Early crustacean evolution and the appearance of epipodites and gills

Abstract

Epipodites are structures on the outer edges of crustacean appendages serving as gills or for osmoregulation. Their evolutionary origin has been debated for a long time. Three major issues are of relevance: 1) the function of epipodites, 2) their development, and 3) the fossil record. While it has long been a problem to distinguish the gill and osmoregulatory functions of epipodites histologically, this has recently become possible based on ultrastructure. A respiratory function has particularly been claimed for the limbs or parts of limbs of early arthropod fossils. Not only rami and cuticular structures, but also entire appendages, have been referred to as “gills”. Among living taxa, the opisthosomal limbs of limulids are called gills or gill limbs, although the numerous leaf-like gill structures occur only on the posterior side of the exopods. It has long been known that crustacean exopods do not serve a respiratory function, which is restricted to structures along the outer proximal edge of the limbs. Three-dimensionally preserved fossil crustaceans from the ‘Orsten’ have contributed much to our understanding of the evolution and phylogeny of Crustacea, in particular limb evolution. The recently discovered Yicaris dianensis from the Lower Cambrian demonstrates not only the presence but also the morphogenesis of three epipodites on all post-maxillulary appendages. Yicaris dianensis may therefore be a valuable model for understanding the evolutionary origin of epipodites, not least since comparative morphology demonstrates that epipodites show a large plasticity among living eucrustacean taxa and may not even be homologous. Epipodites are discussed here in the light of 1) other putative respiratory and osmoregulatory structures in other ‘Orsten’ taxa, 2) morphological and functional variations of epipodites in living eucrustaceans, and 3) the discovery of two new species of ‘Orsten’ eucrustaceans, also showing evidence of three epipodites. This contribution aims to provide a guide for further investigations on the evolution of crustacean epipodites.