Original shell material is rarely preserved in pre-Carboniferous pelecypods. Most early Paleozoic pelecypods are not, however, preserved as simple internal and external molds, but as complex composite impressions preserving superimposed features of both the interior and the exterior of the shell. The term "composite mold" is suggested for this kind of preservation, which is formed by sedimentary compaction during or after removal of the original shell material. An understanding of this kind of preservation is essential in interpreting the degree of distortion of the molds as well as in reconstructing the sculpture, dentition, and internal morphology of the original shell. The fidelity of external detail in most composite molds implies an early dissolution of the shell material while the matrix was still plastic. This further suggests that the shell material of most early Paleozoic pelecypods was dominantly or entirely composed of aragonite.

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