Modern crinoids are dominated by the comatulids (unstalked forms) which range from the intertidal to abyssal depths. Modern stalked crinoids are restricted to depths greater than about 100 m. In the geologic past some stalked crinoids lived at depths of a few tens of meters or less in reef and bank environments. The primary vehicles postulated for the post-Triassic radiation of comatulids are lack of permanent fixation to the substratum and the capacity for mobility. Development of complex muscular articulations has enabled crawling or swimming which serve in habitat selection and avoidance of stress and predators. These and other adaptations may have bestowed on comatulids a higher survival capacity in shallow-water environments compared to stalked crinoids. Modern stalked crinoids lack mobility and complex behavioral adaptations seen in comatulids. Possibly, stalked crinoids in shallow water were unable to cope with the radiation of abundant, predaceous bony fishes in the late Mesozoic and became restricted to greater depths while the more adaptable comatulids gained ascendancy in shallow water.

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Previously undescribed holes on the calyx of single specimens of the blastoids Pentremites and Cordyloblastus are circular in plan view, penetrate the test at a right angle, and are found in the interambulacral region. The same features characterize holes on 23 specimens of the Mississippian blastoid Orophocrinus. The holes are interpreted as biogenic in origin; examples of multiple complete holes on individual specimens and of incomplete holes on some blastoids indicate that the holes represent parasitism rather than predation. The similarity of these holes to those previously described in nucleocrinids and crinoids suggests that they were produced by platyceratid gastropods or closely related taxa.