One thing which I hope this site makes clear is that even though the Cambrian is one of the most
intensively researched periods of geological time, there are still many points of contention about the
nature of the metazoan radiation event, with new discoveries and techniques adding to the picture
all the time. Most researchers agree that a significant biotic event did occur around the base
of the Cambrian. However opinion is divided upon whether this event was a true radiation of metazoan forms
from a single Precambrian ancestor, or if it was simply a revolution in biomineralisation by already
established phyla; an explosion of fossils rather than phyla.
The lists below summerise the main pieces of evidence for each of these viewpoints:
The Cambrian explosion was a true metazoan radiation:
- There is a paucity of Precambrian metazoan fossils.
- There is a sudden dramatic increase in metazoan fossils at the base of the Cambrian.
- The Burgess Shale type assemblages show that this is a radiation of soft-bodied as well as
skeletalized organisms.
- Trace fossils also increase dramatically in both number and range of behaviours.
- The Ediacara show that soft-bodied Precambrian faunas can be preserved, so why are there no
assemblages of 'true' metazoan soft-bodied faunas from the Precambrian?
- Why would many different, already separate phyla independently develop the
ability to synthesize skeletons?
The Cambrian event was the crossing of a biomineralization threshold by already established phyla:
- Soft-bodied faunas preserve only under exceptional circumstances, therefore Precambrian assemblages
are unlikely to be found.
- The Burgess faunas show that the majority of biota are soft-bodied, if there were no skeletalized elements
it is unlikely that anything would ever be preserved from the assemblage.
- Molecular and cladistic analysis show that the divergence times of most Cambrian phyla must
have occured in the Precambrian.
- Precambrian metazoans could have been very small and therefore fossilization would have been
impossible
- There is a decline in stromatolite diversity and abundance from about 1 billion years onwards -
possibly due to grazing by the new metazoan faunas?
These arguments are not clear cut however and many researchers take different viewpoints giving
some pieces of evidence more weight than others. It is also clear that a morphological
radiation of body forms and size does not have to be the same as a taxonomic radiation of
phyla. A lot depends upon the individual's definition of the metazoan radiation event.
For elaboration of some of the arguments plus a summary of recently discovered fossils go to:
