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Too early to decide on (lack of) bilateral symmetry?

Posted by pdeley on 07 Sep 2014 at 14:01 GMT

These organisms are a very exciting find, but on first reading I'm concerned about two points where I feel much of the discussion of their phylogenetic affinities and implications may be premature. Firstly, it is not clear to me why they are emphatically described and classified as non-bilaterian. The overall shape of disc and stalk in the illustrated specimens looks bilaterally symmetrical, and the only point of exception seems to be the junction of the stalk's gastrovascular canal with the branching canals of the disc. Secondly, the absence of any gonad-like structures or primordia suggests that these specimens are not be adults (allowing for example the possibility of substantial change in morphology and lifestyle, including for example possible presence of cnidocytes or collocytes in adults only), and they may not even be part of a sexual segment of the organism's lifecycle if it were to combine alternate morphologies with alternate reproductive strategies (allowing for example the cnidarian-like possibility of these specimens being a semi-sessile form that reproduces asexually through budding or fission, while a substantially differently structured sexual form may develop at some point, become much more motile and inhabit a substantially different environment with different feeding behavior etc).

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RE: Too early to decide on (lack of) bilateral symmetry?

alephreish replied to pdeley on 08 Sep 2014 at 09:02 GMT

"Non-bilaterian" means "not belonging to Bilateria". What concerns the symmetry type: the authors explicitly mention, that Dendrogramma combine elements of radial and bilateral symmetry. Bilaterality is not unique to Bilateria and its elements can be found e.g. corals.

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