Acta
Palaeontologica Polonica
39, 247-313 (1994). Jerzy Dzik Abstract.
The
allegedly sudden appaearance of skeletal fosssils at the beginning of the
Cambrian is a preservational artifact. The most characteristic earliest
Cambrian fossil assemblages are calcareous fossils with secondary phosphatic
envelopes. Such preservation, although less and less abundant, continued to
occur throughout the whole Early Paleozoic. A high organic productivity, low
sedimentation rate, and shallow bioturbation controlled their distribution. It
is proposed that the evolutionary diversification and ecologic expansion of
infaunal detritus feeders resulted in extinguishing the ‘small shelly
fossils’ benthic environments. Gradual replacement of the Cambrian
coeloscleritophoran-monoplacophoran associations by Ordovician
machaeridian-gastropod and then by Silurian and later bivalve-gastropod
dominated ones is evident in the series of the Meishucun-, Mójcza-, and
Kok-type faunas. This is followed by a reduction in diversity of associated
organisms, starting with the anabaritids (possibly of trilobozoan or
nemathelminthan affinities), through tommotiids (possible machaeridians),
palaeoscolecid priapulids, hyoliths (with monoplacophoran-cephalopod
relationships), octactinellis and receptaculitid sponges, and then the
septemchitonid polyplacophorans. |