248
at least constrained to the Permian. If cestodes and monogeneans would be sister
taxa, as in some morphological (Brooks 1989) and less commonly molecular analyses (Laumer et al. 2015b), trematodes and cestodes could be constrained to the
Devonian as inferred from the first appearance of monogeneans in the fossil record.
The ancient record of parasitic flatworms is quite remarkable since the body fossil record of confidently identifiable free-living flatworms (the paraphyletic
‘Turbellaria’) start considerably later. Trace fossils associated with soft-body
impressions of Permian-to-Triassic age have been attributed to polyclad and triclad
flatworms (Knaust 2010). Earlier Ordovician trace fossils associated with putative
worm remains could potentially be attributable to free-living ‘Turbellaria’, but this
assignment awaits further scrutiny (Knaust and Desrochers 2019). The oldest confidently identified body fossil of a free-living flatworm is that of a rhabdocoel flatworm from Baltic amber (Poinar 2003a). Rhabdocoela separated even earlier from
the lineage leading to the obligate parasitic flatworms (Neodermata) than triclad
flatworms (Egger et al. 2015). This establishes the record of more derived
Neodermata to be more ancient and relatively more common than that of their freeliving relatives.
Acoelomorpha are now assigned to their own clade together with Xenoturbellida
and no longer considered platyhelminths (Ruiz-Trillo and Paps 2016; Philippe et al.
2011). Xenacoelomorpha are now considered simplified relatives of the Ambulacraria
(Philippe et al. 2019). To our knowledge, there is no fossil record of parasitic acoelomorphs. Some drop-shaped structure in X-ray analyses of the Early Devonian
Hunsrück Slate (middle Kaub Formation), considered of Early Emsian age (De
Baets et al. 2013), have been interpreted as acoelomorph remains (Fauchald et al.
1988)—but this interpretation (Knaust and Desrochers 2019) and several other of
Stürmer’s interpretations based on such X-ray analyses have been proven to be erroneous (as reviewed in Otto 1999, 2000). Based on trace fossils associated with body
fossil remains—acoelomorphs have potentially been traced back the Late
Ordovician, although these comparisons are hard to verify and remain unconfirmed
(Knaust and Desrochers 2019).
7.4.4 Fossil Record of Parasitic Nematodes
Molecular phylogenetic analyses have shown that parasitism of plants and animals
has arisen at least 15 times independently in nematodes (Blaxter and Koutsovoulos
2015). The sampling coverage of enigmatic nematode lineages is slowly improving
(Leduc et al. 2018; Leduc and Zhao 2019), but as long as not all extant families have
been molecularly sampled, the independent origins of parasitism might be more
numerous (e.g., Sudhaus 2008, 2010; Weinstein and Kuris 2016). The fossil record
can so far not constrain all of these transitions, but it can place multiple constraints
on the origin of parasitism throughout the nematode tree. The fossil record of parasitic nematodes has recently been reviewed by Poinar (2015), who also reviewed the
fossil record of nematodes in general with a focus on amber deposits (Poinar 2011,
K. De Baets et al.
at least constrained to the Permian. If cestodes and monogeneans would be sister
taxa, as in some morphological (Brooks 1989) and less commonly molecular analyses (Laumer et al. 2015b), trematodes and cestodes could be constrained to the
Devonian as inferred from the first appearance of monogeneans in the fossil record.
The ancient record of parasitic flatworms is quite remarkable since the body fossil record of confidently identifiable free-living flatworms (the paraphyletic
‘Turbellaria’) start considerably later. Trace fossils associated with soft-body
impressions of Permian-to-Triassic age have been attributed to polyclad and triclad
flatworms (Knaust 2010). Earlier Ordovician trace fossils associated with putative
worm remains could potentially be attributable to free-living ‘Turbellaria’, but this
assignment awaits further scrutiny (Knaust and Desrochers 2019). The oldest confidently identified body fossil of a free-living flatworm is that of a rhabdocoel flatworm from Baltic amber (Poinar 2003a). Rhabdocoela separated even earlier from
the lineage leading to the obligate parasitic flatworms (Neodermata) than triclad
flatworms (Egger et al. 2015). This establishes the record of more derived
Neodermata to be more ancient and relatively more common than that of their freeliving relatives.
Acoelomorpha are now assigned to their own clade together with Xenoturbellida
and no longer considered platyhelminths (Ruiz-Trillo and Paps 2016; Philippe et al.
2011). Xenacoelomorpha are now considered simplified relatives of the Ambulacraria
(Philippe et al. 2019). To our knowledge, there is no fossil record of parasitic acoelomorphs. Some drop-shaped structure in X-ray analyses of the Early Devonian
Hunsrück Slate (middle Kaub Formation), considered of Early Emsian age (De
Baets et al. 2013), have been interpreted as acoelomorph remains (Fauchald et al.
1988)—but this interpretation (Knaust and Desrochers 2019) and several other of
Stürmer’s interpretations based on such X-ray analyses have been proven to be erroneous (as reviewed in Otto 1999, 2000). Based on trace fossils associated with body
fossil remains—acoelomorphs have potentially been traced back the Late
Ordovician, although these comparisons are hard to verify and remain unconfirmed
(Knaust and Desrochers 2019).
7.4.4 Fossil Record of Parasitic Nematodes
Molecular phylogenetic analyses have shown that parasitism of plants and animals
has arisen at least 15 times independently in nematodes (Blaxter and Koutsovoulos
2015). The sampling coverage of enigmatic nematode lineages is slowly improving
(Leduc et al. 2018; Leduc and Zhao 2019), but as long as not all extant families have
been molecularly sampled, the independent origins of parasitism might be more
numerous (e.g., Sudhaus 2008, 2010; Weinstein and Kuris 2016). The fossil record
can so far not constrain all of these transitions, but it can place multiple constraints
on the origin of parasitism throughout the nematode tree. The fossil record of parasitic nematodes has recently been reviewed by Poinar (2015), who also reviewed the
fossil record of nematodes in general with a focus on amber deposits (Poinar 2011,
K. De Baets et al.