[Taxacom] Origin of arthropods

[John Grehan]

Interesting article Ken. This higher level systematics for arthropods is
not something that I, for one, have kept up with (having no particular
reason to do so), but it was nice to see this. I had not even been aware of
the paraphyly of Crustacea in morphological and molecular studies. It was
interesting to see that there was some incongruence between the molecular
and morphological trees for Crustacean and insects which was discounted as
"This cannot be argued to be an especially strong critique of the
phylogenomic [.e. sequence] groupings because the rival morphology-based
groups have poor branch support and show considerable topological
instability." I wonder what they would have said if the morphology groups
had strong branch support and considerable topological stability? Even with
'poor branch support' the morphological phylogeny may be just as good.
Often just means that not as many characters are involved at particular
nodes, so inherently 'unstable', but if over time no other morphological
contradiction arises the morphological phylogeny may be just as 'robust'. I
wonder what the nature of the instability was and what difference it made,
if any? (just a rhetorical note).

John Grehan

On Thu, Aug 4, 2016 at 11:49 AM, Kenneth Kinman <kinman at hotmail.com> wrote:

> Dear All,      I've catching up on the past few years articles on the
> origin of arthropods.  I am very happy to see a consensus forming that
> Euarthropoda are descendants of a paraphyletic group of bivalved
> arthropods.  That is an extremely important advance.          However,
> almost all of the cladistic analyses (including Figure 4 in the paper
> below) continue to assume that these bivalved arthropods evolved from a
> paraphyletic series of lobopod taxa (tardigrades, onychophorans, Opabinia,
> and anomalocaridids).  I believe that exactly the opposite is true, and
> that lobopod taxa evolved from bivalved arthropods through the loss of
> their bivalves and then the loss of compound eyes and other losses.
>  If this proves true, the bivalved arthropods should be at the base of such
> trees, giving rise to increasingly complex forms (Euarthropoda) on the one
> hand, but also to increasingly "simplified" forms (lobopodans, etc.) on the
> other hand.  If you assume that lobopods are the outgroups and that
> assumption is false, it will tend to prevent further progress on
> determining the real outgroups of these bivalved arthropods (and thus the
> interrelationships of Arthropoda with other phyla).  Anyway, take a look at
> Figure 4 in the following paper:
> https://www.researchgate.net/publication/260912145_
> Frontiers_in_Palaeontology_Origins_and_early_evolution_of_arthropods
>                                -----------------------Ken
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