318
In brief, the oldest chelicerates are Cambrian in age (Fig. 9.1); either middle if
one accepts the Canadian Burgess Shale fossil Sanctacaris as a chelicerate (e.g.
Legg 2014), or late Cambrian if dated on a putative larval sea spider from the Orsten
of Sweden (see below). The oldest horseshoe crabs and eurypterids are Ordovician
in age, while the oldest arachnids (e.g. Dunlop 1996; Waddington et al. 2015) are
Silurian (Fig. 9.1). These comprise scorpions (Scorpiones) from the mid-Silurian
and the extinct spider-like Trigonotarbida from the late Silurian. Harvestmen
(Opiliones), pseudoscorpions (Pseudoscorpiones) and some mites (Acariformes)
are known from the Devonian. Spiders (Araneae), whip spiders (Amblypygi), whip
scorpions (Thelyphonida) and camel spiders (Solifugae) were present by the time of
the Late Carboniferous Coal Measures. The remaining mites (Parasitiformes), palpigrades (Palpigradi) and schizomids (Schizomida) are first recorded from
Cretaceous (ca. 100 Ma) Burmese amber. All three are typically small and rather
cryptic groups, and this late occurrence in the fossil record is probably an artefact of
their low chances of being preserved. Thus a Palaeozoic origin for all the arachnid
orders seems likely (Fig. 9.1) and is also implied by several fossil-calibrated molecular studies (e.g. Rota-Stabelli et al. 2013; Sharma and Giribet 2014: Fig. 3).
Warnock et al. (2012) also inferred that parasitiform mites should have been present
by at least the Carboniferous.
9.2 Sea Spiders
Sea spiders are an enigmatic group whose position within the arthropods has long
proved controversial (reviewed by Dunlop and Arango 2005). Most workers now
accept them as chelicerates (see e.g. Giribet 2018), with which they share the chelate first pair of appendages—termed chelifores in sea spiders—although these are
reduced or absent in some derived groups. All sea spiders are characterised by a
unique feeding apparatus, the proboscis, which they use to ingest food, typically
sucking up nutrients from sessile organisms. While adult sea spiders are not strictly
parasitic, the larval stages of numerous species have been reported as ecto- or endoparasites of other marine organisms; for a review see Brenneis et al. (2017), and
references therein. These authors recognised five basic patterns of development,
three of which involve a hatching (protonymphon) larva which begins its life cycle
as a parasite. It should be noted here that (a) the entire life cycle is not known for all
pycnogonid species and (b) that the observed patterns are not necessarily taxonspecific, such that different developmental pathways may occur even within the
same genus:
1. In developmental pattern 1, the protonymphon lives as an ectoparasite on cnidarians and occasionally molluscs.
2. In pattern 2, there is no parasitic phase; the animals hatch at the later postlarval
stage and are lecithotrophic—i.e., they receive nourishment from a repository of
yolk held in the midgut—before eventually becoming free-living.
J. A. Dunlop
In brief, the oldest chelicerates are Cambrian in age (Fig. 9.1); either middle if
one accepts the Canadian Burgess Shale fossil Sanctacaris as a chelicerate (e.g.
Legg 2014), or late Cambrian if dated on a putative larval sea spider from the Orsten
of Sweden (see below). The oldest horseshoe crabs and eurypterids are Ordovician
in age, while the oldest arachnids (e.g. Dunlop 1996; Waddington et al. 2015) are
Silurian (Fig. 9.1). These comprise scorpions (Scorpiones) from the mid-Silurian
and the extinct spider-like Trigonotarbida from the late Silurian. Harvestmen
(Opiliones), pseudoscorpions (Pseudoscorpiones) and some mites (Acariformes)
are known from the Devonian. Spiders (Araneae), whip spiders (Amblypygi), whip
scorpions (Thelyphonida) and camel spiders (Solifugae) were present by the time of
the Late Carboniferous Coal Measures. The remaining mites (Parasitiformes), palpigrades (Palpigradi) and schizomids (Schizomida) are first recorded from
Cretaceous (ca. 100 Ma) Burmese amber. All three are typically small and rather
cryptic groups, and this late occurrence in the fossil record is probably an artefact of
their low chances of being preserved. Thus a Palaeozoic origin for all the arachnid
orders seems likely (Fig. 9.1) and is also implied by several fossil-calibrated molecular studies (e.g. Rota-Stabelli et al. 2013; Sharma and Giribet 2014: Fig. 3).
Warnock et al. (2012) also inferred that parasitiform mites should have been present
by at least the Carboniferous.
9.2 Sea Spiders
Sea spiders are an enigmatic group whose position within the arthropods has long
proved controversial (reviewed by Dunlop and Arango 2005). Most workers now
accept them as chelicerates (see e.g. Giribet 2018), with which they share the chelate first pair of appendages—termed chelifores in sea spiders—although these are
reduced or absent in some derived groups. All sea spiders are characterised by a
unique feeding apparatus, the proboscis, which they use to ingest food, typically
sucking up nutrients from sessile organisms. While adult sea spiders are not strictly
parasitic, the larval stages of numerous species have been reported as ecto- or endoparasites of other marine organisms; for a review see Brenneis et al. (2017), and
references therein. These authors recognised five basic patterns of development,
three of which involve a hatching (protonymphon) larva which begins its life cycle
as a parasite. It should be noted here that (a) the entire life cycle is not known for all
pycnogonid species and (b) that the observed patterns are not necessarily taxonspecific, such that different developmental pathways may occur even within the
same genus:
1. In developmental pattern 1, the protonymphon lives as an ectoparasite on cnidarians and occasionally molluscs.
2. In pattern 2, there is no parasitic phase; the animals hatch at the later postlarval
stage and are lecithotrophic—i.e., they receive nourishment from a repository of
yolk held in the midgut—before eventually becoming free-living.
J. A. Dunlop