321
3. In pattern 3, the protonymphon is an ectoparasite of nudibranch or bivalve molluscs, or on sedentary polychaete worms.
4. In pattern 4, the protonymphon is an endoparasite of cnidarians or other hydrozoans. Note that the postlarval instars in modes 1, 3 and 4 are all then free-living.
5. In pattern 5, the animals hatch at a later stage (an advanced postlarva) which is
again lecithotrophic and thereafter free-living.
9.2.1 Cambropycnogon
Fossil Pycnogonida are rare (summarised by Bamber 2007), with a handful of
records from the Ordovician, Silurian, Devonian and Jurassic. All of these are based
on adult material. The oldest putative sea spider is a tiny, ca. 270 μm long, phosphatised juvenile instar from the late Cambrian (ca. 501 Ma) ‘Orsten’ of Sweden
(Fig. 9.2) described as Cambropycnogon klausmuelleri (Waloszek and Dunlop
2002). Its pycnogonid affinities were subsequently questioned by Bamber (2007),
although it does resemble the post-protonymphon stage of modern sea spiders, and
other authors (Wolfe et al. 2016; Brenneis et al. 2017) have been more sympathetic
towards its inclusion in Pycnogonida. If Cambropycnogon is an early sea spider
larva, it would indicate that these animals also underwent anamorphic development
back in the Cambrian; in other words early instars do not have the full complement
of adult appendages and add limb pairs with successive postembryonic stages. It is
thus tempting to speculate whether Cambrian sea spiders also had a parasitic hatching stage. Brenneis et al. (2017) noted that Cambropycnogon showed similarities to
living species such as Pycnogonum littorale, which use the (parasitic) type 1 development pattern in their scheme. However, it should be reiterated that the
Cambropycnogon fossils are not at the protonymphon stage—the next set of limb
buds (or anlage) was already developing—and its mode of life remains equivocal.
Further data is needed to assess the extent to which Cambropycnogon preserves
characters from the chelicerate ground pattern, as opposed to adaptions for a parasitic mode of life.
9.3 Horseshoe Crabs and Eurypterids
Excluding sea spiders, the remaining chelicerates are usually grouped together as
the Euchelicerata (reviewed by Giribet 2018). This clade traditionally comprises
Xiphosura, Eurypterida, Chasmataspidida and Arachnida although, as elucidated by
Lamsdell (2013), several fossils traditionally considered to be basal horseshoe crabs
may be better interpreted as basal euchelicerates instead. It should also be noted that
the textbook ‘Merostomata’ grouping of Xiphosura and Eurypterida has not been
supported in more recent phylogenies, which tend to place eurypterids closer to
arachnids. Irrespective of phylogenetic details, the feeding ecology of modern
9 Chelicerates as Parasites
3. In pattern 3, the protonymphon is an ectoparasite of nudibranch or bivalve molluscs, or on sedentary polychaete worms.
4. In pattern 4, the protonymphon is an endoparasite of cnidarians or other hydrozoans. Note that the postlarval instars in modes 1, 3 and 4 are all then free-living.
5. In pattern 5, the animals hatch at a later stage (an advanced postlarva) which is
again lecithotrophic and thereafter free-living.
9.2.1 Cambropycnogon
Fossil Pycnogonida are rare (summarised by Bamber 2007), with a handful of
records from the Ordovician, Silurian, Devonian and Jurassic. All of these are based
on adult material. The oldest putative sea spider is a tiny, ca. 270 μm long, phosphatised juvenile instar from the late Cambrian (ca. 501 Ma) ‘Orsten’ of Sweden
(Fig. 9.2) described as Cambropycnogon klausmuelleri (Waloszek and Dunlop
2002). Its pycnogonid affinities were subsequently questioned by Bamber (2007),
although it does resemble the post-protonymphon stage of modern sea spiders, and
other authors (Wolfe et al. 2016; Brenneis et al. 2017) have been more sympathetic
towards its inclusion in Pycnogonida. If Cambropycnogon is an early sea spider
larva, it would indicate that these animals also underwent anamorphic development
back in the Cambrian; in other words early instars do not have the full complement
of adult appendages and add limb pairs with successive postembryonic stages. It is
thus tempting to speculate whether Cambrian sea spiders also had a parasitic hatching stage. Brenneis et al. (2017) noted that Cambropycnogon showed similarities to
living species such as Pycnogonum littorale, which use the (parasitic) type 1 development pattern in their scheme. However, it should be reiterated that the
Cambropycnogon fossils are not at the protonymphon stage—the next set of limb
buds (or anlage) was already developing—and its mode of life remains equivocal.
Further data is needed to assess the extent to which Cambropycnogon preserves
characters from the chelicerate ground pattern, as opposed to adaptions for a parasitic mode of life.
9.3 Horseshoe Crabs and Eurypterids
Excluding sea spiders, the remaining chelicerates are usually grouped together as
the Euchelicerata (reviewed by Giribet 2018). This clade traditionally comprises
Xiphosura, Eurypterida, Chasmataspidida and Arachnida although, as elucidated by
Lamsdell (2013), several fossils traditionally considered to be basal horseshoe crabs
may be better interpreted as basal euchelicerates instead. It should also be noted that
the textbook ‘Merostomata’ grouping of Xiphosura and Eurypterida has not been
supported in more recent phylogenies, which tend to place eurypterids closer to
arachnids. Irrespective of phylogenetic details, the feeding ecology of modern
9 Chelicerates as Parasites