Origin of body plans (phyla)

Keith B Miller (kbmill@ksu.edu)
Fri, 16 Oct 1998 20:53:55 -0600

An interesting article has just appeared in the journal _Lethaia_ (vo.31,
no.3, p. 197-210) by Graham Budd entitled "Arthropod body-plan evolution in
the Cambrian with an example from anomalocaridid muscle." This article
discusses the evolution of the arthropod phylum from early Cambrian
lobopods. A number of spectacular discoveries in recent years have
provided a surprisingly good series of transitional forms that cross
phyla. The sequence goes from onychophoran-like lobopods, to armoured
lobopods, to gill-bearing lobopods, to anomalocarid-like forms with lobopod
limbs, to anomalocarids, to true arthropods.

An interesting point here is that arthropods possess a jointed exoskeleton
moved as lever arms with internal muscles, and have minimal or absent body
wall muscles. This is a functionally integrated system in which all parts
are required for function. Budd states rhetorically "..it may be difficult
to see how one aspect of that system may evolve without causing failure of
the whole." This sounds precisely like an irreducibly complex system ala
Behe. To make matters even more interesting, the onychophorans are
unsegmented, possess both circular and longitudinal muscle forming their
body wall, have hydrostatic skeletons, and lack a sclerotized cuticle. How
could it be possible for lobopods to evolve into arthropods through a
series of functional intermediates?

With new fossils that actually preserve the muscle fibers of the animals,
we now have a pretty good idea just how this transition did in fact take
place. The sequence is as follows: 1) circular and longitudinal muscle in
the body wall, 2) a muscular body wall with internal muscles that cross the
body cavity, 3) the loss of body wall muscle as internal muscles take over
locomotory functions, and 4) the sclerotization of the external cuticle.
As Budd states, there are no functional discontinuities in this sequence.

With this transition the boundaries of these phyla have become unclear.
The same pattern that is seen during transitions at any other taxonomic
level. Budd concludes: "In other words, the evolution that characterized
the emergence of phyla appears to be qualitatively similar to that which
characterized any other stage of evolution. If no distinction can be made
between this level of evolution - by definition at the level of the
phylotype - and that within the phyla which is supposedly within the
phylotype, then the application of the body-plan concept in the Cambrian
becomes problematic." This is a point I have been trying to get heard when
phyla are claimed to be the unbridgeable categories of life. New
discoveries are occurring apace and the origins of several other phyla are
close to being able to be reconstructed in manner similar to the
arthropods.

Keith

Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www-personal.ksu.edu/~kbmill/