Re: Burgess Shale revisions - current state?

[cnedin@geology.adelaide.edu.au (Chris Nedin)]

>Hello all, from a lurker:
>
>This may not fit exactly in this list's area of expertise, but I'd
>appreciate it if anyone could help. I'm writing a piece about the current
>status of the Burgess Shale taxa, focussing on how Gould's theory about
>decreasing disparity has held up since he published Wonderful Life five
>years ago.

Simple answer, not very well.
Briggs et al (1992) analysed a range of arthropods and a group of forms
from the Cambrian and found no greater disparity in the Cambrian forms than
in modern arthropods. This result has been questioned by Gould, but it
appears that there is less disparity than Gould first suggested. Some of
Goulds most cherished 'flagship oddballs', as you say below, have now been
allocated to extant phyla.
The main problem about disparity is the modern taxanomic system (with the
emphasis on 'modern') The modern system is based, not unnaturally, on
modern forms. Hence the characteristcs which define an arthropod, for
example, were taken from extant arthropods. This was relatively easy to do
since organisms have been evolving along specific body plan morphospace for
a considerable period of time, resulting in a clustering around specific
body plans and very little overlap between bodyplans.
An analogy. Imagine morphospace as an undulating plain with frequent peaks
and various organisms as dots on this plain (this is not a plug for the
'landscape' theory of evolution, just an analogy). These peaks represent
body plan (and hence evolutionary) specialization. Those peaks which
represent adventageous body plans soon become populated with dots to the
detriment of those dots out on the flat (since the peaks represent an
advantage they outcompete the forms on the plain.
Now, after a suitable period of time, dots cluster around and over the
peaks and the plains become relatively pauperate. This is the situation we
see amongst extant forms today - easily recognised and distinct body plans
with very few 'intermediates'.
However, when we look back to the Cambrian, the modern situation has not
yet developed . There are numerous forms which have made it to the
foothills of the various peaks, but wallowing there. There is stilll a lot
of morphospace slack. Those staying in the foothills will eventually be
outcompeted by those farther up the slope (not necessarily the slope they
are around). These forms will not make it to the slopes and their
characteristics will not make it into the modern description of the body
plan. Thus, although the forms we see in the Burgess Shale has started out
on (and up) the winding road which leads to what we call arthropods, they
are destined (by chance or being outcompeted) never to make it and thus
their characteristics are not represented in modern arthropods. Does this
mean that they are not arthropods? I think they are.

 Also, as you mentioned, some of the non-arthropod 'oddballs' have now been
asigned to extant phyla. Therefor I think that the disparity claimed by
Gould is over-optimistic

> I've seen an article in Discover from May 1992, but have found
>nothing more recent. I know that Hallucigenia has been placed in the
>onychophora, and that Wiwaxia and Opabinia might also belong to extant
>phyla.

_Opabina_ is probably allied to _Anomalocaris_. (see Briggs and Whittington
1987)


> But what are the halkieriids that Conway Morris and Peel dug up in
>Greenland? How do they fit?

The Halkieriids are an old and destinguished group of (still) uncertain
affinities. The specimens found by Conway-Morris and Peal have two small
shell-like structures,  which look superficially similar to monoplacophoran
molluscs. However, the jury is still out on that one.


Chris Nedin   cnedin@geology.adelaide.edu.au
Dept. of Geology and Geophysics
University of Adelaide
South Austrlalia 5005

Refs

Briggs, D.E.G. & Whittington, H.B. (1987) The affinities of the Cambrian
animals _Anomalocaris_ and _Opabinia_. Lethaia, 20: 185-186.

Briggs, D.E.G.; Fortey, R.A. & Wills, M.A. (1992) Morphological disparity
in the Cambrian. science, 256: 1670-1673