The evolution of Spiders: Chelicerata
The subphylum (or phylum) Chelicerata (pronounced /kəˌlɪsəˈreɪtə/ or /kəˌlɪsəˈrɑːtə/) (New Latin, from French chélicère, from chél-; chela + -cère from the Greek keras, meaning “horn”) constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from in the Late Ordovician period. The surviving marine species include the four species of Xiphosurans (horseshoe crabs), and possibly the 1,300 species of Pycnogonida (sea spiders), if the latter are chelicerates. On the other hand, there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species.
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The evolution of Spiders: Phalangiotarbi
Phalangiotarbi (Haase, 1890) is an extinct arachnid order first recorded from the Early Devonian of Germany and most widespread in the Upper Carboniferous Coal measures of Europe and North America. The last species are known fron the early Permian Rotliegendof Germany.
The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida has been recently proposed to be sister group to (Palpigradi+Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990).
** the above image is a model