ABSTRACT
The genus Balanocrinus is a common Jurassic crinoid recognised primarily on the pattern of the columnal articulations. Superficially similar columnals are found as early as the Middle Triassic and have been taken as ancestral. Chladocrinus and Balanocrinus share an identical shape and arrangement of stereom lumina on columnal latera, identical arrangement of brachial articulations, and similar very long cirri. Balanocrinus quiaiosensis is a small species showing strong morphological similarities to juvenile Chladocrinus. Derivation of Balanocrinus from Chladocrinus, by paedomorphosis, during the mid-Sinemurian seems probable. Subsequent evolution fits the punctuated equilibria model, with prolonged periods of evolutionary stasis punctuated by brief speciation events which correlate with rapid and widespread facies changes.
