EARLY-STAGE POST-LARVAL CRINOIDS FROM THE LOWER DEVONIAN HUNSRÜCK SLATE, GERMANY
Based on comparison with the size and gross morphology of developmental stages in living comatulid crinoids, these tiny crinoids are judged to be analogous to the pentacrinoid stage of development, just after metamorphosis from the stalked, but armless, cystidean larval stage. Living comatulid crinoids have three larval stages (the uniformly-ciliated and doliolaria stages, and the stalked cystidean stage) before reaching the pentacrinoid developmental stage defined by arm development. Final metamorphosis to the juvenile stage in comatulids is by autotomy of the stem resulting in a free-swimming adult form. Living stalked crinoids become juveniles after metamorphosis from the cystidean stage as there is no pentacrinoid larval stage ending with autotomy of the stem (however, little is known about post-doliolaria larval development in living stalked crinoids). These tiny Hunsrück crinoids are judged to be at this post-larval developmental stage after metamorphosing from the cystidean larval stage.
These post-larval juvenile, crinoids are a further example of the extraordinarily detailed preservation of delicate tissues in pyrite from the Hunsrück Slate. They are most likely juveniles of adult crinoids (30 genera) present in the Hunsrück. Although similar in size, they are not thought to be paedomorphic adult microcrinoids, which lacked arms and apparently fed either by podia through open oral plates, or by direct absorption of nutrients through the epidermis.