> You're grossly oversimplifying things again. Much of that billion years of
> Late Precambrian is dominated by igneous and metamorphic rock, with
> unconformities mixed in. What sedimentary rocks do exist are generally not
> the type that can preserve small, delicate organisms. Besides, there is yet
> to be any kind detailed systematic search of Precambrian strata for the
> kinds of fossils that would have to exist, namely very small, very delicate
> organisms not too much different from sponges, only without the silicate
> scaffolding. As such, it is a bit premature to say that the fossil record
> does not support a claim that complex animal forms had been around for a
> billion years before the Cambrian.
>
Many Precambrian rocks are metamorphosed of course, they have been round longer
and had more chance of being metamorphosed. On the other hand there are many
Precambrian successions in many parts of the world which have not been any more
metamorphosed than those in the Phanerozoic. To use Australian examples I am
familiar with, the Late Archaean (2.6-2.7 Ga) greenstone belts in some parts of
Western Australia have had only greenschist metamorphism and have well
preserved sedimentary structures. Sediments of the Late Archean to
Paleoproterozoic (2.5-2.4 Ga) Hammersley Basin are flat lying. The flat lying
sediments of the MacArthur Basin are of Paleoproterozoic to Mesoproterozoic age
(1.8-1.5 Ga) and contain oil. Both these successions contain contain thick
successions of fine-grained clastics and carbonates, and many delicate
sedimentary structures. You can't use lack preservation through unsuitable host
rocks or metamorphism to explain the paucity of the Precambrian metazoan record.
This brings me to some more general comments that have come gurgling to the
surface as I have watched this discussion unfold. We must not forget that the
"Cambrian explosion" is a colourful figure of speech. We should not let it
blind us to the reality, or let rhetoric over come logic.
These are the facts. We go from the Neoproterozoic and Mesoproterozoic where
the wee beasties are very small, very delicate, and rare, to the Ordovician
where they are large, skeletonised, and abundant. Most of this change occurred
in the Cambrian. This period does not see the appearance of Metazoans, but the
skeletonisation and proliferation (both in diversity and numbers) of metazoans.
It is not due to changes in depositional environment or preservation potential.
There are few differences in lithology between the Neoproterozoic and the
Cambrian, apart from glacials in the Neoproterozoic and metazoan reefs in the
Cambrian younger. This statement does not hold true for Mesoproterozoic and
older successions, where there are some unique lithologies.
Although there are rare traces of metazoans (tubes, trails) in older rocks, the
change began with the Ediacara fauns (~600 Ma) and had ended by the beginning
of the Ordovician (~490 Ma) This is 100 My period is only 2% of the earth's
history. Most of the radiation did occur in the Cambrian (hence "Cambrian
explosion"). In fact, it was concentrated in the second half of the Cambrian as
the first half of the Cambrian is pre-trilobite.
So in about 25 million years, less than half the length of the Tertiary, we see
the appearance of all the skeletonised phyla. There is no other period in the
fossil record like it.
The problem is not intractable. Molecular genetics can give us phylogenies and
mechanisms by which new body plans develop. Stratigraphy can tell us the rate
at which this occurred (I have little faith in molecular clocks except as a
stimulus for people to look for fossils!) and what was happening in the local
and global environments at the time. We know a lot more than we did 20 years
ago, especially about how changes body plans happen at the level of
developmental biology and genetics, about the diversity and distribution of
Neoproterozoic fossils (Ediacara fauna and the like) the stratigraphy of the
Cambrian-Neoproterozoic boundary, and have discovered metazoans in the Late
Mesoproterozoic at least. There is still much to learn, but both optimism and
humility are the order of the day.
Jonathan