Microstructural design of the stalk in the crinoid Seirocrinus supports its pseudoplanktonic lifestyle

During geological history, some crinoids convergently deviated from a benthic lifestyle. Seirocrinus is an iconic example of a crinoid that adapted to a pseudoplanktonic mode of life by living attached to drift logs. While significant effort has been devoted to exploring its functional morphology and palaeoecology, little attention has been given to the microstructural design of its stalk. Here, we describe the stereom microstructure and internal architecture of the Seirocrinus stalk. We demonstrate that its stereom organization closely resembles that observed in extant benthic crinoids (isocrinids): coarse, irregular labyrinthic-like stereom is found in the interpetaloid zones, while the aligned, galleried stereom is extensively developed in the petaloid zones. We show that this galleried stereom displays a distinct Voronoi-like geometrical pattern, enabling strength-to-weight optimization and minimization of calcification costs. In contrast to the stalks of extant isocrinids, however, the seirocrinid stalk exhibits certain skeletal adaptations that reduce the crinoid's weight, such as large intercolumnar cavities. Consequently, the overall weight of these pseudoplanktonic crinoids might have been lower than previously thought, and thus they may not have significantly contributed to the sinking of the raft system, which would have allowed for multi-generational colonization of the log. These results support the hypothesis that the life of the Seirocrinus-bearing raft "colony" was more dependent on the wood structure itself and did not rely heavily on the seirocrinid "colonies" causing overloading.