Ghosts from the past - ancestral features reflected in the jaw ontogeny of the polychaetous annelids Marphysa fauchaldi (Eunicidae) and Diopatra aciculata (Onuphidae)

Species of Eunicida (polychaetous annelids) have complex jaw apparatuses with ventral mandibles growing throughout life, but dorsal maxillae undergoing successive moulting episodes. Jaw ontogeny was studied in the eunicid Marphysa fauchaldi, from the appearance of the mandibles and larval maxillae through two moults during the transformation to juvenile and sub-adult maxillae. Although larval maxillae have been previously known, this is the first description of the juvenile stage for Eunicidae. The immature maxillary stages of M. fauchaldi were compared with those of Diopatra aciculata of the sister family Onuphidae, and, in a novel approach, extended deep into the fossil record by searching among adult stages of extinct eunicidans. Although neither the larval nor the juvenile stage resembles any complete fossil apparatus, each stage has features of separate maxillae known from different geological times. The larval maxilla II of both species is similar to the jaws of the Early Ordovician xanioprinids, the juvenile maxilla I of D. aciculata strikingly resembles those of the adult Palaeozoic Atraktoprion, whereas that of M. fauchaldi resembles those of Brochosogenys and allies. Thus, the extant taxa analysed represent cases in which ontogeny recapitulates phylogeny. It may also suggest that onuphid jaws are closer to the ancestral state than those of eunicids. This study opens a new perspective on evolutionary patterns, discusses asymmetry in jaw apparatuses relating to the basal plate concept, the apparent absence of early ontogenetic stages from the fossil record and cautions the use of architectural types in the interpretation of fossil maxillary apparatuses.