Metazoa		Chelicerata
ARTHROPODA		Chelicerata

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Chelicerata

ARTHROPODA  
|--Trilobita
`--Euarthropoda
   |--Myriapoda  
   `--+--Chelicerata  
      |  |--Merostomata
      |  |  |--Xiphosura
      |  |  `--Eurypterida
      |  `--Arachnida  
      |     |--"Scorpioniformes" 
      |     |  |--Opiliones
      |     |  `--Scorpionida
      |     |     |--Palaeoscorpiones
      |     |     `--Orthosterni
      |     |        |--Chactoidea
      |     |        `--Scorpionoidea
      |     `--"Araniformes"
      `--PANCRUSTACEA  

image from Tree of LifeSalticidae
© 1994-1995, Wayne Maddison

The Chelicerata are distinguished by the absence of antennae, and the presence of fangs or chelicerae (the first pair of appendages) rather than mandibles.  Members of this ancient group also possess pedipalps, a second pair of appendages near the mouth, which have various functions in the different groups.  Usually they are used to manipulate food.  In some chelicerates, such as scorpions and some eurypterids, the pedipalps are modified into large claws.

Whilst insects are defined as having three fused body sections (tagmata), the chelicerates have two. The body is divided into a front part - the cephalothorax or prosoma, and a rear part, the abdomen or opisthosoma.  The prosoma contains six or more fused segments, with at least six appendages (usually one pair of pre-oral chelicera, one pair of post-oral pedipalps, and four pairs of walking legs). The opisthosoma lacks walking appendages but may contain such specialized features as book lungs (pages of gills for respiration), a telson (a long spike for balance), spinnerets (in spiders, for spinning silk), or a sting (in scorpions, which seems to be a modified telson)

The Chelicerates include among their number both the tiniest (the microscopic Acari or mites) and the hugest (the giant Paleozoic Eurypterida) of all the Arthropoda.

Taxonomic Relationships

Chelicerates have traditionally been divided into two major classes, the Merostomata and Arachnida. The Xiphosura (horseshoe crabs and their prehistoric relatives) and the Eurypterida (sea scorpions) have been grouped together in a taxon called the Merostomata, defined as large marine chelicerates with book-shaped gills and a long terminal spine. While the true scorpions have been classified in the Arachnida along with the scorpions, spiders, mites, etc, these being primarily terrestrial.

More recently this classification has been challenged, as it has been argued that the traditionally Arachnid terrestrial scorpions are more closely related to the traditional Merostomata eurypterids or sea scorpions, or that alternatively they represent a three-way arrangement with eurypterids and arachnids, whilst the Eurypterids and the xiphosurids do not appear to be as closely related as previously thought. This makes both the Merostomata and Arachnida artificial (possibly polyphyletic) groups. There is a tendency therefore to split the Chelicerates up into a number of distinct classes, with the old Merostomata replaced by the class Xiphosura, and several additional minor groups, and the Arachnida retained sans the Scorpions, which are now given their own class. (This may perhaps be taking taxonomic inflation a bit far, but these sort of problems show the advantage the cladistic system has over the traditional Linnean hierarchy.)

Finally, there is a little-known group of arthropods that may be Chelicerates, but are only very distantly related to all other forms (they have even been placed on occasion in a separate phylum!) the pycnogonids or "sea spiders".

Phylogeny

<==o CHELICERATA 
   |-- PYCNOGONIDA [Pantopoda] 
   `--o EUCHELICERATA
      `--+--o †Lemoneites 
         `-+--o XIPHOSURA (horseshoe crabs)
            `-+--o †CHASMATASPIDA 
              `-+-o "Higher Chelicerata"
                |-+- †EURYPTERIDA ( sea scorpions) 
                | `- SCORPIONIDA ( scorpions)
                `-- ARACHNIDA (arachnids)

the above cladogram has been modified from the  original to accommodate the findings of Dunlop and Selden

Links

Links

Introduction to the Chelicomorpha

Subphylum Chelicerata

Merostomata - Animal Diversity Web, UMMZ

Introduction to the Cheliceramorpha (Chelicerata)

The early history and phylogeny of the chelicerates - by Jason A. Dunlop and Paul A. Selden (abstract of journal paper)

Books

Treatise on Invertebrate Paleontology - Part P: Arthropoda 2: Chelicerata. Geological Society of America and University of Kansas Press, Lawrence, Kansas. - Technical and now rather dated, still remains the best review of fossil groups. Lots of line-drawings of extinct genera