259
then the molecular divergence estimate (Li et al. 2018). However, the age of
Lagerstätte is still uncertain (Castro et al. 2018) and it might also be just a matter of
time before similar eggs are recovered from older vertebrate coprolites.
In other cases, remains have been recovered from extinct hosts which are phylogenetically placed in between extant hosts suggesting their record might still be
consistent (De Baets et al. 2015) with a deep evolutionary origin of these parasites
in their common ancestor (e.g., oxyurid nematode in cynodont coprolites, monogenean flatworms attached to fossil acanthodians). In yet other cases, fossil helminth
remains are recovered from extinct hosts which are only distantly related to extant
hosts suggesting extinct host relationships which are no longer recovered nor predicted from extant host relationships (e.g., putative pentastomid attached to ostracod, gnathiferan ectosymbionts attached to scalidophorans). This suggests caution
in considering modern evidence alone when constraining the evolution of past parasites and inferring ancient parasite-host relationships (see also De Baets and
Littlewood 2015; De Baets et al. 2015). However, novel methods are currently being
developed which might be able to deal with changes in host repertoire (Braga et al.
2020; Warnock and Engelstädter 2021). Fossil finds and their known host associations should be considered when available and further finds might provide crucial
information in the context of unravelling the evolutionary history of helminth
parasitism.
7.6 Conclusions
The fossil record of soft-bodied helminths is patchy, but more diverse than commonly assumed. This summary provides evidence that ectoparasitic helminths in
various lineages have existed since the early Paleozoic, while endoparasitic helminths arose during, or before the late Paleozoic. Lineages parasitizing terrestrial
plant and animal hosts have been in existence at least since the Devonian and
Triassic, respectively. All major helminth phyla had evolved endoparasitic lineages
at least since the Mesozoic. Data also provide evidence for obvious host switches or
extinctions, which cautions against models relying solely on co-divergence where
use of host calibrations to constrain divergence time estimates may be considered.
In some cases, the fossil record of parasitic helminths is better than that of their freeliving relatives, yielding constraints on the age of their free-living sister-groups. In
at least some cases (Eucestoda, stem-Ascaridoidea), the earliest fossil evidence for
particular lineages falls within the divergence time estimates based on host constraints. The fossil record also provides direct evidence for certain modern or now
extinct host associations (e.g., putative pentastomid on ostracods).
Acknowledgements We thank Holger Herlyn and John Huntley for valuable discussions and
comments which helped to improve the text. This research which led to this work was funded by
Swiss National Science Foundation (2012e141438) and FAU Emerging Talents Initative (SS16_
NAT_11) to K.D.B. P.D.-D. is funded by Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq - 302936/2018-1).
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
then the molecular divergence estimate (Li et al. 2018). However, the age of
Lagerstätte is still uncertain (Castro et al. 2018) and it might also be just a matter of
time before similar eggs are recovered from older vertebrate coprolites.
In other cases, remains have been recovered from extinct hosts which are phylogenetically placed in between extant hosts suggesting their record might still be
consistent (De Baets et al. 2015) with a deep evolutionary origin of these parasites
in their common ancestor (e.g., oxyurid nematode in cynodont coprolites, monogenean flatworms attached to fossil acanthodians). In yet other cases, fossil helminth
remains are recovered from extinct hosts which are only distantly related to extant
hosts suggesting extinct host relationships which are no longer recovered nor predicted from extant host relationships (e.g., putative pentastomid attached to ostracod, gnathiferan ectosymbionts attached to scalidophorans). This suggests caution
in considering modern evidence alone when constraining the evolution of past parasites and inferring ancient parasite-host relationships (see also De Baets and
Littlewood 2015; De Baets et al. 2015). However, novel methods are currently being
developed which might be able to deal with changes in host repertoire (Braga et al.
2020; Warnock and Engelstädter 2021). Fossil finds and their known host associations should be considered when available and further finds might provide crucial
information in the context of unravelling the evolutionary history of helminth
parasitism.
7.6 Conclusions
The fossil record of soft-bodied helminths is patchy, but more diverse than commonly assumed. This summary provides evidence that ectoparasitic helminths in
various lineages have existed since the early Paleozoic, while endoparasitic helminths arose during, or before the late Paleozoic. Lineages parasitizing terrestrial
plant and animal hosts have been in existence at least since the Devonian and
Triassic, respectively. All major helminth phyla had evolved endoparasitic lineages
at least since the Mesozoic. Data also provide evidence for obvious host switches or
extinctions, which cautions against models relying solely on co-divergence where
use of host calibrations to constrain divergence time estimates may be considered.
In some cases, the fossil record of parasitic helminths is better than that of their freeliving relatives, yielding constraints on the age of their free-living sister-groups. In
at least some cases (Eucestoda, stem-Ascaridoidea), the earliest fossil evidence for
particular lineages falls within the divergence time estimates based on host constraints. The fossil record also provides direct evidence for certain modern or now
extinct host associations (e.g., putative pentastomid on ostracods).
Acknowledgements We thank Holger Herlyn and John Huntley for valuable discussions and
comments which helped to improve the text. This research which led to this work was funded by
Swiss National Science Foundation (2012e141438) and FAU Emerging Talents Initative (SS16_
NAT_11) to K.D.B. P.D.-D. is funded by Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq - 302936/2018-1).
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution