Australia: The Land Where Time Began

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Carpoids (aka stylophorans or calcichordates) Middle Cambrian-Middle Devonian

The carpoids, the 4th group of putative fossil chordates, are organisms with a calcitic (calcium carbonate) outer covering of a particular type, the mineral being pierced by many small holes, the creature consisting of 2 parts, a body portion that was compact, and a long appendage that was segmented. The carpoids have been interpreted as aberrant echinoderms by most authors, but have been proposed by one author (Jefferies, 1986, 1997) that they are a mixed group of basal echinoderms, cephalochordates, sea squirts, and vertebrates. According to Benton, there are 4 criticisms of Jefferies proposal.

  • There is agreement between morphological and molecular phylogenetic analyses that doesn't correspond to the 'calcichordate' hypothesis requirements
  • Anatomical structures, the function of which is still being debated, comprise much of the basis for the 'calcichordate' hypothesis. An example is the interpretation by Jefferies (1986, 1997) of the appendage on carpoids as a tail, his critics calling it a locomotory stem or feeding arm. Jefferies calls a major body opening a mouth, while other researchers call it an anus, and a series of openings is interpreted by Jefferies as pharyngeal gill slits, others calling them inhalant respiratory pores.
  • There are also a number of major character losses that are included in the theory, and according to the theory, the carpoid and echinoderm calcite skeleton was apparently lost in 3 lines, along the lines leading to the cephalochordates, tunicates and vertebrates. Benton suggests the calcite skeleton of carpoids and echinoderms evolved once and was subsequently retained, this being the most economical assumption.
  • The carpoids have been determined to be a monophyletic group (Ruta, 1999), that share a number of characters, such as the flattened 'bag-like' shape, as well as the appendage, many details of the plates covering the body, and various openings. If the group is monophyletic, as is debated, Benton suggests it cannot be distributed in different places throughout deuterostomes phylogeny. He suggests that it is potentially interesting that the presence of gill slits that have been postulated in carpoids (Jefferies, 1986, 1997), as they could suggest a deuterostome character that was subsequently lost in echinoderms, saying that if we are not descended from carpoids, what are we descended from?

Sources & Further reading

  1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.

 

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Last Updated 16/08/2011

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading