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.

A relatively diverse chelicerate fauna has been detected in Early Devonian, Siegenian strata of the Westerwald area, Rhineland-Palatinate, Germany. The arachnids, comprising trigonotarbids and the oldest phalangiotarbids, are described and figured here along with the chasmataspidids. To accomodate the phalangiotarbid a new genus and species in the family Architarbidae, Devonotarbus hombachensis, is raised. Devonotarbus n. gen. is characterized by an approximately straight posterior carapace margin, abbreviated and undivided anterior tergites, a large sixth tergite, and fused posterior tergites. The chasmataspidid closely resembles Diploaspis casteri from the Emsian assemblage of Alken an der Mosel, but is readily discernible as a new species, D. muelleri, by a strong tuberculation of its dorsal integument. With only fragmentary opisthosomal remains available, the trigonotarbids cannot be placed in known taxa with any certainty at this time.

A new sea spider (Arthropoda: Pycnogonida) from the Lower Devonian Hunsrück Slate (Germany) is described as Pentapantopus vogteli gen. et sp. nov. This is the fifth pycnogonid species known from this exceptional fossil Lagerstätte. The most conspicuous character of the new species is the presence of five pairs of walking legs. This character, in concert with a reduced abdomen, indicates a phylogenetic position of P. vogteli among the crown group pycnogonids. P. vogteli extends the knowledge of fossil pycnogonid body plans and underlines the significance of the Hunsrück Slate, as this locality shows the highest diversity of sea spiders for the entire fossil record so far.