Crinoid holdfasts from the Silurian of Gotland

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Abstract
Crinoid holdfasts occur throughout the Silurian sequence of Gotland [Sweden], with a marked concentration to reefs and immediate reef surroundings. Four morphological groups are recognized: discoid holdfasts; cirriferous holdfasts, represented by Rhizocrinus-like holdfasts, large radices with stout, often branching cirri and rhizoid holdfasts, formed by a complicated net of pseudocirri; creeping stems attached to the substrate by thin strands of stereome; and coiling stems. Discoid attachments, cirriferous holdfasts and coiling stems show little preference to substrate and adapt morphologically to conditions at hand. Rhizoid holdfasts display the greatest variation and apparently occur within many different groups of crinoids. Rhizocrinus-like holdfasts have been found only in quiet-water depositits, while creeping stems were concentrated to more turbulent environments. Coiling stems were epizoic, attached to rugose corals, bryozoans, crinoid stems and similar supports. Growth, mode of life (attached or free), settling and fixation of larvae and relation to substrate and other organisms are discussed.