Various parameters of the cup and crown of the crinoid Cromyocrinus simplex from different Carboniferous regional substages of the Moscow region have been measured to examine this species’ evolutionary pathways. Statistical tests clearly show differences between specimens depending on geological age, suggesting that there were several Cromyocrinus species, from the same phylogenetic lineage. The ontogenetic series of Cromyocrinus simplex from several intervals have been studied. Allometric growth of brachials was revealed. A comparison of specimens of Cromyocrinus with and without evidence of Platyceras settlement indicates no size suppression, supporting a non-parasitic nature of their interaction.