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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 490

Last Page: 490

Title: Models for Interpretation of Micromorph Faunas in Washita Group: ABSTRACT

Author(s): Ernest A. Mancini

Article Type: Meeting abstract


Micromorph faunas typically are found in limonitic, pyritic, or phosphatic black shales. They consist of individuals that are smaller than those in normal faunas. These faunas are primarily the consequence of stunting, transportation, juvenility, and paedomorphosis. Probable specific factors that might lead to stunting are abnormal salinities, low food supply, and low oxygen. The winnowing out and concentration of smaller individuals of an assemblage can produce a transported fauna. Juvenility is the result of large-scale fluctuations in immature ecosystems. Evolution resulting in paedomorphosis is a response of populations to environmental factors such as instability or substrate fluidity. Faunal and sedimentologic characteristics which are potentially diagnostic of thes mechanisms can be recognized. Models defined by expected conditions of the criteria were established for each mechanism.

To test the usefulness of the models for distinguishing micromorph faunas in the Washita Group, a fauna described as micromorph was studied. The small size of fossils in the Grayson Formation of Texas, Oklahoma, and Mexico has been attributed to excessive iron concentrations, anaerobic conditions, and seasonal fluctuations. Comparison of the faunal and sedimentologic characteristics of the Grayson fauna with those of the established models indicates that the small shell size of some of the Grayson oysters and ammonoids is probably an adaptive strategy evolved by these organisms for survival in a soft-substrate environment. The small size may be a consequence of paedomorphosis. Pyritized Grayson specimens are probably a result of microreducing environments that developed within individ al shells in the soft mud.

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Copyright 1997 American Association of Petroleum Geologists