Regarding the discussion over the stratigraphic succession of the Early
Cambrian: It is nearly 10 years since I worked on such rocks in South
Australia, but as I recall trilobites appear relatively high up in the
sequence. From memory the succession in the Flinders Ranges is: Ediacaran
(Wilpena Group and possibly older), Tommotian (with the usual problematica plus
hyoliths, molluscs, and brachiopods and a range of burrows and tracks) in the
Uratanna and Parachilna Formations, and Faunal Assemblage I (Archaeocyaths,
spiculate sponges, plus the others) in the lower part of the lower Wilkawillina
Limestone. Trilobites do not appear until Faunal Assemblage II (upper part of
the lower Wilkawillina Limestone), where they join the other major groups.
Interestingly, there are trilobite trace fossils (Rusophycus, Cruziana) in the
Uratanna and Parachilna, but no body fossils. This is not a preservational
feature, because there are other body fossils in the same rocks. Clearly there
were trilobites about prior to FAII, but with unmineralised skeletons. The
appearance of trilobite fossils is therefore an indication of their acquisition
of mineralised skeletons, not of their actual appearance. We only know of their
earlier presence because of distinctive trace fossils. Arthropod groups which
do not have characteristic trace fossils will not be discovered (barring
exceptional circumstances such as the Burgess Shale) until they develop
mineralised exoskeletons.
These data are broadly congruent with the model brought to our attention by
Keith Miller.
In Christ
Jonathan
David Campbell wrote:
> >Trilobites are the first found metazoans of certain affinities in most
> >places around the earth. There are a hundred sites where trilobites are
> >found first for every site where another metazoan of certain affinity is
> >found. There are no sites of which I am aware where the presumed ancestors
> >of the arthropods are found stratigraphically below trilobites. You can
> >fill me in if I am wrong.
>
> "Of certain affinities" does restrict it more, though older primitive
> mollusks, sponges, and cnidarians are known from many areas. I do not
> remember whether other phyla are definitely known from the pre-trilobite
> basal Cambrian, though there are now some summary papers on the subject.
>
> However, if nothing of certain affinity is found below the trilobites, then
> nothing is known about when taxa appeared and we cannot say whether the
> trilobites appeared before, after, or simultaneously with the lobopods
> except by extrapolation.
>
> David C.