245
also constrain the first appearance of Ichthyostraca (Wolfe et al. 2016; but see
Haug et al. 2021).
Hard minimum. 497 Ma. The Orsten fossils derive from the late Cambrian Alum
Shale derive from the Agnostus pisiformis zone, which is now considered the
upper boundary of the Cambrian series 3 (Nielsen et al. 2014). The minimum age
of the uppermost stage of the Cambrian 3 series therefore yields 497 Ma according to GTS 2016 (Ogg et al. 2016).
7.4.3 Fossil Record of Parasitic Platyhelminths
Transitions from free-living to parasitism amongst the Platyhelminthes may have
evolved at least nine times (Weinstein and Kuris 2016), but only the divergence of
the Neodermata can be reasonably constrained by fossil evidence (De Baets et al.
2015). Neodermata are a monophyletic group comprising exclusively obligate parasites (Fig. 7.4). Present day associations include the use of vertebrates as definitive
hosts. The internal phylogeny is still debated, but most authors consider ectoparasitic monogeneans as the earliest diverging lineage while cestode and trematodes
are considered sister groups in most molecular analyses (Lockyer et al. 2003; Park
et al. 2007; Egger et al. 2015). The oldest fossils of Neodermata include circlets of
hooks found on early armored jawed fishes (Leung 2017; Upeniece 2001, 2011).
Their appearance, particularly the arrangement of hooks and association with putative sucker discs, are consistent with their identification as monogeneans (Leung
2017; De Baets et al. 2015). Their location near gills and abdominal near fin spines,
would also be consistent with an ectoparasitic mode of life. The larger size of hooks
when compared to extant taxa is consistent with a trend towards smaller size within
monogeneans over time (Poulin 2005).
Node. Neodermata
Fossil evidence. Over 69 circlets of hooks, at least 29 still associated with their fish
hosts in position suggestive of ectoparasitism (Upeniece 2001, 2011).
Phylogenetic justification. No formal phylogenetic analysis on the attachment
organs was performed. However, the circular arrangement of the bilateral symmetric hooks, traces of cuticular disc as well the maximum number of 16 hooks
strongly suggests their assignment as monogenean platyhelminths (Upeniece
2011; De Baets et al. 2015; Leung 2017, 2021).
Host. mostly attached to the acanthodian Lodeacanthus gajicus (24) and some (5)
also associated with juveniles of the placoderm Asterolepis ornata (Upeniece
2001, 2011).
Minimum age. 382.7 Ma. The formation containing the finds was initially assigned
to the Early Frasnian (Upeniece 2001), but currently the Lode Formation is
assigned to the Upper Givetian (Lukševičs et al. 2012, 2014). This corresponds
with 382.7 Ma corresponding to the minimum age assigned to the GivetianFrasnian boundary according to GTS 2016 (Ogg et al. 2016).
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
also constrain the first appearance of Ichthyostraca (Wolfe et al. 2016; but see
Haug et al. 2021).
Hard minimum. 497 Ma. The Orsten fossils derive from the late Cambrian Alum
Shale derive from the Agnostus pisiformis zone, which is now considered the
upper boundary of the Cambrian series 3 (Nielsen et al. 2014). The minimum age
of the uppermost stage of the Cambrian 3 series therefore yields 497 Ma according to GTS 2016 (Ogg et al. 2016).
7.4.3 Fossil Record of Parasitic Platyhelminths
Transitions from free-living to parasitism amongst the Platyhelminthes may have
evolved at least nine times (Weinstein and Kuris 2016), but only the divergence of
the Neodermata can be reasonably constrained by fossil evidence (De Baets et al.
2015). Neodermata are a monophyletic group comprising exclusively obligate parasites (Fig. 7.4). Present day associations include the use of vertebrates as definitive
hosts. The internal phylogeny is still debated, but most authors consider ectoparasitic monogeneans as the earliest diverging lineage while cestode and trematodes
are considered sister groups in most molecular analyses (Lockyer et al. 2003; Park
et al. 2007; Egger et al. 2015). The oldest fossils of Neodermata include circlets of
hooks found on early armored jawed fishes (Leung 2017; Upeniece 2001, 2011).
Their appearance, particularly the arrangement of hooks and association with putative sucker discs, are consistent with their identification as monogeneans (Leung
2017; De Baets et al. 2015). Their location near gills and abdominal near fin spines,
would also be consistent with an ectoparasitic mode of life. The larger size of hooks
when compared to extant taxa is consistent with a trend towards smaller size within
monogeneans over time (Poulin 2005).
Node. Neodermata
Fossil evidence. Over 69 circlets of hooks, at least 29 still associated with their fish
hosts in position suggestive of ectoparasitism (Upeniece 2001, 2011).
Phylogenetic justification. No formal phylogenetic analysis on the attachment
organs was performed. However, the circular arrangement of the bilateral symmetric hooks, traces of cuticular disc as well the maximum number of 16 hooks
strongly suggests their assignment as monogenean platyhelminths (Upeniece
2011; De Baets et al. 2015; Leung 2017, 2021).
Host. mostly attached to the acanthodian Lodeacanthus gajicus (24) and some (5)
also associated with juveniles of the placoderm Asterolepis ornata (Upeniece
2001, 2011).
Minimum age. 382.7 Ma. The formation containing the finds was initially assigned
to the Early Frasnian (Upeniece 2001), but currently the Lode Formation is
assigned to the Upper Givetian (Lukševičs et al. 2012, 2014). This corresponds
with 382.7 Ma corresponding to the minimum age assigned to the GivetianFrasnian boundary according to GTS 2016 (Ogg et al. 2016).
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution