Brachiopod miniaturization and its possible causes during the Permian-Triassic crisis in deep water environments, South China

摘要

Brachiopod miniaturization is both the reduction in size of individuals of a species over time, and where one brachiopod fauna dominated by larger individuals was replaced by a brachiopod fauna dominated by smaller individuals over time. This paper investigates this phenomenon prior to and during the end-Permian crisis in a deep water setting in South China, based on precise measurements of brachiopod body size and on the abundance and diversity of brachiopod species. The process of miniaturization near the Permian-Triassic boundary in deep water environments is characterized by: (1) Larger species, including Martirria sp., Costattamulus dongpanensis, and Anidanthus mucronata, reduced in average body size and abundance over time towards the end-Permian brachiopod extinction event. (2) Small species, including Attenuatella mengi and Spinomarginifera semicircridge, showed a mixed response with either no size change, or a reduction in average body size, while these species increased in abundance over time, indicating they were less sensitive to the stressed environmental setting. (3) Both the total abundance and the diversity of this brachiopod fauna decreased over time towards the end-Permian crisis. Regression, an increased input of terrestrial material and productivity decline are considered to all have contributed to brachiopod miniaturization, based on the biotic replacement and changes in sea-water chemistry. Among these factors, the fall in sea level is perhaps the primary factor, and led to a decline in productivity and an increased input of terrestrial material into the marine setting. (C) 2007 Published by Elsevier B.V.