125
origins in the Neoproterozoic (900–600 Ma) (e.g. Dohrmann and Wörheide 2017),
reliable fossil evidence from these times is extremely scarce (reviewed in
Cunningham et al. 2017). Both the oldest reliable crown-bilaterian and crowncnidarian fossils, Kimberella (555 Ma) and Corumbella (543 Ma), respectively, are
dated to the uppermost Ediacaran (Grazhdankin 2004; Warren et al. 2012). Other
Ediacaran fossils, including the classic Vendobionta (soft-bodied Ediacaran macrofossils like Dickinsonia) and the Weng’an and Lantian biota are difficult to interpret
as to both their exact age and affinities.
Extant Cnidaria comprise the taxa Anthozoa, Staurozoa, Endocnidozoa,
Scyphozoa, Cubozoa and Hydrozoa. Phylogenetic relationships among these taxa
have been subject to extensive debate. The most recent phylogenomic analyses
(Kayal et al. 2018) support a sister group-relationship between Anthozoa and the
clade comprising Medusozoa and Endocnidozoa (Fig. 4.2). Previously regarded as
sister group to Medusozoa, Kayal et al. (2018) recover Staurozoa within Medusozoa,
as sister to a clade uniting Cubozoa and Hydrozoa. The earliest generally accepted
cnidarian polyps, Corumbella and Paraconularia, are from the uppermost Ediacaran
or earliest Cambrian (<550/540 Ma) (Van Iten et al. 2014). Several Neoproterozoic
cnidarian candidates remain questionable, including Haootia, a 560 Ma impression
from the Ediacaran of Newfoundland interpreted as a staurozoan polyp. Lantianella
bears similarity to a scyphozoan medusa, but the Lantian Formation is not dated
precisely, with estimates ranging from 590 to 635 Ma (Wan et al. 2016). According
to molecular clock analyses the origin of cnidarians and their initial diversification
most likely date to the Cryogenian or even earlier (Park et al. 2012; dos Reis et al.
2015; Cunningham et al. 2017; Dohrmann and Wörheide 2017). Given its phylogenetic position, the endocnidozoan stem lineage must have diverged at some point
between the basal branching of Cnidaria and the emergence of crown medusozoans (Fig. 4.2). The earliest unambiguous cnidarian medusae are from the Middle
Cambrian Burgess Shale (Epoch 3, 505 Ma) (Young and Hagadorn 2010 and references therein). Stem medusozoans, perhaps still lacking the medusa stage could of
course be older, but recognising these forms could be highly challenging. Molecular
clock analyses date crown medusozoans to the late Cryogenian to early Ediacaran
(~700–600 Ma) (Park et al. 2012; Dohrmann and Wörheide 2017; Holzer et al.
2018). Holzer et al. (2018) infer ages of 651 Ma (601–700 Ma) and 588 Ma
(540–642 Ma) for the origins of Endocnidozoa and Myxozoa, respectively.
4.5.2 Vertebrate Origins and Fossil Record
Vertebrates, especially “fish”, are the most speciose and best studied endocnidozoan
hosts, and they have a relatively extensive fossil record (Friedman and Sallan 2012).
A number of soft-bodied potential vertebrates are known from Cambrian strata. A
particular problem in the interpretation of these fossils, however, is the lack of
crown-vertebrate characters. Among the more convincing fossil vertebrates are
Metaspriggina from the Burgess Shale and Haikouichtys from Chengjiang. These
4 Evolution, Origins and Diversification of Parasitic Cnidarians
origins in the Neoproterozoic (900–600 Ma) (e.g. Dohrmann and Wörheide 2017),
reliable fossil evidence from these times is extremely scarce (reviewed in
Cunningham et al. 2017). Both the oldest reliable crown-bilaterian and crowncnidarian fossils, Kimberella (555 Ma) and Corumbella (543 Ma), respectively, are
dated to the uppermost Ediacaran (Grazhdankin 2004; Warren et al. 2012). Other
Ediacaran fossils, including the classic Vendobionta (soft-bodied Ediacaran macrofossils like Dickinsonia) and the Weng’an and Lantian biota are difficult to interpret
as to both their exact age and affinities.
Extant Cnidaria comprise the taxa Anthozoa, Staurozoa, Endocnidozoa,
Scyphozoa, Cubozoa and Hydrozoa. Phylogenetic relationships among these taxa
have been subject to extensive debate. The most recent phylogenomic analyses
(Kayal et al. 2018) support a sister group-relationship between Anthozoa and the
clade comprising Medusozoa and Endocnidozoa (Fig. 4.2). Previously regarded as
sister group to Medusozoa, Kayal et al. (2018) recover Staurozoa within Medusozoa,
as sister to a clade uniting Cubozoa and Hydrozoa. The earliest generally accepted
cnidarian polyps, Corumbella and Paraconularia, are from the uppermost Ediacaran
or earliest Cambrian (<550/540 Ma) (Van Iten et al. 2014). Several Neoproterozoic
cnidarian candidates remain questionable, including Haootia, a 560 Ma impression
from the Ediacaran of Newfoundland interpreted as a staurozoan polyp. Lantianella
bears similarity to a scyphozoan medusa, but the Lantian Formation is not dated
precisely, with estimates ranging from 590 to 635 Ma (Wan et al. 2016). According
to molecular clock analyses the origin of cnidarians and their initial diversification
most likely date to the Cryogenian or even earlier (Park et al. 2012; dos Reis et al.
2015; Cunningham et al. 2017; Dohrmann and Wörheide 2017). Given its phylogenetic position, the endocnidozoan stem lineage must have diverged at some point
between the basal branching of Cnidaria and the emergence of crown medusozoans (Fig. 4.2). The earliest unambiguous cnidarian medusae are from the Middle
Cambrian Burgess Shale (Epoch 3, 505 Ma) (Young and Hagadorn 2010 and references therein). Stem medusozoans, perhaps still lacking the medusa stage could of
course be older, but recognising these forms could be highly challenging. Molecular
clock analyses date crown medusozoans to the late Cryogenian to early Ediacaran
(~700–600 Ma) (Park et al. 2012; Dohrmann and Wörheide 2017; Holzer et al.
2018). Holzer et al. (2018) infer ages of 651 Ma (601–700 Ma) and 588 Ma
(540–642 Ma) for the origins of Endocnidozoa and Myxozoa, respectively.
4.5.2 Vertebrate Origins and Fossil Record
Vertebrates, especially “fish”, are the most speciose and best studied endocnidozoan
hosts, and they have a relatively extensive fossil record (Friedman and Sallan 2012).
A number of soft-bodied potential vertebrates are known from Cambrian strata. A
particular problem in the interpretation of these fossils, however, is the lack of
crown-vertebrate characters. Among the more convincing fossil vertebrates are
Metaspriggina from the Burgess Shale and Haikouichtys from Chengjiang. These
4 Evolution, Origins and Diversification of Parasitic Cnidarians