The enigmatic Mesoprosopon triasinum from the Triassic Hallstatt Limestone of Austria, which was once considered to be either a representative of Brachyura or Cycloidea, is re-interpreted herein as a eumalacostracan larva and is the oldest of its type known to date. It shows a mixture of characters that are typical of the zoea stages of certain meiuran ingroups (e.g., Hippidae) or of erichthus-type larvae of stomatopods. Four long spines evidently provided additional buoyancy to counteract the comparatively heavy load of a calcified shield. Additionally, a distinct ventral gape might imply that the animal was able to enrol into a tight ball. Our recognition of specimens of M. triasinum as larval stages, rather than adults, may have a major impact on the re-study of some still poorly known cycloids. In the future, the term "mesoprosopon" may be salvaged as the name of this type of larva. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.