Physical vs. biological disarticulation of two extant stalked crinoids: implications for encrinite composition and depositional interpretation

Crinoid-dominated carbonate factories occurred intermittently from the Ordovician through the Jurassic, resulting in widespread encrinites and “regional encrinites” composed almost exclusively of the fossilized remains of pelmatazoan crinoid endoskeletons. The preservation of fully articulated crinoids is rare, with the bulk sediment comprised mainly of disarticulated stalk fragments. Therefore, understanding the taphonomic and biostratinomic processes affecting the generation, breakdown, and dispersal of crinoidal sediments is critical for interpreting ecological conditions and depositional processes.

Encrinite textures, grain sizes, and bioclast composition reflect both the intrinsic biological properties of crinoids (e.g., skeletal construction and connective tissues) as well as the extrinsic environmental controls, including physical transport processes, predation, and oceanic bottom-water temperature, oxygenation, and chemistry. Together, these factors influence skeletal disarticulation, dispersal, and preservation patterns of crinoid skeletal elements. Physical experiments on modern stalked crinoids provide a quantitative framework for linking laboratory-scale breakdown processes to encrinite textures observed in outcrop and basin scales.

Two extant stalked crinoids, Neocrinus decorus and Endoxocrinus parrae, were subject to simulated biological (bleach bath) and physical (mechanical tumbling) breakdown experiments to replicate dominant sediment-generation mechanisms. The resulting sediments were analyzed for grain-size and grain-shape distributions and qualitative breakdown patterns. Bleach experiments produced 135,661 particles representing five distinct element types and preferentially produced columnals and pluricolumnals characteristic of many encrinites. In contrast, tumbling experiments resulted in stepwise crown disarticulation and extensive stalk abrasion, producing fine-grained (<0.15 mm), morphologically indistinct skeletal particles. These results indicate that biological degradation is a primary control on diagnostic encrinite sediment production, while fine skeletal material may record subsequent physical transport, providing new constraints on the sedimentary origins and depositional histories of crinoid-dominated carbonate systems.