Monstrocrinus is a most unusual crinoid from the Devonian of Germany. It has long, simple to complex spines attached to most calyx plates. A stem was not associated with the initial specimens on which this genus was named. Thus, it was assumed that Monstrocrinus was a stemless crinoid that sat directly on the sea floor and, when alive, rolled around on its spines. For a variety of reasons, this seems unlikely, but the rolling crinoid lifestyle interpretation has persisted in the literature. More thorough cleaning of historical specimens and, more importantly, a new specimen with an attached column clearly refute the classical lifestyle interpretation and demonstrate that Monstrocrinus was a typical crinoid attached to a long column that was presumably anchored to the sea floor. In addition to revising the lifestyle of this crinoid, the varieties of spine types are redefined with names that more accurately reflect their morphology. Monstrocrinus occurs in Europe and South America, and one previously described species is placed in synonymy so that the genus is now comprised of three species.

The diplobathrid camerate crinoid genus Monstrocrinus is morphologically reinterpreted on the basis of new finds from the upper Emsian (Lower Devonian) to lower Eifelian (Middle Devonian) of Germany (Rhenish Slate Mountains, North Rhine-Westphalia, and Rhineland-Palatinate). The most complete, new specimen has a long segment of column projecting outward from the basal concavity, which confirms that the Monstrocrinus crown was attached to a column as an adult. Thus, a free-living life mode of a passively rolling or drifting crinoid is refuted and Monstrocrinus is reinterpreted as an attached, stalked echinoderm. This finding is supported by re-examination of the historical type material. A column attachment with a central pentalobate axial canal was recognized for the first time in the calyx of the holotype of the type species Monstrocrinus securifer. The holotype of Monstrocrinus granosus is far more complete than previously thought. It is a partially disarticulated crown embedded together with an associated longer part of the column. The extraordinary spines on the calyx plates of Monstrocrinus can be divided into eight morphological types and into two superordinate categories: “Category A” is a physical part of the calyx plate, whereas “Category B” is an attachment onto the calyx plate. Taxonomically, M. aliformis from the upper Emsian of Spain is treated herein as a subjective junior synonym of M. securifer.