Palæos:		Arthropoda
INVERTEBRATES		Basal Arthropoda

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Basal Arthropoda

Near the base of the arthropod family tree are a number of enigmatic forms that have turned up at the famous Cambrian lagerstätten at Chengjiang, Yunnan, China (Atdabanian Age) and the Burgess Shale, British Columbia, Canada (Amgan Age). These are intermediate between the very primitive and lobopod like forms and more typical arthropods.

This was part of a diverse Cambrian fauna that flourished briefly, dominating the Cambrian, but apart froma few holdovers died out at the end of the period.

Superclass Proschizoramia Hou & Bergström, 1997

Many forms that occur in the Chengjiang lagerstätten have been described by Jan Bergström and Hou Xianguang (the discoverer of the Chengjiang fauna), who follow an intuitive "key-character" approach that provoides an evolutionary-linnean classification, rather than the standard cladistic approach. As a result, their taxonomy is rather original. My own position is that the cladistic and linnean approaches, while incompatable, nevertheless are both equally useful

In their monograph, Hou & Bergström present the following subdivisions of the Proschizoramia:

Superclass: Proschizoramia
          class Yunnanata
          class Paracrustacea
          class Megacheira 
          class Sanctacaridea

Regarding the above schema, the relationships of these forms is very controversial. The Canadaspidida for example, considered by Hou & Bergström an early side-branch of arthropod evolution ("paracrustacea"), are more often interpreted as early true Crustacea. The Sanctacaridea are generally considered very basal Chelicerates (or Arachnomorpha, as oppsoed to true Chelicerata)

The following is a cladogram adapation of the above

<==o PROSCHIZORAMIA Hou & Bergström, 1997 
   |--o †YUNNANATA 
   |  `--o †FUXIANHUIIDA
   |     |-- †Fuxianhuiidae
   |     `-- †Chengjiangocarididae
   `--+?- †Habeliidae
      |?- †Molariidae
      |?- †BRADORIIDA
      |?-o †ACANTHOMERIDIIDA
      |  `-- †Acanthomerididae
      |?-o †TUZOIIDA
      |  `-- †Tuzoiidae
      |?-o †VETULICOLIDA Chen & Zhou, 1997
      |  `-- †Vetulicolidae
      |--o- †PROSCHIZORAMIA
      |  |--o †PARACRUSTACEA
      |  |  `--o †CANADASPIDIDA
      |  |     |-- †Canadasipididae
      |  |     |-- †Hymenocarididae [jr. synon. to Canadaspididae?]
      |  |     `-- †Perspicarididae
      |  `--o †MEGACHEIRA
      |     |--o †LEANCHOILIIDA
      |     |  |-- †Leanchoiliidae
      |     |  |-- †Actaeidae
      |     |  `-- †Alalcomenaeidae
      |     |--o †YOHONIIDA
      |     |  `-- †Yohoiidae
      |     `--o †FORTIFORCIPIDA
      |         `-- †Fortiforcipidae
      |--o †SANCTACARIDEA 
      |  `--o †SANCTACARIDA
      |     `-- †Sanctacarididae
      `--+-- LAMELLIPEDIA 
         `-- CRUSTACEOMORPHA

Note: Problematic taxon, Vetulicolida Chen & Zhou, 1997, has been removed from Schizoramia by Shu, et al., 2001 and replaced to a new phylum Vetulicolia [Metazoa: Deuterostomia:].