Vachonisia rogeri LEHMANN, 1955, is redescribed on the basis of 5 previously known and 15 new specimens. V rogeri exhibits a dorsal shield, which covers the full length of the body. In cross section, the shield is shaped like a broad and low "V", probably with a very obtuse angle. Characteristics of the dorsal shield are a median ridge, which ends in a small anterior projection, an anterior indention of the shield, postero-lateral edges, and a ventral cavity that accommodates the appendages. The outer margin and the inner margin of the ventral shelf are strengthened. The limbs of V rogeri are arranged in two tagmata: a head and a trunk region. The head region possesses a hypostome, one pair of short and stout antennae (A1) and four additional limb pairs. Each of the first three post-antennal pairs of appendages has an endopod and an exopod. The exopods are walking leg-like and vary considerably in length and in the position and number of spines. The endopods of A2 and A3 bear a terminal chela. The endopod of the third post-antennal appendage (A4) is short and walking leg-like. A5 is a thin, extended, multi-segmented limb. The second branch (endopod?) was not found, but probably exists. Mature individuals developed between 50 and 80 biramous trunk appendages. The exopods consist of numerous short podomeres, each carrying a flattened seta with strengthened margins. The endopod podomeres (up to six) are fairly short, each bearing an endite. There is evidence from CT scans that endites occur on only the most anterior endopods (presumably the first to the fifth). The chelate condition of some appendages in the head, the notable increase in size in the exopods from A2 to A4, and the development of a flat ventral shelf on the ventral shield parallel the morphology of xiphosuran chelicerates. These characters are taken as indicators of a similar mode of life. V. rogeri was able to capture and comminute larger food particles. The trunk appendages and the shape of the shield of V rogeri indicate some swimming capability. The new material includes three ontogenetic stages. It is probable that V rogeri developed a stable number of head appendages early while the number of trunk appendages increased during ontogeny. The outline of the dorsal shield of the earliest ontogenetic stage is rounded in contrast to that in later stages. A close phylogenetic relationship of V rogeri with the recently described Silurian Xylokorys chledophilia (SIVETER et al. 2007) is proposed. Both taxa share a dorsal shield covering the full body. Additionally, they have the same number of head appendages and a very similar structure of the appendages in the head and trunk region. Both form the sister group of a taxon including Marrella splendens from the Cambrian, Mimetaster hexagonalis from the Devonian and presumably species of Furca so far described from the Ordovician. The monophyly of the often debated Marrellomorpha is confirmed based on several synapomorphies. The Marrellomorpha are interpreted as early stem lineage representatives of the Euarthropoda. The low degree of cephalisation of the Marrellomorpha is comparable to other early, stem Euarthropoda.
