Difficulties with the interpretation
When interpreting the fossil as a larval form of a barnacle or one of its relatives, three possible aspects need to be discussed:
The fossil is comparably large, at least for a nauplius, as most eucrustacean nauplii are rather small. Nauplii of representatives of Cirripedia are mostly in a size range between 200 µm and 1 mm (Walossek et al., 1996; Walker, 2001; Høeg et al., 2004). Yet, also nauplii reaching astonishing sizes have been reported (Rybakov et al., 2003). In fact, shield sizes well over 1 mm seem not to be uncommon among modern forms (Fig. 2), resulting in total lengths of about 6 mm in Lepas anatifera (Moyse, 1987) or in Lepas pacifica (Ryusuke Kado, unpublished data).
The only fossil example of a possible cirripede larva is that of Rhamphoverritor reduncus (Briggs et al., 2005; see also further below). This larval specimen is not a nauplius, but may represent a settling stage, a so-called cypris, hence the stage following the last nauplius stage. Among modern forms the lengths of cypris larvae are difficult to infer from the literature. The fossil cypris has a total length of 4 mm.
Crustaceans usually increase their size by up to 30 % within a single molt (see discussion in Kutschera et al., 2012). The largest known cirripede eggs can reach up to 400 µm (Korn et al., 2004). All extant representatives of Cirripedia develop through at most six naupliar stages (nauplius I – nauplius VI; Høeg et al., 2015). By calculating this example, the possible maximum size of a nauplius VI would result in an overall size of about 2 mm.
However, the 30% rule seems to be less strict in certain crustaceans. The size increase between nauplius I and nauplius II in e.g. Lepas pectinata, is in average 150% (Moyse, 1987). Consequently, nauplius VI could reach overall lengths of more than 7 mm. Taking this into account, a shield length of 4.7 mm in the fossil specimen described herein is quite reasonable (but see also further below).
2) Position of the fronto-lateral horns:
In most cirripede nauplii the fronto-lateral horns arise right from the fronto-lateral corners of the shield (Fig. 2 B–C, E). This seems not to be the case in the fossil specimen. Here the shield rim is further drawn out, forming a set-off ring. Interpreting the horns differently is difficult, other possible structures such as frontal filaments, which occur within Thecostraca in all representatives (Walker, 1974; Grygier, 1987), are tiny and soft and hence unlikely to be preserved in a fossil. Also they are not horn like. In some naupliar stages, e.g. of the rhizocephalan Peltogaster paguri, the fronto-lateral horns are fully covered by a round extension of the shield (Fig. 2D; Høeg, unpublished data). These structures in the fossil specimen described herein are blunt at the tip and might therefore end in a pore as do true fronto-lateral horns. This observation supports the interpretation of the spine-like extensions as fronto-lateral horns and not as frontal filaments.
3) Interpretation of the set-off ring:
Examples of extensions of the shield, so-called ‘floating collars’, occur in some ingroups of Cirripedia, more precisely of Rhizocephala (exclusively parasitic forms). Such a floating collar has been considered as floatation device, enhancing the buoyancy of the nauplii (Veillet, 1943; Høeg et al., 2004). Such a type of floating collar (Fig. 2D) is known from the rhizocephalan ingroups Peltogastridae and Lernaeodiscidae, but could be part of the rhizocephalan ground pattern (Høeg et al., 2004; Glenner and Hebsgaard, 2006; Høeg et al., 2009).
The floating collar in rhizocephalans is shed separately from the rest of the cuticle and is made of exceedingly thin cuticle (Fig. 2D; Høeg et al., 2004). This seems to be quite different in the fossil specimen. Also in the fossil the possible floating collar seems to be positioned under the horns, while in modern forms it is over these. Still the structure and position of the ring in the fossil could still indicate an at least comparable function in the fossil. It could also be speculated that this could be indicative of a closer relationship to Rhizocephala.