Arthropod Evolution Alternative

[Jason at]

Dear Ken,

My apologies for my long delay.


Manton’s views on Arthropod phylogeny were ambiguous. As I interpret her
ideas she believed that the different lineages “arthopodised” independently
i.e. they acquired their exoskeleton after they diverged. Hence my comment
that your “independent mineralisation” hypothesis is somewhat similar to
Manton’s. Of course, lacking fossils or a deep understanding of the genetics
underlying exoskeleton formation, arguing about arthropodisation or
mineralisation of the Arthropoda is just a philosophical exercise.

Pertaining to the placement of the Myriapoda and its monophyly, it is still
very much in the air considering the evidence as a whole and in particular
molecular evidence. Nuclear ribosomal genes support, albeit weakly, a basal
placement in the Mandibulata. However in a total evidence approach with
morphological data it is not strong enough to overturn the
Uniramia/Tracheata. Hence, as it stands, the Mandibulata is still very much
a polytomy. Personally I think that the Crustacea is a much more likely
sister clade to the Hexapoda, but it is just an opinion. In any case, a
Pancrustacea clade (Hexapoda + Crustacea) with a basal Myriapoda
(monophyletic or not)does not point either to a “bivalved” ancestor but
rather to the classic “annelid” form.

A polyphyletic annelida does not change this either. You are probably right
that the Annelida is not monophyletic and that the characters used to group
them are plesiomorphic. But, considering the list of most likely candidates
for sister to the Arthopoda (Nematoda, Priapulida & Annelida), the most
parsimonious reconstruction is an undifferentiated anatomy and not a shelled
(and therefore regionalised) body plan. Also don't I see why the shell
development of the Brachiopods could not follow the pathway as proposed by
Morris rather than the opposite.

The defense mechanisms that you mention as ulterior support are probably
influenced more by habitat than phylogenetic placement. After all many
parasites are rather simple but highly derived.

Cheers

Jason






>From: Ken Kinman <kinman at HOTMAIL.COM>
>Reply-To: Ken Kinman <kinman at HOTMAIL.COM>
>To: TAXACOM at USOBI.ORG
>Subject: [TAXACOM] Arthropod Evolution Alternative
>Date: Mon, 5 May 2003 02:02:35 +0000
>
>Dear All,
>      The morphology of the first arthropods is a long-standing mystery
>that
>is critical to the proper rooting of arthropod phylogeny, not only of the
>many living taxa, but also placing its rich fossil record in its true
>context.  Therefore, I would like to encourage anyone with interests in
>this
>subject to consider the following alternative rooting of arthropods, even
>though it may sound a bit bizarre on first hearing.
>      It is my belief that the first arthropods lived within bivalved
>shells
>somewhat similar to those of modern ostracodes (or their bradoriid
>ancestors).  Just as I believe all molluscs arose from the bivalved
>molluscs
>with bodies gradually "evolving beyond their shells" (and some even
>shedding
>the shells for good), I believe that all arthropods probably ultimately
>evolved from related bivalved arthropods also growing beyond their shells.
>      And I am using the term "shells" in a broad sense, since there was
>little if any mineralization of such shells before the latest PreCambrian.
>What I am saying is that mineralisation of shells occurred independently in
>many different lineages, but that the evolution of the original
>unmineralised bivalved shells was an extremely rare event.  The notion that
>various arthropods evolved bivalved shells many times convergently is not
>only unparsimonious, but to me has always seemed rather nonsensical.  It
>seems far more likely that multiple losses of shells would  have occurred.
>Again this parallels my view of mollusc evolution (gradual shell reduction
>and loss).
>     If this hypothesis is true, it will require the rerooting of arthropod
>phylogenies as well (crustaceans in particular, and this is one reason I
>did
>not recognize a formal taxon "Crustacea" in my 1994 classification).  That
>the insects arose from a paraphyletic "Crustacea" has become increasing
>clear.  The relationship of myriapods is much less clear, but they may have
>simply evolved from a different group of crustaceans (as opposed to a
>direct
>relationship with either insects or chelicerates, both of those proposed
>hypotheses seeming much less likely to me at the present time).
>     The arachnomorphs (trilobites, chelicerates, and allies) arose so
>early
>from some sort of protocrustacean group that they COULD (for most purposes)
>be considered sister group of a mandibulate clade.  However, if
>protocrustaceans evolved from protomolluscs (early bivalves), their shared
>orientation within the shell seems to be the plesiomorphic state, while the
>trilobitic body reorientation (resulting in dorsal-ventral shells rather
>than left-right shells) could be an arachnomorph synapomorphy (depending of
>course on one's particular definition Arachnomorpha).
>      Interestingly, my view is somewhat intermediate to the traditional
>view of agnostids (as closer to trilobites), and on the other hand some
>more
>recent views that regard agnostids as closer to crustaceans.  I simply
>regard agnostids being neither one nor the other (along with other
>"protrilobites" like helmetiids, naraoiids, mollisoniids, etc.).
>       But then, I am always on the lookout for solutions that lie between
>what I see as opposing extreme viewpoints.  Once again I reiterate that
>such
>intermediate viewpoints are not always simplistic "lowest common
>denominators" that lack merit, but real solutions that perhaps seem just
>too
>good to hope for.  I only latch onto those that I believe represent the
>most
>probable hypotheses, even if they are presently viewed as improbable from a
>conventional perspective.
>          ---- So it continues,
>                      Ken Kinman
>P.S.  Does anyone know if the term "De-arthropodization" has ever been
>used?
>  This is a term I plan to start using for arthropod descendants that have
>lost various arthropod characteristics secondarily.  Pentastomids are
>perhaps the best example.  Who knows, perhaps tardigrades and onychophorans
>are also NOT protoarthropods, but secondarily de-arthropodized taxa
>instead.
>  And wouldn't it be really weird if nematodes turned out to be tardigrades
>that had secondarily lost their legs (analogous to snakes evolving from
>lizards).  Something to think about.
>
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