; Verneau
et al. 2009; Badets et al. 2011; Cuthill et al. 2016). Some of the most commonly
used examples for host or biogeographic constraints include those made for the
monogenean group Polystomatidae (Verneau et al. 2002, 2009; Badets et al. 2011;
Héritier et al. 2015; Tinsley and Tinsley 2016). However, both approaches might
involve circular reasoning (Kodandaramaiah 2011; De Baets et al. 2015, 2016;
Warnock and Engelstädter 2021). Fossil associations are among the only direct
sources of evidence for the existence of particular lineages in the geological past
(De Baets and Littlewood 2015). In addition, the fossil record can provide additional information on morphology, host association and/or life history.
K. De Baets et al.