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Evolution of Chordata
Topic Started: Feb 2 2018, 03:48 AM (247 Views)
Fishfreak
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Friend of the fish
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Hey guys,

was curious if anyone was interested in the evolution of the Chordates as well as the Deuterostomes. Particularly the question of whether the Craniate sister group is Urochordata (forming Olfactores) or Cephalochordata (forming Euchordata), The evolution of commonly accepted chordate traits (e.g. pharyngeal slits, post-anal tails and of course notochords) and the position of Carpoids as either a stem chordate or echinoderm.

Thoughts?
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Claudiu Constantin Nicolaescu
Heterotrophic Organism
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But what is your opinion? And what are the questions? What are the "Carpoids"?
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Jinfengopteryx
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Aspiring paleontologist, science enthusiast and armchair speculative fiction/evolution writer
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I cannot respond to the OP question, but I can respond to the above:
http://en.wikipedia.org/wiki/Homalozoa
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Claudiu Constantin Nicolaescu
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Thank you very much for your answer, Jinfengopteryx!
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Fishfreak
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I was mostly interested in hearing what people thought of the 'Calcichordate theory'. It's the idea that the latest common ancestor of all Deuterostomes had a fairly complex structure containing both a calcite ectoderm (as in echinoderms), pharyngeal slits (as in chordates, and possibly hemichordates?). The argument is that carpoids such as:

Cothurnocystis
Posted Image

seem to contain all these traits. Usually regarded as a stem echinoderm, under the calcichordate theory it is thought to be an early stem chordate. The theory further goes on to argue that the Carpoids do not represent a single clade, but instead a number of different stem chordates groups, with the mitrates serving as the closest known sister group to extant chordates (not including hemichordates, which are regarded as ambulacrarians (Hemi-chordates + Echinoderms)). Cornutes are thought to be a group diverging off earlier than mitrates but before homalozoans. The reasoning for this is there is a general tendency towards bilateral symmetry. Carpoids also seem to possess primitive notochords in their 'tails'.

Compare:
Homalozoans
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Cornutes and Mitrates
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Some variations place some of the mitrates as stem cephalochordates, stem urochordates or stem craniates. The main argument against the theory usually is that it would requite calcite skeleton to be lost at least twice (in Hemi-chordates and chordates), but potentially more times (depending on the proposed identity of mitrates), which would violate parsimony. In the case of carpoids as stem echinoderms though it would imply that either a notochord-like structure developed twice independently in chordates and homalozoans.

Anyone got any thoughts on this or anything else regarding stem chordates?
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Claudiu Constantin Nicolaescu
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This is a very interesting and difficult topic, Fishfreak! Thank you very much for the information you posted! I did not know until now about these animals.
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