Cathodoluminescence of Late Triassic terebratulid brachiopods: implications for growth patterns

The distribution pattern of CL lines that correlates with growth rate fluctuations potentially reflects a short-term environmental instability on the time scale of an individual life span. The data show correlation between CL line pattern and different bathymetric settings. Regular repetition of CL lines is preserved in Rhaetina gregaria, Rhaetina hybensis and Rhaetina pyriformis that occur in relatively uniform depositional settings below a normal storm wave base. In contrast, irregular pattern of CL lines is typical of R. gregaria from highly variable deposit associations derived from very shallow, subtidal settings above a normal storm wave base. In the case of R. gregaria, this difference reflects intraspecific variations in growth dynamics among bathymetric settings, indicating environmental control on their growth rates. Rhaetina gregaria is mostly the only one brachiopod species present in benthic assemblages from very shallow, marginal marine environments in the Western Tethys. The implication is that R. gregaria, in contrast to most other brachiopods, could live in physically stressed habitats characterized by higher environmental instability/unpredictability.