EARLY-STAGE POST-LARVAL CRINOIDS FROM THE LOWER DEVONIAN HUNSRÜCK SLATE, GERMANY

Several stemmed crinoids with crowns as small as 1 mm wide by 2 mm high are newly recognized from the Hunsrück Slate of southwestern Germany. Some of these tiny crinoids have a stalk up to 4 mm long with a holdfast. The presence of erect arms distinct from the calyx is uncertain, but they may be tightly infolded. Taxonomic identification of these crinoids is not possible, even to the subclass level, although they are preserved with larger juveniles that are most likely the cladids Propoteriocrinus and Lasiocrinus. The larger juveniles exhibit pyritized calcite plates, whereas the tiny crinoids appear to be preserved as pyritized dermal tissues enclosing poorly-calcified ossicles. Although a rare type of preservation, pyritized soft tissues have been found in a wide variety of taxa in the Hunsrück Slate.

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.