Coaptative devices, enrollment, and life habits in Paralejurus, a particular case in scutelluid trilobites

Abstract

The first completely enrolled adult scutelluids assigned to the Paralejurus brongniarti group (Pragian) and to Paralejurus rehamnanus (Emsian) from southern Morocco are described. The Pragian taxon exhibits volvation in the sphaeroidal mode and a simple morphological configuration of the external thoracic pleurae with edge to edge fit between each other and against both the posterior edge of the genae and the anterior edge of the pygidium, respectively. Evolutionary trends towards perfect tightness of enrollment and the avoidance of lateral shearing are obvious in later Moroccan Paralejurus such as P. rehamnanus and allies from the Lower/Middle Devonian transition. In these forms, coaptative devices, such as submarginal grooves on the posterior edges of the genae and external thoracic pleurae, ventral fulcral processes and sockets, as well as bulging of the anterolateral pygidial doublures and corresponding depressions in genal doublures, were developed. The possible role of terrace ridges to assist in coaptation and/or to obtain hermetic closure of articulation gaps during volvation is discussed. Morphological and behavioural traits point to a semi-endobenthic lifestyle of Paralejurus with enrollment being an exceptional defence strategy against predatory attack. These traits that differentiate Paralejurus from all other scutelluid taxa are regarded as ecological rather than phylogenetical in origin. Consequently, the assignment of Paralejurus to the subfamily “Paralejurinae” within the Scutelluidae is favoured.