A brief response:
The fossil record indicates that the Late Precambrian was dominated by
solitary and colonial coelenterates that may have included all four living
cnidarian classes (Conway Morris, 1993a). Excellent sponge fossils are
also present in the late Precambrian. There is also evidence for the
presence of mollusks as well as echinoderms before the beginning of the
Cambrian. The other major component of these ancient communities were
burrowing and trail-making worms of unknown affinity. Worms may also be
represented by the mineralized tubes of Cloudina and the large
multi-segmented Dickinsonia. Annelid, priapulid and palaeoscolecid worms
may be represented by body fossils. Remembering that nearly half of all
living phyla are worms, and that only the annelids have a significant
fossil record, it is clear that the phylum-level diversity of the Late
Precambrian may have been much greater than I have indicated.
Two aspects of the Early Cambrian fossil record will be emphasized here.
Firstly, with important new fossil discoveries and the redescription of
previously known forms, the many peculiar Cambrian taxa are now being
grouped into coherent phyla. These phyla include living phyla as well as
groups interpreted as ancestral to living phyla. Secondly, many of the
Early Cambrian taxa have morphologies that bear similarities with more than
one living phylum (eg scale and plate bearing slug-like metazoans). That
is their morphologies are mosaics of phylum level characters. There is
also a good fossil transition that crosses phyla that shows a change in
fundamental body plan - this is the lobopod to arthropod transition that I
have mentioned previously on this listserve.
The Early Cambrian fossil record of the metazoan phyla shows the same
pattern as that of class and order level taxa in the Phanerozoic. Near the
origin of these higher level taxonomic categories, the boundaries between
the taxa become blurred and fossils become difficult to classify. Moving
back in time toward their presumed point of diversion from a common
ancestor, organisms belonging to separate phyla converge in morphology.
The argument that the morphological disparity of Cambrian life exceeded
that of living organisms is disputed by many workers. The primary problem
is the retrospective nature of classification. Those morphologic features
that are used to define higher taxonomic categories are consequences of
history. Stable character states within modern higher taxa (ie. those
characters used to define these higher taxa) are often more variable in
organisms near the time of their origin and early diversification. Some of
these originally highly variable character states appear to have become
stabilized in subsequent evolution, while others disappeared. In many
cases the defining characters of higher taxa may have had no special
adaptive ecological significance, relative to other character states, at
the time of their appearance. Thus, characters used to distinguish modern
taxa cannot be used to assess the disparity of Cambrian forms. The
disparity of the Cambrian must be assessed as though one were living during
the Cambrian, without reference to classifications based on living forms.
Such an analysis was completed by Briggs and Fortey (1992) and Wills and
others (1994) who found that the morphologic disparity of Cambrian
arthropods was no greater than that of living arthropods, and that Burgess
Shale problematica fall within the range of morphospace delimited by living
taxa. Furthermore, they conclude that the extinct problematic Cambrian
forms do not represent body plans of equal status to modern arthropod
classes.
An interesting study by Valentine and others (1994) uses the number of cell
types as a useful measure of morphological complexity. They plot the
estimated times of origin of major body plans against their cell type
numbers. The resulting plot shows that the upper bound of complexity has
increased steadily and nearly linearly from the origin of the metazoa to
the present. Furthermore, they conclude that "...the metazoan 'explosion'
near the Precambrian/Cambrian transition was not associated with any
important increase in complexity of body plans..." (Valentine et al., 1994,
p.131).
Briggs, D.E.G. and Fortey, R.A., 1992, The Early Cambrian radiation of
arthropods. IN, J.H. Lipps and P.W. Signor (eds.), Origin and Early
Evolution of the Metazoa: Plenum Press, New York, p. 335-373.
Valentine, J.W., Collins, A.G., and Meyer, C.P., 1994, Morphological
complexity increase in metazoans: Paleobiology, vol. 20, p. 131-142.
Wills, M.A., Briggs, D.E.G. and Fortey, R.A., 1994, Disparity as an
evolutionary index: a comparison of Cambrian and Recent arthropods:
Paleobiology, vol. 20, p. 93-130.
Keith
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506
kbmill@ksu.ksu.edu
http://www-personal.ksu.edu/~kbmill/