A new aquatic crustacean is described from the Early Devonian Rhynie chert. The small, broadly oviform carapace consists of a univalve symmetrical about its longitudinal axis, formed by a continuous cuticle with no fold or hinge. In transverse cross-section, the ventral surface exhibits a broadly 'W'-shaped outline. Areas of fine, setose objects are preserved, positioned antero-ventrally on either side of the mid-line axis. In transverse sections, a ring-shaped internal structure is also visible. In longitudinal cross-section, this structure is resolved into an axially positioned, internally subdivided tube occupying the mid- and posterior part of the carapace. A small, anteriorly positioned rostrum has a deep pit on each side, perhaps indicative of the socket of an antennal appendage. The arthropods are commonly found clustered together around plant axes, comprising groups of up to 25 or more similarly sized individuals. The chert texture enclosing many individual specimens indicates a sub-aqueous preservational environment. The same organism has recently been discovered in preparations from the nearby and equivalently aged Windyfield hot spring deposit. The morphology of this new arthropod suggests affinity with Branchiopoda and it is tentatively placed within the Diplostraca (an order which contains both 'conchostracans' and 'cladocerans', or water fleas). The presence of a univalve test suggests a cladoceran affinity, but the lack of diagnostic appendages in the present material does not allow us to take the classification any further. This is potentially the earliest known occurrence of the group and would extend their fossil record back from the Early Cretaceous. The remarkable similarity between these arthropods and their present-day representatives strongly indicates morphological stasis within the group from early on in their evolutionary history.
