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CHELICERATA

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1. Chasmataspids have a genital appendage

Chasmataspids appear to represent a distinct, and extinct, clade within the Chelicerata. They have the diagnostic character (or autapomorphy) of a nine-segmented postabdomen. What are their realtionships within the chelicerates? They were orignally thought to be xiphosurans, but my recent work (Geodiversitas, in review) has identified two significant features in chasmataspids; namely a genital appendage, and a metastoma. Both of these features were thought to be diagnositic only for eurypterids, but their presence in a eurypterid strongly suggests that both these extinct groups, i.e. eurypterids and chasmataspids, are very closely related.


2. What should we do with poor little mites ?

Mites have proved to be one of the most difficult arachnid orders to place in phylogenetic studies. It has even been suggested that they are not a natural (i.e. monophyletic) group. Ongoing studies suggest that the mouthparts of mites, solifuges (camel spiders) and pseudoscorpions have similarities of the mouthparts, i.e. a kind of beak or rostrum, which have not been widely recognised as homologous, at least not in mites. The same structures appear to have been given different names within the different groups! If the mouthparts in these orders are homologous then these three orders could be interpreted as a clade.


3. Pycnogonids probably are chelicerates

Pycnogonids, or sea spiders, are a controversial group which look like spiders but which have not always been acepted as chelicerates or even relatives of them. The problem is that no-one has demonstrated convincingly that pycnogonids are more closely related to any other arthropod groups. Only pycnogonids and chelicerates have the chelate appendage respectively called a chelicera or chelifore. I think pycnogonids deserve to be placed as sister group of the other chelicerates, the latter of which have a derived pattern of embryological development (Wallosek & Muller, 1997).
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