From benthic to pseudoplanktonic life: morphological remodeling of the Triassic crinoid Traumatocrinus and the Jurassic Seirocrinus during habitat change

Abstract

The pseudoplanktonic driftwood-attached crinoids Traumatocrinus (late Ladinian—early Carnian) and Seirocrinus (Norian–Toarcian) are compared with their Middle Triassic benthic ancestors of the families Encrinidae and Holocrinidae, respectively. The morphological transformations allow five major adaptations of stalked crinoids for pseudoplanktonic life to be specified within the constraints of the ancestral clades: (1) Lightweight construction to reduce the load of the driftwood, maintaining its buoyancy for as long as possible. This was achieved by pore systems of intercolumnal fossulae and longitudinal tubuli in Traumatocrinus, and by a distally tapering column in Seirocrinus. (2) Peramorphic lengthening of the column to reach plankton-rich water levels (Traumatocrinus and Seirocrinus), and the possibility to make the column flexible or stout by mutable connective tissue (Traumatocrinus). Different column lengths in a colony of Traumatocrinus allowed filtering of a larger water column. (3) Solid and persisting anchoring to the drifting substrate to prevent breakage and detachment from the log by adhesive, anastomosing radicular rootlets in Traumatocrinus and—less successful—in Seirocrinus by a tuft of flexible cirri. (4) Increasing the length of the food grooves to compensate for the disadvantage of drifting along with the plankton by peramorphic augmentation of endotomously branching arms (both forms). (5) Enlargement of the functional cup by intercalation of tegminal or/and pinnular elements (both forms). Exclusively ligamentary (granosyzygial) brachial articulation made the Traumatocrinus crown rather inflexible while Seirocrinus retained the ancestral flexible muscular articulations. A prerequisite for the pseudoplanktonic lifestyle was the availability of large driftwood logs that became available in late Middle Triassic times after the recovery of large gymnosperms. After 50 Ma, the niche of pseudoplanktonic drifting of stalked crinoids was closed in the Middle Jurassic by the rise of shipsworms.