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

Order Opiliones - Harvestmen

Order Solifugugae - Solifuges (=Solpugids)

Order Pseudoscorpiones - Pseudoscorpions

'Order' Acari - Mites and Ticks

Order Ricinulei - Tick spiders

Order Palpigradi - Palpigrades

Order Araneae - Spiders

Order Amblypygi - Whip spiders

Order Thelyphonida - Whip Scorpions (=Vinegaroons)

Order Schizomida - Micro-whip scorpions

Species numbers and fossil record from Dunlop (2002).


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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2003-4