Modern Groups in Chelicerata

Xiphosura

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Limulus polythemus (courtesy of AMNH)

Xiphosura: Horseshoe crabs The only living xiphosurans are four species of horseshoe crab belonging to a single family. As such, they are the only living aquatic chelicerates and can tell us much about chelicerate evolution and the palaeobiology of extinct groups. Modern horseshoe crabs live in shallow marine environments but come ashore in large numbers, to reproduce and lay their eggs in the sand. They swim upside down (exploiting the vortex resulting from the shape of the prosoma) , or walk on the seabed, and are also able to burrow. They are predatory, feeding on small invertebrates, which they grind up with their spiny leg bases (=gnathobases). The fossil record of Xiphosurida extends back to the Carboniferous (350 million years ago), with earlier records of related taxa (see diagram). The group is generally rare in the fossil record, but famous examples include Carboniferous freshwater genera such as Euproops and Belinurus, and the Solnhofen (Jurassic) Mesolimulus, exceptionally preserved at the end of its 'death trails'.

Arachnida

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Order Scorpiones - Scorpions	Order Opiliones - Harvestmen	Order Solifugae - Solifuges	Order Pseudoscorpiones - Pseudoscorpions
'Order' Arari - Mites	'Order' Acari - Ticks	Order Ricinulei - Tick spiders	Order Palpigradi - Palpigrades
Order Araneae - Spiders	Order Amblypygi - Whip spiders	Order Thelyphonida - Whip scorpions*	Order Schizomida - Micro-whip scorpions*

(*Sometimes united into Order Uropygi)

All arachnid drawings from Berland (1932)

Order Scorpiones - Scorpions

• Scorpions are familiar because of their striking appearance, which they owe to their exaggerated pedipalpal claws and curving tail with telson and aculeus (sting). They are well adapted for their terrestrial lifestyle, with book lungs and effective walking gait. There are around 1250 extant species.
• They extend over a range of habitat types, from forest to desert. They have low metabolic rates and can stand extremes in temperature.
• Their fossil record is good, with mid Silurian examples (430 million years old) showing that they were important early terrestrialisers (see diagram).

Order Opiliones - Harvestmen

• Harvestmen are small-bodied, long-legged arachnids that have lost lateral (compound) eyes, have a fused prosoma-opisthosoma. Males possess a penis used for direct insemination. There are about 5000 extant species.
• Harvestmen occupy a wide range of terrestrial niches.
• The earliest harvestman has been reported from the Rhynie Chert of Scotland (Devonian, around 410 million years old) (Dunlop et al., in press).

Order Solifugugae - Solifuges (=Solpugids)

• Medium to large arachnids (up to 7 cm long), desert-dwelling and with large jaws for catching invertebrates. Around 900 extant species.
• Very poor fossil record, known from Carboniferous (c. 300-350 million years ago).

Order Pseudoscorpiones - Pseudoscorpions

• Small (less than 7 mm long), with large pedipalpal claws but no 'scorpion tail'. No median eyes. Display the unusual behaviour of 'phoresy', which involves attaching themselves to a passing insect as a dispersal mechanism. Occupy leaf litter or bark crevices, etc. Around 3000 extant species.
• Very poor fossil record, but good examples known from the Devonian (c.400 million years ago).

'Order' Acari - Mites and Ticks

• Small to tiny chelicerates. 'Acari' is an enormous group with around 40,000 extant species; it may consist of many orders, and may not be a natural group. Worldwide distribution encompassing decomposing, predatory and parasitic niches.
• Fossil record poor (due to small size) but known from Devonian (c. 400 million years ago).

Order Ricinulei - Tick spiders

• A rare and unusual group, without eyes, but with sperm-transferring penises. Inhibit forest litter. Around 60 extant species.
• Fossils recorded from the Carboniferous (c. 350 million years ago).

Order Palpigradi - Palpigrades

• Tiny and rare arachnids, without eyes. Inhabit soil, caves or littoral sand. Around 80 living species.
• Very poor fossil record; not known before Caenozoic (post -65 million years ago)

Order Araneae - Spiders

• The most familiar arachnids, and a diverse and important group. Defined by their silk glands and spinnerets (silk-weaving organs), the sperm-transferring male pedipalp, and poison glands in the chelicerae. Small to large (1 mm to 25 cm), and abundant in a range of habitats. Web-spinning or free-living forms. Around 34,000 extant species.
• Relatively good fossil record, first known from Devonian (c. 400 million years ago), and extensively known from Mesozoic and Caenozoic ambers.

Order Amblypygi - Whip spiders

• Medium-sized (up to 4.5 cm), with a flat body and long legs. Mainly subtropical to tropical. Around 120 extant species.
• Fossils known from the carboniferous (c. 300-350 million years ago).

Order Thelyphonida - Whip Scorpions (=Vinegaroons)

• Medium to large arachnids (up to 7 cm body length). Distinctive for their telson 'flagellum'. Can also spray defensive liquids (hence the name 'Vinegaroons'). Around 85 extant species.
• Fossils rare, but known from the Carboniferous (c. 300-350 million years ago).

Order Schizomida - Micro-whip scorpions

• Miniature whip scorpions, only a few millimetres long and blind. Inhabit warm, humid habitats (mostly tropical), typically with very restricted geographic ranges. Can be associated with ant or termite colonies. Around 180 extant species.
• Fossils known from the Caenozoic (post-65 million years ago).

Species numbers and fossil record from Dunlop (2002).

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