Interpretation of the present case
Cirripede nauplius larvae represent dispersal and growth stages that can last short or long (Høeg et al., 2015). A short larval span is only possible if the larvae find a suitable habitat in close distance to their parents (Buhl-Mortensen and Høeg, 2006). In environments that have a patchily distributed settlement habitat, it is more likely that larger larvae are adapted for long-distance and long-time dispersal as it has been reported for some deep-sea cirripedes (Buhl-Mortensen and Høeg, 2006; Yorisue et al., 2013). The Solnhofen limestone Lagerstätte represents a Jurassic back-reef lagoon (Barthel et al., 1990), where suitable habitats for cirripedes might have been rare and nauplii must have searched for a long time for their settlement site. Additionally, in modern cirripedes, lecithotrophic nauplii are more rounded and larger than planctotrophic nauplii, but show more simple setae and reduced development of the appendages and the labrum (Anderson, 1965; 1987; Høeg et al., 2004). However, the fossil specimen described herein is generally large and rounded, but show at the same time well developed appendages and possibly a well developed labrum (Fig. 1D). Hence, it is likely that the fossil specimen described herein could store lipids and ingest food for its metabolic needs at the same time to survive a long-term dispersal phase. As pointed out above, modern cirripedes seem to be restricted in the number of molts as a nauplius. It seems therefore most likely that the larva represents a case two, i.e. an obligate dispersal larva. This is also in accordance with a supposed floating rim of the shield.
It might be seen as special that we have a highly specialized nauplius larva as the first fossil report of a cirripede nauplius. Yet, it is in overall concordance that we tend to find giant larvae. Moreover, the finding is also important because it provides us a rare look into the Mesozoic plankton of which our knowledge is still very incomplete.