237
coprolites using microscopic (Zangerl and Case 1976) or thin section approaches
(Dentzien-Dias et al. 2013; Francischini et al. 2018; see fig. 1; Dentzien-Dias et al.
2018)—a common approach to study fossil coprolites (Chin 2002). The assignment
of coprolites to their hosts is less straightforward than commonly assumed. It is
often done based on composition, fossil content (providing clues to environment or
diet), morphology and size. However, these methods are not flawless and identification is best achieved when they are still associated with their owner. Even if the
coprolite owner can be identified, there is still a need to establish that putative parasitic remains are not simply present in the coprolite because the microorganism (or
its host for that matter) is microbophagous (Poinar 2015) or coprophagous (Bajdek
et al. 2016).
Sclerotized hooks from helminth attachment organs have also been identified
from fossil remains (Upeniece 2001; De Baets et al. 2015). The best evidence for
this is a collection of 69 circlets of microscopic hooks, mostly still attached to their
acanthodian and placoderm fish hosts in the Devonian Lode Formation (Upeniece
2001). Similar remains from acanthocephalan or cestode hooks have yet to be
found, although the presence of their attachment organs have been interpreted to be
present in remains of developing larvae within putative parasite eggs (Cardia et al.
2019a; Dentzien-Dias et al. 2013).
Other types of evidence involve characteristic trace fossils and pathologies left in
their hosts (e.g., Huntley et al. 2021), although they are hard to tie to particular parasite taxa with much phylogenetic precision. In some cases, various phylogenetically
distantly related modern or now extinct groups with similar behavior could have led
to similar structures. More confidence is gained when characteristic parasitic
remains or at least signs of in-situ infestation are associated with these structures.
Such evidence is however comparatively rare and in some cases, the host structure
might be unsuitable for preservation (e.g., early decay of isopod parasite within
decapod pathologies: Klompmaker et al. 2017).
7.4 Time Constraints on the Origin
of Eumetazoan Helminths
The fossil record can only provide meaningful hard minimum time constraints on
the timing of divergences of a clade. Various authors have used records of nearrelatives to place soft maxima on the existence of a certain clade at a certain time.
This is typically based “on the understanding that they could have been there but are
not based on the existence of facies preserving their near relatives, among which are
at least some ecological analogues” (Benton et al. 2015). In the case of Eumetazoa
a meaningful maximum constraint would be the maximum age of the Lantian biota
(Benton et al. 2015). This together with Doushantuo Formation represent the oldest
Konservät-Lagerstätten which have yielded putative eumetazoans in similar preservations which have yielded eumetazoans in younger formations. Consequently, a
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
coprolites using microscopic (Zangerl and Case 1976) or thin section approaches
(Dentzien-Dias et al. 2013; Francischini et al. 2018; see fig. 1; Dentzien-Dias et al.
2018)—a common approach to study fossil coprolites (Chin 2002). The assignment
of coprolites to their hosts is less straightforward than commonly assumed. It is
often done based on composition, fossil content (providing clues to environment or
diet), morphology and size. However, these methods are not flawless and identification is best achieved when they are still associated with their owner. Even if the
coprolite owner can be identified, there is still a need to establish that putative parasitic remains are not simply present in the coprolite because the microorganism (or
its host for that matter) is microbophagous (Poinar 2015) or coprophagous (Bajdek
et al. 2016).
Sclerotized hooks from helminth attachment organs have also been identified
from fossil remains (Upeniece 2001; De Baets et al. 2015). The best evidence for
this is a collection of 69 circlets of microscopic hooks, mostly still attached to their
acanthodian and placoderm fish hosts in the Devonian Lode Formation (Upeniece
2001). Similar remains from acanthocephalan or cestode hooks have yet to be
found, although the presence of their attachment organs have been interpreted to be
present in remains of developing larvae within putative parasite eggs (Cardia et al.
2019a; Dentzien-Dias et al. 2013).
Other types of evidence involve characteristic trace fossils and pathologies left in
their hosts (e.g., Huntley et al. 2021), although they are hard to tie to particular parasite taxa with much phylogenetic precision. In some cases, various phylogenetically
distantly related modern or now extinct groups with similar behavior could have led
to similar structures. More confidence is gained when characteristic parasitic
remains or at least signs of in-situ infestation are associated with these structures.
Such evidence is however comparatively rare and in some cases, the host structure
might be unsuitable for preservation (e.g., early decay of isopod parasite within
decapod pathologies: Klompmaker et al. 2017).
7.4 Time Constraints on the Origin
of Eumetazoan Helminths
The fossil record can only provide meaningful hard minimum time constraints on
the timing of divergences of a clade. Various authors have used records of nearrelatives to place soft maxima on the existence of a certain clade at a certain time.
This is typically based “on the understanding that they could have been there but are
not based on the existence of facies preserving their near relatives, among which are
at least some ecological analogues” (Benton et al. 2015). In the case of Eumetazoa
a meaningful maximum constraint would be the maximum age of the Lantian biota
(Benton et al. 2015). This together with Doushantuo Formation represent the oldest
Konservät-Lagerstätten which have yielded putative eumetazoans in similar preservations which have yielded eumetazoans in younger formations. Consequently, a
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution