257
place the origin of this group considerably earlier as far back as the Cambrian
(Sielaff et al. 2016; Herlyn 2021). Enigmatic hook-like fossils (Cambroclavida)
from the Cambrian have been attributed to Acanthocephala (Qian and Yin
1984; Amin 2013), but their size and morphology make this assignment
unlikely (Herlyn 2021).
The age of the gnathiferan crown group is complicated by the phylogenetic
position of chaetognaths, which have historically been difficult to place in animal
phylogeny. Chaetognaths have recently been argued to belong either within, or as
the sister taxon of Gnathifera (Laumer et al. 2019; Marlétaz et al. 2019) based on
phylogenomic analyses. Furthermore, macrofossils with morphological similarities with both chaetognaths and other gnathiferans have been identified from the
middle Cambrian Burgess Shale (Vinther and Parry 2019; Caron and Cheung
2019) further bolstering the hypothesis that chaetognaths and gnathiferans are
closely related.
The oldest fossil evidence for chaetognaths are protoconodonts, identified as the
remains of grasping spines (Bengtson 1983; Szaniawski 2002; Vannier et al. 2007),
which can at least be traced back to the Fortunian (Yang et al. 2016; Kouchinsky
et al. 2017). The grasping spines of chaetognaths have been argued to be homologous with the jaw apparatus of gnathiferans (Vinther and Parry 2019). These fossils
therefore provide a minimum age for the split between chaetognaths and their sister
taxon, which may be all other gnathiferan phyla (e.g., some analyses in Laumer
et al. 2019), a clade of rotifers and gnathostomulids (Marlétaz et al. 2019) or Rotifera
(Vinther and Parry 2019).
Ectoparasitic, or at least ectosymbiotic, relationships were likely present since
the Cambrian (Cong et al. 2017) in forms which are by some considered as possible
gnathiferans (Vinther and Parry 2019).
Node. Total Group Gnathifera
Fossil taxon. Protohertzina anabarica
Phylogenetic justification. No formal analysis incorporating protoconodonts has
been performed, but based on microstructural and gross morphological similarities, protoconodonts have been assigned to the total group of chaetognaths
(Bengtson 1983; Szaniawski 2002; Vannier et al. 2007) and would therefore be
crown group gnathiferans.
Hard minimum. The oldest protoconodonts assigned to Protohertzina anabarica
(Bengtson 1983) derive from the middle Fortunian Anabarites trisulcatus—
Protohertzina anabarica assemblage zone or its correlatives (Kouchinsky et al.
2017; Yang et al. 2016). This would yield a minimum age of 532.7 Ma according
to GTS 2016 (Ogg et al. 2016).
Node. Archiacanthocephala
Fossil evidence. Unnamed remains of acanthor larva (Cardia et al. 2019a).
Host taxon. Probably a crocodilian based on the characteristics of the coprolite
(Cardia et al. 2018, 2019a, 2019b), but this is probably only a false or facultative
host-parasite based on modern host-parasite relationships.
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
place the origin of this group considerably earlier as far back as the Cambrian
(Sielaff et al. 2016; Herlyn 2021). Enigmatic hook-like fossils (Cambroclavida)
from the Cambrian have been attributed to Acanthocephala (Qian and Yin
1984; Amin 2013), but their size and morphology make this assignment
unlikely (Herlyn 2021).
The age of the gnathiferan crown group is complicated by the phylogenetic
position of chaetognaths, which have historically been difficult to place in animal
phylogeny. Chaetognaths have recently been argued to belong either within, or as
the sister taxon of Gnathifera (Laumer et al. 2019; Marlétaz et al. 2019) based on
phylogenomic analyses. Furthermore, macrofossils with morphological similarities with both chaetognaths and other gnathiferans have been identified from the
middle Cambrian Burgess Shale (Vinther and Parry 2019; Caron and Cheung
2019) further bolstering the hypothesis that chaetognaths and gnathiferans are
closely related.
The oldest fossil evidence for chaetognaths are protoconodonts, identified as the
remains of grasping spines (Bengtson 1983; Szaniawski 2002; Vannier et al. 2007),
which can at least be traced back to the Fortunian (Yang et al. 2016; Kouchinsky
et al. 2017). The grasping spines of chaetognaths have been argued to be homologous with the jaw apparatus of gnathiferans (Vinther and Parry 2019). These fossils
therefore provide a minimum age for the split between chaetognaths and their sister
taxon, which may be all other gnathiferan phyla (e.g., some analyses in Laumer
et al. 2019), a clade of rotifers and gnathostomulids (Marlétaz et al. 2019) or Rotifera
(Vinther and Parry 2019).
Ectoparasitic, or at least ectosymbiotic, relationships were likely present since
the Cambrian (Cong et al. 2017) in forms which are by some considered as possible
gnathiferans (Vinther and Parry 2019).
Node. Total Group Gnathifera
Fossil taxon. Protohertzina anabarica
Phylogenetic justification. No formal analysis incorporating protoconodonts has
been performed, but based on microstructural and gross morphological similarities, protoconodonts have been assigned to the total group of chaetognaths
(Bengtson 1983; Szaniawski 2002; Vannier et al. 2007) and would therefore be
crown group gnathiferans.
Hard minimum. The oldest protoconodonts assigned to Protohertzina anabarica
(Bengtson 1983) derive from the middle Fortunian Anabarites trisulcatus—
Protohertzina anabarica assemblage zone or its correlatives (Kouchinsky et al.
2017; Yang et al. 2016). This would yield a minimum age of 532.7 Ma according
to GTS 2016 (Ogg et al. 2016).
Node. Archiacanthocephala
Fossil evidence. Unnamed remains of acanthor larva (Cardia et al. 2019a).
Host taxon. Probably a crocodilian based on the characteristics of the coprolite
(Cardia et al. 2018, 2019a, 2019b), but this is probably only a false or facultative
host-parasite based on modern host-parasite relationships.
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution