External morphology of Lightiella monniotae (Crustacea, Cephalocarida) in the light of Cambrian ‘Orsten’ crustaceans

The species-poor meiofaunal Cephalocarida have played an important role in discussions of the phylogeny and evolution of Crustacea since their discovery in 1955. One reason may be that the morphology of cephalocarids includes some aspects of putatively ancient appearance, such as the simple roof-shaped head shield, the anterior three head appendages resembling those of a nauplius larva, or the trunk-limb-like second maxilla. Cephalocarida have even been suggested to represent the sister taxon to all other Eucrustacea. Presence of possibly plesiomorphic characters, however, does not necessarily point to a basal position in the system. Growing evidence demonstrates that the modification of the fourth post-antennular cephalic appendage, the ‘maxilla’, into a “mouth part” may have occurred independently in the different eucrustacean lineages, so a trunk-limb-like maxilla is an ancient feature that does not hold only for cephalocarids. Retention of its plesiomorphic shape and function in the Cephalocarida remains, however, noteworthy. Cephalocarids are still little studied and incompletely known, especially their external morphology. By examining several adults and one young specimen of Lightiella monniotae Cals and Delamare Deboutteville, 1970 from New Caledonia, we aimed to a) document as many details as possible, and b) compare these data with other species of Cephalocarida. We also aimed to reconstruct aspects of the ground pattern of Cephalocarida, which is a pre-requisite for any comparisons in a broader perspective of crustacean phylogeny. Among the new findings or conclusions are: (1) Lightiella is in need of a revision since several assumed differences between the species are questionable or subject to intra-specific variability; (2) the cuticle of the trunk-limb basipod is sub-divided into a number of smaller sclerotized areas as in various exceptionally 3D preserved fossil crustaceans from Cambrian ‘Orsten’ faunal assemblages; (3) a small transitional portion on the post-maxillulary limbs in the area where the endopod and basipod connect is discussed as either a reduced, proximal endopod segment or as an evolutionary new joint of the basipod to enhance its flexibility; (4) the so-called pseud-epipod is interpreted as an outer branch of the exopod; (5) compared to ‘Orsten’ crustaceans many characters of the Cephalocarida are more modified than previously assumed, including the morphology of the trunk-limb basipod, and the unique, ring-shaped appearance of the abdominal segments. Also the development is not as plesiomorphic as sometimes assumed, at least not compared to that of the strictly anamorphic series of the ‘Orsten’ eucrustacean Rehbachiella kinnekullensis. The application of SEM techniques has again proved to be especially appropriate because of the small size of these animals, and because it permits direct comparisons with other similarly small crustaceans and the ‘Orsten’ crustaceans and their larvae.

► External morphology of a cephalocarid (Crustacea) is studied in detail by scanning electron microscopy. ► Homologies of limb parts such as the ‘pseud-epipod’, endopod, and exopod are discussed. ► Cephalocarida are compared to similarly looking Cambrian ‘Orsten’ crustaceans. ► Cephalocarida are with respect to ontogeny and morphology of thoracopods and abdomen more derived than previously assumed.