234
recent phylogenomic analyses (Weigert et al. 2014) have confirmed that myzostomids are modified annelids, but their precise phylogenetic position within the phylum remains uncertain.
Nematoda are closely related (sister) to Nematomorpha and are grouped together
in the clade Nematoida. Most studies place Nematoida and other Cycloneuralia
(Scalidophora) together with Panarthropoda (Onychophora, Tardigrada and
Arthropoda) in a clade named Ecdysozoa (characterized by a growth strategy
involving periodic moulting of an external cuticle; but see Giribet and Edgecombe
2017). Acoelomorpha are long since removed from the phylum Platyhelminthes
which now consists of the paraphyletic “Turbellaria” and the parasitic Neodermata
(Egger et al. 2015). The relative position of Platyhelminthes and Rotifera within the
Metazoa is less well resolved (Laumer et al. 2019), but they are usually assigned to
Spiralia (Struck et al. 2014; Laumer et al. 2015a; Marlétaz et al. 2019). The Spiralia
also contain Lophotrochozoa including phyla as diverse as Cycliophora, Entoprocta,
Mollusca, Nemertea, Annelida, Brachiopoda, Phoronida and Bryozoa. Most recent
studies recover the mostly macrofaunal clade Lophotrochozoa as a sistergroup to a
platyhelminth—gastrotrich clade (Rouphozoa sensu Struck et al. 2014), both of
which are sister to a clade populated by Gnathifera (Gnathostomulida,
Micrognathozoa, Syndermata). The assignment of Chaetognatha as sistergroup to,
or in a nested position within Gnathifera, is still debated (Marlétaz et al. 2019).
Even within these phyla, parasitism may have evolved multiple times independently. Weinstein and Kuris (2016) reviewed incidences of parasitism in light of
modern phylogenetics and suggested that parasitism might have evolved as many as
143 times in Arthropoda, 18 times in Nematoda, 13 times in Platyhelminthes, nine
times in Annelida, seven times in Rotifera (= Syndermata), six times within
Acoelomorpha, but only once in the Nematomorpha. These analyses, clearly suggest that parasitic helminths have evolved convergently. This might not only have
resulted in similarities in morphology and ecology, but also in their preservation
potential, which is largely restricted to sites of exceptional (soft-body) preservation
(Konservation-Lagerstätten sensu Seilacher 1970; see De Baets et al. (2021) for a
review).
7.3 Preservation Potential
The fossil record of various groups of helminths, or the phyla they belong to, was
recently reviewed by various authors (Maas 2013; Poinar 2011, 2014, 2015; De
Baets et al. 2015; Klompmaker and Boxshall 2015; Parry et al. 2014, 2019b, Herlyn
2021). We discuss the bearing of these fossils and alternatives for constraining date
estimates of the origins and diversification of parasitic helminths.
As many helminths are small and soft-bodied their fossil record is quite patchy
and largely restricted to rare finds in conservation traps (e.g., amber) or other sites
of exceptional preservation (Littlewood and Donovan 2003). Ectoparasitic forms
often use attachment organs (suckers, hooks) and during fossilization might be
K. De Baets et al.
recent phylogenomic analyses (Weigert et al. 2014) have confirmed that myzostomids are modified annelids, but their precise phylogenetic position within the phylum remains uncertain.
Nematoda are closely related (sister) to Nematomorpha and are grouped together
in the clade Nematoida. Most studies place Nematoida and other Cycloneuralia
(Scalidophora) together with Panarthropoda (Onychophora, Tardigrada and
Arthropoda) in a clade named Ecdysozoa (characterized by a growth strategy
involving periodic moulting of an external cuticle; but see Giribet and Edgecombe
2017). Acoelomorpha are long since removed from the phylum Platyhelminthes
which now consists of the paraphyletic “Turbellaria” and the parasitic Neodermata
(Egger et al. 2015). The relative position of Platyhelminthes and Rotifera within the
Metazoa is less well resolved (Laumer et al. 2019), but they are usually assigned to
Spiralia (Struck et al. 2014; Laumer et al. 2015a; Marlétaz et al. 2019). The Spiralia
also contain Lophotrochozoa including phyla as diverse as Cycliophora, Entoprocta,
Mollusca, Nemertea, Annelida, Brachiopoda, Phoronida and Bryozoa. Most recent
studies recover the mostly macrofaunal clade Lophotrochozoa as a sistergroup to a
platyhelminth—gastrotrich clade (Rouphozoa sensu Struck et al. 2014), both of
which are sister to a clade populated by Gnathifera (Gnathostomulida,
Micrognathozoa, Syndermata). The assignment of Chaetognatha as sistergroup to,
or in a nested position within Gnathifera, is still debated (Marlétaz et al. 2019).
Even within these phyla, parasitism may have evolved multiple times independently. Weinstein and Kuris (2016) reviewed incidences of parasitism in light of
modern phylogenetics and suggested that parasitism might have evolved as many as
143 times in Arthropoda, 18 times in Nematoda, 13 times in Platyhelminthes, nine
times in Annelida, seven times in Rotifera (= Syndermata), six times within
Acoelomorpha, but only once in the Nematomorpha. These analyses, clearly suggest that parasitic helminths have evolved convergently. This might not only have
resulted in similarities in morphology and ecology, but also in their preservation
potential, which is largely restricted to sites of exceptional (soft-body) preservation
(Konservation-Lagerstätten sensu Seilacher 1970; see De Baets et al. (2021) for a
review).
7.3 Preservation Potential
The fossil record of various groups of helminths, or the phyla they belong to, was
recently reviewed by various authors (Maas 2013; Poinar 2011, 2014, 2015; De
Baets et al. 2015; Klompmaker and Boxshall 2015; Parry et al. 2014, 2019b, Herlyn
2021). We discuss the bearing of these fossils and alternatives for constraining date
estimates of the origins and diversification of parasitic helminths.
As many helminths are small and soft-bodied their fossil record is quite patchy
and largely restricted to rare finds in conservation traps (e.g., amber) or other sites
of exceptional preservation (Littlewood and Donovan 2003). Ectoparasitic forms
often use attachment organs (suckers, hooks) and during fossilization might be
K. De Baets et al.