233
The oldest known unambiguous fossil helminths are isolated remains of pentastomids which are known since the Cambrian-Ordovician (Klompmaker and
Boxshall 2015). Older finds have been reported (Cong et al. 2017) although their
phylogenetic assignment is poorly resolved and they are interpreted to be likely
ectosymbiotic rather than ectoparasitic. There is also fossil evidence that nematodes
already parasitized early land plants in the Early Devonian (Poinar et al. 2008) and
monogeneans parasitized early gnathostomes in the Middle Devonian (Upeniece
2001, 2011). The first confidently identified evidence for intestinal helminths
derives from the Permian (Dentzien-Dias et al. 2013)—although older Carboniferous
finds have been reported (Zangerl and Case 1976). These finds suggests that the
fossil record of helminths is more ancient than commonly perceived.
Here we review the fossil record of helminths with the aim of providing suitable
minimum and maximum constraints for dating their origins and key host transitions
during their evolution. The fossil record of helminths is reviewed at greater length
in other publications (Poinar 2011, 2015; Huntley and De Baets 2015; Parry et al.
2014, 2019b). We focus particularly on similarities and differences in the fossil
record of helminths and in a phylogenetic context. For this purpose, we first briefly
review the progress made in disentangling their phylogenetic position within the
Metazoa since a major review on the topic (Zrzavý 2001).
7.2 Phylogenetic Affinities and Distribution
of Parasitic “Helminths”
Since the early 1990s, the rapid accumulation of molecular sequence data has revolutionized our understanding of animal phylogeny and evolutionary history
(Edgecombe et al. 2011). Bilaterian animals are now usually subdivided into three
principal clades (Laumer et al. 2019), Deuterostomia, Ecdysozoa and Spiralia
(including Lophotrochozoa), with the latter two groups forming the Protostomia.
Protostomia contains the most animal phyla and body plans and likewise contains
the highest diversity of groups that have evolved a parasitic lifestyle (Weinstein and
Kuris 2016). A possible exception are Myxozoa—some of which are worm-like—
now considered to be derived cnidarians (Okamura et al. 2015; Okamura and Gruhl
2021 for reviews). Various helminths are so divergent from their close relatives that
they have previously been considered separate phyla. Examples include
Acanthocephala and Pentastomida, which are now recognized as highly modified
members of Rotifera or Syndermata (Struck et al. 2014; Sielaff et al. 2016; Herlyn
2021) and Crustacea in Euarthropoda (Sanders and Lee 2010; Oakley et al. 2012),
respectively. In general, parasitic lineages have elevated rates of molecular sequence
evolution, which often makes them hard to place robustly in phylogenies, where
they often form long branches. Myzostomida are a key example, having been placed
as close relatives of flatworms (Eeckhaut et al. 2000), rotifers (Zrzavý et al. 2001)
and annelids (Bleidorn et al. 2009) based on different molecular datasets. More
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
The oldest known unambiguous fossil helminths are isolated remains of pentastomids which are known since the Cambrian-Ordovician (Klompmaker and
Boxshall 2015). Older finds have been reported (Cong et al. 2017) although their
phylogenetic assignment is poorly resolved and they are interpreted to be likely
ectosymbiotic rather than ectoparasitic. There is also fossil evidence that nematodes
already parasitized early land plants in the Early Devonian (Poinar et al. 2008) and
monogeneans parasitized early gnathostomes in the Middle Devonian (Upeniece
2001, 2011). The first confidently identified evidence for intestinal helminths
derives from the Permian (Dentzien-Dias et al. 2013)—although older Carboniferous
finds have been reported (Zangerl and Case 1976). These finds suggests that the
fossil record of helminths is more ancient than commonly perceived.
Here we review the fossil record of helminths with the aim of providing suitable
minimum and maximum constraints for dating their origins and key host transitions
during their evolution. The fossil record of helminths is reviewed at greater length
in other publications (Poinar 2011, 2015; Huntley and De Baets 2015; Parry et al.
2014, 2019b). We focus particularly on similarities and differences in the fossil
record of helminths and in a phylogenetic context. For this purpose, we first briefly
review the progress made in disentangling their phylogenetic position within the
Metazoa since a major review on the topic (Zrzavý 2001).
7.2 Phylogenetic Affinities and Distribution
of Parasitic “Helminths”
Since the early 1990s, the rapid accumulation of molecular sequence data has revolutionized our understanding of animal phylogeny and evolutionary history
(Edgecombe et al. 2011). Bilaterian animals are now usually subdivided into three
principal clades (Laumer et al. 2019), Deuterostomia, Ecdysozoa and Spiralia
(including Lophotrochozoa), with the latter two groups forming the Protostomia.
Protostomia contains the most animal phyla and body plans and likewise contains
the highest diversity of groups that have evolved a parasitic lifestyle (Weinstein and
Kuris 2016). A possible exception are Myxozoa—some of which are worm-like—
now considered to be derived cnidarians (Okamura et al. 2015; Okamura and Gruhl
2021 for reviews). Various helminths are so divergent from their close relatives that
they have previously been considered separate phyla. Examples include
Acanthocephala and Pentastomida, which are now recognized as highly modified
members of Rotifera or Syndermata (Struck et al. 2014; Sielaff et al. 2016; Herlyn
2021) and Crustacea in Euarthropoda (Sanders and Lee 2010; Oakley et al. 2012),
respectively. In general, parasitic lineages have elevated rates of molecular sequence
evolution, which often makes them hard to place robustly in phylogenies, where
they often form long branches. Myzostomida are a key example, having been placed
as close relatives of flatworms (Eeckhaut et al. 2000), rotifers (Zrzavý et al. 2001)
and annelids (Bleidorn et al. 2009) based on different molecular datasets. More
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution